Hi Rick,
Thanks for that - good point about Inverse chebyshev. and the toroids were dust
iron…
We’re fortunate in not having mega strong signals near the top of the MW band
here.
Jeremy
> On 17 Apr 2020, at 05:24, Richard (Rick) Karlquist <richard@karlquist.com>
> wrote:
>
>
>
> 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
Jeremy Maris
140 Edward Street, Brighton BN2 0JL
jeremy@maris.plus.com
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