500 - 515 kHz Preselector
The 500 - 515 kHz preselector described here was built for use ahead of the station receivers at WE2XGR/2 - primarily to reduce mixing/intermodulation products from AM broadcast stations. Although no interference was noted when using the high Q Marconi transmit antenna for receive, it was a different matter when using broadband, receive-only antennas such as the LF/MF K9AY. Small receive only antennas typically require a preamplifier in order to hear weak signals. While the robust, low-noise broadband preamplifier in use showed no signs of overload, the additional gain from the preamplifier caused mixing/intermodulation problems in the receivers that followed - especially when operating adjacent to the AM broadcast band. This filter is used between the Marconi transmit antenna or amplified receiver only antennas and the receivers.
Spectrum analyzer/tracking generator measurement of the preselector. Horizontal scale is 150 kHz/division and vertical is 10 dB/division.
Close in spectrum analyzer/tracking generator measurement of the preselector. Horizontal scale is 5 kHz/division and vertical is 10 dB/division.
10 kHz bandwidth is about as narrow as one can make this filter since inductor unloaded Q is the limiting factor. The inductors used here are wound on 61 type material which yields the best inductor Q of any of the ferrite materials. The Q of these coils measure approximately 300. Don't substitute different core materials or prewound coils unless you can verify that inductor Q approaches 300. A three-section, 365-pF broadcast variable is the main tuning element although only a small amount of its tuning range is actually used. These capacitors are fairly common items and are still available new from several sources.
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