Re: RFI or feedback issue

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From: Mr Mike Caughran (kl7r@yahoo.com)
Date: Thu Mar 20 2003 - 19:29:36 CST


A couple things I have learned by experience on the
subject:

Clip on rf chokes from Radio Shack work great.
Try them on your speaker and power supply leads.

Balanced antennas are the way to go.
Change your random dipole into the biggest loop
you can put up in the same area. If you cant
put up the loop try to choke off the imbalanced
rf at the antenna by using a balun or 10 turn
coax choke. You might think you have a
balanced antenna since you have a dipole and twinlead
but half of the antenna might be coupling into your
house wiring better than the other half. Make it
into a loop and this cant be a problem.
(I hear some hams just wrap a loop of wire around
their houses under the eves)

Feed line lengths are a big deal.
If you happen to have a length that presents a
virtual short or an open to your tuner, you will
get rf in the shack. Try adding 8 feet of twinlead
and see what happens.

Artificial Grounds work great.
MFJ 931 artificial ground or the MFJ 934 tuner
with artificial ground. If you have an old tuner
or an old L network laying around. Try putting
that old tuner and wattmeter between your rig/tuner
and whatever ground you have. First tune for min swr
on your main tuner then tune for max current in your
ground. If you dont have a station ground at all, Try
laying a 33 foot piece of wire out on your lawn or
living room and using that as your counterpoise.

Lastly check for shorts and opens...
I put up a horizontal 80 meter loop in the dark and
couldnt understand why I had rf in the shack. In
the morning I found that both legs of the loop were
shorted to my metal roof :) Another time that
I had rf in the shack one of the leads of my feeder
broke loose at the antenna.
 

--- "Upton, Shawn" <SUpton@ALLEGROMICRO.com> wrote:
> This past weekend, I set up my radio, but I seem to
> have an RFI issue. I
> have a 40m dipole squeezed into a 30' run (zig-zag
> setup), with about 10' of
> 300 ohm twinlead to an MFJ-969 tuner, to my Scout.
> The Scout is powered by
> some off-brand switching power supply, which seemed
> quiet enough. No
> grounding is used. The radio is about 6' or so from
> the feedpoint of the
> antenna.
>
> The issue: My SSB audio comes through the Scout
> speaker with some
> distortion. And on 20m, if I just push the PTT, the
> radio will "howl". I'm
> not too surprised that 20m has issues--antenna is
> the wrong length--but all
> the other bands too?
>
> The Scout is left running at 50W, and when I tune
> the tuner, I am acheiving
> 1.5:1 SWR or so (as tuned by a MFJ-207--the Scout
> sees very low SWR, which
> seems to in step with the MFJ-969). I tried
> swapping coax (from tuner to
> radio) with no change, to make sure it wasn't a bad
> piece of coax. I don't
> have snap on ferrites, but I tried wrapping 20 turns
> of the power cord
> around a metal screwdriver, and had no change. This
> RFI does not seem to be
> band dependant. As for feedback on CW, well, the
> sidetone keeps me from
> checking.
>
> I tried out my FT-817 on the setup, and had no
> issues (with the radio
> setup). But that is only transmitting at 5W, not
> 50W. Also, when I did it,
> I did run the RF first through an Z-11, and then to
> the 969 on bypass, and
> just using the 4:1 balun in the unit.
>
> I'll try tonight changing power supplies to a linear
> that I have, and to see
> if the feedback exists when transmitting into a
> dummy load. Any
> suggestions? I really don't want to mess with
> grounding, as the only AC/DC
> ground is the ground pin in the receptical, and the
> RF ground would be kinda
> messy (a 1/4 wave of wiring running around the
> house). Plus, I find it hard
> to belive that I need an RF ground, what with the
> balanced antenna setup and
> the band-independant RFI issue.
>
> Thanks.
>
> KB1CKT
> Shawn Upton

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