[TowerTalk] BCB Intermodulation

[Don Moman VE6JY]

Your questions raises more....

Is the hi pass filter the VERY first thing in the line after the switch? Try
taking everything out of the line (esp power meter sensors).

Has it always done this? If not, what was the last thing you did to the
system before you noticed it?

Does it go away (even momentarily) if you tx with hipower on the 80m or
160m antenna? I'd look at the low band stuff first as they would be
gathering the most bcb energy.  That can indicate a bad (nickel plated??)
connector somewhere.

Tower guyed or self supporting? (I'm resisting the obvious temptation here).
A bad joint/contact in a self supporter is less likely, but guyed towers can
be problematic.  The guy wires need to broken up with at least one
insulator, preferably close to the tower.  Pull on the guys or antenna legs
and see if anything changes.....

GL!  I fight this stuff every winter (most rectification type products seem
to melt away in the spring) and it can be a never ending battle.  Any bad
joints in and around the antennas that gather enough mw energy can cause
rectifciation and put these mixing products everywhere. And I'm 80 km away
from the local MW stuff here.

73 Don
VE6JY

On Fri, Jan 9, 2009 at 3:02 AM, Ed Richardson <ed_richardson at shaw.ca> wrote:

> Just wondering if anyone has experienced intermodulation caused by some
> component in an antenna system? I am getting severe broadcast band
> intermodulation across the 80m band and have ruled out the receiver.
>
> The intermodulation takes the form of (2x Frequency 1) + frequency 2. I
> have no less than 6 strong AM stations that all interact with each other
> to basically wipe out most of 80m. The intermod products vary from S9 to
> S9+20dB.
>
> I have installed a high pass filter in line that reduced the
> fundamentals at least 40 dB but the intermod remained unchanged. Added a
> 30 dB pad on the input, no change in intermod except being reduced by 30
> dB.
>
> I have a single feedline (7/8" Heliax) with a jumper to the radio
> connected to a RATPAK tower mounted switch. Switching between multiple
> antennas yields intermod with slightly varying levels. When selecting an
> open connector port, Intermod drops to almost nil but is just slightly
> detectable.
>
> Any suggestions where to look next?
>
> Ed
> VE4EAR
>
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