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[TowerTalk] Finding roof beams, determining impedance of coax cable.

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Subject: [TowerTalk] Finding roof beams, determining impedance of coax cable.
From: (Flanders, Jim)
Date: Thu, 26 Jun 1997 10:09:07 -0500
This is a late reply because twice my email program crashed in the 
middle of typing.  Then I got busy.

Determining the location of roof beams:
        I have a few rent houses.  I'm also a gadgeteer addict.  So some of 
the tools I bought to refurbish rent houses 
are ultrasonic stud finders.  Do not confuse these with magnetic stud 
finders that only find the nails.  I have two of the
$19.95 variety that work well for studs behind drywalls.  And I have 
one that cost $125.00 that even shows me the wires
behind the wall.  After a recent post, I went on the roof between the 
thunderstorms here in Texas and tried it for finding
roof beams.  The $19.95 variety could work eventually, but it is very 
frustrating and time consuming because of the var-
iations of composite shingle thickness.   The professional model has a 
calibrator that errors out these variations, and works
well.   I thought I bought it at Grainger, but I just checked their 
catalog at and couldn't find it.

Determining impedance of unknown coax cable.  (also handy to determine 
ratio of unmarked balun):
        It's been a long time since I've done this myself (~50 years), but 
here goes.
Take a half wave length of cable and terminate it with a non-inductive 
resistor such as 50 ohms.
Place a swr bridge at the TX end.  Tx into the cable with just enough 
power to get a reading on the meter.
If the swr is 1 to 1 the cable is the impedance of the resistor.  Note 
that the MFJ antenna analyzer will also
do this for you, but not all of us have one, and we all should have a 
swr bridge and a 50 ohm dummy load.
If you don't, then build them, it will take less than a weekend. (1 to 
4  hours each)
        In the case of an unknown balun.  If you suspect that it is a 4 to 1 
balun, then simply put a 200 ohm
resistor at the other end.

Jim Flanders

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