Re: interference from SW broadcasters above 7200

Cor Beijersbergen
 

Hi,
Based on the 0.2496 factor given by Allard I assumed that this could have to be 0.25, the inverse of 4. So there might be a 4th harmonic (or is that the third harmonic?) that is a factor of 4 higher.
I played a bit with this number and found the following:
If you take the VFO frequency that goes with the image frequency, multiply that by 4 and mix it with the true frequency of the incoming broadcast station you get something very near the BFO frequency.
F vfoimage = F bfo- F image
F true = 4*F vfoimage - F bfo

These formulas work for the list of F image and F true that Allard supplies.
It also works for F true = 7199 and F image = 7199. Only the F true must be generated inside the BITX40, probably according to the calculation by Hans.

Am I doing anything wrong?

73, Cor PA4Q

-----Original Message-----
From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Allard PE1NWL
Sent: maandag 13 februari 2017 2:21
To: BITX20@groups.io
Subject: Re: [BITX20] interference from SW broadcasters above 7200

This evening I noted down the exact image frequencies (f image) of some SW broadcast stations, and compared them with their corresponding 'true' frequencies (f true). I put them in a table and found a perfect linear regression equation:
f image = -0.2496 * f true + 8996
Guess what: 7199 kHz is exactly on the regression line. See attached table and graph.

So the birdie at 7199 is probably also causing the images of the SW broadcast stations in the ham section of the band.
If we could fix the birdie, then we would probably also get rid of the images!

Any ideas?
73, Allard PE1NWL

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