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 www.grainger.com 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
W0OOG/5
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