Central States VHF Society Conference August 17, 1979 Irving, Texas K5CM F2 backscatter If you're fortunate to live on a 5000-foot mountain and have ... pages of JA's and ZL's you probably wont be interested in this, W6XJ! But, if you're like the rest of us and have to do something besides call CQ, this might keep you busy for a while. OK. F2 backscatter has been a little bit of a mystery to me for some years, and I really didn't get into it until this year and into figuring out really what happened. On the map up there we have northeastern Oklahoma to southern California, a typical F2-backscatter circuit which develops quite often in March and April and February. The transmitting signal leaves going out green, you probably didn't know that you had green r.f., and I just show the F layer as a blob there. Of course, everybody knows that it gradually spreads out and the ionization is not necessarily like I show it, but that's as good as any. And the ionization, one important point to remember, the ionization does not have to be continuous between - well, I show a break in it. OK, what I wanted to show was there would be a break, you don't necessarily have to have to have a continuous F-layer ionization between the two backscatter points. Of course, the first most interesting thing to notice is that really, of course, the backscatter does not occur off the F layer. It occurs off the ocean. Probably the rougher the ocean is probably the better the backscatter is. As the waves lap up, the signals get reflected in several directions all the way back to the transmitter, so you can have a backscatter circuit in lots of different directions. In this particular one we have between southern California and Oklahoma would be about 12 or 1300 miles so the angle is very sharp right here. But you could have a backscatter circuit between here and South America, however, there would have to also be another 50-MHz "blob" over here to support that backscatter circuit. It gets rather complex on what's really going on when you're talking to a station that's way off of the ... path. OK, what I want to show next are some slides of backscatter radar. The legality of this is probably legal because I don't do anything rather than transmit with a normal transmitter, with a normal key-click filter, and it's not really pulse - it's really bursts. So, I'm to expect the first question you'd ask so I was going to answer that before you asked it. But, we'll discuss that ... OK, you have to excuse the ... I was more interested in 6-m at the time. OK, the leading burst here will be first. This is a 5 msec/cm and it's from a Techtronix 454. And, I didn't realize how ... it doesn't look this bad on the Polaroid picture, but when you look at it in the slides it is bright. OK, the leading edge is the transmitted signal lasting about 5-6 msec. OK, then this is the echo off of the F layer. If you count the time divisions approximately 30 msec before the first return occurs. Now, ionospheric sounders or backscatter radars have been around for a long time, but most of them have sounded straight up, and it doesn't do a whole of good for us to know what is going on straight up, and it only measures the critical frequency - this measures the absolute MUF. This was taken on March 4th, I believe we had quite good backscatter that day. So you can see that the strength of the signal is actually pretty good; if you were to actually listen to that signal it would probably move the s-meter 2 or 3 units. And what I'm trying to get across here is not necessarily go home and buy a $5000 scope and do this. It only takes a $200 audio scope. Nothing I've showed here so far is r.f.; this is all audio. As a matter of fact, about two stages before the final audio amplifier in the SB-110. However, even without a scope it is still possible to do this, even with a really simple station. All you have to do is to disable the avc or agc, turn the transmitter on continuously, disable the sidetone, and when you turn the transmitter off, if your relays switch fast enough, you will actually hear the return signal. Many of you maybe have tried this by now. It's quiet easy on 10-meters, in particular. With the newer transceivers you don't need a $5000 scope, you just simply turn the transmitter on and you can actually hear the return signal. And in many cases the return signal will be more complicated than that and will last longer. So, you know, rather than sitting around calling CQ all the time and trying to figure out which way to point your antenna, you can simply sweep your antenna from side to side while listening for the little "blip" to come back. It's not quite like a moonbounce echo, but it's there. OK, the distance here is 186 miles/msec, so the total distance would be about 6,000 miles, which would indicate, of course, the signal goes up, hits the F layer, goes down, hits the ocean, comes back, hits the F layer, comes back to your transmitter for a total of 6,000 miles. There is a slight angle there that you have to calculate, but it's close enough. OK, next. OK, the signal is starting to get a little stronger now. The