On 4/16/2020 2:45 PM, Jeremy Maris wrote:
Make your own!
Use this site to design your filter.
http://www.iowahills.com/9RFFiltersPage.html
Attached are the values I used back in 2016 when the G4AQG FT-1000MP had
intermod problems with a new Beverage.
I built a 9 pole Chebyshev high pass filter, designed with the excellent RF
filter design package from Iowa Software, and used an LC meter to get the
capacitor and inductor values correct. Caps were made from polystyrene and
inductors wound on small ferrite torroids..
Difficult to see from the (ancient!) spectrum analyzer picture but the filter
response was almost exactly as the design showed.
No labels for stop-band but it was 10dB down at 1579kHz, 50dB down at 1000kHz
and at 693kHz was in the noise, at least 70dB down, almost undetectable
compared to 60dB over S9 or more without the filter, and the intermod was gone.
The good filters use INVERSE Chebyshev designs,
made with mica or C0G capacitors, and powdered iron cores.
10 dB down at 1579 kHz is not good enough for many QTH's.
I have a very strong local station at 1700 kHz,
for example, and another at 1530 kHz.
Rick N6RK
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