[Cover of Sullivan's book 2009, Cosmic Noise]
Sullivan's Cosmic Noise, Cambridge University Press, 2009


NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with John V. Evans
At Helsinki URSI meeting
7 August 1978
Interview time: 55 minutes
Originally transcribed by Pamela M. Jernegan (1979), retyped to digitize by Candice Waller (2016)

Note: The interview listed below was originally transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009). The original transcription was retyped to digitize in 2016, then reviewed, edited/corrected, and posted to the Web in 2016 by Ellen N. Bouton. Places where we are uncertain about what was said are indicated with parentheses and question mark (?).

We are grateful for the 2011 Herbert C. Pollock Award from Dudley Observatory which funded digitization of the original cassette tapes, and for a 2012 grant from American Institute of Physics, Center for the History of Physics, which funded the work of posting these interviews to the Web. Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event.

Click start to listen to the audio for tape 110B of the 1978 interview.

Begin Tape 110B

Sullivan

This is talking with John Evans on 7 August 1978 at Helinski at the URSI meeting. Could you tell me, first of all, what your educational background was and how you first came in contact with radar or radio astronomy?

Evans

Yes. In 1954, I went to, no, 1951 it would be, went to the University of Manchester into the Physics Department working for, what in England would be called, an Honors degree in physics. That was awarded to me in 1954, three years later. At that time in Britain, there were a number of scholarships available to people to stay on go for PhDs and it was virtually unknown for people to do this without receiving one of these scholarships. You really have to have a fair amount of independent means to do that. In our particular department about eight of these, just (?) and the graduating class was about eighty. So they interviewed the top ten in the class, gave out eight, and I well remember this because I wasn’t near the top of the ten, so I was one of the doubtful ones – whether I’d get one or not. I was quite anxious. An interviewing committee consisting of professors of the department, which were about four or five, there was Lovell, who was head of radio astronomy, and there was (?) who was head of astronomy and there were others, there was the head of theoretical physics, another the head of cosmic ray physics group and so on. And they asked me what I’d like to do and I said, “Well, I wanted to go to Jodrell Bank and do radio astronomy.” And that was fine, I explained my reasons – that I’d specialized in doing electronics in my last year. Our particular system there was specializing in the third year. So I’d really spent my third year in the electrical engineering department. Well, they said, “What’s your second choice?” And I said I didn’t have a second choice, if I couldn’t have astronomy, I didn’t want it. Now, I’m not sure this is a wise strategy or not. But I turned out to be wise, Lovell was looking for somebody who claimed to know something about electronics. Moon radar had just been built, Murray and Hargreaves had both left, so there was this huge room full of equipment – he thought he’d never run again, nobody would ever get it to work again.

Sullivan

Could you tell me just what you do know, since I’ve talked to neither of them, about what they did?

Evans

Yes. Murray was an older guy, he’d been in the Navy, anyway, he’d done his national service, which in those days meant two years’ service, and his was the first successful attempt to get moon echoes. There had been prior attempts by Bromley (?) and others at Jodrell, but without success. The real lack was of an adequate sized antenna. Murray built an antenna which only allowed it to move for an hour or so, a fixed due south pointing, a phased ray really. Just in the phasing of it, you could set the elevation, but you could only see moon at transit, meridian transit. And this gave him the sensitivity to detect moon echoes fairly reliably.

Sullivan

Because he didn’t have to physically move the whole antenna, you mean?

Evans

It was small enough, though a big enough array. It was close to the ground, it was actually an (?long rating) Anyway, this -

Sullivan

What was Murray’s first name?

Evans

Well, we all called him Sandy. His initials were W.A.S., what they stand for, I don’t know. But his nickname was Sandy – Sandy Murray. One amusing story. Sandy was pretty uptight; he’d been struggling along to get this moon echo, and building this antenna was sort of the big last throw, you understand. He really did a nice job, but he was building all of the antenna himself, with the help of the technicians, it was all open wire transmission lines, (?) copper wire. And it was a complicated phasing network, sliding sort of trombone systems to adjust the phase.

Sullivan

What sort of frequency was this?

Evans

This was 120 megacycles. And fortunately, while Sandy was building the antenna was where a path had cut from one of the labs across to another lab on (?). On several nights, when Sandy came back the next morning, he’d found that some of his wires were broken and he concluded that people were trying to use this path despite the fact that he was building his antenna there. And he got very upset. One lunchtime to the assembled room he sort of stood up and said, “The next bugger that walks through my antenna, I’ll kill him.” Deathly silence. And Tom Kaiser, who was at Jodrell Bank in those days, quietly took Sandy aside and said, “Maybe Jack Frost was doing it” And that, indeed, was the explanation. The construction wire was so tight that it cooled off at night and would snap. (laughter)

Sullivan

Now, you say that there had been efforts before to detect the moon, but were these with antennas especially built for that, or were they just taking the meteor radar and taking a look at the moon?

Evans

No. I think they took the thirty-foot dish.

Sullivan

Oh, the one that Rod Davies used for HI?

Evans

Used (?) transmission lines running out from the moon hut to it. And that was totally inadequate. Anyway -

Sullivan

Murray, you say, actually did get his degree?

