On the Elecraft-KX mailing list, an item popped up on a topic that I’ve been pondering for a while. Titled Balun necessary? Barry N1EU asked the question:
I’m planning some 40M-15M portable operation with a 44ft doublet fed with 25ft of twinlead into the KX2 ATU. Is it, or is it not, necessary to use a balun between twinlead and KX2 antenna terminals for this application?
I’ve been pondering this very question myself, as my go-to antenna for portable operation with my KX-3 is a 66-ft. doublet fed with twisted pair. I’ve just been connecting the twin-led feed line directly to the radio with a BNC-binding post adapter, but it occurred to me last summer that I should perhaps be connecting the unbalanced output of the KX-3 to the balanced feed line. The antenna seems to work just fine without the balun, but I wonder in the antenna system would be more efficient with one. At 10 – 15 W out, you want your antenna system to be as efficient as possible.
As you might expect, this question generated a lot of replies. Many of the replies simply said, “A balun is preferred,” without really giving a reason for this. Many write that using a 1:1 balun or current choke would prevent common-mode currents on the feedline. The purpose of this being to reduce the chance of the feedline radiating and causing RFI or to reduce noise pickup.
I jumped into the conversation when it was well underway and mentioned that I have been using a doublet with twisted-pair feed line. That prompted a private reply from Barry, and we struck up a conversation on the relative merits of twisted-pair feedline vs. 300 Ω twinlead and whether or not to use a balun. In one of the emails, he wrote:
FYI Dan, did a little experiment. 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.
Other equipment used included an Elecraft KX2/ATU and Android phone running WSPR Beacon. The 300-ohm twinlead was connected using a homebrew dual FT140-43 core 4:1 Guanella current balun and the twisted pair was connected using a homebrew FT140-43 common mode choke (12 turns).
I should probably do some of my own testing. While scouring the shack for stuff to take to a hamfest a couple of weeks ago I ran across some quality 300 Ω twinlead, so I could make up a 66-ft. doublet with the twin lead feedline. I also have ferrite cores and try winding the twisted pair feedline around it to see if that makes any difference. Barry’s test show that the twin lead is more efficient, but twisted pair is easier to handle and does wind up into a smaller and lighter package for transporting to and from a POTA or SOTA site.
It would be interesting to do some testing with the 44-ft. doublet. In general, of course, the more wire in the air the better, but less wire again makes it more manageable. I’m going to have to download WSPR Beacon to my Android phone, too. In any event, it looks like I’ll have a lot of fun stuff to play with this summer when I can get out and do some activations.
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