[TowerTalk] Antenna Problem - Solved

[Tom Rauch]

> Date:          Mon, 15 Jun 1998 13:34:19 -0400

Hi Guy,

> I have the same MFJ unit. I got one of those cheap bead choke balun kits
> from The Wireman, and always have it on the MFJ analyzer. All the wierd
> problems disappeared. Guess it was unbalance: My hand attached to the
> case, therefore one side of the feedline and not the other. I also have
> the other analyzer (RF-1? It's at home...) and don't use it without the
> bead choke either.

What antenna type were you measuring? If you add the beads and it 
changes something, you have a balance problem somewhere.

If adding the beads fixes the analyzer, it indicates a need to fix 
things elsewhere in the system.

> As a matter of fact, in general, killing the current on the outside of
> coax shields has fixed all kinds of wierd stuff around the shack. From
> the above to the KW high isolation chokes on the input & output of the
> AL1200, rf on coax shields is a real pain.

True enough. I use beads on the coax feeders of all my low noise 
receiving antennas, in conjunction with ground rods and short 
radials. It prevents stray noise from following the cable shield to 
the antenna, and looping back in to the system. 

My TX antennas are generally not a problem (although occasionally 
they are), but I use air wound choke anyway to increase the impedance 
along the shield for lightning hits. 

I'm surprised the AL-1200 requires such beads, unless your antennas 
are less than perfectly UN-balanced or close to the rig/ shack.

Could you have a piece of equipment with poor RF shielding around the 
amp? Some tuners have pretty poor cabinet designs and poor 
panel grounding. The AL-1200 has about one zillion screws (OK, 
about two dozen) and the cover should have no paint under each 
screw. 


73, Tom W8JI
w8ji.tom@MCIONE.com

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