Steve,
Having read the other responses to date, I'd like to toss in one more.
First, by all means, follow G3SEK's advivce about getting some sort of RF
current detector. That will hewlp separate the true ingress paths from the
"maybe" ones. However, while in the process of doing that, you might want
to consider your friend's suggestion about RF riding on the outside of one
(or more) coaxial shields. The VHF/UHF hard lines sound like prime
candidates for the job.
First, in accordance with good lightning safety practice, those lines
(which you say go directly to your attic ham shack) should be grounded
outside of the house. If they are not grounded outside, they should be.
The Andrew grounding kits can be used. You can tie all of the separate
ground wires to one main downlead, preferably made of strap, as wire will
have too much inductance from the top of your house to ground. An
alternative would be to fit a metal entrance plate where the lines enter
the attic, install Type N bulkhead connectors on the plate, and attach all
lines to the plate. Again, a heavy grounding downlead is needed to attach
to your external ground system. You could then run jumpers from the plate
to your gear in the shack.
Both approaches not only enhance the issue of shunting lightning currents
before they enter the shack, but also will shunt any RF riding on the
exterior of the cable shields to ground. Luckily, the wavelength at 160m
is quite long, so getting an effective RF ground for that band from the
attic to your grounding system should not be too difficult.
You don't say how far the tower is from your house. If more than 10 or 15
feet away, plan to drive in an additional ground rod adjacent to the house
foundation and tie it to the tower ground with some buried #2 (or heavier)
bare copper. Depending upon your soil conditions, you may wish to add
more ground rods around the tower, too. See the excellent 3-part series in
QST this past year (June through August, I believe) on station grounding
and lightning protection.
Changing to a balanced type of antenna (dipole or inverted-V with a balun
feed) does get the direct RF ground current off of the tower and other
parts of your system. However, if the problem is coax shield-coupled RF
because of the cables on the tower being in the 160m near field, then
changing antenna type will probably not help. However, these are good
reasons to check to see where the actual RF currents are flowing.
Good luck.
73, Dale
WA9ENA
STRutledge@aol.co
m To: rfi@contesting.com
Sent by: cc:
rfi-bounces@conte Subject: [RFI] 160 Meter RFI
Help Needed
sting.com
01/26/03 02:34 PM
Hi folks. I have a problem with RFI on 160 meters. I just put up a 1/4
wave
sloper on my 65" Rohn 45 tower at the 35' level. It is fed directly to an
Ameritron switch-box at the base of the tower. I have a C3E, CC D3W and
yagis for six, two, 222, 432 and 1.2 on the tower as well as a Diamond
vertical and inverted vee for 80/40. So, the HF stuff all goes through the
switch box. The UHF/VHF goes direct to the shack in my attic via 7/8" to
1/2" Andrew Heliax. Tower is grounded with an eight foot rod on one leg.
It
hasn't been up very long. I get NO RFI of any sort on any other bands,
even
when using amp.
The 160 meter sloper is getting into the two control panels on my home
security system. Anything over about 30 watts causes the "beep" in the two
panels to "beep." You should hear CW. I am guessing and a friend is
telling
me that the RF is coming down the outside of one of my pieces of
coax??????????
I don't even have a clue where to start. Do we think this is a function of
the antenna, band, ground system or both?????? If I went to a different
type
of antenna, i.e., inverted vee on 160, do we think it might make a
difference?
Thanks for your help.
Steve, N4JQQ
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