Jeff DePolo wrote:
Maybe I'm not understanding right. Are you saying that by varying the cable
length between the transmitter and the duplexer that you can affect the
insertion loss of the duplexer?
No.
Because the impedance is not matched between the transmitter and
duplexer, the 'apparent' loss of the duplexer is greater than the
manufacturers stated loss of the duplexer. Changing the cable length is
not changing the loss of the duplexer, it's changing the power that is
accepted at the transmitter port of the duplexer by matching the output
impedance of the transmitter to the input impedance of the transmitter
port of the duplexer.
And also that by varying the cable length between the transmitter and the
duplexer that you can vary the reflected
power on that same line?
Yes.
In a situation where the duplexer and transmitter have differing
impedances, and a cable optimized in length matches these impedances,
the mismatch at the duplexer is minimized, therefore the power reflected
by the duplexer is minimized.
Please tell me I'm reading this wrong...I've been
on the road a long time and working a lot of long hours, so it's quite
possible...
--- Jeff WN3A
I have found that when you get a transmitter that is 'picky' about the
length of interconnecting cable, power being read at the output port of
the duplexer is low and you cannot alter the tuning of the cavity
closest to the transmitter to make things right. In other words, the
place where lowest VSWR and maximum power transfer occurs is at two
completely different places, and power transfer is not up where it
should be (transmitter makes 100 watts into a dummy load but only shows
50 watts on the output port of the duplexer that has a stated 1.5 dB
loss (29 %)). As you get close to the 'optimum' cable length, the
lowest VSWR and maximum power transfer occur near the same place when
tuning the cavity closest to the transmitter.
I usually pay more attention to what is coming out the antenna port of
the duplexer - first. Then, when things are right, comparing forward
power going to the duplexer and power going to a good dummy load will be
very close the same, since matching the impedance of the transmitter to
the impedance of the duplexer was accomplished by some means.
I use a compensation cable along with my Bird so that the samplers
transmission line length is nullified in the line under test. This
compensation cable results in exactly 1/2 WL including the samplers
transmission line. Compensation cable lengths are outlined in the
manual for the Bird 43.
Kevin
