Find coax cable velocity factor using an antenna analyser with OSL calibration
A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that supports OSL calibration, an AIMuhf in this example.
Whilst this seems a trivial task with a modern antenna analyser, it seems to challenge many hams.
There are a thousand recipes, I am going to demonstrate just one that suits the instrument and application.
We will use a little test fixture that I made for measuring small components, and for which I have made test loads for OSL calibration. We will find the frequency where reactance passes through zero at the first parallel resonance of an O/C stub section, this is at a length of approximately λ/2 (a good approximation for low loss coaxial cables above about 10MHz).
The text fixture used for this demonstration is constructed on a SMA(M) PCB connector using some machined pin connector strip and N(M)-SMA(F) adapters to connect to the instrument.
Above is a pic of the test fixture with adapters (in this case on a AA-600).
Before measurement is made of the inductor, the test fixture must be calibrated with an OPEN, SHORT and LOAD.
Above are the OPEN, SHORT and LOAD components, the load is two 100Ω 1% SMD resistors in parallel. The reference plane is a point about 1mm below the top of the pins. If one was obsessive, 1mm should be trimmed from the OPEN… but the effect will be very hard to measure at these frequencies.
The OSL calibration is performed using the AIMuhf.
OSL calibrated measurement
A sample of the coaxial line is prepared by cutting both ends clean. The sample was measured at 661mm and will have a half wave resonance somewhere below 200MHz.
The O test load (the pair of open machined pins is plugged into the adapter, and the cable end is carefully pushed just 1mm onto the pins to connect the inner and outer conductors to inside and an outer pin of the adapter (as shown earlier). The DUT is scanned in search of the consistent sensible first half wave resonance which in this case was at 183.6MHz.
A half wavelength in free space H0 at that frequency is found by dividing c0 by twice the frequency, so H0=299792458/183600000/2=816.4mm.
Velocity factor is found by dividing the DUT length by H0, vf=661/816.4=0.81.
Now that length can be used in designing tune line lengths such as phasing lines etc. Do not ignore the effect of connectors in such applications.
Analyser software often has utilities like the one above in Aim910B which I could not get to work and has zero documentation. Give them a miss and just use a hand calculator to perform calculations that you understand.
By all means, design and use your own fixtures but be aware of any change to connection lengths to the DUT that will invalidate the calibration. attempting to measure sample line sections with connectors on them of unknown electrical length, and worse, ones of unknown Zo as well (eg UHF connectors) can introduce significant error.