[TenTec] RFI Issue

[Joel R. Hallas]

Shawn,

It does have a definate RFI sound to it! I would be surprised if it were
power supply related, but would rather focus on grounding.

My guess is that the combination antenna/tuner is not well balanced,
resulting in currents flowing on the shield of the coax back to the radio
area. It could also be that the antenna is close enough that direct
coupling from the antenna to microphone or other cables is the culprit.

You are planning on using a dummy load, and I would think that should tell
the story. I would guess that with a properly cabled dummy load on the
radio, and then moved to the tuner, you will find that the problem
disappears. If so, then one or both of the above problems is very likely.

If it is the balance, a good solid ground on the tuner should help. If
your shack is close to the service entrance and connected by short cable,
a 3-wire ac plug with a short heavy wire on the ground pin may even work.
I wonder why you don't have any grounding as part of your configuration?

If direct radiation, filtering of all the wires coming to the radio,
including the coax from the tuner (using a turns on toroid) should help,
but best is to get the antenna further away!

Also check the obvious, is the mike shield connected at the plug end?

Good Luck & 73, Joel Hallas, W1ZR

Shawn Upton wrote:

> I set up my radio over this last weekend, 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 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--but the issue appeared
> on 80m, 40m, 20m, and 15m.
>
> The Scout is left running at 50W, and when I tune the
> tuner, I am acheiving 1.5:1 SWR or so.  I tried
> swapping coax (from tuner to radio) with no change.  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.
>
> 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.
>
> 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?
> Thanks.
>
> KB1CKT
> Shawn Upton
>
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