Antennas and Preselectors

The KRS AA-1 Active Antenna was sold through Radio West in the early '70's, and was a good performer. The only complaint was that the frequency response was from about 200 kHz to 150 MHz, and the VHF pickup might overload some HF receivers. I find it convenient to be able to use the same antenna for shortwave and for the VHF/FM bcst band. Power is fed to the whip antenna section through the coax cable, simplifying installation. Values are not critical. Gain can be increased by changing the turns ratio of T1 if there are no local stations which might cause overloading/cross-modulation.
AA-1 Active Antenna Schematic.

A preselector is a device placed between an antenna and a receiver which has both gain and selectivity.

The strength of the desired signal, as measured on the receiver S-meter, has very little to do with how well that station is received. What does matter is how much stronger than the noise level the station is. There are two types of noise to be considered here, atmospheric and manmade noise picked up by the antenna, and noise generated inside the receiver or preselector by the amplifier stages themselves. A PRESELECTOR CANNOT IMPROVE THE RATIO OF DESIRED SIGNAL TO NOISE PICKED UP BY THE ANTENNA. Whatever combination of signal and noise is picked up by the antenna will be amplified equally by by the preamplifier.Raising the S-meter reading from S5 to S9+20 db for a signal barely above the noise level will not increase the signal's readability. A preselector will help a weak signal from the antenna overcome the noise generated inside the receiver only if the noise figure of the preselector is better than the noise figure of the receiver. Noise figure is a measure of the noise internally generated within an amplifier stage or other circuitry. The lower, the better. If the noise figures of the preselector and receiver are the same, the ratio of desired signal to internal noise will be the same, and thus the readability of the station will not be improved. The same circuits are available to a receiver designer as to the preselector designer, and high sensitivity is one of the easiest parameters to build into a receiver, so most modern receivers have as good a noise figure as could be expected out of a modern preselector design. This level of performance is only obtained with a receiver which is kept properly aligned and serviced; if your receiver doesn't seem as hot as it used to be, the answer may be to have it fixed rather than add a preselector. If you have an older tube type receiver, particularly military surplus, a preselector is likely to help your receiver's sensitivity. Military receivers have long featured freedom from overloading rather than maximum sensitivity.

With a good preselector design, overloading of the preselector itself will occur only with extremely high input levels. On the other hand, a high gain preselector can cause overloading in even a quality receiver designed to be resistant to this. If a particular receiver has an overloading problem only when received signal levels reach S9+60 db, and a preselector with 30 db gain is placed before the receiver, a received signal with a level of S9+30 db will now cause overloading. There are not many S9+60 db signal levels encountered. S9+30 db levels are much more common. Thus the chance of having a receiver overloading problem is greatly increased. If the preselector has an RF gain control, the signal level presented to the receiver can be decreased to the point where overloading does not occur. The RF gain control on many receivers is not useful in preventing overloading from occurring because most of the gain reduction occurs in the IF stages instead of ahead of the mixer, where the overloading is occurring. For best results, use only enough gain so that the noise figure of the preselector predominates over that of the receiver. The proper gain of a preselector can be set as follows: Tune the receiver to a clear channel. Decrease the preselector gain until only the internal noise of the receiver can be heard. Now increase the preselector gain until the antenna noise masks the receiver noise. (This may take a little practice-receiver noise is a very smooth hiss. Atmospheric and manmade noise picked up by the antenna is much rougher sounding.) This maximum necessary gain point will vary according to frequency and band conditions. It may be helpful to adjust the RF Gain control of the preselector when straining to hear a weak signal. If the desired station becomes more legible when the RF gain is decreased, the receiver is most likely suffering from some form of overloading (any form of degraded performance - cross modulation, intermodulation, desensitization, etc.) Adjust the gain until a balance is achieved between receiver overloading and loss of sensitivity. Receiver overloading may show up as a high background noise level or as recognizable signals on the desired station. If the interference decreases as the preselector gain is decreased, or the preselector switched out of the circuit, you're using too much gain.

In the '70's, this circuit was developed suitable for use as a antenna preamplifier or as the amplifier for a box loop antenna.
FET Preamp Schematic.

A fancier version, with a dual-gate FET, featuring an RF gain control. Dual-gate FET's can still be found, but not easily. This schematic is closely related to a preselector sold as the KS Preselector.
Dual Gate FET Preamp Schematic.

Better nulling is obtained with a loop antenna when it is center tapped and and amplified with a balanced amplifier.
Differential FET amplifier Schematic.


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Last revised May 8, 2003
Copyright © 2003 John Kolb