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1-10 of 13 messages
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RF in Transmit Audio
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by N4AWP on October 9, 2009
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I have a severe problem with RF in my transmit audio.
This is a list of things I have done:
Ground for equipment is a buss bar on the back of the desk with all equipment to it w/ #12 solid wire and a #6 solid copper wire through the wall 12' to a 8' ground rod.
The tower is 5' away from this rod and 3' from the building (building has a metal roof). It is on a seperate ground rod.
The power panel is on a separate ground rod.
All ground rods have #6 solid wire and I drilled a hole in the top, put the wire in it and brazed it to the rod. No worries about a good connection.
I bonded all 3 rods together and then disconnected them after it made no difference.
I tried 3 different microphones. 2 Heil and the stock hand mic.
Tried 2 different radios. Icon 718 & a 756 Pro II
Connected the antenna direct to the radio. Nothing but the power supply and antenna on both radios.
Connected the power to my truck battery.
Grounded the metal roof to the equipment ground and then to the power ground. This made it WORSE!!
It gets worse the longer I transmit.
Tried 2 different power supplies, both switching.
Thinking about putting the roof on a separate gnd rod and also moving a rig and a different antenna outside to try that. I have ordered some clamp on ferrite chokes to put on the connecting lines. I had one and put it on the power supply but no difference.
The only thing that has changed it is when the roof is grounded,it was worse. Acts like something charging and then after no xmit for a few minutes it is not as bad. They say the audio is very raspy and the longer it goes it gets to the point you can only understand a few words.
I would appreciate any GOOD ideas.
Thanks
Art N4AWP
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RE: RF in Transmit Audio
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by WB5JEO on October 9, 2009
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The first thing that comes to mind is one or more ground loops. This is always a suspicion with a ground bus run linearly behind the equipment, with each piece of gear having its own ground lead to the bus. I would run the equipment ground leads to a single point, literally no loops formed. It just seems to me the most obvious thing to explore, especially since you've pretty well eliminated things like one or another mic or one or another mic connector being poorly grounded at the shield. Hopefully, that's it, because it's an easy fix. It's really a classic description of a ground poop from a bus with equipment grounded at different points along it.
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RE: RF in Transmit Audio
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by N4AWP on October 9, 2009
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TNX Gerald
Don't need to watch for the "COWS". The buss bar is 6" long and is from a breaker box. I can change to copper??? Or,I can bond them together.
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RE: RF in Transmit Audio
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by WB5JEO on October 9, 2009
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I doubt the physical nature of the bar is at work. But you do have to make good connections at every point. I've been thinking about your report that it gets worse as an operating session goes on. That kind of makes me wonder about connections in the RF path, but I don't think I have a specific thing to suspect. I still think it's an RF ground problem, but that takes in both your RF ground system and your feedlines, especially cox shields, since that's often the route that completes the loop. How about disconnecting all grounds, and see if it persists. Maybe even go into a dummy load and monitor with the other rig. If it goes away, come back on with one part at a time.
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RE: RF in Transmit Audio
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by N4AWP on October 10, 2009
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Thanks
I tried with no grounds and the same results. Also two antennas, OCF dipole and a Triband beam. Coax direct to the radio with no change. I have on my "Try" list the dummy load suggestion. I have a intercom between the shack and house. It is one of thoes that plugs into the house AC. A friend brought that to my attention yesterday. I will try these ideas today.
Thanks for your help.
Art
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RE: RF in Transmit Audio
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by WB5JEO on October 10, 2009
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I have doubts about the intercom, since you tried it on battery power and got the rotator out of the line with the OCF. Try the dummy load and at least get the antennas cleared. Do report, even if you fix it, so the cause can be mentally filed for reference in the future.
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RE: RF in Transmit Audio
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by AA4PB on October 10, 2009
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The OCF dipole is always suspect for bringing RF back down the outside of the coax shield but the triband yagi is not normally a problem. If the dummy load check is good then I'd try getting the OCF dipole out of the way or at least disconnecting it from the coax. Just wondering if it might be picking up RF from the Yagi and bringing it back into the shack, either on its coax or coupling into the Yagi coax and rotor control cables.
Make sure all coax connections are good on the Yagi and that the SWR is reasonable (under 2:1) without using a tuner.
Actually, you should'nt need any "RF" grounds with the Yagi. The grounds are only a lightning/safety issue. I've run many a Yagi and center-fed dipole without a ground of any type without experiencing any RFI problems.
By the way, the NEC requires that you connect the radio system grounds to the electrical system ground with a #6 wire, minimum.
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RE: RF in Transmit Audio
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by N4AWP on October 14, 2009
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Thanks to all.I put up a 20M dipole and connected the 718 to it. The radio was on a table at the door of the shack, about 10' from the desk. Transmited the ProII into a dummy load, no help. Changed positions and the problem followed the ProII. I can't explain the problem being reported in both radios but the 718 is clear. This was the first time I listened to it. It sounds very raspy. I am going to send the ProII to Icom MI for repair. I also bonded all the grounds together again.
Thanks for all your help
Art
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RE: RF in Transmit Audio
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by KF7CG on October 14, 2009
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By the way, running all those devices to a ground bus and then running the buss to ground is a good way to ensure that there is rf on all the chassis if there is on any. The chassis are all at an RF potential above ground that is determined by the Impedance of the buss to ground connection and the summed RF currents to ground.
A separate wire to the gound point helps eliminate this.
From last repotts it sounds like a microphone connection problem.
KF7CG
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