Evans

Yes, he got his degree. But, he actually had a nervous breakdown over the whole thing.

Sullivan

And what about this other fellow, Hargreaves?

Evans

Hargreaves was essentially only working for a Master’s degree. We had this system in Manchester where they admitted you and sort of evaluated you, at least that was the way it (?) you were supposed to write a Master’s thesis and then you’re allowed to go for a PhD. And that took two years, no that was done at the end of the first year, I’m sorry. Those people, Lovell felt, were going to make it anyway, he’d tell them not to write a Master’s degree thesis.

Sullivan

I see.

Evans

And he wrote a letter to the head of the department, excusing this guy and he was going on to do a Ph.D. That’s what happened to me. But in Hargreaves’ case, Lovell must have either said, it was either known at the outset to be going for a Master’s degree, or Lovell sort of told him that he didn’t think he was suitable for Ph.D.

Sullivan

I see. So he was assisting Murray.

Evans

He was essentially assisting Murray.

Sullivan

So you came on the scene, now it’s 1956 before you have any publications, of course, but you were a fresh graduate student in 1954 so (?)

Evans

My thesis supervisor was Ian Browne. He came from Oxford and had a little bit of radar experience. He and I were told, by Lovell, essentially to do something with the moon radar.

Sullivan

So Browne was a member of the staff?

Evans

Yes. Browne was a member of the staff, a lecturer. And what Ian thought we should do was measure the electrons in the ionosphere, because Murray had correctly identified the Faraday rotation effect as giving one of the causes of fading. At that time, it was known that moon echoes fade, but the full spectrum of causes was not known, and Murray identified a slow, fading mechanism as being caused by the plane in rotation.

Sullivan

Right.

Evans

(?) rotating in the ionosphere. Now the trouble is that you need to know the number of rotations in the ionosphere, so what we had to do was switch the frequency of the radar to try and observe the change in the polarization and from that deduced how many (?) rotations there would be. And so it was my job as electronics expert to make a radar switch off tube and that really proved very difficult. It was a very narrow band system with lots of frequency multipliers – it wasn’t a, what you really would have like would have been an up-down heterodyne system where you develop both in the receiver and in the transmitter the required frequencies for local oscillators, and then you could shift one or more of these. Well, the receiver was of course a heterodyne, but the transmitter was mostly frequency multiplier. This had two very bad effects. It meant you couldn’t tune it very readily because the multipliers are very, very (?) And also meant that the pulses were quite badly (?) because your amplitude modulator and multiplier was a bad thing to do. So there was a lot of gain lost in the system because of the way it was built.

Sullivan

How far did you want to switch in frequency?

Evans

We wanted to shift about, well, as much as we could really. We would have liked five or ten percent; we ended up shifting plus or minus 1.5%. Fortunately, (?) the square of the frequency, so this gave us something like five percent effect.

Sullivan

So, it seems to me, then, that your whole effort was designed, not so much to study the moon, but the ionosphere, is that correct?

Evans

Well, not entirely. We also wanted to properly identify the (?) fading mechanism which we sort of surmised was due to the multiplicity of scattering (?) on the moon.

Sullivan

What sort of scales, now, do you mean when you say, “rapid and slow?”

Evans

In this particular radar it used to set up (?) roughly every second, and there’d be correlation times of a couple of seconds. In other words, there’d been some correlation between (?) amplitudes, but very little. And then the slow fading would take periods of half an hour or so.

Sullivan

I see.

Evans

To see maximum go to minimum. Well, anyway, based on the data then available, Ian Browne did an analysis of the rapid fading. By studying essentially those correlation functions. Tried to compare that with the fading pattern you would have expected for a moon that had scatters, say, distributed all over it. And if you could see it, it would appear uniformly bright. Or a moon that was actually limb-bright as if it had (?)

Sullivan

(?)

Evans

Yes, that’s right (?) scatters on the limbs and one that was bright in the middle. And he actually made a mistake in his calculations, factor of 2 pi or something (?). And there was no good fit to the data and we sort of, in that Murray-Hargreaves-Evans paper, published rather inconclusive story on the rapid fading.

Sullivan

This is the one in 1956?

Evans

Yes.

Sullivan

By then you had two years’ worth of lunar echoes according to the abstract here.

Evans

All right. A year had been, more or less, Sandy Murray’s work and I’d spent a year rebuilding the radar until it barely got accurate. So, the first thing I did was to rebuild the transmitter to get ten times as much power – and that took some time because I had to do it by myself and I wasn’t very skilled. I’d had a lot of experience making television sets and things, but not in high powered transmitters. And making the receiver more sensible from tens, and so on, so that we increased the sensitivity of the system by a factor of 20, and then threw away a large fraction of that, a factor of five or so, perhaps when I switched off tune, so that the antenna was fairly narrow band too, but we then had a workable system with reliable results when we were switching off tune which is what we couldn’t do with the original system. That took about a year. At the end of that year, I started to get some results on the effect of the switching. And so that paper contains the first results on the effects of the switching and Ian’s analysis of Sandy Murray’s rapid fading echo plus a few that we collected ourselves.

Sullivan

I see.

