[amsat-bb] 70cm high-pass filter designs

Glen Zook gzook at yahoo.com
Thu Apr 16 20:55:29 UTC 2015

Don't discount the strength of the 3rd harmonic.  It doesn't take much to cause problems.  Remember, a 3/2ths wave-length antenna radiates as well as 1/2 wave-length antenna and your 2-meter antenna will radiate fine on the 70 cm band.  Also, with "normal" coaxial cable, you are going to get radiation from the line.  Commercial two-way installations use at least double-shielded cable for things like duplexers and cavities to basically eliminate interference (as much as possible).
Having both a high-pass filter for 70 cm and a low pass filter for 2-meters should reduce the problem even more. Glen, K9STH AMSAT 239/LM463

Website: http://k9sth.net
      From: Daniel Estévez <daniel at destevez.net>
 To: 'Amsat BB' <AMSAT-BB at amsat.org> 
 Sent: Thursday, April 16, 2015 12:41 PM
 Subject: Re: [amsat-bb] 70cm high-pass filter designs
Many thanks to everybody for all the suggestions. I'll have a look at
the filter designs.

There are some people that have suggested that the problem might be the
3rd harmonic from the 2m TX, instead of being overload by the 2m signal
somewhere in the receiver. I'm also suspicious that this could be the
case, but I did yesterday some tests that seem to indicate that the
level of the 3rd harmonic is fine.

I tested with different antennas for TX and RX, with about 2 or 3 meter
separation between them (my normal antenna is an arrow type antenna,
with on coax for the 2m yagi and other coax for the 70cm yagi). Fist I
connected the dongle to the 70cm port on the diplexer (and the antenna
to the common port). With this setup I got a bit of desense as usual.
Then, I connected the dongle to the 2m port on the diplexer. With this
setup the desense is very big.

This makes me think that the problem is with the 2m frequency and not
the 3rd harmonic, because the distance between the antennas and the 2m
port of the diplexer should be more than enough to kill any 3rd harmonic.

However, it's possible that I am seeing two different effects with this:
the 3rd harmonic when the dongle is on the 70cm port and overload from
the 2m signal when the dongle is on the 2m port.

What do you guys think?


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