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A Simple Preselector for Use Between Antenna & Radio Receiver

Author: R.J.Edwards G4FGQ © 14th July 2003

A preselector attenuates strong unwanted signals in the same band as wanted weak signals. This prevents overload of high-gain receivers, improves signal-to-interference and signal-to-noise ratios, and reduces signal distortion. This preselector is a parallel-coil-and-capacitor tuned circuit with both the random length wire antenna and receiver tapped into the coil. The bottom end of the coil is grounded and it then becomes an impedance-matching transformer.

The antenna is tapped into the coil such that loss plus radiation resistance is matched to the coil loss resistance. By itself, this maximises signal level in the coil but halves coil Q. The receiver input resistance is also independently matched to coil loss resistance by tapping the receiver into the coil. The overall result is not an exact antenna-to-receiver impedance match but circuit operating Q is reduced only to 1/3rd of intrinsic coil Q. It is important to remember the higher the intrinsic Q of the coil the greater the operating selectivity.

The circuit is intended to tune over a wide frequency band. It should be stated the program's computed performance accurately applies only over a narrow band of frequencies in the vicinity of a "mid-frequency". Nevertheless results are a good guide to performance over a broad band covering a 3-to-1 frequency range.

The actual frequency range available depends on antenna length and input impedance.

Computed coil tapping turns are optimum in the sense that if tapping turns are increased (further up the coil from grounded end) signal input to the receiver will not increase significantly and selectivity will decrease. And if tap turns are less than computed value then selectivity will increase but received signal strength will decrease. In practice, tap turns are not very critical.

For the medium and longwave bands a 500pF or greater value tuning capacitor is suitable with a coil 1.5" diameter by 5" long (38 by 130mm), or larger. Coil Q is roughly proportional to a coil's linear dimensions. Change both dimensions in the same proportion and Q will change by the same amount. It is assumed the coil is close-wound (touching turns) with enameled copper (magnet) wire.

It is suggested coil taps are at 1,2,3,5,9,15,26,44 turns or a similar sequence to which the antenna can be connected by a rotary switch or by alligator clips. The receiver tap can be more permanent if always used with the same receiver. On the other hand, experimenters can have a fully flexible arrangement and make all coil connections with alligator clips. Keep a very short connection between the ground end of the coil and the receiver grounded input terminal.

The antenna input impedance is displayed for interest.

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