You're using a transistor amp in the base. It's oscillating at low drive levels (carrier condition). As you apply more drive (modulate) it breaks out of the oscillation and the SWR returns to where you would expect even though there is much more power than at a dead key where the SWR is bad. Tune the antenna without the defective amp inline. Send the amp to a qualified tech to have the real problem fixed.
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Astrobeam
- Thread starter RT2020
- Start date
You're using a transistor amp in the base. It's oscillating at low drive levels (carrier condition). As you apply more drive (modulate) it breaks out of the oscillation and the SWR returns to where you would expect even though there is much more power than at a dead key where the SWR is bad. Tune the antenna without the defective amp inline. Send the amp to a qualified tech to have the real problem fixed.
Will that SWR change after moving the antenna from 15 feet to 48 feet? Yes, there will be some change in the SWR. It may or may not be much, and it may even not be a bad change, but you can bet that there will be one. SWR is a measurement of impedance of the antenna system. Since almost any/every thing around the antenna can affect it's input impedance it means that when there's some significant change in the antenna's environment that the impedance of the antenna will also change. That move from 15 to 48 feet is a significant change. So what do you do? If the change isn't much, then you just don't worry about it. If the change in SWR is very much, as in going from maybe a 1.2 - 1.5 to something closer to or over 2:1 then you take it down and re-adjust it, put it back up and check again. Still not good? Up/down till it is acceptable. Lot of trouble? Yep, it can be. Everybody goes through the same thing, it's normal. Didn't say 'nice', but normal.
About that SWR change with an incrfease in mic-gain. You can not read SWR accurately when 'modulating', or if the signal isn't steady/constant. If there's any significant signal change, as when modulation is applied to an AM signal, then the meter is no longer accurate at all. SWR doesn't change with power levels but the accuracy of an SWR meter certainly does. That's why you have to calibrate/set the meter before reading it. The thing about SWR is that it's an impedance balancing thing. If it's off in either direction the meter doesn't know what 'direction' it's off, only that it is off, so that with any imbalance the meter reads higher. There are several things that affect that balance, not just one or two. That meter can't tell you what's causing the imbalance, only that there is one. It's up to you to figure out the 'why' part of it and correct it.
This @#$ is such fun, isn't it??
- 'Doc
About that SWR change with an incrfease in mic-gain. You can not read SWR accurately when 'modulating', or if the signal isn't steady/constant. If there's any significant signal change, as when modulation is applied to an AM signal, then the meter is no longer accurate at all. SWR doesn't change with power levels but the accuracy of an SWR meter certainly does. That's why you have to calibrate/set the meter before reading it. The thing about SWR is that it's an impedance balancing thing. If it's off in either direction the meter doesn't know what 'direction' it's off, only that it is off, so that with any imbalance the meter reads higher. There are several things that affect that balance, not just one or two. That meter can't tell you what's causing the imbalance, only that there is one. It's up to you to figure out the 'why' part of it and correct it.
This @#$ is such fun, isn't it??
- 'Doc
Hey 2020, I am still thinking about your problem with your AstroBeam. I just read an article in ARRL Antenna Book, 13th edition that was written by Dr. Uda, one of the original inventors of the Yagi-Uda beam idea. The article was captioned, "The Three Element Beam," and was under the sub title, "Impedance."
It said the resonant length of the driven element, by itself, is very close to the same length as it would be as a beam...and when the two parasitic elements are tuned properly for maximum gain. He further stated this was due to those two parasitic elements, the director and the reflector, being out of phase with each other, thus canceling some of their effects on the driven element. He also said the two element beam did the same, and this sounds to me like he is making a unique distinction and we should pay attention.
It went on to say, when the match is close to 50 ohms the gain is at or near its minimum, so that would suggest to me that the match should be showing a low resistance for max gain. If this is so, then maybe the tuning harness will work as intended to help raise the match up toward 50 ohms only when the antenna match shows a very low impedance at the feed point. At least this may be something to consider.
This idea maybe a stretch, and Uda was not talking about the A/B, but maybe this principal might still apply. I don't think I've ever read this same idea suggested...regarding multi element beams.
I found another article in a Google search for Yagi-Uda beams, where their white paper report described a three element beam as specifically having a 28 ohm match, plus a balun phase shifter for maximum gain and match. Maybe this is why Avanti included this transforming harness, and maybe this is why the radiator is so short...so it can yield a low impedance. The little A/P radiator is very broadbanded by itself, and maybe Avanti found the driven element had to be shorter in order to lower the resistance and get maximum gain out of the beam, which is probably not so uncommon. See here: http://yagi-uda.com/three_elements_yagi-uda.php
Hey Bob85, if you read this thread, does this idea sound plausible, in light of the fact you have noted that the Avanti engineers didn't know how this beam worked.
2020, when I make my A/B model with a shorter radiator I see the match go from 37 ohms down to 32 ohms, so maybe the AstroBeam does need that short top radiator, and that harness...to raise the match back up closer to 50 ohms.
If this is not the case, what do you think the dual 59u coax is all about? How long is it? Is the cable marked RG59U? Does it look like dual cables, or is it just one piece of coax?
Bob, would dual pieces of RG59u, 75 ohm coax in parallel transform, 28-30 ohms up to about 50? Or, am I thinking wrong again?
2020, I saw you antenna way off in the back ground, but maybe you can give us some close up pictures, including this jumper. I'm still curious about this antenna.
It said the resonant length of the driven element, by itself, is very close to the same length as it would be as a beam...and when the two parasitic elements are tuned properly for maximum gain. He further stated this was due to those two parasitic elements, the director and the reflector, being out of phase with each other, thus canceling some of their effects on the driven element. He also said the two element beam did the same, and this sounds to me like he is making a unique distinction and we should pay attention.
