Hot Mic

[ckmcdonald]

I picked up a used 400W HF amp. I dropped it into my system between my multi-band 80m OCF dipole and my 100W HF radio. My dipole passes overhead the shack about 20' above me and my gear. Prior to getting the amp I had no trouble in the shack running barefoot.

The EMP calculations (assuming I'm doing them correctly) say I'm OK on any band the amp supports as they all calculate safe distances less than 20', conservatively at 500W.

However, I'm picking up a lot of energy in the shack. Twice I have damaged electronic devices in the room when in CW mode and more than a few times I have been shocked by my mic.

Am I too close to the antenna?

 

[N9PBD]

My guess is that you're bringing RF back into the shack via the coax/transmission line. Do you have a RF choke at your coaxial entry point? If not, you should look into one. That bite you got from your microphone was almost assuredly an RF shock. You can wind your own coaxial 1:1 current balun to keep RF (or at least reduce it) from getting into your shack.

If you don't feel like making one, there are several sources available, including MFJ's MFJ-915 ($30).

 

[ckmcdonald]

Thanks for the response Greg!

My antenna has both a balun and choke - as supplied by the manufacturer. The choke is directly next to the balun followed by 50' of coax to my amp.

Ant/Balun: OCF Dipole
Choke: Maxcon Line Isolator

At 7.2MHz where I use the station the most the antenna is 1.1:1 measured and verified by both my radio and an antenna analyzer.

 

[jonwienke]

Assuming your line isolator is working (you should check it), I would look into the grounding of both your antenna and your AC power. The antenna and your AC power should each have their own ground rod, but they should be tied together with #6 or heavier wire.

 

[iMONITOR]

Prior to getting the amp I had no trouble in the shack running barefoot.

However, I'm picking up a lot of energy in the shack.

Run faster!

 

[prcguy]

Direct radiation off the antenna overhead can get into mic circuits but if you are getting stinging shocks or burns off the metal parts of your mic that's from RF on the coax.

An OCFD lights up the coax with RF by design and you need a really effective RF choke to make it play nice. I suspect your model of choke is simply a string of ferrite beads over coax and the best I've measured of that type will reduce common mode currents by about 20dB and that's at its best frequency. They are typically worse on 80m and below.

Your probably getting lots of RF past your choke and you can buy or make a much more effective one. The best common mode chokes I've seen are from MyAntennas and the CMC-130-3K has the most restive choking of anything out there. If you only operate 80 through 10m the CMC-330-1K is very good.

You can make your own choke that is much more effective than your Maxcon for about $10 by wrapping 10 turns of RG-58, LMR-240 or the best, RG-142 coax around an FT-240-31 ferrite core. I suspect if you improve your choke the hot RF problem will go away.

I have a 133ft 80-10m OCFD that is about 20ft above my roof going right over my operating station and I can run full legal limit on any band and nothing happens, no RF into my radio or my telephone or anything. The type of balun used in my OCFD is known to reject common mode current on the feedline and I have a home made version of the MyAntennas CMC-230-5K choke on the roof and a CMC-130-3K at the radio.

If your anywhere near Los Angeles I would loan you a choke for testing to see if that fixes your problem.
prcguy

 

[jwt873]

I was running a 160 meter OCF (AKA Windom) antenna. I was getting burns off my key and mic..

I added an extra layer of isolation by installing another choke just outside the house where the coax came in. It's an MFJ product. It's nothing more that an white plastic tube with connectors on either end and a row of ferrite donut's in the middle. MFJ Enterprises Inc.

I can't guarantee that it will work for you, but it did the trick for me.

 

[SCPD]

Hi CkMcDon--
.
As you are experiencing, RF can give you quite a bite...
I think you were probably very close to feeling them at 100 Watts-but the problem only appear'd as you QRO'd.
.
I take this issue seriously- and as you said, you'd already experienced some of what that stray RF can do.. ie: damaging electronics in your house. I have a little scar on my left thumb where it was sliced as clean as a scalpel by such an errant arc- the same effect as a medical cautery.... Draw one of those off a mic and your nose.......!
.
That Off Centre Fed antenna is a bugger for common mode currents,-- among other issues-- (I am not a fan of OCF's.) I would seriously consider using something else- something like a resonate dipole.... but that's me.
.
That said, be careful in trying to patch up the problem with chokes- They will/maybe work, but using them like this is like putting BandAids on the issue. You really want to correct the problem. I am not being Holier-than-Thou- in my work we deal with this frequently.
.
One thing I do as a quick-fix in the field is carry a big bag of stainless steel wool. When we have an common mode issue, I can usually wrap a section of the offending cable with it and continue the test project- but it is (one of my favorite sayings)- a "Mickey Mouse" fix. I regard chokes a just a step above steel wool.
.
Of course I'm sure you've looked for ground loops and other grounding issues.
.
Good Luck, and until this is resolved, watch your nose !....
.
.....................CF

 

[rfradioconsult]

Hot Lips wasn't just a nurse on MASH. Welcome to the club. In college my transmitter was a Johnson Viking II with jury rigged antennas, hit the right combination and you became a member of the hot mic club.

 

[N9PBD]

What jwt873 said, you need your choke at the entry point to your shack/house. Stop the RF before it comes inside. I have a end-fed antenna (another kind of OCF antenna), and I use an air-wound choke made from RG-58. Here's another link to a discussion on air-wound rf chokes: Air wound RF choke

I was running a 160 meter OCF (AKA Windom) antenna. I was getting burns off my key and mic..

I added an extra layer of isolation by installing another choke just outside the house where the coax came in. It's an MFJ product. It's nothing more that an white plastic tube with connectors on either end and a row of ferrite donut's in the middle. MFJ Enterprises Inc.

I can't guarantee that it will work for you, but it did the trick for me.