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Richard Clark · July 25th 06, 11:39 PM posted to rec.radio.amateur.antenna

On 25 Jul 2006 13:54:49 -0700, wrote:

I decided to try one of the simplest of antennas, a quarter wave
vertical for 20m supported by a roach pole.

I live on the 9th (top) floor of an appartment block, with a balcony.
The balcony is surrounded by a a metal railing which I decided to use
as the counterpoise.

I have a portable communications receiver with a small helical antenna.
Obviously this gives limited performance with this antenna...

Anyway, hooked up the antenna to 50 Ohm coax and connected the other
end to the comms receiver.

Result? Noise at about 8 S units, all but drowmning out the signal.
If I remove the counterpoise, it's even worse, a howling sound. If I
short the counterpoise and vertical, the signal is actually BETTER. If
I go back to the little helical that comes with the comms receiver,
then the noise VANISHES and the signal's not bad either!!!

Question: What am I doing wrong? How can I improve the vertical
(without moving)?

Question: Are there any other antennas that would be better in this
situation (9th floor appartment)? Magnetic Loop?

Any help anyone can give would be MOST welcome!!!!

Tim

Hi Tim,

You give a very good description of your problem. There's more that
could be said, but it starts very well.

First, we can start with what is good. You get no noise and a good
signal with your little helical. I presume this is a battery operated
set, or could be battery operated. If not, connect it to an extension
line (but try to go battery operated).

Move your set to the balcony and see if the noise returns. If so, use
the set to try and locate the noise. If not, then your problem is
between where you are listening and where the new antenna is going to
be.

Try tracing that path back while monitoring on your set to see if any
noise sources are along the way (this is unlikely, and shielded coax
should take care of it). If so, we will discuss this below. If not,
then my bet is your noise will be discovered in the next test.

Put your set back into its usual operating area. Connect the coax to
the set. Disconnect the coax from both the balcony and the antenna
(the far end of the coax is just floating - don't let it touch any
metal) and check for noise. If not, you are ready for the next test.
If so, the problem may still be related to the next test, but I would
bet your coax is connected to something metallic along the way.

Connect JUST the coax shield to the rail of your balcony. If you get
noise AND your set is plugged in (try this too, if you normally use it
with AC power); then you have a ground loop, or conducted noise. You
can retain the "ground" side of the antenna with the rail, if you
insert a capacitor between the coax shield and the rail. However,
this is a Hail Mary kinda of solution and will not be entirely
satisfactory. If you don't get the noise through this one connection
and finishing with a connection to the antenna does, then there is one
more thing you can do.

At the antenna end, coil the coax with about 8 - 12 turns around a
coffee can or large canister or 2 liter pop bottle just before the
bitter end where connections are made. If this reduces the noise,
that noise is arriving from somewhere along the path and being coupled
into the line at the antenna. Alternatively, your set may need
grounding too. Problem here is that you are a long way from RF ground
(9 floors away), and that need for ground is more a need for shielding
around the set. All of this argues local noise, so it serves your
interest to find it.

One quick way is to kill power to the entire building and successively
add it back until the noise reappears. This, of course, is a fantasy;
but you could do it for your own apartment at the breaker panel. If
you still have noise in a dead apartment (battery operation of
course); then you have problematic neighbors. This still demands that
you haul that set around snooping for the source of noise. If the
noise went away, sometimes it arrives from a distant source (in the
apartment) that SHARES the same breaker (other side of the wall?). If
so, experiment with plugging your set into other outlets on different
breakers (use an extension line so that you don't have to move the
set).

If we have to return to the balcony rail as the culprit, then build
two antennas and connect one to the coax shield, one to the coax inner
conductor, make the coil described above (or look for a 1:1 current
balun), and make SURE that nothing touches the railing, electrically.

73's
Richard Clark, KB7QHC

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