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Mythbusters: V/I ratio is forced to Z0
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! -- |
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