Evans

But reaching rather inconclusive sort of position on what the actual scattering mechanism was.

Sullivan

The nature of the lunar surface.

Evans

Yes.

Sullivan

Can you tell me how this effort related to the others at Jodrell Bank? I assume the meteor radar people you were probably working closely with, but -

Evans

Totally independent. In fact, each of the groups was really, the meteor people had some common interests, there were several meteor groups, but moon radar was really off totally on the side. It was just regarded as a bloody nuisance because (?)

Sullivan

(?)

Evans

Yes. We had these horribly long pulses with chunks of about thirty millisecond pulses, once every second – it just clobbered everybody else. So they just hated us.

Sullivan

I assume you worked out when you were going to be transmitting versus when people were going to be doing at least critical observations.

Evans

Yes. Anyway, when, I spent, having made these changes in the radar, spent sort of eighteen months or so clutching echoes in the switching frequency mode. Unfortunately, the range of declinations we could cover was limited and so you could only see the moon two weeks out of every lunar month. So, you’d see the moon for one hour a day, for two weeks a month, and the hour a day shifted roughly by an hour a day. So, what I was trying to do was connect records together to try and get the actual diurnal variation of total electron content, and it was difficult.

Sullivan

At least you had your long-term variations also in the ionosphere.

Evans

Yes, well the day to day variations didn’t repeat perfectly, so ambiguities that you didn’t wholly resolve. Nonetheless, it showed that the technique did work and you could, if you followed the moon, gather fairly good total content measurements. Which, in fact, we did, when the 250ft. dish was built some years later. Anyway, I also took a look at the rapid fading problem discovering Ian Browne’s problem was two-ply. I wrote a paper which gave the correct result, namely, that the lumen was very limb-dark – it has a bright spot in the middle that does most of the scattering.

Sullivan

Let’s see, this would be Proceedings of the Physical Society, 1957? “(?) From the Central 10% of the Disc?” [Note added 2016: Evans: The Scattering of Radio Waves by the Moon]

Evans

Yes. That was the paper that set that right. Ian sort of offered to commit hara-kiri when he heard that. (laughter)

Sullivan

Let me also ask about this period of 1954-’57, what other groups were there working in lunar radar that -

Evans

That we knew about, none. Although, it turned out that the Navy had a fairly active group, and what’s the guy’s name, begins with a ‘T’.

Sullivan

Not at the Naval Research Lab?

Evans

Yes.

Sullivan

You mean the fellow using it for communications primarily?

Evans

Yes.

Sullivan

Oh, what is his name, I know who you mean.

Evans

He had built a 220ft. section of a parabola which illuminated obliquely. And had done the brute force experiment, sending a very short pulse and measuring how much it was lengthened. And that’s what we would have like to have done, but we didn’t have the sensitivity. We were obliged to measure from the fading path the distribution of the scattering (?) and infer something about, you know, about whether there was limb brightening or not. In other words, if it were uniformly bright, then you have a wide spectrum because the Doppler broadening is from the whole width of the moon and that would give you a more rapid fading than if you just have scatter in the middle – there was less Doppler broadening in the slower fading. So we were really measuring the transform of the spectrum. We had to go back from the spectrum to the distribution.

Sullivan

Right. Trexler.

Evans

Trexler, yes.

Sullivan

But his work was all classified, you didn’t know about it.

Evans

His work was all classified and we didn’t know about it. Lovell went off to this meeting at URSI in 1957, in Denver, taking with him two of my papers, one on the scattering characteristics of the moon and the second on the total actual content. The second one he never gave, but when he arrived, by this time the U.S. Air Force had been partially supporting our work and the contact was Jules Aarons with AFCRL, and so Lovell probably told Aarons about this. And Aarons, who knew what was going on in the United States, went and told the Navy people and chortled, for the small $10k or so keeping (?) at Jodrell Bank, he had the answer to the problem, but he knew that they were spending a million or so for that answer, and they were about to be presenting at this meeting. Whereupon, the Navy people wired home and got permission to release (?) the results.

Sullivan

I see. So Trexler was actually at that meeting.

Evans

I believe both Trexler and Yaplee were and gave papers releasing some of their information. So Lovell was able to come home and tell me that I was right.

Sullivan

Now, how does one get U.S. Air Force money in England?

Evans

In those days, AFOSR, Air Force Office of Sponsored Research had an office in Brussels and supported a lot of research throughout European community. What had actually happened was that one of our guys, Jerry Hawkins, had come over to the States to work at the Smithsonian on meteors and somehow had come in contact with Jules Aarons. And Jules Aarons had sort of made him his agent and told him that the next time he was back at Jodrell to scout around and see if there was any good work going on that deserved to be supported. And Hawkins came back and persuaded Lovell to accept some money; Lovell was very suspicious, especially when he was dealing with the Americans, he’s rather anti-American – anyway, Hawkins got round him. I put some pressure on, because we could really use the money. We had no money to buy instruments with.

Sullivan

So it wasn’t a matter of you guys looking around for money, it sort of came to you.

Evans

It actually came to us. And it was like $2,000 – useful amount to buy us (?)

Sullivan

But then there was also a lot of NASA money that came into Jodrell later on, so Lovell must have gotten over this.

