I am not an expert, just an older, wiser ham, but vividly remember an intermod
elimination project years ago done by a Master RF Engineer Dr. Dick Alder,
then K3CXZ, at the Navy School's K6LY repeater tower on Huckleberry hill in
Pebble Beach CA. LOS to the Pacific Ocean about about 3 miles away. Over time
we noticed the Intermod seemed to get worse with long dry spells, and usually
disappeared with heavy rain.
Dick brought a spectrum analyzer, set it up inside the repeater building,
watched a broad spectrum, in the ham bands, I think VHF mostly, and dispatched
the young climbers of the club up the tower, instructed to bring a small
hammer, and look for and bang or tap on, loose, rusty or corroded joints
anywhere in metalwork on he tower, which had many, transmitters, including high
power commercial transmitters, all over the spectrum. MW broadcast station
transmitters were nearby. I was in the site shelter watching and listening to
the Master comment on the spectrum display while hearing climbing and tapping
noises above us on the tower.
At some stage, we saw the whole spectrum analyzer screen jump wildly upward.
Darn, I cannot remember the Frequency limits set on the screen. Professor
Adler yelled: Whoa! What are you guys doing? Where are you? Then we
stepped outside the building an looked up. A couple of the guys were standing
on a large, very rusty, triangular I beam guy wire torque amplifier, about 80
ft up. Rust flakes showered down whenever the guys hopped on the I beam brace.
Looked something like this photo:
http://www.arec67.org/img1214/1214-11.jpg
Dick (Reverently called Radio Richard) had an AHA! moment, and smiled. He
yelled... jump a little on that bracket you are standing on... Sure enough, the
the spectrum analyzer screen went VERY animated with each jounce. I remember
distinctly the repeated harmonic nature of the rectified intermod signals on
the display.
Radio Richard found the problem he predicted existed, (rusty diode junctions,
and rectification) and prepared the young guys with the proper tools, climbing
gear, and small hammers. I think the commercial tower owner replaced all
together the rusty steel guy brace.
The core prediction/preparation that Radio Richard briefed us on, was to look
for "rusty stuff" and loose hardware, at all Metal junctions, and tap them
with hammers while he/someone watched the spectrum analyzers.
I think Don's response to Ed, below is right on target. If you have the
luxury of spectrum analyzer access, it can paint a really vivid picture of what
is going on, especially when watching and jiggling antenna system components.
All the Best, Pat Barthelow,
AA6EG aa6eg@k6bj.org Skype: sparky599
Jamesburg Moon Bounce Team
http://www.jamesburgdish.org
"The most exciting phrase to hear in Science, the one that heralds
new discoveries, is not "Eureka, I have found it!" but:"That's funny..."
----Isaac Asimov
> Date: Fri, 9 Jan 2009 03:29:16 +0000
> From: ve6jy.1@gmail.com
> To: TOWERTALK@contesting.com
> Subject: Re: [TowerTalk] BCB Intermodulation
>
> 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@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
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
