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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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  #1 (permalink)  
Old 09-12-2017, 7:46 PM
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Default Is my Dipole too Close to the Shack?

Need some help...

I have been fighting RF in the shack for several months now. I have ruled out the radio, amp, feed line and very likely my grounding scheme. I'm running an 80m straight (not inverted) OCF dipole 28' off the ground. I have a "factory" provided voltage balun and a "factory" provided 600-1000 ohm choke directly below the balun. "Factory" meaning purchased, and with a lot of other customers using this same setup without problem. I have 75' of LMR-400UF between the choke and the radio.

I'm getting enough RF in the shack that my mic is shocking me. Noticeably warms my skin at 100W (barefoot) and shocks me pretty bad at 500W (amp). At 400W-500W out I have fried several electronic devices in the shack - fortunately nothing very expensive and all were sitting within 2-3 feet of the radio gear. It does it on at least 2-3 bands, most notably around 7.2MHz. The raw SWR of the dipole at this frequency is 1.3:1.

The short leg of the OCF dipole hangs directly overhead the shack 21 feet above the radio gear.

If my calculations are correct the distance to the antenna is well within the MPE limits. I'm too inexperience to have a feel whether this much radiation can be expected from a dipole at that distance.

I can't determine if the radiation is getting in the shack on the feed line (the choke is supposed to prevent that) or getting in through the air from the antenna (or some other way). The radio (and amp) is connected only to a battery (no AC connection).

All equipment is grounded with 8 feet of 6 AWG copper wire to a 6 foot copper ground rod (all installed new less than 4 months ago).

Can I get comments please on if my antenna is too close to the shack, or if this is feed-line RF?

Thanks
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  #2 (permalink)  
Old 09-12-2017, 8:16 PM
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The only thing it could be is common mode currents down the coax right from the antenna and your common mode choke is not doing much good. You probably need 5,000 ohms or more of resistive impedance across all the bands you operate to tame most OFC dipoles. Generally you can't pick up enough direct radiation off the antenna in the shack to give hot mic.

I run a 133ft 80m OFC dipole about 30ft high and horizontal running right over my house/operating station and with full legal limit on any band nothing happens, no hot mics, no RFI into phones or TVs or anything. I have a MyAntennas CMC-130-3K at the radio/amp end and a home made version of his CMC-230-5K at the antenna end. The worst choke has a minimum of 7,500 ohms resistive impedance on 80m and the other has at least 8,000 ohms on 80m. On the higher bands the resistance of one choke balun goes upwards of 14,000 ohms.

My 80m OFC dipole is also by MyAntennas and the type of balun used has very good common mode rejection to start wtih. Many OFC dipoles use a voltage balun which has very poor CM rejection and lights up the coax as part of the antenna.

Am I starting to sound like a MyAntennas ad?
prcguy

Quote:
Originally Posted by ckmcdonald View Post
Need some help...

I have been fighting RF in the shack for several months now. I have ruled out the radio, amp, feed line and very likely my grounding scheme. I'm running an 80m straight (not inverted) OCF dipole 28' off the ground. I have a "factory" provided voltage balun and a "factory" provided 600-1000 ohm choke directly below the balun. "Factory" meaning purchased, and with a lot of other customers using this same setup without problem. I have 75' of LMR-400UF between the choke and the radio.

I'm getting enough RF in the shack that my mic is shocking me. Noticeably warms my skin at 100W (barefoot) and shocks me pretty bad at 500W (amp). At 400W-500W out I have fried several electronic devices in the shack - fortunately nothing very expensive and all were sitting within 2-3 feet of the radio gear. It does it on at least 2-3 bands, most notably around 7.2MHz. The raw SWR of the dipole at this frequency is 1.3:1.

The short leg of the OCF dipole hangs directly overhead the shack 21 feet above the radio gear.

If my calculations are correct the distance to the antenna is well within the MPE limits. I'm too inexperience to have a feel whether this much radiation can be expected from a dipole at that distance.

I can't determine if the radiation is getting in the shack on the feed line (the choke is supposed to prevent that) or getting in through the air from the antenna (or some other way). The radio (and amp) is connected only to a battery (no AC connection).

All equipment is grounded with 8 feet of 6 AWG copper wire to a 6 foot copper ground rod (all installed new less than 4 months ago).

Can I get comments please on if my antenna is too close to the shack, or if this is feed-line RF?

Thanks
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  #3 (permalink)  
Old 09-12-2017, 11:01 PM
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Hi CK
.
I'm sorry about your 'hot mic' issue. I too think you have a common mode (CM) problem, centre'd on your Off Centre Fed antenna.
.
Anyone whose read my comments knows I'm hardly a fan of the OCF's- not that they are a bad antenna by design, but they can be a real bugger to get them to work *as* design'd.... with common mode problems being a never ending, recurring theme.
.
I am not in favor of using chokes to correct CM- considering them a 'band-aid' patch up on a more serious problem. Sure, they can work, but to me it isn't a true fix. That said, and short of trying a new antenna (recommend'd)---> in which case I'd highly suggest a resonate dipole--- you should re-address the chokes you have along the feed line; also look at you matching transformer....and, as PRC mention'd, a voltage balun?....they are notorious for CM..... adding/replacing these is a starting point. But don't try to co-exist with the RF in your 'shack.' That is a dangerous situation, as anyone who has been really RF burn'd will tell you (ie: me.)
.
I'm going to suggest something that you may wish to try. It doesn't always work for CM, but we use it all the time in my work, .... with fairly good results. Its Steel Wool (preferrably stain steel.)
.
Wrap a goodly length of it about your coax at varying intervals... do not connect it to the cable's shield. It forms a high impedence trap to RF currents running back along the cable- and it *May* take the sting out of your microphone. If it improves things, (no promises !)-- you'll know the next steps to take.
.
I carry a big box of it on field trips just for such issues.... and if it doesn't stop RF from coming into your radios, it works great for sealing those cable holes in equipment hut walls that mice love to climb thru.
.
Good luck
.
.
........................CF
.
.

Last edited by Coyote-Frostbyte; 09-12-2017 at 11:06 PM..
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  #4 (permalink)  
Old 09-19-2017, 8:37 PM
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I purchased and installed a MyAntennas CMC-130S-3K choke. That appears to have cleared up the RF in the shack - at least what's detectable without measuring equipment. All hot mic is gone - even at 500W, interference with other devices has stopped. I still had a little noise on the PC speakers and our capacitive-touch switch lamp toggles on-off every time I key up but I fixed both those with ferrite cores - they were picking it up from the antenna I believe. But, for now all is well.

Of course the choke changed the impedance of the antenna, SWR is better in some places and worse in others. However, it completely took out the 80m band. The SWR starts at 3:1 at 3.5MHz and quickly rises to over 10:1 at 4.0MHz. That's not making me very happy.

How I have my eye on a tuner. I sure like the specs on the KAT500.

Wish I could just have a real antenna, ... but for now the OCF dipole will have to do.

Thanks for the help guys ... and gal!!

Last edited by ckmcdonald; 09-19-2017 at 8:42 PM..
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