Evans

Well, he was essentially forced to. The whole business of costs of the telescopes put him in a very awkward position.

End Tape 110B

Click start to listen to the audio for tape 111A of the 1978 interview.

Begin Tape 111A

Sullivan

Continuing with John Evans on 8 August 1978. So you were referring to, I asked you about later on Lovell accepted quite a bit of American money.

Evans

Yes, well, essentially what happened was that the telescope, as it was nearing completion, it was clear that it was a large over-run and was going to be some kind of public scandal. And Lovell was in a very vulnerable position because at least the Public Accounts Committee took the view that he had colluded with, was in collusion with, the architects, the consulting engineers in changing the design. Now what actually had happened was that Husband’s, the consulting engineers, had told Lovell that the change in the design would actually reduce costs. He was probably right, but the combination of strikes and the fact that the thing had taken much longer to build than anybody had anticipated, really ran up the bill. And so Lovell felt, I’m sure, that his neck was on the chopping block. And what really proved to be a savior was the launch of Sputnik, although Lovell didn’t quite realize it at the time. The Sputnik was launched the telescope was essentially all but finished in that the surface was there and the motors were installed, but we didn’t have servo control of it. So you could move it essentially by throwing on the breakers and then pulling them off again – you couldn’t steer it or anything. There was an intense interest in Britain in this because it represented Britain’s only contribution to the space age, which suddenly became banner headlines, you know, overnight. And reporters descended on Jodrell Banks to see what we were doing about space.

Sullivan

Right, what are you doing about it?

Evans

And we were doing nothing. And Lovell sort of said the telescope wasn’t finished and we couldn’t. The consulting engineer, Husband’s, put out that a counter public release saying that it was finished and the only reason the people at Jodrell Bank weren’t doing anything was that their equipment wasn’t ready. This kind of put us on the spot, so we hurriedly connected both the moon radar to it by a very long transmission lines, lost about ten degrees of power, and installed on it a small transportable meteor radar, and this happened, more or less, over the weekend. Sputnik was launched on Friday and the next Wednesday we got echoes with the meteor radar.

Sullivan

I see.

Evans

From the carrier rocket. And about by Friday, a week after it had been launched, we got (?) with the moon radar on the carrier rocket.

Sullivan

But there hadn’t been any urgency before Sputnik and the press and so forth.

Evans

Anyway, the amount of news coverage that all of this generated, served, I think, over a period of that week, convinced Lovell that here was a tool that could be exploited, namely, the press. And British patriotism, and so he kept us working on tracking Sputnik and the carrier rocket, particularly by radar – we were the only group doing it, and kept us in the attention of the press. And would use times when he was talking with the press at Jodrell to complain about the niggardliness of the British government, saying he couldn’t even pay his phone bill to sort of bring public opinion around his side so that when the full extent of the scandal became clear, he was, more or less, in an impregnable position. (laughter) And he never did go to jail.

Sullivan

So Sputnik really was a savior then. So I also suspect that he probably also got to like this mode of dealing with the press and that when the space age kept on going on he was sort of -

Evans

Yes, my view was that he had previously tended to shun press, but he sort of developed almost the qualities of ring-master of a circus – “now for my next trick” kind of thing. And he began to enjoy it – he became essentially in Britain, the spokesman for what was going on in the space age, and every few months or so say whether the British, sorry, the Russians, or the Americans are first in the race to get to the moon.

Sullivan

And of course, his stunt of intercepting the Russian photographs and so forth. Well, okay, this is straying a little bit. Let me see now, you worked on this lunar radar – are there any other aspects of what you did at Jodrell Bank on lunar radar that we should cover?

Evans

Very little. The big thing that we did after the telescope was finished was move the lunar radar – putting it out physically on the telescope – primarily for the satellite tracking work but it did enable us to do the same experiment continuously tracking the moon.

Sullivan

You were doing the satellite tracking via radar?

Evans

Yes. We were the first group probably anywhere to get radar returns from the carrier rocket. And that was really only a stunt, but the implications, the political implications were that this was a, you know, the booster for a war head that had put the satellite into orbit and could you actually track such a thing?

Sullivan

Right and what was its size and so forth.

Evans

Well, anyway, so we did some work. I, with a guy called Nick Taylor, did some work where we would track the moon from moon rise to moon set essentially on many days, and he and I wrote a paper on (?) winter.

Sullivan

Oh yes, in Nature?

Evans

No, it was a paper in JATP [Journal of Atmospheric and Terrestrial Physics]

Sullivan

I don’t have one here with Taylor on that. At least after 1960.

Evans

It was after 1960.

Sullivan

Oh, that’s why it’s not here.

Evans

The actual work was done just before I left and we jointly wrote the paper while I was over here. That was the last, essentially, big piece of work I did there.

Sullivan

By this time this telescope was tracking and you were following.

Evans

Yes, yes.

Sullivan

Following the moon. What about this… well, before we get to Venus, let’s see if there’s anything else in the lunar radar. You had a paper in JATP in 1957 on the Farraday effect and so forth. I assume that, what were you able to actually find about the ionosphere from that?

