On Fri, 8 Feb 2013 10:57:03 -0800, (Dave Platt)
wrote:

In article ,
Jeff Liebermann wrote:

Any ideas how to eliminate cross channel intermodulation with good antenna?
Or right antenna?

You're antenna isn't going to do much for removing excessively strong
signals, such as paging. The single best improvement you can do is to
lose your scanner, and get a better receiver with a better 3rd order
intermod (IMD3) specification. By the nature of the beast, scanners
are highly susceptible to intermod mixes in their front ends.

This is also a problem for many (most?) modern ham HTs, which have
broad-as-a-barn front ends. Their "DC to daylight" reception is both
a feature-advantage and a robustness-disadvantage. Older single-band
radios often have better front end filters.

Yep. Some receivers and scanner at least have tracking filters in the
front end. These have their own collection of problems, such as the
varactors going nonlinear when overloaded and creating a nifty mixer.
However, for most applications, they're much better than a barn door
front end.

There are cavity and crystal notch filters, that will reduce the
signal levels around the paging transmitter frequency, without
affecting the operating frequency (much). Search for crystal VHF
notch filter or cavity VHF notch filter.

http://www.parelectronics.com/amateur.php

I can offer a thumbs-up for the PAR Electronics VHFTN152-158. I had
terrible pager intermod problems with my Yaesu VX-5, whenever I had it
hooked to a "real" antenna (roof, bicycle-mobile flag J-pole, etc.)
rather than a rubber duck.

I have a VX-5 and know what you mean. Totally useless receiver with
an outside antenna. Good to know that the PAR notch filters work. I
may have an application at a repeater site. I usually make my own
notch filters, but I'm tend to get lazy when I'm short on time.

The pager-notch filter eliminated the problem, and as far as I can
tell it hasn't had a significant effect on ham-band receive
sensitivity or transmit power on either 2 meters or 440. I assume I'm
losing some signal and power due to insertion loss but it hasn't been
noticeable.

The Par site claims 0.5dB loss on both 144 and 440Mhz. That's about
5%, which is barely noticeable. No clue what's inside the Par
filters, but from the trimmer, my guess(tm) would be some helical
resonators.

I usually use single port cavities, which are generally useless for
anything except traps, instead of 1/4 wave stubs. I also have a few
adjustable coax sleeves, which work great, but tend to leak a bit.
Lots of ways to build a notch filter.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558