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RE: Another interesting preamp story



Thanks Kevin, Tony, et. al., for the clarity on this often-discussed subject.  I would like to offer a few additional comments related to actually using a diplexer in a two-antenna setup to reduce 2m de-sense.  Like Dominico, I used to experience de-sense only when the antenna elevation was low and pointed over certain portions of my roof.
 
 - the diplexer must be installed between the antenna and the preamp (and, of course, the preamp should be as physically close to the antenna as you can practically get it).
 - if you don't have a preamp and you still have 2m de-sense, get another radio.
 - Installing a diplexer on the 70 cm input is a cheap and effective solution, but there is a penalty in system performance as you are introducing a couple dB loss before the first active device (the preamp).  Although using high quality coax and connectors helps, the only practical remedy for this small system performance degredation is more gain at the antenna.
 - I found installing a diplexer on the 2m output (connecting low port and common) to have no observable impact on the de-sense problem, thus supporting the front-end-overload v. 3rd harmonic concept.
 
Several years ago, Mike, KE4AZN, and I bought several V/UHF diplexers from a Japanese company (had to mail order them over the I-net) that were built with N-connectors and housed in weather-proof enclosures, solving two of the practical installation problems with the $30 MFJ variety.  I don't have them with me now (in storage) and I cannot seem to find a reference for them on the I-net, but they are out there somewhere :-)
 
73,
Jerry, M0MOE
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