Hi Ed,
You can have a multitude of problems causes by the high RF levels.
What you want to do (as we all do) is make sure none of the
equipment has a RF potential difference between it and other
equipment, or the power lines.
Whether you ground that equipment or not is secondary to the real
problem, that you have current flowing between the chassis' of the
gear and power line leads and so on.
First, get the equipment as close together as you can and bond
ALL the cabinets together (even the tuner) to a common point. The
best conductors are smooth large-surface-area conductors, and the
leads should be very short in terms of the wavelength.
Bring the power leads to that point, and use a bypassing-type of
brute force line filter to RF-ground the power line to that same
common point.
This alone will cure 99% of the RF problems...without adding a
ground. The reason is all the equipment and the power line are tied
together for RF.
If you still have problems, choke the antenna lead with a common
mode choke (choke balun) a short distance away from your
equipment. Whatever you do, DON'T make any equipment on your
desk be isolated from other equipment.
Be sure the cabinets have good RF grounding, especially on
tuners. Many tuner manufacturers are sloppy with cabinet
grounding, and one even floats the cover by using nylon inserts on
the screws! These floating cabinets can be a real source of RFI. If
the cover in not securely grounded at multiple points it will act like
a tiny antenna right next to the radio.
In severe cases, you might need to make a ground plane "under
the rug". I rolled up the carpet in a friend's radio room and laid
aluminum foil down criss-crossing the floor. I laid a length of brass
foil across the aluminum, and let the carpet pressure hold it
together.
That totally cured his radio installation, even though he was using
an end-fed longwire hanging out a 10th floor window and trying to
run a half-kilowatt!
> SITUATION:
> -- My station is on the 6th floor of a 6-floor apartment block.
> -- My antenna is a vertical dipole, cut for 7.15 MHz, and fed by an
> antenna tuner to force it to accept RF on 80m thru 10m. The feedline
> is coax. There is no balun at the antenna feedpoint. -- There is no
> safety ground in the local power system in this building. 220 VAC is
> provided as a pair of wires. -- I can not bring an earth ground into
> the apartment from outside. The apartment overlooks the grounds of
> the French embassy, which I can't enter. In any event, a 20m long
> "ground lead" will certainly be floating by the time it enters the
> apartment.
>
> COMMENTS:
> -- I suspect the coax between the antenna and tuner is hot with RF on
> its shield. -- Since the chassis of the tuner can not be earthed, it's
> likely that the coax between the tuner and transceiver is also hot
> with RF.
>
> Your ideas?
>
> Eric R3/K3NA
> ....................
> PGP fingerprint: F40B A402 795B 3F52 43B0 47B4 3576 BA88 3A34 5058
>
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com Administrative
> requests: towertalk-REQUEST@contesting.com Problems:
> owner-towertalk@contesting.com
>
73, Tom W8JI
W8JI@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|