We had a discussion about using small feedlines such as RG-174 or alternatives a while ago. I recently came back from a hamfest some bits of coax cable with different colored stripes, but no data, so I set about measuring them, and a number of other types that I had on hand. This rather lead me down a rabbit hole…
The result is an article on adventures with small feedlines:
This not only discusses several different cable types (with plots of the loss vs. frequency compared to RG-174 and RG-58), but also links to how to measure the characteristics of cables, so you can evaluate unmarked cables or other options such as speaker cable or twisted pair.
Of course I haven’t included every possible option, and I still have further work to do on parallel-conductor lines (the individual twisted pairs from Ethernet cables show promise). So it will get extended over time.
And, while there are cables with lower losses available, my 1978-vintage piece of RG-174 is still holding up pretty well. I’ll be trying out some other types more out of curiosity than a need for lower losses in a short piece of cable.
For those who want to experiment:
I tested a length of Radio Shack (Tandy) flat 4-conductor telephone cable, the type designed for RJ-45 modular plugs (remember when phones used wires?).
With alternate wires connected in parallel, it made a nice 50 ohm balanced line with less loss than RG-174 up to about 20 MHz. (SWR less than about 1.2 : 1 up to 40 MHz when terminated with a 51 ohm resistor.)
With the two wires on each side in parallel, it was about 100 ohms.
As an unbalanced line, with the two outer wires as the shield and the two inner ones as the center conductor, it was around 60 ohms.
Still more characterization to do, however, as the characteristic impedance changes with frequency, especially above 20 MHz.
But something to consider if you still have a bunch of old telephone cords around. (Sorry, don’t have a sample of British / European wire to test.) I do have some of the older round 4-conductor cable, and will try the same approach with bits of ribbon cable. That’s in addition to all the old Ethernet cables you have saved away in the garage since the switch to wireless networking…
If you look closely at CAT5/6 cable pairs, you will see that each pair has a different twist rate. This is to minimise crosstalk between pairs.
However it also means that the length of transmission line (per metre of CAT5) is slightly longer where the twist pitch is smaller. i.e. the effective velocity factor is slightly different for each pair.
At some length & frequency the phase difference will affect paralleling up two 100Ω pairs to get 50Ω
Good work on the telephone wire, it’s one I had never considered in my quest for stingyness. I have some here that appears to be PE for the wire insulation, but PVC for the jacket.
Interesting… Thank you for bringing it to my attention.
I notice that some Belden CAT 6 types are described as “low skew”, perhaps to minimize the difference?
My approach would probably be to use a single 100 ohm pair for the feedline (half the weight of trying to use two in parallel) and design the antenna to work with the 100 ohm line, and/or cut the line to 1/2 wavelength or multiple thereof. But if one were to terminate one line in a mis-matched load, measure the impedance, and then adjust the length of the second line to give the same impedance when terminated with the same resistor, that should get them pretty close.
Of course, if one is using an ATU, the feedline doesn’t need to be close to 50 ohms.
I don’t expect everyone to have access to the Archer 278-365 telephone cable that I tested, of course (or even find some still on the original spool, with the part number readable on the faded label), but I was quite surprised to see how that particular piece could be adopted to make a 50 ohm line. (I still need to do more work to understand quirky behavior above 20 MHz - differences in conductor might explain some of it, and something to be aware of going forward.) It certainly wasn’t characterized for RF use, so the actual impedance may vary with different spools or batches. The point was that, with the tools for measuring such lines, others can test the various types of cables that may be available to them locally.