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.

Google for "intermod suppression panel".
http://www.google.com/search?hl=en&q=intermod+suppression+panel&um=1&ie= UTF-8&tbm=isch
http://www.repeater-builder.com/antenna/ant-sys-index.html
http://antennasystems.com/product/sinclair-PC3/PC3113.html
http://www.telewave.com/pricelist/impanels.html
http://www.sinclairtechnologies.com/catalog/group.aspx?id=32
http://birdtechnologies.thomasnet.com/item/components/intermodulation-suppression-panels/81-series?
http://www.ferrocom.com/products.htm
http://www.taradios.com/IM_Supression_Panels.html
etc...

Incidentally, my Rotomola UHF MSF-5000 repeater has *THREE* isolators,
connected in series, in addition to a band pass cavity filter. Lossy,
ugly, but clean.

Next, silver plating is not a magic fix. Getting rid of bright nickel
plated cheap connectors *is* a magic fix. I've cleaned up several
systems by simply getting rid of cheap connectors and adapters and
replacing them with silver plated brass connectors (or just brass as
in Heliax connectors).

Google for "Passive Intermod Distortion":
http://www.amphenolrf.com/simple/PIM%20Paper.pdf
Materials: Ferromagnetic materials
such as nickel or steel must be
eliminated from the current path due to
their non-linear characteristics. Brass
and copper alloys are generally
accepted as linear materials. Tests
have shown that nickel plate under gold
on the center contact will typically result
in a 40 to 50 dB increase in PIM.
Stainless Steel in the body will usually
give a 10-20 dB increase in PIM.

Mo
https://engineering.purdue.edu/IDEAS/PIM.html (nice video)
In case you've seen magnets taped to coax connectors, this might
explain why.

More on PID.
http://aeroflex.com/ats/products/prodfiles/articles/8814/Intermod.pdf


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