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Author Topic: Antenna problem-still puzzling me  (Read 2539 times)
AF5CC
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Posts: 800




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« on: April 20, 2013, 02:18:16 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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N8YQX
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Posts: 66




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« Reply #1 on: April 20, 2013, 11:28:48 PM »

Sounds like you have some oxidation in your connector.  I suspect when you transmit, you burn up the oxidation and you make good contact, but the oxidation builds back up when you are idle.

I had similar problem with my car audio system.  One speaker would drop out, but it would come back once I cranked up the volume for few seconds.  In my case, I cleaned the contacts with Flitz and waterproofed the connector with dielectric grease.

Good luck,

N8YQX
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73,
N8YQX
AF5CC
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Posts: 800




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« Reply #2 on: April 21, 2013, 07:19:52 AM »

I agree that is probably the problem, but why would it occur on some bands and not others?

73 John AF5CC
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N8YQX
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Posts: 66




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« Reply #3 on: April 21, 2013, 08:36:53 AM »

I'm taking an educated guess here, without any evidence to prove my theory.

Assuming the oxidation theory is correct, oxidation creates insulation.  Placing insulating material between two conducting surfaces creates capacitance (basic mechanism of capacitor).  My theory is that the capacitance being created is resonant on certain bands, but not on others.

Again, just an educated guess, without any proof or experience.

73,
N8YQX
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73,
N8YQX
PA3GOS
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Posts: 43




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« Reply #4 on: April 21, 2013, 04:45:32 PM »

My guess would be that you are using a tuner in the shack too.
In that case, some connection in it may be intermittent (by oxidation?) as well.
Most probably the inductor switch or the contact on a roller inductor.

This would explain why you are experiencing this effect more one some bands than on others.
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AF5CC
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« Reply #5 on: April 21, 2013, 07:38:27 PM »

It can't be the tuner, though, as the problem was still there when I took the tuner out of line.

73 John AF5CC
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K1CJS
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Posts: 5830




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« Reply #6 on: April 22, 2013, 04:32:11 AM »

What about the antenna switch?  It could well have oxidation buildup on the switching contacts, or the connections to it may be bad (SO239s/PL259s).  Try bypassing the switch and feeding one antenna directly.

Don't forget that the connections to the jumper co-axs may also be problematic, as in the braid or the center could be intermittent on one of them.  Metering them while moving the ends around can usually find a problem for you..

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K0JEG
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« Reply #7 on: April 22, 2013, 06:20:28 AM »

Check your RG-11 connectors, if you're using F fittings. Most of them have a "blind" connection to the center conductor, and are notorious for causing problems if it isn't exactly centered when installed. If the coax isn't supported the center conductor can work itself out of the connector as well (called a suck-out).
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K8AXW
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Posts: 3602




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« Reply #8 on: April 22, 2013, 10:00:21 AM »

John:  Do you have a dummy load?  They're great for determining if you have a radio or antenna system problem.

With that being said, a great way to troubleshoot a 'system' is to bypass whatever you can with jumpers..... rather it is coax or a wire jumper on the work bench.

I understand you won't be able to bypass the tuner so if you bypass everything else and still have the problem, then I would go to work on the tuner, the coax connectors and everything else between the wire antenna and radio. 

The name of the game is to eliminate possibilities. 

My favorite is the ham who has his rig upside-down on the work bench and is telling his buddy, "I have it narrowed down to a fuse or something else."
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N2MG
Administrator

Posts: 121



« Reply #9 on: April 23, 2013, 07:16:31 AM »

You really need to isolate the problem.  Your described steps don't seem to do that definitively. It seems your antenna arrangements too often share key components.  Can you set up something with just the radio and an antenna?

Just because the new(er) radio exhibits the receive-comes-back-after-I-key-the-radio problem doesn't mean both rigs don't have the same problem.  Radios have a T/R relay and they often develop this problem.  "Wiping" the contacts by keying the relay briefly cleans the surfaces and restores the receive signal.  Other weak connections can exhibit the same problem.  The difference is the internal relay problem often benefits from simply keying the radio with NO actual transmit signal. Bad connections further away (like in a PL259 or whatnot) don't "wipe" when you key, they only improve when actual RF power passes through them.

Here's what I'd do:  Take a long wire (say 100 feet of 24- or 18-gauge or so) run it outside and take one end, strip an inch or so, twist it and fold the bare part in half and insert it in the SO-239 in the back of the radio.  Turn on the rig and listen. When the receive the signal goes down, momentarily key the radio in Sideband mode. (You can turn all the power down if you want). If the receive signal restores, then I'd bet your paycheck it's the T/R relay.

For a well written trouble-shooting guide, see
http://www.w8ji.com/relay_cleaning_and_life.htm

A snippet:

Intermittent connections causing sporadic weak receive signals can occur anywhere in a receiving system. The bad or intermittent connection might be inside the antenna system, in a coaxial connector, or anyplace between the antenna and receiver input components.

Connections are often healed by momentary application of transmit signal through the poor connection.

Intermittent receive is almost always caused by a poor pressure-connection, where the receiver signal path depends on pressure to form a good low-resistance electrical bond. This might be unintentional, perhaps a poor solder joint in an antenna or connector. It also might be in an intentional pressure connection, like a crimped connection or even in a connector.

There are multiple sources of trouble like this, and we are always much better off using rational, logical, trouble-shooting techniques to locate the bad connection.


Tom goes on with more excellent advice.  Please read that.

Mike N2MG
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AF5CC
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Posts: 800




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« Reply #10 on: April 23, 2013, 04:20:48 PM »

Thanks for all of the advice so far!

I have taken the coax jumpers, the tuner, and the low pass filter out of line, and just connected the antenna to the radio.  Still had the same problem.  I then connected my 6m antenna directly to the radio and listened on the affected bands, didn't have the problem, so I have pretty much isolated it to the dipole antenna.

73 John AF5CC
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AF5CC
Member

Posts: 800




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« Reply #11 on: April 29, 2013, 07:44:47 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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KQ6Q
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Posts: 963




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« Reply #12 on: May 08, 2013, 09:05:19 PM »

I've found that some SO-239 double female connectors lose their grip after a number of years. Your lightning arrestors likely had similar problems. When you have one coax run feeding one antenna on several bands, and have issues on one band, the problem can also be that you have an odd multiple of 1/4 wave in the length on that band. Add a foot or two using a jumper and see if the problem goes away.
Also - with feedline junctions out in the weather, be sure to carefully waterproof them.
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