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Author Topic: Antenna problem-still puzzling me  (Read 1495 times)
AF5CC
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« on: April 20, 2013, 02:08:18 PM »

A couple of weeks ago I posted an item about the receive in my Icom 706 occasionally dropping down on some bands, and coming back after you click the mic or key putting the rig into transmit.  After some testing of things in the antenna system-coax jumpers, low pass filter, tuner, I concluded (with help from the forum) that it must be in the rig.  I sold the rig as a fixer upper and got a good deal on a replacement.  Well it has the same problem!  Same symptoms, same bands (different brand and model of rig) everything!  Talk about frustrating!

One test I made in the past was to switch over to the 6m yagi and see keying the rig would bring the receive back, since it is on a different feedline from the dipole I am running.  It did, which is one reason I concluded it must be a rig problem.  I now see that there probably was a little energy making it to the dipoles feedline anyways, since both antennas were on an antenna switch in the tuner, and I know that isolation isn't perfect.  Tried hooking the 6m antenna straight up to the rig and didn't witness the receive level dropping at all.  That tells me it must be something in the antenna system.

Here is my antenna system-105 feet or so of copper wire, attached to Radio Shack 300 ohm twinlead, about 20 feet or so of that.  The twinlead runs to a 4:1 voltage balun, which transitions to RG-11 75ohm coax, which then runs to the shack.  Given this system, where is the problem likely to be-and more importantly, why would it occur on some bands and not others.  It happened mainly on 12m and 15m, and was starting to happen more on 17m and 30m.  I never saw it occur on 10m or 20m, and only very rarely on 40m.

Anyone have an idea as to what might be going on?

73 John AF5CC
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KE3WD
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« Reply #1 on: April 20, 2013, 02:18:32 PM »

Bad connections/solder joints on the antenna might be the culprit...
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AF5CC
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« Reply #2 on: April 20, 2013, 04:21:39 PM »

Bad connections/solder joints on the antenna might be the culprit...

That is what I kind of suspect, but why would it affect some bands and not others?  I use the same antenna on all bands.

73 John AF5CC
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KE3WD
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« Reply #3 on: April 20, 2013, 06:15:46 PM »

Might be that the amount of resistance of a bad solder joint affects things differently on different bands. 

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AC5UP
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« Reply #4 on: April 20, 2013, 07:11:02 PM »


That is what I kind of suspect, but why would it affect some bands and not others?

The position of the oxidized joint relative to the nearest RF null or node on the wire.  Visualize a resonant dipole. Typically center fed where voltage is at its lowest and current highest, if a connection at that point was less than good the current demand would likely reveal the fault. Flip side of this would be the ends of the dipole where voltage is highest and current lowest. At a Hi-Z point a poor connection can pass enough current to work well provided it doesn't try to arc over. In your case of a multi-band wire it's possible the poor connection(s) is/are near enough to a current node on some bands that a quick blip of the TX flashes it back into conduction, while on other bands they're at a Hi-Z point that's much less noticeable............
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AF5CC
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« Reply #5 on: April 20, 2013, 07:45:12 PM »

AC5UP, THANKS!  That is the best answer I have received yet.  I was figuring it was something like that, like maybe the corrosion acted like a diode or capacitor and passed RF at some frequencies but not others.

73 John AF5CC
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N8NSN
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« Reply #6 on: April 21, 2013, 04:34:47 AM »

The balun may be intermittently problematic. Could be any one or even more of the solder connections throughout the system.

Hope all is fixed soon. 73
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AF5CC
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« Reply #7 on: April 29, 2013, 07:44:15 PM »

Well I think I fixed it!  Yesterday I went up on the roof (and didn't fall off, that is always a good sign) and removed the RG8X coax choke I recently made, along with the SO239 barrel connector that attached it to the feedline.  I also took out one of those lightening arrestors that I had had installed for 10 years or so.  It is just a PL259 on one end, a SO239 on the other end, with a screw used for a grounding wire to bleed off anything that develops on the feedline due to storms.  Now the antenna seems to work fine again.

John AF5CC
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