Hum on 70cm and 2m transmit

[KC3ITD]

I'm trying to track down the source of background hum during transmission on local 70cm nets (and 2m), and have been trying various fixes, to no avail. Wondering if you fine folks have some other suggestions. Here's my environment:

• BTech UV-25X4 radio
• Ed Fong TBJ-1 tri-band antenna, mounted in garage attic (~18' elevation)
• 60' run of RG6 70 ohm coaxial cable (from garage attic to basement office)
• Radio Shack switching 13.8v DC power supply

I realize that RG-6 cable is not ideal and that my TBJ-1 antenna should be mounted outside with some elevation. But at this point, I'm just trying to reach local repeaters <15 miles away, and my scenario has been delivering on that.

There is very little electrical wiring in the garage attic, but I do have an HD TV antenna mounted up there, probably less than 10 feet from the TBJ-1 antenna. The TV antenna is an Antennas Direct - ClearStream Indoor/Outdoor HDTV Antenna (not amplified).

Here are the easy fixes I've tried, and none of these have eliminated the hum.

Power

• cheap AC/DC converter
• 12v battery

Environment

• shut off over-head LED lights
• close office door (to reduce noise from server room)

Connectors

• eliminated a BNC to PL259 pigtail
• re-terminated the F-type plug
• now using F-type to BNC to PL-259 adapter

The next fixes I'm contemplating include the following:

• try different mobile radio (the QYT 8900R from my truck)
• move the TBJ-1 antenna to the far side of the garage attic
• replace the RG6 cable with Belden 9913 cable (w/ new BNC and PL-259 crimp plugs)
• (last resort) move the TBJ-1 antenna outside

Could the ohm mismatch in the RG6 cable or HDTV antenna be causing the background hum? Any other ideas to try?

Thanks!

 

[W9WSS]

Is Ed Fong Industries still in business? The antenna you have described and its companion model are listed as "sold out."

 

[KC3ITD]

Is Ed Fong Industries still in business? The antenna you have described and its companion model are listed as "sold out."

Not sure about his status. I bought mine several years ago.

 

[mmckenna]

What is the SWR like?

RF getting back into via the mic can cause weird issues. High SWR can result in RF riding on the outside of the coax.

Power supply would have been my other guess, but you've eliminated that with the battery test.

Got a hand held radio you can try with, to eliminate the radio?

 

[prcguy]

I would start with the radio run from a battery into a service monitor or 50 ohm load and monitor on another radio. Do all your channels have CTCSS running? If so disable that on a simplex channel and see if the hum is still there, could be a misaligned CTCSS deviation.

I don't think its external from the radio but you might check to see if there is a 75 ohm to 300 ohm balun on the TV antenna. If so those are not intended to transmit through and probably won't handle much more than 1 watt.

 

[KC3ITD]

What is the SWR like?

I don't have a SWR meter, but I'll get one.

RF getting back into via the mic can cause weird issues. High SWR can result in RF riding on the outside of the coax.

I forgot to add, lowering the mic gain a little was an early part of my trouble shooting.

Got a hand held radio you can try with, to eliminate the radio?

Yes, I plan to try both a hand held and a different mobile radio this evening. Will advise.

 

[KC3ITD]

Do all your channels have CTCSS running? If so disable that on a simplex channel and see if the hum is still there, could be a misaligned CTCSS deviation.

I think I have a couple of GMRS frequencies programmed without any CTCSS, I can try that too.

check to see if there is a 75 ohm to 300 ohm balun on the TV antenna.

OK, I'll add that to the list of things to check when I get back up into the attic.

 

[mmckenna]

I forgot to add, lowering the mic gain a little was an early part of my trouble shooting.

Mic gain won't fix that.

The CTCSS suggestion from prcguy is a good one.

 

[tweiss3]

Do you have a spare mic cable? It pops up ever few years from use, but I have a handful of new mic cables to swap in when a buzz or no transmit audio pops up.

 

[AB5ID]

Are you hearing the hum on your own receiver, or are others reporting that they hear a hum when they talk to you?

 

[KC3ITD]

Do you have a spare mic cable? It pops up ever few years from use, but I have a handful of new mic cables to swap in when a buzz or no transmit audio pops up.

I have another mobile radio that has a RJ-45 connection mic. But I assumed the mics were proprietary, since the key pad layout is different on mine.

 

[KC3ITD]

Are you hearing the hum on your own receiver, or are others reporting that they hear a hum when they talk to you?

I was alerted by other users on repeater that they hear a hum when I transmit. I don't hear it on my own receiver, but I notice the hum when I've been testing (i.e. key up on GMRS simplex frequency), and listen with a handheld nearby.

 

[AK9R]

But I assumed the mics were proprietary,

The pin-out of the mic connector may be different between brands. Check your owners manual.

 

[ecps92]

I was alerted by other users on repeater that they hear a hum when I transmit. I don't hear it on my own receiver, but I notice the hum when I've been testing (i.e. key up on GMRS simplex frequency), and listen with a handheld nearby.

is this on all channels or just one ?
Curious what the PL on this one freq/channel is ?

 

[KC3ITD]

I have a SWR meter and a 50 ohm dummy load arriving tomorrow. Will do some more testing with multiple radios and antennas this weekend.

 

[K6GBW]

If you have a hum on your transmitted audio, then it's not being caused by an external source. You probably should immediately change that coax to a 50-ohm variety and make sure it's grounded. Put some ferrite beads on your power leads coming from the power supply. If your antenna is very near the radio then you might be getting RF on the mic cord so a torrid bead on the mic cord might help, but in that situation the correct answer is to move the antenna. If all else fails, it might be the radio. I've had radios that had the PL tone levels were too high and they create a hum.

 

[tvengr]

Unplug the AC adapter from the outlet and run on the battery only. Transmitting requires much more power than receiving and your cheap AC/DC converter probably is not capable of filtering adequately at high current output. That can induce hum into transmissions. You will not notice the hum on receive which doesn't overtax the converter. If that doesn't solve the problem, try an AC ground lift adapter on the AC/DC converter if it has a 3-prong grounded plug. You may have a hum loop.