Evans

Well, the major thing we found out was that the extent of the ionosphere was such that from the bottom side with a swept frequency sound you are only seeing about one third of the total electrons, or one quarter of them. There’s three times as many electrons above the peak of the layer to the lull and this had implications about the actual formation of the F layer. It was, in fact, a, what Ratcliffe chose to call, a ton of (?) type layer, that is a layer formed essentially because the fact that production fell off less rapidly than loss above the F1 peak. The loss in the ionosphere turns out to be something that depends on the abundance of molecular ions like N2 and O2 – they fall off with a scale high which is half that of atomic oxygen.

Sullivan

I see.

Evans

(?) while the production is proportional to the abundance of atomic oxygen. So both the level of peak production which is about 180 kilometers you have loss falling off less rapidly than, no I’ve put it the wrong way around. Production is falling off less rapidly than loss, and so if it were just a chemical equilibrium situation the density would actually grow – it doesn’t because there’s downward diffusion so you actually have a layer in which production loss in double diffusion of the ionization transform its own scale height distribution of the three forces. What we should have been able to recognize, well, what was obvious was that a 3 to 1 ratio (?) was not what you’d expect from even for such a layer, you’d expect about 2 to 1, and that meant the temperature of the particles and the top particle layer had to be fairly high. And we didn’t actually realize what the cause of that we a later (?) determined that, in fact, the electrons are twice as hot as the ions.

Sullivan

I see. Let me ask about the relationship between I notice there was a paper at the Jodrell Bank Symposium in 1955 and at the Paris Symposium in 1958 on your work. What was the relation between the radio and radar astronomers? Was it really a pretty distinct little sub-field or was it just another specialty?

Evans

It was probably viewed as a specialty within radio astronomy at the time. There was the Paris Symposium, I think, had as its first session, radar astronomy – they really didn’t seem to object to being included in the program.

Sullivan

But it was the sort of thing where you were part of the community, it was not a separate thing?

Evans

Yes. The ionospheric work was distinctly separate but the radar astronomy wasn’t.

Sullivan

And at that time, if someone had asked you what are you, what do you do, at a cocktail party, what would you have answered?

Evans

I would have said I was a radar astronomer.

Sullivan

So that term was in vogue?

Evans

Yes, it was in vogue, and… that would have allowed several more moments chatting, explaining what the difference between radar and radio is.

Sullivan

Well, that’s still true, of course. Well, let’s move on beyond the moon. There’s a paper there with Evans and Taylor, 1959, Nature, on a 2½ sigma result for Venus echoes.

Evans

Yes, which proved to be wrong.

Sullivan

But could you tell me about that experiment?

Evans

Yes, well, the other ambition Lovell had besides getting echoes from the moon was to get echoes from Venus, and so after I got my Ph.D. or as I was about to get it, he gave me the job of building a Venus radar. That was a major step, because you needed about another 70 db to get from the moon to Venus. The total round trip path loss, you had a 1-watt signal radiated isotropically from the moon, was something like 240 db and now if you were going to do Venus, was 310 or something db. Picking up 70 db even though we had the big telescope, was a big challenge. We had to go up in frequency to get more antenna gain. We went to 408 megacycles, but we also had to have a high power transmitter. For that we depended on clystron, a homemade clystron the physics department people built from a semi-kind of accelerator. And it was a horrible beast; it was supposed to give us about 100 kilowatts peak power with a 30 millisecond pulse, but it was made of brass and copper and was continuously pumped and the people in the department had managed to sway, I think, it was English Electric to give us a modulator for it, which consisted of an enormous delay line which charged up to something like 5 kiva and this produced a pulse which went into an ordinary big 50 cycles main transformer, stepped up from 5 kv to 60 kv by the whopping 60 kv pulse to this. And we had to put this whole thing up in the top of one of the towers of the telescope. But we only put the clystron up there and we left all of the modulating things down on the ground. We piped the 5 kv pulse up in coax to the pulse transformer which was also up in the tower, and had to bring liquid nitrogen for the vacuum pump up the elevator and then up vertically two flights of steps, it was just really a nightmare. Well, we really had hoped to get echoes in 1957 when the nearest conjunction occurred, and it was totally impossible. (?) group over at Lincoln Lab claimed to get a detection, and so when the next chance came around, we had the high powered radar working, almost, we chose to try and integrate, what you have to do is make a hypothesis about what the total size of the solar system is, the astronomical unit, because we had only a limited range window we had to integrate in the noise to find the echoes. In addition, we had to track those range window along at the rate you think the distance of Venus is changing and you have to tune the receiver in Doppler. Doppler shifts were on the order of 30-40 kilocycles and we had a pulse that only had a width of 30-40 cycles. So we had to precisely tune the receiver off precisely set range gauge and precisely keep each of those adjusted with time.

Sullivan

Right. And the standard values for the AU were considering the extent that -

Evans

The two covalues each of which claimed an accuracy of one part ten to the fourth but differed by one part in ten cubed. Anyway, we chose to adopt the Lincoln estimate.

