(Note: this involves the specific application of a balanced feedline with a balanced antenna - here I’m using a 44ft non-resonant doublet, AKA “NorCal Doublet”, fed with 300-ohm twinlead or PTFE twisted pair)
I set up a 44ft doublet on a summit yesterday and ran some 1 watt WSPR transmissions on 40, 20, and 15m using first a 300-ohm twinlead feedline and then a twisted pair feedline (PTFE insulated, harvested from surplus CAT5e cable). I’ve always been intrigued by the super lightweight twisted pair and several years ago used it once with very good results (high score in 2014 QRPTTF/SOTA event). But I always wondered about the loss involved, especially with the high SWR encountered in a non-resonant antenna.
I lacked sufficient time on the summit to be very thorough and methodical but I believe the data is valid. 30-45 minutes separated the transmissions for the different feedlines.
40m - 24 reports for both - twisted pair averaged -3.75dB down from twinlead
20m - 27 reports for both - twisted pair averaged -1.56dB down from twinlead
15m - 13 reports for both - twisted pair averaged -4.15dB down from twinlead
Perhaps one day I’ll do a more thorough and methodical test but I think these results show that PTFE twisted pair is a viable balanced feedline and makes for an ultralight option for SOTA activation, although twinlead is preferred when weight is not an issue.
73, Barry N1EU
P.S. other equipment used was Elecraft KX2/ATU and android phone running WSPR Beacon. The 300-ohm twinlead was connected using a homebrew dual core FT140-43 4:1 Guanella current balun and the twisted pair was connected using a homebrew FT140-43 common mode choke (12 turns).
Interesting results. Who measured the differences between setups, and how much time passed between testing the two antenna / feedline combinations ? Ex: you have 24 reports on 40m, did each station give you an S meter reading in db for each antenna ? Did you take the measurements of the received signal ? I have used lamp chord in a pinch as a feed line. It worked, but I have no idea what the losses were.
Perhaps you’re not familiar with WSPR. Go to https://www.wsprnet.org/drupal/wsprnet/spots and you’ll see spotters all over the world are constantly spotting all signals received with SNR in dB. By searching on my callsign, I have the reports that I can drop into a spreadsheet.
As I stated above, 30-45mins passed between xmsn on different antennas
pretty sure zipcord would have much higher loss due to its dielectric. Bear in mind that the PTFE insulated CAT5e twisted pair is designed to operate at the very high frequencies of Gbps ethernet.
Thanks for running this test. Much appreciated.
Here is some interesting data on CAT5e.
Read the entries by Hatchet. https://www.belden.com/products/cable/ethernet-cable/category-5e-cable/7930a
Belden’s specs for 7930a.
As you mentioned in a post once (I read it about 18 months late, hi!) I’ve been using 100’, 66’ and 44’ doublets with CAT5e single twisted pair feed line for a few years now. The results have been good. I make lots of QSOs and S2S contacts.
More convincing is the number and extent of RBN spots I receive. And two European stations tell me that certain recent 17 and 15 meter QSOs with them were via long path. The higher SFI has much to do with results like this. But my feed line loss can’t be terrible.
The Belden data and Hatchet’s measurements lend credence to my conclusion.
The KX3 tuner matches the 44’ doublet on 40 through 10 with very low SWR, even on 6 meters, with no power fold back (5 watts). I connect via a double banana to BNC adaptor, no balun.
I mount the antennas inverted on a single pole or horizontal when possible. A good sloping drop off, usually northeast, is a priority in choosing my operating spot.
The antenna is so light that even a small center insulator withstands the horizontal strain.
One caveat. Josh, WU7H, tells me this feed line does not work well in wet conditions,though I don’t have much experience in this situation.
Here is a picture of one of my antennas. The 44’ antenna works well at 18’. The 66 footer works better on a 30’ mast.
Barry,
Interesting report, and especially the other referenced article. I have used CAT5 in baluns and it works well up to 100 W on HF.
I would have preferred to see not just A - B tests but A - B - B - A tests with the average of the A tests compared to the average of the B tests. This reduces the effect of changing propagation but takes twice as long.
If not a correct RBN S/N test, i.e. with all devices under test at the same time, then the apparent accuracy could perhaps be improved a little with this test scenario:
As well as the possibility of changing propagation you also changed the balun. Can you be sure the losses through these are the same? Why did you feed the 300 ohm twinlead via a 4:1 balun but not the twisted pair?
Tests on feedline are best carried out on the bench. On air tests will have large margins of error unless carried out very carefully. These tests don’t fall into that category. Good fun but don’t rely on the results to be meaningful.
Yes, A-B-B-A would be nice, but I lacked the time. It definitely needs a follow-up test. Unfortunately a contracted a severe case of poison oak dermatitis from my activation and I won’t be returning to a summit any time soon.
I first connected the twinlead directly to KX2 binding posts but the ATU wouldn’t tune a low SWR on 40m. So I tried the 4:1 balun and it matched fine on 40m as well as all other bands so I stuck with it.
Both “baluns” were carefully researched, including consulting directly with subject matter experts, and carefully constructed. I have confidence that they are both low loss and, if a factor, are at least an order of magnitude less contributory than the feedine losses due to naturally high SWR characteristic of the 44ft doublet.
I like it because it’s easy to setup, fairly light, super quick band changes (with KX2-auto tuner) and performs well on all bands.
I want to duplicate your test because the original version of the antenna used 450 ohm ladder line. Here’s another video showing setup of that version: SOTA Activation W7U/TO-045 Tabbys Peak - YouTube (That’s a long video… but the link will take you right to the antenna setup… the setup is less than a minute.)
Subjectively, it seems like the twinlead fed version of this antenna doesn’t perform as well as the ladder line version. It seems like the s/n ratio of received signals is worse. They just don’t seem to “pop-out” like they did with the ladder-line version. It also seems S2S calls take longer to break pileups.
However, it’s much lighter than the heavy ladder-line, so I’ve put up with it.
I know my experience is subjective… it might be my imagination.
It’s time to learn about WSPR, etc… then, when the weather is better, compare twinlead to ladderline. I might as well try some CAT-5 twisted pair while I’m at it! I have all of this in my junk box.
Bruce, the learning curve on WSPR xmsn is fairly easy. I use the WSPR Beacon android app. I also use the Atomic Clock app to keep my phone’s clock accurate. You need to wire a cable with 3.5mm TRRS connectors on both ends (assuming an Elecraft KX2/3). You’re connecting the phone’s audio jack to the rig’s MIC jack. Wire it as follows:
Connect the tip (left audio) of the phone output to the tip (Mic audio) of the KX2/3.
Connect ring 2 (Ground) of the phone output to ring 2 and sleeve of the KX2/3.
Do not connect anything else.
USB mode on the radio with VOX on (compression off), adjust VOX gain so rig goes into TX mode and MIC gain for 4-5 bars ALC when WSPR app is transmitting. Set DISP to PA Temp so you can watch temp of your finals - you don’t really want to go much over 50degC during the 2-minute long xmsn. Lower the power level if temp is too high.
