RF in Transmit Audio

Problem: I have severe RF in my transmit audio if I use anything other than a direct hand-mic into the front of the radio. I need to run a W2IHY EQ and a RIGBlaster (the W2IHY box at a minimum since my mic is a Gold Line Pro with an XLR connector).

I don't have access to an earth ground since I'm in an apartment 3 stories up, but for the last 4 years, that was never an issue -- I'd never had RF into the TS-570D(G) that I'd used all that time. Now the Mark V is completely unusable at any power level over about 40W or so on phone. I've been using it mostly for CW and a little FSK in the interim, but I'll do anything to get this SSB problem solved. I'm trying to achieve ESSB audio.

I've tied everything together and run a line to the radiator feed pipe in the shack, and it made absolutely zero difference. I tied audio ground to the connector shell on all my mic connections and it made no difference either. On a dummy load it's mostly clean (still a little something but acceptable for the moment), but the instant I go over to a live antenna it's so bad at times that I'm almost completely garbled.

My antennas are dipoles on the rooftop above my head, coax baluns and a W2DU current balun on each one. Nothing except the radio changed in the last 2 months.

Solution 1: Courtesy of Heil Sound www.heilsound.com


Many amateur radio stations today are experiencing terrific RFI (radio Frequency Interference) that is impeding their audio signals and causing very garbled and distorted audio. Careful listening of MANY SSB signals on the air today exhibit RFI - not sometimes enough for their receiving stations to notice as they listen to a 3" speaker of a transceiver but careful listening in a wideband receiver with VERY high quality receiving equalizers and studio monitor speakers allow this slight interference to become VERY annoying. But then there are the signals that have terrific problems with RF getting into their audio lines and cause all sorts of problems.

We, here in the Heil Sound lab have discovered a very interesting fact. Most of the major transceivers today do NOT ground their microphone shields! That's correct - the mic shields FLOAT! Now wasn't this one of the FIRST things we learned about building RF transmitters with speech audio sections? GROUND those shields!! So, we came up with a very simple fix that just about anyone can make to their rig...don't have to get inside...don't void any warranties...simple....effective.

This applies to the 4 and 8 pin Foster (that's the Japanese company that builds those dang little mic connectors!!) microphone connectors so common on today's rigs. Plug your Heil (what - you don't have one yet! ) microphone cable into the front panel of the transceiver.

Remove the two small #4 Phillips head screws and the cable clamp they hold. Then remove the tiny Phillips head screw that holds the rest of the metal sleeve. Slide that back onto the mic cable. Now...cut off the end of a resistor or get a #20 solid tinned wire about 3" long. Locate the mic pin that has the shield of the mic cable soldered to it. With a small iron, carefully solder this solid wire to that shield and pin. Bring the sleeve back up the cable and attach to the connector with the small screw. This leaves the solid wire coming out the back of the connector. Replace the cable clamp and as you do that, tightly wrap that solid wire around one of the #4 Phillips head screws and tighten the clamp assembly very well.

What you have accomplished is grounded the shield to the transceivers chassis ground through the ring on the mic connector. (Make sure that ring is tight). This has been a big help to many stations with RFI problems and should help you clean up your signal.

Solution 2: There are some things you could try out: - Your problem seems to be caused by improper HF grounding and relatively high RF fields in the shack. Try to improve the grounding as the first step. A radiator pipe is practically useless as HF ground which is a completely different animal compared to DC ground. Use a counterpoise as grounding system on the radio system. A length of tuned wire laying on the floor in the apartment could work. You need one wire length for each band. It is of paramount importance to have a perfect grounding system for all equipment for an ESSB station.

Alternatively you can try the MFJ artificial ground match box. - Use quality audio isolation transformers with RFI decoupling to block HF in all lines from the microphone to the radio (it may also be necessary between the different ESSB boxes- maybe that is already the case with the -W3IHY EQ). The W2IHY MURF-box is an excellent item. I have built one myself in a small metal box. - Study the NU9N information pages about ESSB. There you will have a lot of ideas on how to battle RFI and build your ESSB station.




This site was last updated 08/21/07