[TowerTalk] finding rf on coax

[Jeff AC0C]

I've done some work with attic antennas and can offer some advice.

1. Use a balanced resonant antenna if at all possible.  That means dipole. 
Using a non-balanced antenna is a recipe for having tons of RFI problems in 
the house.

2. Use a serious coax transmission line choke with this antenna right at the 
feedpoint.

3. If you want a multiband antenna, make it balanced as well, and use a 
remote tuner in the attic.  I had good luck with a zig-zag shaped 88' dipole 
+ attic mounted tuner as my first attic antenna.   #2 still applies.

4. Regardless of the power level you are running, be prepared to spend money 
on type 31 ferrites.  The more power or the closer the antenna, the more 
ferrites you are going to need.

5. As much of a hassle as putting up a stealth dipole outside may be, the 
work will be a lot less than trying to do something in the attic...

An end fed antenna at 200W is going to light up all kinds of stuff in the 
house...

Good luck!

73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie

-----Original Message----- 
From: Jim Brown
Sent: Tuesday, November 08, 2016 12:25 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] finding rf on coax

On Mon,11/7/2016 6:23 PM, David Voit wrote:
> I
> have an end fed wire with a 9:1 balun with about 15 feet of coax to the
> banun and 90 feet of wire with no counterpoise.

No counterpoise is your problem. RF current MUST have a return. If we
don't provide a good return path, mother nature will find one, like the
wiring in our home. ANY end fed wire without a counterpoise is only half
of an antenna. A connection to earth is NOT a return -- the earth is big
resistor! Indeed, a connection to earth does NOTHING useful for a
transmit antenna.

However -- security products and systems are notorious for having
TERRIBLE RF immunity. Virtually any wired security system is likely to
have problems with RF, regardless of how it is radiated or conducted.
And yes, problems like this CAN be the result of the antenna itself
radiating -- that's what it's supposed to do -- but those devices SHOULD
be built to reject RF.

For some practical ideas about counterpoises, check out the slides for a
talk I did at Pacificon several years ago and for several ham clubs
about how to get on 160M from small lots. It's mostly about vertical
antennas and counterpoises. http://k9yc.com/160MPacificon.pdf

73, Jim K9YC

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