In article ,
"Dave" wrote:

I recently hooked up a thirty-six foot (plus or minus a couple of feet)
piece of four-stranded wire with alligator clip to the internal "whip"
antenna of my portable shortwave receiver, for the extra performance such a
device offered. It works so well that I now cannot usually use my "DX"
setting because of all the background noise (sounds like hundreds of other
broadcasts vying for attention.) I don't know the frequency source of all
this background noise, but would like to filter out as much of it as I can.
One manufacturer of a similar "wind-up" antenna adds a capacitor to the wire
in order to lower the resonance frequency of the wire. If I were going to
try something similar (adding a capacitor, in series) in an attempt to bring
the resonance of the wire down into the 30 MHz range, what size (roughly)
capacitor should I use? Should I just try a few with different ranges, or
does anyone here have any suggestions?

Since you have cross posted to sci.electronics.basics lets try to look
at this logically and as non technically as possible.

The are two things that you need to accomplish to hear a station on your
radio in the way of signal strength.

1. The signal must large enough for the radio to amplify it and
reproduce it at the speaker.

2. The signal must be stronger than the noise floor of the radio and any
external noise the antenna picks up by some margin over the station you
want to hear. Usually this is something like 10 dB.

You can't do anything about the noise floor of the radio unless you want
to modify it. The basic sensitivity of the radio is a decision you made
when you bought it.

That leaves the antenna. What you did was to put up the most basic type,
which is called a Marconi or common mode antenna. For a simple antenna
it is about as non-selective as you can make hence the noise level is
high on the radio. Worse you might be over loading the radio and the
radio itself may be generating some of the noise. Portables are designed
to be sensitive and simple so they can't handle much signal. A strong
signal out or in band could be causing you additional trouble.

Whether 36 feet of wire is to much or not depends on where you live but
for most radios generally won't cause the overload problem it's just
that it is picking up everything well including lots of locally
generated noise.

That is the basically where you are at.

What can you do about this. You want to pick up more of the signals
(stations) you want to hear without hearing noise from other electrical
appliances or stuff out of band.

You need a more complex antenna design that will not pick up as much
noise as the signal you want to hear. Noise is on all frequencies and
comes from all directions.

A more complex antenna design can do things like:

1. Limit the direction it picks up signal or noise. You can benefit from
this by pointing the antenna at the signal you want or conversely
attenuating a noise source.

2. Changing the type of energy the antenna picks up. The antenna type
determines whether it picks up common mode or differential mode.

3. The antenna type also determines whether it is sensitive to the
electric, magnetic fields or both.

4. The antenna type also determines the band or bands of frequencies it
will pick up well.

All the above will limit the total amount of noise energy it will
present to the radio so it has less to deal with. Basically you use the
antenna design to preselect the signals you want to pick up. The
downside of this is short wave covers a wide range of frequencies so you
will need more than one antenna. For some type of resonant antenna the
smallest number of antennas you need are two and better would be three.

To get started with a more complex antenna and to see if you are really
improving your reception start with a weak signal using just the radios
whip antenna. Use a station on a high band (smaller antenna) during the
daytime.

Make a simple resonant antenna like a dipole cut for that frequency
connected to a coax and determine how to connect the coax to your radio.
If it is a portable radio try operating on the batteries as some of the
wall wart power supplies are noisy or noise on the house wiring is being
conducted to your radio through the power cord.

Now to test the antenna to see if it really helping you can disconnect
it from the radio and extend the radios whip antenna and collapse it
again reconnecting the external antenna to see which works the best.

You can put the external antenna outside away from noise generating
electrical equipment or switch them off.

Once you have a dipole making an improvement on weak signals you can
make other antenna types and antennas for other frequencies.

There are plenty of antenna sites on the web and ideas on finding local
noise sources.

--
Telamon
Ventura, California