It’s Not a Doublet, It’s a Derblert

While a doublet sometimes refers to a simple dipole, this application adds an additional requirement – balanced feed line. The advantage of a balanced feed is ease of tuning at frequencies higher than the dipole’s resonant frequency. This, in turn, confers an added benefit of additional gain when operating at frequencies for which the dipole’s length is a multiple wavelength. In locations where an antenna farm cannot be built, this design is an acceptable compromise. One disadvantage of the doublet is the requirement of an antenna tuner to match the impedance of the feed line to the radio when changing frequencies. 

I recently acquired an old manual antenna tuner and, having learned about the doublet antenna, decided to make an attempt at building it. There was just one catch: I had no balanced line with which to feed the elements! Confident that I could assemble a length of ladder line, I began searching for suitable build components. I then realized there was another catch: the old tuner, a Kenwood AT-200, did not have outputs for balanced line! After some research, bugging other hams, and many cups of coffee, I opted to try adding a current balun between the tuner output and the feed line.
The parcel of land which houses my station was once home to several successive parties of squatters, all of whom left the place a mess with copious amounts of household trash and random junk. To create the feed line, I took a long length of outdoor power extension cable, presumably used by the squatters to steal electricity from the neighbors. I also found an old broadcast AM receiver, from which I extracted a ferrite rod to use in the current balun. At least one party of squatters used the place for clandestine cultivation of contraband flora. Seemingly, a later party used their irrigation lines as rope to bind some framing for their clandestine cultivatory operation. I removed the tubular irrigation line, hoping to use it for spacers in the ladder line.

Taking stock of my available zip ties, I realized I’d have to find some other way to bind the spacers to the feed line. After some failed attempts with twine, I found that the outer insulation of the outdoor extension cable, cut into strips, was suitable enough to keep the spacers in place. I used the few remaining zip ties at the feed point end of the line, but they were probably unecessary. For the balun, I used some speaker wire to wind around the scavenged ferrite rod and connected a little bit of coax to one end in order to pass the line through my shack’s metal wall and connect to the tuner.

After about half a week of leisurely construction, I had nearly 50 feet of ladder line. While this length would not be enough to feed the dipole at the desired point (strung between two large trees), I am impatient boor. I quickly took down my crummy 26 foot VHF mast (made from waiawi sticks lashed together), erected it at a location within the limits of the ladder line, hoisted VA7EEX’s 40m dipole (fed with my ladder line) up to the top, and tied the ends off as high as possible.

While the “derblert” does indeed provide a 1:1 match on the 40-10m bands, its low height led to some less than ideal results. My assumption is that, at 40m, ground attenuation contributed to poor DX performance. On the other hand, NVIS results were no worse than previous antenna installations. On higher bands, the radiation patterns differ, presenting multiple lobes – and, in turn, nulls. Additional consideration to that fact may be needed when orienting the derblert in future installations. 

As a first attempt at working with (and fabricating) ladder line, I’d say the results were acceptable and illuminating. Since coaxial cable is expensive and 50 ohm lengths are unavailable locally, I plan to use homemade balanced lines more often in future projects.

Getting Twisted – The Long Forgotten Transmission Line

Twisted pair has been used in signal transmission for several decades. Today, it is most commonly used in network cabling. In the distant past, it was not uncommon to see it employed as a balanced antenna feed line. Modern radio handbooks give it little or no mention. As I understand it, the regular twists serve to cancel out electromagnetic interference, a benefit not inherent in other balanced lines.

I received some unlabeled twisted pair cabling from two local hams, neither of whom claimed to know its intended purpose. This mysterious cable comes in a single twisted pair and is insulated with cloth-over-rubber (and in one case, enamel just beneath that). The stranded 22AWG conductors appear to be steel, with what appears to be a tinned copper center strand.

I had initially used the wire as elements for various HF and VHF antennas, which yielded satisfactory results despite challenges that come from not knowing its velocity factor. While it’s safe to assume that it’s not the best radiator, the wire’s inherent durability is an acceptable compromise for my purposes. Untwisting enough of the cabling to make a 40m loop is a bit of a chore, and soldering the stuff is a bit more difficult than copper of the same size, but the it was free – and I am poor!

After spending half a week cobbling together some ladder line (more on that another day), I realized the 50 feet I made was about 100 feet too short. I began looking for something else to use for feed line while I scrounged enough material to make more ladder line when it dawned on me: I might have suitable feed line – and large amounts of it – right here!

Is it safe? I don’t know! Assuming the only danger was overheating and causing a fire, I checked the line for hotspots and heat damage after each session until I was confident that I wasn’t going to burn the forest down.

