[TowerTalk] Antenna analyzer overload from BC

[dan hearn]

Hi Pete: you are one of many who have had this problem, including
myself. When I attempted to gamma match my 131 ft crankup on 160, the
259 meter was pinned regardless of what I did. I am 20 miles from the
local BC stations. I went to my old reliable resistance bridge,
sometimes called an "Antenna Scope" when I first saw it described in HR
magazine many years ago. It has been described many times and a version
is in the ARRL antenna book. I have a 250 ohm Ohmite type AB pot in
place of one of the 50 ohm fixed resistors which is calibrated with
various values of resistors on the output. I drive it with an old Tek
190B oscillator. Vary the oscillator freq and the pot until a perfect
null is obtained. You read the feed point resistance from the pot dial
and the resonant freq from the oscillator dial. It works every time and
seems to be bullet proof to BC interference. This resistance reading
gives you a very good idea how effective your radial system is. If you
see less than 40 ohms for a 1/4 wave vertical, you have a very efficient
radial system.
  Unfortunately, this rig is awfully cumbersome to carry up your tower
but it was done once to solve a difficult problem, Hi. 73, Dan, N5AR

Pete Smith wrote:
> 
> Yesterday I tried to use a borrowed MFJ 259B to tune the omega match for my
> 160m shunt fed tower.  Readings were all over the map, followed no logical
> pattern, and would jump dramatically from time to time -- an R of 24 would
> suddenly go to >600, for example.
> 
> By pure dumb luck I managed to get the match set up almost perfectly -- SWR
> 1:1 at 1830 -- but I'm intrigued.  The nearest AM broadcast station is ~5
> miles away and runs only 500 watts on 1550 kHz.  It pins the s-meter on my
> Mark 5, barely -- 6 dB of attenuator takes the meter off the pin.  Could
> that possibly be enough power delivered to the antenna analyzer to make it
> behave erratically?
> 
> 73, Pete N4ZR
> 
> The revised World Contest
> Station Database
> is online at www.pvrc.org
> 
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________________________________________________________________________
This list is sponsored by the new eHam.net Store.  When you buy products
like ICE filters, Array Solutions StackMatch, or M-Squared Antennas from
the eHam Store for the same price you pay direct, a portion of the sale
price goes to support this list.  Check it out at http://www.eham.net.

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