Exploiting your antenna analyser #18

Measure velocity factor of open wire line

One of the measurement tasks that one often encounters is to measure the velocity factor of a transmission line.

Often this is an indirect task of tuning a tuned line section, my method is to often measure some line off the role, find the velocity factor (vf), and use that to cut line for the tuned section making appropriate allowance for connectors etc.

Measuring vf for an open wire line includes all that is done for measuring vf of coax, but requires measures to ensure that common mode current does not affect measurement significantly.

To minimise common mode current effects, I will use two measures:

  • a high common mode impedance Guanella balun; and
  • form the line section being measured into a loose helix supported on some fishing line to spoil any common mode resonance.

Aa600BalunRg316

Above is the balun used, it is described at Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Jaycar LF1260 suppression sleeves.

Measuring the resonance of the line section is simplest if the reference plane is adjusted to the terminal screws on the balun. The AA-600 allows OSL calibration to that reference plane, but a simpler method that is sufficient for the task and available on a wider range of analysers is to use the facility in the software to back out the length of transmission line equivalent to the balun and adapters.

Screenshot - 08_05_16 , 14_39_52

Above is a scan of Z with a s/c on the balun screw terminals. It shows the classic response of a short s/c stub, the balun comprises a length of RG316 and some adapters.

Lets use the “subtract cable” facility of Antscope to back the line out.

Screenshot - 08_05_16 , 14_39_38

Above is a scan having backed out some length of RG316. The length was adjusted until the green X line was a flat as possible, essentially the scan is indicating a near s/c over 1-100MHz… which is what we have at the screw terminals.

Screenshot - 08_05_16 , 14_39_23

Above is a screen shot of the cable parameters set.

Now lets measure a sample of open wire line, in this case an 8.6m sample of the line on the roll, nominal 300Ω slotted TV ribbon.

Screenshot - 08_05_16 , 15_02_28

Above is a scan about the first half wave resonance of the o/c line section.

Noting that the first half wave resonance of this line section is 16.8MHz, we can calculate the electrical length as c0/f/2=299792458/16800000/2=8.922m, and since its physical length is 8.286m, vf=8.286/8.922=0.93.

For a half wave at 14.15MHz, we need c0/f/2*vf=299792458/14150000/2*0.93=9.852m of this line.

I have performed this demonstration using the AA-600 as the associated software Antscope provides a convenient means of finding the length of cable to back out. My other analysers / VNAs either can’t do it, or are very inconvenient (AIMuhf).

References

  • Duffy, O. 2001. RF Transmission Line Loss Calculator (TLLC). VK1OD.net (offline).

Watch the blog for continuing postings in the series Exploiting your antenna analyser. See also Exploiting your antenna analyser – contents.