Sullivan

Based on their 1957 -

Evans

Results, right. And run the experiment having a lot of problems we kept blowing up the, well, we had a problem of a rotary joint for the elevation axis. We had to get the power out and we accomplished this with a, just essentially taking some flexible cable through a hole in the elevation bearing, and there was a coupler there that just wouldn’t stand the power. It kept blowing this coupler out. And we had other problems. The net result was we never got any very long runs in, and if you took what you thought were to be the reliable portions of the runs we did have, and I did them altogether, then you got this 2½ sigma bump at about the right range. And I debated for long whether to believe this was an echo or not. Lovell began to put pressure on me because the Duke of Edinborough was coming or something and he wanted to either tell the Duke that we had or had not – Evans, will you make up your mind. It was rather unfair, actually, and looking back on it, it was the one big mistake I’d made – I sort of caved in and said, “Well, yes, we did, I think we got echoes.”

Sullivan

And the Duke was duly informed?

Evans

Duke was duly informed and so was the press and we had to write up a (?) but it turned out we didn’t, and the length of the work itself was spurious, they had an even larger return of about 5 sigmas, so it wasn’t until -

Sullivan

Now in 1959 they also had a spurious return?

Evans

No. In 1959 they tried again and failed to repeat their results.

Sullivan

Okay. So you were comparing with their 1957 results?

Evans

Yes, yes.

Sullivan

Even though you came out with a significantly smaller signal, you were still under the pressure and so forth -

Evans

Yes, although it was discouraging because our radar was more sensitive than theirs and if they really had it right, we should have seen a much bigger echo. And the fact that at that time, I didn’t know they had failed to repeat their results, but when it became obvious that they hadn’t and noting the disparity of their claimed cross-section and our cross-section, I began to doubt that we really had achieved an echo, long before we disproved it.

Sullivan

Now, you say that you used ephemerous values that agreed with their claim from 1957, now how did that agree with the standard optical -

Evans

It was quite dissimilar.

Sullivan

From either of the two.

Evans

Yes, yes.

Sullivan

I see.

Evans

Ours proved to be the real value when it was finally (?).

Sullivan

But still somewhat different from the 1957 one, since that wasn’t real. Okay. Let’s see. You reported this at an RAS meeting I have here also in an Observatory in December 1959. Obviously it seems that the radar astronomy was integrated in with the traditional astronomers as much as radio at that time.

Evans

Yes.

Sullivan

So at this stage, you’re perking along quite a bit at Jodrell Bank and what was the next step?

Evans

Well, because we’d got this large telescope, Jodrell Bank was kind of a Mecca for visiting Americans of whom, in those days, there were large numbers. And for reasons I never quite understood, Lovell frequently gave me the job of escorting them around. Maybe he thought I was more entertaining than some of the other members of the staff. And I got to know a fair number of people, including the likes of Gordon Pettengill and got an invitation essentially from Vaughn Eshleman to consider going to Stanford, and he arranged under the subterfuge of my going over to an URSI meeting, a Fall U.S. URSI meeting, which was going to be held in San Diego, the chance to go to Stanford and discuss this with him in 1958. That I did, and whilst I was there I also visited the group at Boulder, Colorado headed by Ken (?) who were in the process of starting up their big radar to (?) in Peru.

Sullivan

Ah, yes.

Evans

And indicated to Ken my interest in inchoate scatter, it seemed to be a very powerful technique and something I dearly wanted to get into. And then there was another meeting in the U.S. in the Spring of 1959 on radar astronomy held at MIT, which I went to. I may be getting these confused. The four events, as a result of people coming through Jodrell and result of my going over there to the States a couple of times, I ended up with three job offers, four really, one at Stanford, one at SRI, one at Lincoln Lab, and one at Boulder. And chose the Lincoln one and ended up going there in the summer of 1960.

Sullivan

This meeting at MIT I haven’t heard about before – were there any published proceedings of that?

Evans

I don’t believe so. It was a workshop, it was attended by all the people in the field at the time, and was held at Endicott House which was a sort of house -

Sullivan

Yes, I know about that.

Evans

I’ve got the story wrong. That was in the Fall of 1958, I think.

Sullivan

Still, I’ve never heard about that. I wish there were some proceedings. But now, it must have appeared to you that a future at Jodrell Bank in terms of radar astronomy was not going to be as bright as in the U.S.

Evans

Yes, there were two things that consistently bothered me. One was the unreliability of our transmitter, I mean , the corresponding transmitter at Lincoln was a 2½ megawatt versus our 100 kilowatt transmitter. Their average power was greater than our peak power. Our 150 kilowatts of average power was very flexible, you could adjust the pulse line from 10 microseconds to about 4 milliseconds. And our very long pulses plus the phasing stability within the pulsars meant that we couldn’t do the kinds of experiments we wanted to even if we did know what the right astronomical unit was. Plus, the other thing we lacked was a digital computer. They had used, a general purpose digital computer for doing the integration, and we had a home built integrator which only covered a very small number of range windows. They could cover as many as they wanted. And I consistently campaigned with Lovell, essentially, that we’d have to compete in kind if we were to stay in the race. Either he couldn’t or he wouldn’t accept this; he just felt, I guess, that he was so over extended on the telescope that he didn’t want to try and lay any further claims to public money for the large radar, and sort of kept putting me off with arguments of, “You know, the British have a genius for getting around troubles (?)” kind of thing.

