Jeff Liebermann wrote:

On 09 Sep 2011 19:50:43 GMT, dave wrote:

Jeff Liebermann wrote:
Yep. If the repeater is going in any of the buildings where I have
equipement, it will need a cavity and ferrite isolator to prevent
intermod problems. I know of one building that demands Heliax, no
braided coax including LMR-xxx coax, all silver plated connectors, and
other draconian anti-intermod measures. Tempest like packaging on all
computahs. All I can say is that it works (if you can afford it).

I've never seen that anywhere. Intermod is a math problem. No amount of
silver plating will fix bad coordination.

Sounds like you've never had to deal with transmitted intermod.
Consider yourself lucky as getting rid of it is a PITA. Also, not all
intermod comes from mixing in the receiver. TX intermod is real and
preventable. The xmit cavity and ferrite isolator prevent any RF from
adjacent antennas from going down the antenna, into the power amp,
mixing there with the xmit signal, and having the power amp amplify
the intermod.


An intermod study compares transmit freqs as well as input freqs.
Circulators are accepted practice. The intermod study will tell you if
you need more traps, BPF, etc. There is a lot of pseudoscience in
technology (and a comparable amount of "overkill"). By far, the worst
problem I encounter is XM radio on 2.5 gHz and ground radar from
airplanes (they like to use tower sites as benchmarks).

I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South
Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John
Hancock building, the router room at Channel 4, etc.) and I have never
seen a blanket ban on LMR because it leaks.