The myth: Measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50
ohm line on both sides of itself.

I have performed a test using components at hand where the Bird 43 has
75 ohm line on both sides of itself, and the test configuration is
designed to present a 50+j0 ohm load at the point where the Bird 43
sampling element is located. The question is, how does the Bird
respond?

The test in detail. Each component is in a list from the source to the
load:

IC706IIG
1m RG58 with UHF connectors
MFJ949E ATU
3m RG6 (Zo=75 ohm) with BNC connectors
Bird 43
5.27m Belden 9275 (Zo=75 ohm, vf=0.83) with BNC connectors
50 ohm dummy load

Short adapters were used to connect to the Bird's Type N connectors,
the dummy load's Type N connectors and the MFJ949 UHF connector.

The half wave resonance of the 5.27m length of RG6 was determined by
s/c one end and connecting the other end via an adapter to a MFJ259B
and finding the impedance dip at 23.05MHz. The calculated vf from this
test is 0.81, which reconciles reasonably with the specs. Free space
wavelenght at the test frequency is 13m.

The transmitter was set to 23.05MHz, and the ATU tuned to develop
rated power output. The ATU is only used to present the rated load to
the transmitter so as to obtain 100W for the test, to suit the Bird 43
element. It is inconsequential to the DUT (the Bird 43).

With this configuration, it is expected that the impedance at the Bird
43 is approximately 50+j0, and that there would be almost zero
reflected power.

The Bird 43 indicated 100W forward power and a quarter of a needle
width detection on reflected power.

The Bird 43 would appear to provide valid readings for the conditions
on the Bird 43 Thruline section in this case, notwithstanding that
there is not any 50 ohm transmission line attached to the Bird 43 +
N-BNC adapters.

The myth that measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50
ohm line on both sides of itself is BUSTED.

Has anyone experimental evidence to the contrary?

Owen

PS: I hasten to add / apologise, I do not watch much television, but I
was forced to endure Mythbusters when visiting a friend recently. It's
about as scientific as what goes on here, so I thought the style
appropriate!
--