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@destevez.net To: 'Amsat BB' AMSAT-BB@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?