In a message dated 2/23/2007 6:36:24 P.M. Pacific Standard Time,
[EMAIL PROTECTED] writes:
In regards to the question of a wattmeter adding length to a cable, in
places where I use Z-matchers, I actually leave an N-barrel connector in line
that
can be removed for inserting the wattmeter without adding any cable. So
when you put that barrel back, you're pretty darn close. ...
Not necessarily. At low band VHF frequencies, you could probably discount
the error produced
by the Bird. But in high band VHF and up, it becomes progressively more
critical. The constant is the physical length of the Bird line section which is
3
13/16 inches but including the terminating N connectors, the length is 5 1/8
inches. This is where it gets interesting. The line section is air line and
has a
different velocity constant than RG142 or RG214.
Section 3-35 states:
Using the THRULINE you will be inserting a 4 inch length of
50 ohm air line
and the load on the transmitter will be changed from its
original condition
without the THRULINE.
But Section 3-40 states:
Since the length of line between a mismatched load and the
source transforms
the impedance of the load as seen at the source, line length
now becomes
critical. If the adjustments for maximum power transfer were
made with the
Model 43 in place, removing it shortens the line by four
inches, plus two
connectors. (emphasis mine).
So one section says to allow 4 inches while the other section says to allow
5 1/8 inches.
Go figure. In any event, the use of a single barrel would miss the mark at 2
meters and
above.
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