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..............
----- Original Message -----
From: "Robert Shohet" <kq2m@mags.net>
To: <towertalk@contesting.com>
Sent: Sunday, August 19, 2001 8:24 AM
Subject: [TowerTalk] Wattmeters and 75 ohm Coax
>
> Hi Guys,
>
> I was wondering if anyone out there knows if the Bird 43 wattmeter and
> Coaxial Dynamics peak reading wattmeter work with 75 ohm coax or just 50
> ohm?
>
> If they don't work properly with 75 ohm coax, is there anyway to "offset"
> the mismatch to get more accurate readings?
>
> Are there any wattmeters that can work properly with both 50 & 75 ohm coax?
>
> Thanks for your help.
>
> 73
>
> Bob KQ2M
>
>
>
>
> List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
> Trylon Titan towers, coax, hardline and more. Also check out our self
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>
> -----
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>
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 96 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
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