I have been a
shortwave listener for 55 years and have never found an antenna that
works any better than a long wire or inverted L antenna. At most
locations it is atmospheric and/or local man made noise that limits the
ability to receive weak signals. Any antenna should be located as
far from man made noise sources as practical and fed with coax.
A good ground at the antenna also improves reception by reducing the
noise pickup by the coax shield. There are times when noise is
low and hence, reception is much better.
Coax Most people use
standard RG58 or RG59 for their feed line, including me. I once
had an antenna fed with about 70" of RG58. To test the
effectiveness of the coax shield, the antenna was disconnected from the
coax. With only the feedlline connected to the receiver, many
stations could still be heard loud and clear. If the feedline is
receiving stations, it is also receiving noise. Perfict coax
would not pickup anything. The best coax
feedline that is available for a receiver, is the pre-made lengths of
quad shielded RG59 used for TV. It will greatly reduce any noise
pickup by the feedline. Home Depot carries it.
Pre-Amps The ability of a
radio receiver to receive weak signals is affected by two things,
atmospheric and man made noise, and the noise figure of the
receiver. Noise Figure is a technical term that represents the
amount of the noise produced by the receiver, that the signal must
overcome. The lower the better.
Atmospheric
noise is affected mostly by local weather and weather conditions at the
location of the station you are trying to receive. It is very
difficult to reduce this kind of noise. There is always some
amount of atmospheric noise which decreases as frequency increases.
Man made noise
refers to noise from electrical appliances, ignition noise from cars,
florescent lights, and other things like that. This type of noise
can sometimes be minimized by locating the antenna as far as possible
from the noise source and using good coax feedline to the receiver.
A low
noise preamp between the antenna and the receiver will improve
reception with most receivers, when noise is low. The most
improvement will be above 10MHz since noise
decreases as frequency incerases and often the receiver
sensitivity decreases at higher frequencies. Especially with
vintage tube receivers.
AGC The distortion
caused by frequency selective fading can be noticeably reduced by
making the receiver AGC much slower than it normally is. The AGC
is intended to hold the audio relatively constant with different
strength signals. The problem is that it tries to do that by
reacting to the carrier strength. During a frequency selective
fade, when the carrier fades, the AGC, mistakenly thinks the audio
level has decreased, and increases the gain, which makes the distortion
sound much worse than it is. You can test this with your receiver
if you have an RF gain control. Find a station that is fading and
distorted, then turn the AGC off and set the RF gain to the same
loudness. You will find that the distortion is much less
annoying. If possible it is a good idea to modify the receiver
AGC to allow an additional much slower setting, or modify the existing
slow setting. A 3 or 4 second recovery time seems to work
well. It is usually as simple as adding a capacitor to the AGC
line. I added a 2.2mf to my HQ-100. If modifying the
receiver is not practical, just switch the AGC off and use the manual
RF gain control when experiencing distortion caused by fading.
Tuning AM Using SSB Mode
By tuning AM signals using the SSB mode, some types of fading can be
reduced, because the received carrier is not used. The carrier is
provided by the beat frequency oscillator (BFO). Also, only one
sideband, which is selectable, is needed, so if one sideband has
interferance, select the other one. A receiver that is very
stable and has good selectivity is best. The BFOs for selecting
USB of LSB must be on precise frequencies for good results, so they
should be crystal controlled.
Audio A simple circuit
that I have used to reduce the loudness of static crashes from local
lightning during storms, uses only two parallel back to back diodes and
a resistor. The diodes are connected across the speaker terminals
and the resistor is in series with the hot lead of the speaker.
The idea is to adjust the audio level just below the point that
distortion is caused by the diodes. Then when a lightning crash
occurs that causes the diodes to conduct, it is muffled and is much
less annoying. My version has a transistor and LED that flashes
when the static crashes occur. In this case the AGC should be
short so the gain of the receiver recovers quickly from the lightning.