At 10:19 PM 02/23/07, you wrote:
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.
I read it as he opens up one of the male-to-barrel connections and inserts
the combination of a Bird with the jumper cable that makes it plus the cable
a half wave. This way nothing changes.
Mike WA6ILQ