band is open, this at 1803, 210-deg from Oklahoma, there is backscatter to southern California and to a lot of the southern states as I see this. I might add ... slide, when I would first start hearing the backscatter echoes usually no one would be on the band. Usually everyone would be on 10-meters waiting for someone to say that band's open. I would get on 10-meters and I'd say, "Gary, the band's open. Let's go make a contact.", and probably 5 or 6 times of the times we tried it we made a contact in 5 minutes. And before that no one was on the band. So it does indicate that the band was open and immediately when the band is open. The cut-off is very sharp also. The band will be open at 50, not be open at 50.110. I usually sometimes start at 50 rather than 50.110 as it gives you a little bit of a head start. If the MUF's coming up fast then it doesn't make any difference if at 50 or 50.1. It will go up very quickly, and usually seldom have I seen it go above, at least at my place, 51. Seldom did I get good echoes above 51. But occasionally it does happen. Because the break-off is very sharp, that amazing to me that the break-off would be so sharp within 100 kHz from receiving a good strong echo like that one to receiving nothing just 100 kHz away. OK, so the signal's a little stronger there. Next slide. OK, just another typical picture, if I could show these in order I was trying to show that the distance was changing a little bit here. Can we go back to No. 14, and we'll go thru them pretty quick here and maybe see the difference. OK, you observe where the signal starts on the cm-marker. OK, the next now. Now, OK, the next one. Now I've got them out of order. OK, the next one. OK, you see that it's starting to move away, getting further away. Now, that's a multi-sweep. I let the scope sweep several times to build up a good signal. I simply opened the shutter on the camera and the scope go. Well, that also gives you an indication of the fade rate. The rate's pretty high right there. From one sweep to the next it'll fade several db. All of there that I've shown you so far have been ... to my one-hop F2. OK, the next one. OK, here is an interesting day. It started off very boring. This is 2:16 pm ... 165-deg toward South America. I've changed the time scale. This is now 10 msec/cm, there is NO 1st-hop echo, which was interesting for a while. When the MUF is below 50 at the first-hop point, which would normally be right there, it will not necessarily, of course, return to earth. However, the rays will still be bent, and they will return to the ionosphere somewhere else. In this case they just barely bent them enough to keep them in to, to return to the ionosphere at the 2nd-hop distance away, 6,000 miles away, and at that point it was strong enough to go ahead and return it to the ocean, and the path coming back was the same. So I'm starting to see a little bit of this 2nd-hop echo, no 1st-hop; remember my time scale has changed from the pictures I was showing you. If we kept it at the same time scale this would be over there, off the sweep of the scope, which faked me out for a while. OK. OK, as we continue on we're starting to see a little bit of the 1st-hop, the 2nd's really looking good. OK, this is only 2 minutes later. Still beaming 165-deg, and we're starting to look good right now ... so now is the time to start thinking, "Well, maybe it's about open." OK. OK, really looking good now. This is about another 2 minutes later. OK, still beaming 165-deg, which is not too far off from Oklahoma to South America. Sam was doing the transmitting for me here. The strong burst is his transmit signal, and when I saw this start coming back I said, "Sam, the band's open, call 'CQ'"". His first answer was, what? LU3DCA? EX? And all this happened before the band was noticed to be open by anyone. The first contact made actually predicted that the band was open and where it was open to. This doesn't always happen. OK, so then, even without a $5000 scope you can't really get the resolution you can get here by listening to it by ear, but at least you can tell which way it is open and where the strongest echo is coming from. Sometimes you'll get multi-peaks. Then you'll get faked-out on this when you're working F2-backscatter stations because both of you peaking your antenna and not really necessarily on the right peak. With the backscatter sounding you can pick out the strongest ones. Of course, many times when there's no 1st-hop, there's a good part of the days when no 1st-hop appears, and the backscatter is rather difficult then because you're dealing with signals that have traveled quite a bit further, and they're quite a bit weaker, and due to this I've noticed that only the higher-power stations can make backscatter contacts on those days. There is one day in particular I remember Gary was there. We were on 10-meters and noticed the long echo - no 1st-hop, and Gary was playing my signal back thru 10-meters, and the delay was so bad that listening to myself I couldn't send. OK, what do we have next? OK, now this starts to be pictures of when I'm actually transmitting