[TowerTalk] RFI Broadcast Station Mixing in Crank-Up tower?

[Earl Morse]

At first glance of that video it doesn't sound like a mixing problem.

That's demodulation of 5th harmonic of KKOH and very intermittent.  3900 kHz is 5th harmonic of 780 kHz KKOH.

Since it is breaking up I would look for intermittent, unintentional PN junctions ala rusty bolt effect.  Since it changes when you move your tower it tells you that the intermittent connection is on the tower.

Then you tell me it is every 10 kHz.  Haven't a clue as to why it is every 10 kHz.  I live in the shadow of 3 AM transmitters and I am treated to birdies at every order of mixing that falls in the ham bands (2f1 + f2, etc), but not every 10 kHz.  Maybe it seems like it is every 10 kHz because it is multiple orders of multiple AM stations mixing.  Tune through the whole 80M or 160M band and mark each 10 kHz where the interference occurs.  Go and look up every AM station within a few miles of you, strongest and closest will be of the most interest.  Get an IMD calculator on the web and punch the numbers.  See which stations are adding up to hit on the frequencies you are having problems with.  There is more than one thing going on here and you will need to separate them out in order to understand it well enough to fix it proper.  Or just leave the tower cranked down, that works too.

I would follow the intermittent aspect.  Crank the tower down a little at a time.  See if it goes away totally or if the level just gets reduced.  That's the difference between an intermittent contact and reducing the amount of energy picked up because you made the antenna smaller.  

It's times like these that I am glad I don't have a crankup.  No moving parts to make intermittent contact or break.  Of course when I am getting ready to climb my 100' stick the crank up starts looking pretty good.

Earl 
N8SS

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Message: 4
Date: Tue, 20 Oct 2015 16:45:09 +0000
From: Rich Hallman - N7TR <rich at n7tr.com>
To: "k1ttt at arrl.net" <k1ttt at arrl.net>
Cc: "towertalk at contesting.com" <towertalk at contesting.com>
Subject: Re: [TowerTalk] RFI Broadcast Station Mixing in Crank-Up
	tower?
Message-ID: <F1F4E0A0-C51B-416E-92EB-734C311CD484 at n7tr.com>
Content-Type: text/plain; charset="us-ascii"

I have tried shorting the rotor wires to the base section of the HDX689 with no changes.  I also would turn the antenna with no changes in the mixing, thinking it might have been mixing in the thrust bearing.  The rotor is a M2 OR2800.

The broadcast station is on 780kc (KOH) and I hear their audio on the mix I'm seeing.  I also did a video of what I am seeing earlier this year.  I thought the mixing was from a ticRing but that was not the source of the mix as that ring has been removed then replaced with an Alfa Ring.

https://m.youtube.com/watch?v=rZ8gXRKGkxE

Thanks....



Rich Hallman N7TR

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