Radio or Antenna? Troubleshooting an annoying little problem.

[Realtor MA]

I have an issue on 20 meters. I'm hoping the brain trust here can help me figure something out. I'll try to lay out the facts and see what you all think.

When operating on 20 meters SSB my radio will slowly begin to mute itself. In other words, if the noise is a S3 it will slowly work it's way down to S1. When I key the mic the receive level comes back up.
Now here are the confusing facts...

a)it only happens with my tri-band yagi. It is a trap antenna, the Cushcraft A4S with 40meter add on.
b) it only happens on 20 meters
c)if I key the mic into a different antenna it does not bring up the receive level
d)If I key the mic on a different band into the same antenna it does not bring up the receive level

ETA I have eliminated all pieces of equipment except for the radio, the antenna, a stretch of coax and a DX engineering balun. I have a few ideas but I want to see what you all come up with.

 

[drgrant]

What kind of a radio is it? Are you running an internal ATU? If so, turn it off when the rig goes deaf and see if the RX level comes back up.

-Mike

 

[scouter-rick]

Do you have a way of determining the SWR during the decline to see if the match is drifting?

 

[Realtor MA]

What kind of a radio is it? Are you running an internal ATU? If so, turn it off when the rig goes deaf and see if the RX level comes back up.

-Mike

I have the Yaesu Ft950. I don't use the tuner so it should be off but I'll check

Do you have a way of determining the SWR during the decline to see if the match is drifting?

I guess I could attach my MFJ 259 analyzer to the antenna lead and see if it drifts

 

[n1oty]

20 tends to be noisier that 15 and 10, so I'd suspect something is pumping the AGC circuit. Can you disable the receive AGC on that radio and see if that has an effect??

 

[cockpitbob]

One other possibility is that the antenna has a bad 20M trap (wet, corroded, flakey capacitor...) and hitting it with power helps for a while.

Do you get the same improvement with 10W that you get with 100W?

 

[Realtor MA]

OK guys. Thanks for your ideas. The internal tuner is out of the circuit so that should not be a factor. The AGC in or out has no effect. I haven't monitored the SWR yet but I will do that. As for the amount of power it takes ; I typically have the radio set at 40 to 50 watts going into the amp but at that point I only key the mic in SSB so there really isn't much power going to the antenna.
Interestingly enough it isn't doing it tonight. Maybe it's because i played with the switches?? I'll let you know as soon as it starts up again.

 

[scouter-rick]

In the world of Electrical Engineering it is the ED circuit that you have just tested. ED is "Engineer Detector" and it is a circuit that is dormant until an issue occurs and then activates once troubleshooting is detected to hide the original issue. Congratulations that your ED is working perfectly! <grin>

 

[cockpitbob]

Is that anything like the Customer Proximity Detector that lets the smoke out of the box during the demo of your prototype?

 

[scouter-rick]

Is that anything like the Customer Proximity Detector that lets the smoke out of the box during the demo of your prototype?

Very closely related.

It correlates to Rule #4 of Engineering which states "Routine Maintenance should not include the Fire Department".

 

[radioman]

Any chance water could be the culprit? With all this melt and rain your balun or coax might have been infiltrated? Water can make antenna systems do crazy things. I'd also check your grounding system and make sure it's at 100%.

 

[Realtor MA]

Any chance water could be the culprit? With all this melt and rain your balun or coax might have been infiltrated? Water can make antenna systems do crazy things. I'd also check your grounding system and make sure it's at 100%.

I was wondering if there might be water in the traps or the balun. I believe the grounds are ok and all the coax connections appear to be tight and weather proof. Although I've been wondering about the mast and it's relation to ground. It's 'connected' throught the rotator, thrust bearing and of course to the booms of the antennas which in effect grounds it via the coax shield so that should be adequate. I think.

 

[Lucas McCain]

OK guys. Thanks for your ideas. The internal tuner is out of the circuit so that should not be a factor. The AGC in or out has no effect. I haven't monitored the SWR yet but I will do that. As for the amount of power it takes ; I typically have the radio set at 40 to 50 watts going into the amp but at that point I only key the mic in SSB so there really isn't much power going to the antenna.
Interestingly enough it isn't doing it tonight. Maybe it's because i played with the switches?? I'll let you know as soon as it starts up again.

Try taking the linear out of the equation, see if the exciter does it when just connected to the antenna.

 

[Realtor MA]

Try taking the linear out of the equation, see if the exciter does it when just connected to the antenna.

I tried it with pretty much everything out of the line and it still does it. Although it did not misbehave last night which makes me think I may have stumbled upon the cure unknowingly. I'll toy with it tonight and let you know.