myself. I'm actually turning my transmitter on with a short burst - not a pulse - right here, and this is the time it takes the relays to recover - at 10 msec/cm, and this is NOT a 6-meter picture. If this was like this on 6-meters we'd be working half way around the world. You can see about how many 1st returns and 2nd returns, some more returns -- it's hard to figure out where they all come from. Most likely sporadic E has something to do with it, especially at these frequencies. Echoes are too close together in some cases to be 2nd-hop F layer, so I assume they are sporadic E. OK, at now even later we have ... echo here and this runs out almost 100 msec at 186 mi/msec ... this is 15-meters at 235-deg running a TS-820 transmitting whatever output it transmits with into a TH-6. So it doesn't take a lot of power. The echoes are good with low power. If you start switching high-power amplifier relays the switching time kills you right here. The TS-820 and some of the other newer rigs will switch fast enough. Unfortunately, because of this problem it is difficult to observe sporadic E yourself. You have to have someone to transmit for you. Back up the slide. OK, here there's no dead time simply because as soon as the transmit station quits transmitting you start listening immediately. As long as the pulse that you're transmitting is short enough and doesn't spill into the return echo, you can see the sporadic E, and I have seen the sporadic-E echoes, though they're few and far between. Sporadic-E echoes are hard to find. I've listened and seldom do I hear them. I've even listened on 2 meters, but I really didn't expect to hear them there since I don't hear them on 6 very often. I'm not real sure why this is except for the fact the clouds may be small, your attenuation ... might make a comment on this later. The clouds are smaller, there's more attenuation than you have at F layer where the F layer is usually very large, and when the F layer ionizes you've got, you know, several hundred square miles up there to bounce your signal off of - that's not the case in sporadic E, if I understand ... then ... So, you can't definitely use this method for sporadic E simply because the opening may not be intense enough to give you the backscatter echoes, there may be too much attenuation on the path at any one time, and, also, it greatly helps to have the sporadic E occur over where the backscatter will hit the water. The water is a very good reflector. The earth doesn't make a very good reflector, the earth itself but compared to water. OK, I've just got some more pictures of ... here's another, let's see the first F-layer backscatter, and this one is close enough I would assume that it's sporadic E. I'm not really sure of that, however. There's no way to really be sure. Then you have more multi-echoes ... thru here. It really turns into a mess when you start looking at it at a frequency that is not near the MUF. The pictures on 6-meters are near the MUF. You get a nice clean picture, and you can almost say what's happening. Some of these pictures are not that way. After you see that first one you know it's an F-layer echo, that 2nd one you think is sporadic E getting back into it, but you're not sure. Again, all you have to do is listen for these - you'll hear these just right out of the speaker if you disable the avc. It will sound simply like a little "blurp" or just a ping. And you can verify that your receiver is not oscillating simply by tuning the receiver while the transmitter is on the same frequency and tuning the r.i.t., whatever you have. OK, this is some more 6-meter stuff. Nothing in particular; we'll just run thru these. Go ahead; just run thru them all. Here's another that shows 10-meters, OK. Here's one that shows my transmit burst - see the nice rise time on it. It doesn't come straight up - I keep emphasizing that -- we're awfully close to the FCC here! Again, you can see the transmit signal in the case of the scope I'm using here, you wont be able to do this with a cheaper scope; this is the actual r.f. right here. I simply put an antenna on the scope and it picks up the r.f. and also picks up the audio coming out of the speaker then. You start to see some spreading of the signal also, you see the burst is very narrow and very clean, some of the later returns are spread pretty bad so that the signal starts smearing and echoing I was just trying to show you how you can tell if the band is open rather than sitting there calling CQ all day, and also that the ... W3IWI: I think that we've all observed ... kind of backscatter that the later echoes from ... the ionosphere and so forth tend to spread out with Doppler. Have you tried moving the r.i.t. around to see where you end up accentuating on with the 2nd and 3rd and 4th hops ... up ... K5CM: All of these pictures have been with just the normal 3-kHz bandwidth. Are you saying that you think that the signal has shifted in frequency also? W3IWI: You will get some Doppler which will be ... since each one of these clouds is sitting there moving sort of passes along the added little bit of Doppler to