Understanding maintaining re tuning antenna duplexers a band pass duplexer using six band pass type cavity resonators is shown in schematic form in figure 1.
Duplexer cable length calculator.
The cable lengths from the repeater transmitter and receiver to the duplexer are usually not critical but making them a half wavelength or multiples of a half wavelength in coax with velocity factor is not a bad idea.
Put the meter after the duplexer and before the dummy load.
I ve made dozens and dozens of cables to duplexers only making them a short but comfortable length and have never had a problem.
The 1 4 wave length cables between the tx and rx to antenna t are critical not for impedance matching but to present a high impedance between tx and rx to isolate one from the other.
The next post will cover the use of the variable length cables and include screen shots from instruments and photos.
I have acquired a like new sinclare 4 cavity uhf duplexer but without the interconnecting jumper cables.
Now remove the jumper cable and dummy load and connect the feedline and antenna.
Note that each branch has three cavities connected together with cables.
A tee junction and two more cables connects the two branches with the antenna feed line.
The impedance in the pass range of the duplexer is hopefully in the 50 ohm range and cable length makes no difference to the impedance in the pass range.
If you do accidentally connect the wrong tx to the wrong duplexer port you probably have enough cable slack to fix it without specifically needing to cut to precisely equal length.
You do not put the thruline meter between the z matcher and the duplexer because the change in electrical length the presence then the absence of the wattmeter and its jumper cable will change the tuning.
The formula is 300 000 000 28 500 000 10 526316 meters or directly in mhz it would be 300 28 5 in order to calculate the answer in feet you could use either 984 frequency in mhz or a more accurate formula the one i use is 983 6 frequency in mhz.
The cable uses m f sma connectors to vary the length.
75 c for pvc and 90 c for xlpe 90 c and 110 c for xlpe 110 c.
For example let s calculate the length of a radio wave at 28 5 mhz.
In this video i show the theory and practical methods at determining inter cavity duplexer lengths.
I can totally see a practical use like that getting urban legended into a duplexer in a suitcase needs equal length feedlines.
A variable length phasing cable for use in determing duplexer cable lengths has been described as an cheaper alternative to a set of phasing cables with a 10mm length increment.
