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Author Topic: Interesting antenna problem  (Read 655 times)
VO1GXG
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Posts: 60




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« on: December 12, 2009, 03:14:27 PM »

Hi all.

My station : 40M dipole, 80M dipole, Cushcraft A3S 20/15/10 beam, Icom IC-718 and a Drake MN-4 tuner.

Problem: I can load up on 40, 80 and 20 no problem but when i load up on 15 and 10 i have either infinite swr or the radio folds back and cuts off the power. If i take the tuner out of line the radio will transmit a 40W carrier on both 15 and 10meters with an swr of between 2:1 and higher. Someone mentioned capacitance in the antennas coils. If this was true wouldn't it effect 20 meter operations? I'm feeding the antenna with RG-8X coax, there is a 1:1 balun made of RG-8X (previous owner made it), the antenna is 30 feet off the ground and the coax is about 35feet long. The ground is about 5 feet long. Any help would be great, this happens no mater the radio, or the tuner.


Thanks 73 VO1GXG
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WB5JEO
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« Reply #1 on: December 12, 2009, 07:44:34 PM »

If I read this right, you're having no problem on 40 and 80 with the dipoles, the beam loads okay on 20, but on 15 and 10, the problem is trying to feed the beam. With the A3S, resonance on 10 and 15 depends on proper function of the traps on the driven element. It's possible to get resonance on 20 without getting any effective trapping to electrically size the element for 10 and 15. (The reflector and director traps can screw up the pattern and gain but shouldn't have this kind of effect if they are wrong.) Do you have any history with this beam? In other words, has it ever worked okay for you?

I have a vague notion that you might have one bad trap, giving you very messed up antenna on 15 and 10. I say this because I suspect it might explain the rather odd difference between using the tuner and not. If it's unbalancing on account of that, you could be fighting an unwinable battle for the tuner. If this was a newly assembled beam, I'd suspect maybe some traps were inadvertently switched, like a trap TB switched with a TC (or more likely something worse, since I don't think a TB mistakenly mounted on the driven element would be so drastic, although the trap hardware makes something worse hard to do), or one or more defective from the factory.

For a quick, no instruments kind of check, I'd bring it down and check for the right traps. And, of course, to make sure all the right tubes were used in the right positions. And if they were the right ones, I'd use the four traps from the reflector on the driven element in place of those on there now and see if I could get something close to resonance on the driven element with the reflector off. If it loads reasonably well, it would be time to check the original driven traps after making an internal visual inspection to make sure there's nothing loose inside and all the inductor connections intact, no insect nest inside, no water inside, etc.

Maybe someone will have something else that sounds likely, but that's what I'd look for. The A3S normally resonates very nicely and needs no tuner.
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VO1GXG
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Posts: 60




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« Reply #2 on: December 12, 2009, 09:00:45 PM »

Thanks for the reply.

This antenna is very old..from the late 70's and has been near salt water for most of its life. I used it on fieldday this year and it worked fine on 15 and 20. I suspect a trap might be bad as well. Could have some kind of salt build up inside as i highly doubt it has been cleaned in the last 30 years. It started acting up only on 15 meters on October 24th as I was in CQ WW SSB.
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WX7G
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« Reply #3 on: December 13, 2009, 09:24:14 AM »

You have two issues going on. The Tribander VSWR is higher than you would like on 15 and 10 meters and the tuner does funny things.

The tuner should tune the beam just fine. By power shutting off I don't know if you mean the RF output or that the rig shuts OFF completely - no DC power.

In either case you might have RF in the shack bothering the rig. Attach two radials to the grounding post on antenna tuner and lay them on the floor in opposite directions. 8'4" and 11'0".
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WW5AA
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« Reply #4 on: December 14, 2009, 07:47:20 AM »

It's amazing the amount of bugs that invade the traps on my A3S every year. Have you checked the traps?

73 de Lindy
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AI7RR
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Posts: 164




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« Reply #5 on: December 17, 2009, 02:05:54 PM »

This is curiously strange.

A trap is essentially a low pass filter. So, if the antenna is working as before on 20m, then your traps are not open or shorted. If there is any evidence of problems on 20m then you have a possible trap issue.

When you "take the tuner out of line", are you completely removing it or just bypassing the tuning circuit with the front panel bypass switch?

You stated "40 watts out", what do you expect to see and where are you reading this power indication?

Have you tried tuning up on 15 and 10 thru the tuner into a dummy load? Your problem could reside in the final tuning circuit of the radio or at the band switch.

I would start at the radio with minimal components, short known good cable and dummy load and test all bands, then progress outward from there.

73,,Roger
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VO1GXG
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Posts: 60




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« Reply #6 on: December 17, 2009, 07:44:45 PM »

40 watts out via  the bird meter

the radio is fine i've tested using the dummy load to everything except the main antenna feed.
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WB6BYU
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Posts: 12983




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« Reply #7 on: December 18, 2009, 01:17:33 PM »

What is the SWR on 20m without the tuner in the line?

Just because you can match it doesn't mean it is working.
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WX7G
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« Reply #8 on: December 20, 2009, 08:28:58 AM »

A trap is essentially a band reject filter. It is a parallel resonant circuit.
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VO1GXG
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Posts: 60




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« Reply #9 on: December 23, 2009, 12:39:51 PM »

Thanks for all the help.

My ground is a 5foot run of #14 stranded wire to the ground on the tower. I do not notice to much RF only place i see it is when my computer speakers are turned on and sometimes through the external speaker of the radio ( Kenwood SP-23). There might be a coil in the wrong place or they might be dirty.

Here is a video i made this afternoon just to show exactly what is going on.

http://www.youtube.com/watch?v=IPg35zf_FQE

Thanks.
VO1GXG
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VO1GXG
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Posts: 60




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« Reply #10 on: December 23, 2009, 02:32:31 PM »

AI7RR:

I've tried tuning the dummy through the antenna tuner and you're right the problem happens there for 15 and 10 meters. Leads me to wounder what my options are.

" Have you tried tuning up on 15 and 10 thru the tuner into a dummy load? Your problem could reside in the final tuning circuit of the radio or at the band switch. "
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