SOME TIPS FOR BETTER SHORTWAVE RECEPTION



Antenna
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.


email us       home page


Electronic Specialty Products •  3054 Moore Drive •  Oviedo, FL 32765
Phone  407-366-4859