In Common mode current and coaxial feed lines, I mentioned that common mode current is easily measured,
Measuring common mode current on coax
Category: Transmission lines
Common mode current and coaxial feed lines
An antenna feed line is intended to convey energy from the transmitter to the antenna, and usually without giving rise to radiation itself.
The term “common mode” comes from consideration of the currents on an open two wire line, and it refers to the net or unbalance current, ie the current that would give rise to external fields, to radiation.
This article looks at the equivalent common mode current in a coaxial transmission line.
RG174 for portable HF operation
There are frequent recommendations of RG174 for portable stations (eg SOTA), usually running QRP, principally because it is light and easy to wind up into a small package to fit into a pack. RG174 commonly uses silver coated steel (SCS) centre conductor, sometimes copper clad steel (CCS), sometimes copper.
Speaker cable as RF transmission line
Speaker cable and similar Figure-8 twin in various sizes is popular as RF transmission line, particularly with QRP operators, especially those operating portable in the field (eg SOTA activators). Indeed, SOTA seems to have triggered a revival in the use of these cables. With the large number of online recommendations, one could be forgiven for thinking that a dipole and feed line formed entirely from #24 speaker wire is the antenna of choice.
With that in mind, this article looks at the performance of Jaycar WB1702 speaker cable, 14×0.14mm conductors (#24) with PVC insulation, cost $0.50/m.
Measuring matched line loss
Matched Line Loss (MLL) is the loss or attenuation of a transmission line terminated in its actual characteristic impedance, and usually give for some length, eg dB/100′, dB/100m, dB/m.
One method that occurs in ham radio articles is to measure the input resistance of a resonant s/c or o/c stub, and to calculate MLL as 8.686*Rin/Zo/length dB/unitlength. (This is not the only method, or even a good one, but it is commonly used.)
Windowed ladder line loss – G3TXQ
G3TXQ has published (undated) a set of measurements on commercial windowed ladder line at Wet ladderline (accessed 10/01/14).
His measurements are of interest as there is a dearth of credible measurement data for these types of lines below 50MHz. Continue reading Windowed ladder line loss – G3TXQ