I bought a nice 23cm bandpass filter from John G3ZTR at Aerial Parts of Colchester to fix a very nasty intermod problem with very loud local signals in the 1GHz range causing mixing products with DTV transmissions from Emley Moor, 51km away.

A homebrew version also worked, bit it wasn’t stiff and stable enough and really not suitable for masthead installation.

Response of the filter is good, I checked the passband using a GPIB/SCPI script in Perl using my SGLabs Prologix IEEE488 to USB controller.  Using the HP E4421B as the signal source, I put a 3dB pad either side of the filter as usual.  Output was measures using my HP436A using 8481A sensor. I set the generator level to give 0dBm with the filter replaced with a back to back SMA female adaptor.

Close-in results using this code snippet are in the first graph.

for($f=1100;$f<1510;$f+=2)
{
   chdir(“C:\\Program Files (x86)\\KE5FX\\GPIB”);
   system(“talk 7 \”SOUR:FREQ $f MHz\””);
   $r=`query 13 T`;
   chomp $r;
   $r=~/..D(.[0-9]{4}E.[0-9]{2})/;
   print “$f,$1\n”;
}

Measured insertion loss at 1296.200MHz is 0.08dB, certainly well within the quoted <0.3dB at 1296.  At 950MHz, measured loss is 38.9dB (quoted >35dB) and at 700MHz is it 41.6dB, again, better than the quoted figure. At the 648MHz subharmonic, it is -40.9dB.

Wider span to follow.