It went on to say, when the match is close to 50 ohms the gain is at or near its minimum, so that would suggest to me that the match should be showing a low resistance for max gain. If this is so, then maybe the tuning harness will work as intended to help raise the match up toward 50 ohms only when the antenna match shows a very low impedance at the feed point. At least this may be something to consider.
This idea maybe a stretch, and Uda was not talking about the A/B, but maybe this principal might still apply. I don't think I've ever read this same idea suggested...regarding multi element beams.
I found another article in a Google search for Yagi-Uda beams, where their white paper report described a three element beam as specifically having a 28 ohm match, plus a balun phase shifter for maximum gain and match. Maybe this is why Avanti included this transforming harness, and maybe this is why the radiator is so short...so it can yield a low impedance. The little A/P radiator is very broadbanded by itself, and maybe Avanti found the driven element had to be shorter in order to lower the resistance and get maximum gain out of the beam, which is probably not so uncommon. See here: http://yagi-uda.com/three_elements_yagi-uda.php
Hey Bob85, if you read this thread, does this idea sound plausible, in light of the fact you have noted that the Avanti engineers didn't know how this beam worked.
2020, when I make my A/B model with a shorter radiator I see the match go from 37 ohms down to 32 ohms, so maybe the AstroBeam does need that short top radiator, and that harness...to raise the match back up closer to 50 ohms.
If this is not the case, what do you think the dual 59u coax is all about? How long is it? Is the cable marked RG59U? Does it look like dual cables, or is it just one piece of coax?
Bob, would dual pieces of RG59u, 75 ohm coax in parallel transform, 28-30 ohms up to about 50? Or, am I thinking wrong again?
2020, I saw you antenna way off in the back ground, but maybe you can give us some close up pictures, including this jumper. I'm still curious about this antenna.
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I might be able to 'bet' on it, but not more than I'd be willing to loose! An oscillating amplifier isn't the only reason that 'drop' in SWR can happen.
- 'Doc
Yes Doc, about the only other thing that could cause this is a loose RF connection that arcs to complete the path under power. Mind you, this scenario is far less reactive to modulation and much more to wind or physical movement and would continue to deteriorate rapidly as it arced. Experience told me he was using a troubled solid state amp and the fact it turns out to be a Texas Star pretty much confirms the oscillation. That's what happens when you power your bias circuit through the same relay contact as the RF output. Extremely poor design that insures a healthy dose of RF will be feedback through the DC bias.
Hey Greg! http://www.worldwidedx.com/amplifiers/145724-texas-star-dx500-high-swr.html <gotproof>
IIRC, you got your 667V around the same time I got my DX500? Maybe a common issue?
73,
RT307
IIRC, you got your 667V around the same time I got my DX500? Maybe a common issue?
73,
RT307
OK I will try to answer all the questions, I apologize if I miss some or do not give the answer your looking for.
Shockwave- the SWR is 1.3 without it.
Marconi- my matching pigtail is RG83U @ 6' long , one single piece of coax male on one end and female on the other, about the size of RG8U coax.
If I get time and weather permitting this weekend I am gonna try the matching harness with the coax going into the shack and see what that does, just looked at my notes (I did take notes this time Marconi....lol) I see I didnt try that with my shack coax just the 18' run.
I will update when I try and thanks.
Shockwave- the SWR is 1.3 without it.
Marconi- my matching pigtail is RG83U @ 6' long , one single piece of coax male on one end and female on the other, about the size of RG8U coax.
If I get time and weather permitting this weekend I am gonna try the matching harness with the coax going into the shack and see what that does, just looked at my notes (I did take notes this time Marconi....lol) I see I didnt try that with my shack coax just the 18' run.
I will update when I try and thanks.
Thanks for the info 2020.
I just found an old thread on another forum: Copper Talk: Info on RG 83/U coax? I was asking questions on this antenna back in 2002, just like today, and I'm still looking.
In one of your posts you say you had 3 of theses and also your friend has one of yours and has good luck, now you have had 4 and they are junk and I should scrap it. I also think I remember you telling me to scrap my AX144. You say I should buy new, well i dont have that kind of cash laying around. Do you go buy a new car when yours breaks down or do you fix it?
If your friend has good luck with the one you gave him tell us how he has his up or what he did to make it work right. I am trying to learn new things here.
That is what I remember about the Astrobeam from way back in the 70's. There were SO many operators with base stations back then, and knocking them down with the backside of the beam made it possible to carry on a QSO. I always heard they were phenomenal for their back signal rejection, and to the sides wasn't bad, either.
73,
RT307
If a 1.3:1 SWR is good enough for you (and it is certainly a good match), I'd say you're antenna tuning is done. The extra 2.5:1 SWR you see with the amp on is the Texas Star amplifier malfunctioning. You will never be able to tune the antenna to show a good match with this amplifier oscillating. Tune the antenna without the amp inline and get the amp fixed as outlined in another thread here.
The fact you have a 1.3:1 without the amp and a 1.3:1 with the amp on under full modulation, confirms it's not the antenna or coax. The problem only shows up when the amp is at low drive in a carrier condition. I also remember many operators used the Astroplane years ago with good results. I'd argue it was the best performing 3 element vertical yagi ever designed for this band.
The fact you have a 1.3:1 without the amp and a 1.3:1 with the amp on under full modulation, confirms it's not the antenna or coax. The problem only shows up when the amp is at low drive in a carrier condition. I also remember many operators used the Astroplane years ago with good results. I'd argue it was the best performing 3 element vertical yagi ever designed for this band.
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