the next one, and the clouds are moving so you'll get a Doppler shift from it. K5CM: It's not very bad though. It's not like sporadic E. You know if you listen to WWV or listen to any low-band signal that's transmitted you can measure the frequency of it, you know, what? 10 to the 7? W3IWI: But, I might suggest it might be interesting for you to ... seeing the 2nd, 3rd, or 4th hops coming in some way, flick on the r.i.t. and move it around a little but inside your ... and eventually you'll see if the ... hops starts to pick up off frequency. K5CM: All right, that could be. I'm still not convinced I could hear it, but ... W3IWI: You might have to ... an earphone might not hear it, but I bet you could see it. K5CM: OK. W3XO: Connie, another thought of something that I've noticed particularly in the last few days that you might try on 2-meters. People don't talk very much about backscatter or whatever it is from meteors. But, I had the occasion the other morning to want to find out if WB2DIN/8 worked WA4CQG, so I turned the antenna down toward Dale and went off to breakfast, and I could barely hear Bob in there off the side of the antenna. And along came a meteor burst, and I started hearing both Dale and Bob very well. Bob is like 50-75 miles to the northwest of me. So it shows the possibility of some pretty good returns that either is specular reflections from the meteor trail itself or it's backscatter. I suspect it's the former, by measuring the time delay ... K5CM: You mean coming directly off the meteor itself, not coming back to the earth first? W3XO: Yes. That's what I think it is. But that would be speculation on my part. K5CM: Well, that very well could be since meteors sometimes support 432. W3XO: Right. Just another interesting thing to measure, however. K5CM: Yeah, you would have to be patient to do that it's true! W3XO: A very good shower, you know, you might do it. ?: Connie, when you returned ... triangle ... to that. K5CM: Fading. The echoes fade very fast. The multi-sweep I had on there a while ago showed how bad the fade rate was. Sweeping you know at 100 msec/cm, in a 100 msec the signal would fade several db. Sometimes you see fading even on the signal, on the burst. ?: Connie, during the high solar flux times ... K5CM: I've never seen it on 50 Megs like it is on 10-meters, where you can get all sorts of garbage like we showed on the pictures. I've not seen that on 6-meters. I have been different places I could point the antenna and get good returns, good strong returns completely different areas, but not like I've seen on 10. When you're near the MUF it's pretty clear. ?: (inaudible) K5CM: OK, I'm simply looking at the detected audio. Now on some of the pictures of 10-meters when I was doing the transmitting I'm actually looking at the transmit burst at 28 MHz simply because I have an antenna on the scope as well as having it connected to the speaker leads or to the 2nd or 3rd stage in the receiver. So you do not have to have an expensive scope at all to do this. ?: (inaudible) K5CM: OK, well, a storage scope would even be more interesting. OK, I'm using a 454. ?: Are you using a single burst or do you use multiple bursts? K5CM: Well, of course, the pictures here were taken, well most of them as they appear on the photographs, were simply a burst. However, the transmitter is running continuously, and I simply set the shutter speed on the camera or either single-sweep the scope. In the case of all the 10-meter pictures and a lot of the 6-meter stuff, I simply start transmitting, get ready to take the picture, open the shutter, put the scope on single-sweep, and you get one clean sweep. ?: What kind of repetition rate did you use? K5CM: Every 100 msec. You can run slower, but you know you don't get as good a picture. ?: You're using a keyer, aren't you? K5CM: Yeah. In the case of the TS-820, if you have a TS-820, the easiest way to do this when you go home and try it, you disconnect the key jack from the key, put it in the cw position, make sure there is no key plugged into the jack at all, connect your keyer up to the push-to-talk lines - don't connect it to the key jack - and that lets your keyer turn on the push-to-talk lines so that the transmitter comes on immediately, and there's no sidetone that way, so you don't get confused hearing the sidetone. Or, if you really want to do it simple, you simply pull the key out, put it in the cw position, push the send switch, turn the send switch off, and you'll hear it. W6XJ: (inaudible) K5CM: The 600, doesn't it? OK. But do you switch relays also, or just the TS-600? W6XJ: Put the TS-600 on cw with key hold down, push down the push-to-talk button, let loose, echoes up to 52 Megs. Driving the amplifier. W3XO: Up on Palomar that is! K5CM: You are switching relays in the amp, too. OK. That's taking quite a bit of time. ?: One way you save that, Connie, is you put up a separate receive antenna. That's essentially what you'll see. K5CM: Yes, that's true. Is that all? Thank you. ============================================================================== Taped and transcribed by Pat Dyer, WA5IYX.