Sullivan

Great amounts of money.

Evans

Unfortunately, there was no simple, you know, the radar equation is there and if you can’t adjust any of the terms just by being clever. So, in fact, the writing was on the wall, we couldn’t persist. And, in fact, that proved to be the case.

Sullivan

Well, I think when you left, the whole effort closed down, didn’t it?

Evans

Well, not immediately. John Thompson, who I’d worked with, carried on and they did succeed in getting Venus echoes with the radar that I had built.

Sullivan

In what year was that?

Evans

In 1960 or 1961, I forget whenever it was, probably 1961. So the radar did work. We told them what the astronomical unit was; we determined it at Lincoln, gave them the value. We’d run all our ephemerides to look at that particular range, found an echo and published it in Nature as if they’d done it themselves, which galled us enormously. Gave us no recognition whatsoever.

Sullivan

Let me ask about the business about integration. Was it all done on-line and there was no first processing?

Evans

Well, that was one thing we tried to change, the first effort was all done on-line and we actually had a voltage to frequency converter which was terribly crude, it was simply consisted of a large amplifier which took the video signal and drove it into an RC network. The capacitor in the RC network was discharged when the voltage got big enough by a little (?) which left. And so you had a variable frequency multi-vibrator, if you will. The number of pulses this produced was roughly proportional to the voltage being applied. It was spread through a number of gates into frequency counters which were these dachatron things (?) little dots going around.

Sullivan

Right.

Evans

Finally, to post office type relay counters, and so we just had something like eight channels of this – eight sort of adjacent range windows. Very, very crude. But before I left, we had taken steps to buy a good tape recorder so that we could tape record the signals along the timing marks and if we failed to get an echo in real time try other hypothesis non-real time.

Sullivan

Another question that occurred to me was that you said you were seriously considering going to Boulder, which would have been purely ionospheric work. So at that stage you were quite willing to do either? You didn’t think of yourself purely as a radar astronomer.

Evans

Yes. I sort of oscillated back and forth on that. Even at, when I came to Lincoln, there was a prospect of doing incoherent scatter because (?) radar had already done some.

Sullivan

I see.

Evans

And so I knew there was that opening, that possibility.

Sullivan

Can you tell me now about the detection at Lincoln, I guess in 1961, it was?

Evans

Yes, well, we essentially repeated the experiment we’d done before but by this time, the Lincoln radar was really shaken down and was running at full power. And in its most sensitive mode, transmitted a four millisecond pulse, something like 15 times a second. There was really a race going on between the Lincoln group and the JPL Group who had an 85 foot dish and the C-W radar at S-band.

Sullivan

And your frequency was?

Evans

Four hundred and forty megacycles.

Sullivan

And this was also an 85 foot V-Millstone?

Evans

84. All right, the JPL people got echoes first, and they had the advantage that it was strictly a Doppler system with no range, so they were only searching in one coordinate; we were searching in range and Doppler. We did have the advantage over the Jodrell Bank people that such a short pulse as the four millisecond pulse meant that the frequency window was 250 cycles. So we were not as sensitive to a misalignment in Doppler as the Jodrell Bank group was with their very long pulse, but we were much more sensitive to the astronomical reaction value of the JPL people were only searching in Doppler, and have a frequency window. So as soon as they got echoes, they saw them, and we learnt them over the telephone that they were seeing something like, I gather, twenty or thirty cycle offset in Doppler.

Sullivan

From the expected?

Evans

From the expected Doppler. They were using a nominal value, some nominal valve for the astronomical unit. And when we finally got echoes, we found, we had an ambiguity, we didn’t know how many pulses there were in the total flight track, so we spent the first couple of days, essentially, repeating the observation but with a different (?) rate to see whether the pulse moved. And from that reduced, very quickly, the astronomical unit, and it agreed with that twenty cycle offset. It supported our interpretation. So we very quickly came up with the value of the astronomical unit. They hadn’t taken their Doppler offset and re-sort of plugged it back in to see what that meant about the astronomical unit at that time. So we cabled both them and the Jodrell Bank people with this new value, which proved to be sort of very close to the actual value.

Sullivan

So, before you heard from JPL, you’d been searching a very wide volume of parameter space?

Evans

Well, we’d been running, more or less, on some nominal estimate which proved to be wrong, and it was only when Venus got close enough that, even though we were off in Doppler by some amount, we could begin to see it, did we actually start to get it.

Sullivan

So that was in the spring of 1961 and then, of course, every Venus conjunction after that, the work continued on.

Evans

Yes, at Lincoln, it gradually transferred over to the big Haystack antenna, which gave us much more sensitivity. One of the exciting things was to explain why the echoes at X-band tend to be weaker than they were at UHF.

Sullivan

Was that discovered in the first season in 1961?

Evans

It was actually discovered with a smaller instrument than the Haystack antenna – no it was discovered much later. It was done around 1964 by a guy called Smith, who used the 60ft. x-band system and got a cross-section which was ten-db less than the Venus cross-section at UHF or s-band.

Sullivan

This is at Lincoln Lab, now?