Is it efficient? I don’t know! I plan to try a few experiments with my meager tools to solve some parts of the mystery. If I make some dummy loads, I might be able to use them to determine the characteristic impedance of the cable.Beyond that, I am unsure how I can investigate further. I’ll have to learn a lot more before I can confidently say much more about this mystery wire – but for now, it’s getting me on the air.

Deedle Fizzle, or WPX RTTY Failures

Over the weekend, I participated (as planned) in the CQ WW WPX RTTY contest. The planned station improvements took much longer and were less effective than expected. While antenna improvements showed decent results, efforts within the shack proved less than satisfactory. As for the contest, my results were poor. In all, it ended up being a weekend full of lessons.

  • Antenna Improvements Took Too Long!

I planned to get my derblert a little higher and add a ground screen before the event. However, between my own laziness and some unplanned obligations,  those efforts were not even partially completed until about four hours into the competition. Had I used known good methods, this time may have been reduced. Pro tip: a really long stick is no good way to thread a line into a tree.

  • Software Settings Need Work.

I’m told that contesting on Windows is preferable to Linux due to its superior logging and RTTY software. My little Atom netbook, which serves as my station computer, currently runs Archlinux. It doesn’t run WINE with windows apps very well, so I opted to try native linux software. Having failed to compile linpsk (the software I planned to try out) in time, I decided to work with my kludgey fldigi setup. Without rig control, fldigi would not populate the log field denoting operating frequency. After enabling fldigi’s xmlrpc control server, I was able to populate the log field by sending a call to the server with curl. It was clunky, and capturing incoming exchange data needs work, but it worked okay.

  • A 3kHz Waterfall Leads to Tunnel Vision.

I suspect I spent too much time on a tiny sliver of the band during my sessions. I’d spend some time spinning the dial, find a signal,  send fldigi that kludgey call, and try working the heard station. After either working the station or giving up, I’d pick an open spot nearby and call CQ. I’m sure I missed a lot of stations outside my narrow 3kHz viewport. I’ll have to get an upconverter cobbled together for HF reception on my RTL-SDR.

  • RBN’s Telnet Servers: TMI.

Since spotting assistance was permitted for this competition, and since I had such a narrow spectral display (and a netbook that can’t do anything else while using a modern browser), I gave the telnet feed from reversebeacon.net a try. I quickly discovered that I needed some kind of filter, since there was a flood of useless (for my purposes at the time) information on the incoming stream. I’ll have to explore parsing and filtering solutions before next time.

With only about 35 contacts, I don’t expect to be the “Best Hawaii Station” for this year’s WPX RTTY contest. Having only operated for about a quarter of the allowed time, I don’t even expect to be the “Best Hawaii Rookie.” Despite the poor score, the experience has provided a wealth of learning in terms of contesting and data mode operation.

WPX Deedles, anyone?

CQ World-Wide WPX RTTY is a contest sponsored by CQ Magazine. It is directed by W0YK, a ham I spotted frequently while chasing a recent DXpedition.

  • Start Time: 2017-02-11 00:00 UTC
  • Duration: 48 hours
  • Bands: 80-10m, excluding WARC
  • Exchange: RST + Serial
  • Scoring: QSO points x QSO count

Station Improvement Required

    As I plan to participate was a Single-Operator, Low-Power Rookie station, I expect to be outdone with my meager dipole. With some minor changes, I hope to lessen a bit of the losses perceived in the current system by raising the antenna and installing a ground screen beneath it. If time and the chosen area permit it, I would like to add a reflector wire in the hope that some horizontal gain may be achieved.

My station computer is not contest-ready. I plan to try linpsk against fldigi and see which will work for my station.

As a single operator station, my participation is limited to 30 of the contest’s 48 hours. This should permit me to sleep whenever conditions become unfavorable! While it may be possible to make NVIS contacts when DX propagation is poor, I don’t expect to hear many new Hawaii stations after the first 12 hours.

If I understand the rules correctly (and maybe I don’t), my station will operate from Oceania as its continent. As such, I plan to maintain the bit of east-west directivity provided by the dipole’s orientation. This should provide me with a better chance to reach stations in Asia and the Americas – which should be worth more than Oceanic stations.

This will be my third attempt at contest play of the year, and so far, my results have been horrible. Here’s hoping for a score better than 1!

Got solder?

With my solder dwindling, I’m considering ways to avoid using it in a number of applications. While mechanical connections may work in most of my upcoming projects, they will undoubtedly require more materials than previously envisioned.