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Re: [TowerTalk] RF in the shack

To: towertalk@contesting.com
Subject: Re: [TowerTalk] RF in the shack
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 7 Mar 2017 10:15:05 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Tue,3/7/2017 5:30 AM, Timothy Holmes wrote:
1. It seems that I have some issues with RF in the shack – Im running a 3
element tribander at 35 feet (practically directly over head of the shack)
and a 125 foot dipole fed with just north of 31 feet of ladderline
(technically its window line bought from AES before they closed – the stuff
in the brown plastic insulation with windows in the line). There is a LDG
4:1 Balun fed by about 20 feet or so of coax that feeds the window line,
and the balun is connected to my LDG AT200Pro Autotuner. My Rig is an
IC706mkIIG running barefoot at 100w.

Hi Tim,

Antennas close to the shack DO put RF in the shack if they're working. :) The feedline to that dipole will carry common mode current from the antenna to the shack unless there's a good common mode choke at the feedpoint. It's not practical to choke 2-wire transmission line like you're using. The feedpoint Z of a low 125 ft dipole is close to 50 ohms on 75M, making it a good match to coax, and low impedance feedlines that are matched to the antenna CAN be choked. See k9yc.com/RFI-Ham.pdf

That 2-wire line you bought is not a good thing for most ham stations, because it cannot be choked. Without a good choke, RF couples to the shack and noise picked up on the line couples to the antenna and from there back down the line to your receiver. That noise coupling is a BIG problem, because we're surrounded by RF noise sources.

The symptoms that I am having are as follows:

on some frequencies waveyness on the LCD computer monitor
the 706 does not like to operate with the front panel extension cable in
place, it shuts down when I key up

That's not surprising. The 706 is a super-cheap rig, and the front panel extension cable makes a great receiving antenna to couple RF into the radio.

occasionally, the tuner will show a reading event though I am using the
Tribander
Also not surprising if it's connected to the dipole and the dipole is close to the tri-bander. The dipole is picking up the RF radiated by the tribander and sending it down the feedline to the tuner. That's how antennas work! I suggest that you don't use the extension cable.
My Father’s computer speakers at the other end of the house relay my
signals QUITE efficiently
Computer speakers are notorious for picking up RF. If they're picking up RF, they are junk. Toss them in the trash.

the touch lamp in the living room goes nuts when I transmit.

This is a common problem with touch lamps. Their detection circuitry is high impedance, and probably not filtered for RF. Again, badly designed. Throw them in the trash or unplug them when you're transmitting.

Im not really sure what other information you may need to help me
troubleshoot and solve these problems, but I would appreciate any help I
can get. Hamvention is coming up, so if I need some parts, I might be able
to get them there, also, DX Engineering is just down the road, and I am
ALWAYS happy for an excuse to go there HI, HI.

If you have money to spend, I'd suggest a better radio. Get the advice of experienced contesters to chose a good used radio like a TS850 or an Elecraft K2/100. And whether you spend money or not, work on good grounding and bonding for your station. It won't help the computer speakers or the lights, but it can help with issues in your shack. Study http://k9yc.com/GroundingAndAudio.pdf

73, Jim K9YC
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