Thanks, Ron. You just reminded me that I have an
MFJ-934 (ATU with artificial ground) stashed
somewhere.
No real need for it just now, but who knows? Now
I just need to find it!
73, Phil W7OX
On 7/16/16 10:39 AM, Ron D'Eau Claire wrote:
One approach to reduce "RF in the shack" using an end-fed wire is to
resonate the "counterpoise" for the band you are using. MFJ made just such a
"tuner" (model 931 IIRC) but any tunable L-network will work. The idea is to
adjust the "tuner" for maximum current flowing into the counterpoise
(meaning the counterpoise is offering the lowest possible impedance to the
equipment it is connected to.) I have successfully used that in a 2nd story
apartment feeding an end-fed wire running outside. I know one Ham who bought
the MFJ box and found it worked FB.
For a homebrew "counterpoise tuner" you'll need an RF ammeter but a good
cheap one is an incandescent flashlight bulb in series where the
counterpoise connects to the rig chassis. Tune for the counterpoise for
maximum brightness with the rig running at the lowest power that will light
the bulb, then short-circuit the bulb for higher power. Of course, the
adjustments will interact with the tuner settings for the end-fed wire. But
the "tuning" of the counterpoise is usually broad enough (low-Q) that noting
the settings for each band you can return to them without doing the whole
retuning thing again. (The MFJ box my buddy bought has an ammeter built in.)
My "counterpoise" was a thin white wire run around the baseboard of the
shack and down the hall - about 30 feet total. White wire + apartment white
wall = invisible counterpoise. Worked FB 40 through 10 meters.
While that will reduce the RF voltage on the rig itself, you will be sitting
in a high-level RF field from the end fed wire, which limits your maximum
power to meet radiation exposure limits, depending upon how far from you the
antenna is located and the band you are using.
73, Ron AC7AC
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