[TowerTalk] Wattmeters and 75 ohm Coax
K7GCO@aol.com
K7GCO@aol.com
Sun, 19 Aug 2001 16:37:50 EDT
<< n a message dated 8/19/01 9:52:28 AM Pacific Daylight Time,
W4EF@dellroy.com writes: <<
Hi Bob,
I don't know about 75 ohm options for the Bird 43 or Coaxial Dynamics
wattmeters. You might want to call the factory and ask them if they can
modify the directional couplers for a 75 ohm characteristic impedance. They
probably don't get much call for this as 75 ohm cable is typically used in
CATV and video applications where RF power levels never get much beyond 100
mW. Of course, the other question is do you really want to do that? If you
are trying to match your antenna to 75 ohm CATV hardline in order to
minimize mismatch losses along
a long cable run, then you would want to normalize your instruments to 75
ohms.
The other option would be to measure the
impedance at the input to the 75 ohm line with an impedance analyzer like
the
MFJ-259/269 and then plug the impedance reading into the formula for VSWR
normalized to 75 ohms (this is a simple algebraic equation). You can also
place
a 50:75 ohm UNUN at the input to your 75 ohm cable. This will allow you to
read
the 75 ohm VSWR/reflected power directly with the 50 ohm instrument.
On the other hand, if you only concern is the match that your amplifier
sees, stay
with the 50 ohm instruments - WYSIWYG. Power output is just Pforward -
Preflected.
Mike, W4EF..............
>>
Very good suggestions. There are many applications for a 75 ohm SWR bridge.
I have a roll of 3/4'' 75 ohm hardline I'm eager to use. As I just suggested
in a previous post, it would be ideal to have a center conductor in the Bird
that could be replaced with a smaller diameter center shaft for 75 ohms.
Hopefully no other major adjustements would be needed. I'll talk to Bird
about that.
In the older MFJ Analyzer I replaced the 50 ohm resistor in the RF
Wheatstone Bridge with a 75 ohm resistor. It was a big and difficult job for
several reasons but it did work. I plan to do it on my latest one and with a
switch so I can either have 50 or 75 ohms. I'll write it up when I get time.
k7gco
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