Investigating the enabling stages for homebrew Rx design leads to the need for input filtering. The GQRP Club, of which I am a member has a handy recipe available, so why not take a look:
A hand full of parts was selected and some ugly style bread boarding was commenced:
The silver-mica capacitor lead ends were kept too long with a view to knocking it down for proper rebuild. It is a 50 Ohm design, hence I connected appropriately. Unlike using an oscilloscope, when using a spectrum analyser, proper terminations for test points are mandatory. This avoids spurious responses all over the gaff:
General response up to 400MHz, centred on 200MHz (ref freq, the dotted line) and horizontal divisions of 40MHz. Our target area is at the far left of the graticule:
Centred on 10MHz and 2 MHz per div. Showing off now by selecting infill:
Ref 3.650 MHz and 500KHz per div:
Noting from the page, "The filters are designed for 50 ohm input/output impedance and may be cascaded for better roll-off outside the passband."
I got out an 80 low pass RF filter that you have seen several times before:
And stuck it inline:
I spy with my little eye something that begins with "Cheby dip" :P Apart from that, the response looks very usable. The LF skirt, being a "straight through," is not touched. Note that although the higher skirt is very much steeper and tighter in, it appears that the ultimate rejection, about 70 dB'ish, is only a bit more, say, 5 dB, than before.
I feel that for a few popcorn parts that this is a very workable solution to the input of cheapie Rx's Obviously the design work and effort was done in the first place; but nice to see a reproducible, real circuit, using real preferred values. No slug core tuning was attempted at all; used "as is" out of the bag!
Two cascaded identical filters would be good. Also, perhaps with more attention to detail, such as shorter leads, preventing the input "seeing" the output with shielding, core tuning, and etc, further improvement may be had. I am not sure what the insertion loss would be, but perhaps the cascade could be switched in/out for difficult conditions.
I will progress this some more; perhaps knock it up in the morning :)
Thanks to GW0OAJ for this article
80m bandpass filter
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