Evans

Yes. And the moon, of course, showed no effect, it has a cross-section which very slightly, but not significantly (?) frequency and so the first guess, well, the other observation that Smith made was that echoes were very broadened so the thought was that it was actually scatter from the atmosphere. It turned out his Doppler broadening was instrumental, and we showed with the Haystack radar in about 1966 in a couple of experiments, we managed to simultaneously arrange from Millstone and Haystack that showed that the echoes were identical distance and were really scattering from the surface. And the Doppler broadening was not very great at all – it was comparable to what you’d expect from the frequency difference. So we really were scattering from the surface, and then we were forced to believe that the factor of 10 was due to absorption from the atmosphere. And that was one of the key ingredients in recognizing the enormous atmosphere that Venus has, that the only way you could explain a ten db change was to consider something like 100 atmosphere of CO2 and (?)

Sullivan

That’s right, there really wasn’t any value for the atmosphere of Venus. I mean, one would assume it was thick, but you didn’t know if it was ten -

Evans

Well, there was the problem of explaining the temperature variations on the surface of Venus. There were people who wanted to -

Sullivan

On microwave.

Evans

- wanted a greenhouse. But you could probably get the ten atmospheres, you could probably get the greenhouse to work, but when the radar measurements showed this ten-degree change, then that really -

Sullivan

Did that report, in fact, a hundred atmospheres?

Evans

Yes, it did. Hundred atmospheres, a nice round number that it gave you.

Sullivan

Which then was confirmed with the (?) measurements?

Evans

Yes.

Sullivan

What about the retrograde rotation of Venus – where did that fit in?

Evans

Both the first observations at Jodrell, no, at Millstone and JPL, measured the spectrum and one observed the total spectra width was as Venus went by, and we would try and fit this to solutions for the rotation range. And both of them, independently essentially, gave the fact that it was retrograde. I think it was the JPL people who recognized that the actual retrograde motion was very close to the synchronous value, you could get if Venus were locked sort of face the earth every time it went by. I don’t think Millstone recognized that.

Sullivan

But this was quite startling to see it be retrograde, you must have been worried about a missed sign somewhere.

Evans

Well, yes, except that we were willing to accept the results. We couldn’t fit a normal pattern to it, it had the wrong, you know, sort of spectra with Doppler variation could do this (?) time (?) the other way.

Sullivan

Right. Did you find any resistance from the traditional solar system astronomers?

Evans

They tended to ignore us. You know, the astronomy books still came out with Venus having a (?) rotation for a long time afterwards. And, of course, we now know that the clouds go around the planet in about four days and this is what was misleading the optical astronomers.

Sullivan

But what about the slow rotation period – I mean that also was totally unexpected. Once again, not much made of it in terms of -

Evans

Well, it excited people like Tommy Gold. It excited the free-thinkers who began to worry about just what could do that. And when a little while later, Mercury was found to be rotating at one and a half time (?) people recognized that there was no reason for a planet to lock in at exactly synchronous. There are higher harmonics you could lock in to, and so people searched for a harmonic that would lock Venus retrograde, that is, if it started out retrograde and slowed down, it could spin down, perhaps, to some state where it got captured. But it still is a mystery, as far as I know, nobody’s come up with a good explanation of Venus’ rotation. It’s one of the all-time cosmic mysteries.

Sullivan

Okay. Just let me ask one final question and that is, in meteor radar astronomy, which you were not in in the late fifties, but nevertheless, it seems to me that the field pretty much sort of died. I mean, there are still some studies going on, perhaps in communications and ionospheric point of view, but the astronomy end of it sort of died. In your view, what was the reason for this demise?

Evans

Well, the major problems were solved. People at Jodrell and elsewhere had established that meteors were, in fact, of solar origin and were not, you know, interstellar. And they’d established the orbits of most of the intense streams; they’d done a lot of work on establishing the orbits of the random meteors. Similar physics had not been worked out, they didn’t really fully understand the rapid diffusion of meteor trails in those days. But the astronomical aspects had largely been worked out and meteor radar was continued into the sixties partly to study the physics of the problem and partly for, as it is now, for studies of winds in the upper atmosphere. That, today, remains the primary motive for people doing meteor echo work. The experiments I did were using the 440 megacycle radar at Millstone and I did that before I got into this Venus thing I was just describing.

Sullivan

Oh, you mean when you first got to Lincoln?

Evans

When I first got there, that was the first thing I worked on.

Sullivan

I see.

Evans

And essentially showed that this very short wavelength trails appeared to be very short in length. Which implies that they rapidly expand to a fairly large diameter.

Sullivan

At 440 megahertz, did you say?

Evans

Yes. And the one interesting thing we did do, which I don’t think had been done before or since, was actually point the radar up in the direction the meteor would come if it were in a meteor shower, and actually measure number of meteors as it decelerated down through the atmosphere, which would instantaneously get the Doppler shift and see that changing.

Sullivan

I see.

Evans

That’s all written up in JGR about 1963 or 1964.

Sullivan

Okay, well thank you very much. That ends the interview with John Evans on 7th August 1978.

End Tape 111A


Modified on Wednesday, 12-Oct-2016 16:06:03 EDT by Ellen Bouton, Archivist (Questions or feedback)