Adding an Antenna Port to an Ultralight

John H. Bryant <bjohnorcas@...>

This started out to be a brief explanation of the subject and an announcement that there is a new photo album showing the steps needed to install an input port for external antennas to one of our ultralights.  That brief note morphed into a rough draft of 2/3 of the article that I'll complete next week for and the clubs. The photo album is the first one in our Photos area at

I'm going to do the same mod on the e100 and then complete the article. In the meantime here is the rough of the front part of the article:

For those of us working with the Unlimited Class of receivers, my Nirvana has always been trying to connect these little beasties to external antennas. My primary DXing interest is Trans-Pacific reception and a couple of people have shown it possible to go TP with a Barefoot Ultralight. However, to receive a goodly number of TPs, the assistance of a major external antenna is probably required. Coupling an external antenna to an ultralight can be achieved by inductively by wrapping the lead of the external antenna around the entire radio (very inefficient) or by wrapping the lead around an auxiliary "booster bar" antenna that can even be tuned to boost efficiency. Radio Plus+ even produced a commercial version of the latter called "The Q-Stick." The weakness of all of these strategies, though, is that there is a major antenna (the unshielded lead itself or the ferrite booster bar) right in the shack with you, just waiting to pick up every growl of RF noise from your computers, digital read-outs, etc.

What I really wanted was a method of attaching a shielded 50-ohm coax lead-in DIRECTLY to the ultralight radio, just like the antenna arrangement of a regular communications receiver. Frankly, I did not think that was possible. There were too many problems: impedance mis-match and, if the stock ferrite bar remained in the radio, having it pick up RF interference from the shack environment were the two that worried me the most.

Early on in the my ultralight work, I decided to explore an SRF-39FP, to learn more about the guts of this seminal receiver and particularly to explore the little 1.75" ferrite bar antenna. At the time, I was mostly thinking of REPLACING  the stock bar with something: a larger ferrite bar, air-core loop or, maybe, some sort of direct connection to a long wire, possibly using a ferrite toroid transformer. I disassembled an SRF-39 and mounted the circuit board on a large piece of perf board.  I removed the stock ferrite bar antenna from the circuit board to decode the coils and connections. By that time, others in the ultralight community had discovered that separating the bar antenna from the circuit board often improved signal to noise ratio, so when I was finished with the measurements, I re-mounted the stock bar on a set of screw terminals, just off the edge of the main circuit board. This would allow me to easily remove the stock bar and experiment with other antenna arrangements. 

After mulling things over for a few days, I decided that my first experiment ought to be to try to see just how bad the RF problem was and to eliminate the simplest solution from further consideration. The simplest solution was to place a small coupling coil around the existing bar antenna.  I had no reason to think that this strategy would work very well, since it ignored impedance mismatch issues and left that darn stock bar in place where it could listen to my computer grind out RF interference by the bucket full. I wrapped 13 turns of 28 ga. insulated wire wrap wire around the now-outboard SRF-39 bar and soldered the ends to a chassis-mount BNC jack. I took the radio over to my computer/DXing position and turned it on.  All that I could hear was computer and digital buzzing, along with my two strongest local stations.  Then, I plugged the radio in to my external antennas: a miracle occurred! All and I mean ALL of the buzzing went away, to be replaced with the complete MW band as it should be! To confirm what had happened, I switched to the Wellbrook directional phased array and flipped through the four directions: sure enough, there was a different station mix in each of the four directions.  I went to open frequencies just above/below the band and there was NO RF INTERFERENCE, even on those station-less frequencies. I unplugged the breadboarded SRF-39FP from the external antennas and, instantly, the awful buzz returned. Since the 13 turns of the coupling coil had been determined totally by "what looked right," I decided to remove the turns a few at a time to find the least number that still caused what I now think of as the "swamping effect." I quit at  five turns without noticing any diminution of the effect along the way. 

I do know that impedance mis-match issues are not very serious in a receive only situation, so that may be one aspect to explaining why my little coupling coil worked. Why the stock bar did not continue to "hear" the computer interference, even on an open frequency, when plugged into an external antenna is simply beyond my understanding.

The original breadboarded SRF-39FP eventually went on to become the heart of my table model "National SRF-39" that I've discussed elsewhere. However, my long range goal had really been to couple external antennas directly to two of my digital Ultralights: the Atkins-modified DT-200VX and a stock Eton e100. I started with the DT-200VX, since Guy Atkins had already been inside one and had taken excellent photographs. I examined those and spotted a place to mount an antenna input jack, thankfully rather close to the ferrite bar antenna.

I poured over a 2" thick Mouser catalog trying to find the smallest possible two-conductor jack for the new antenna input port, since most ultralights are almost solid with internal components and open space is at a premium.  I ended up ordering several different plug/jack combinations from Mouser so that I could carefully examine them before making a final selection.  I finally settled on a conventional monaural 2.5 mm mini-jack/plug pair by Kobiconn. (Mouser p/n 171-3304-EX and p/n 16PJ100) In simpler times, these were known as 3/32" phone jacks.

The modification process is really quite simple. I took the back half of the clam shell radio case off, revealing a stack of two circuit boards, interconnected by a set of slip-connect pin/jacks. By the way, three small switch plates that cover switches on the edges of the radio fell out. Not to worry: each switch cover is slightly different in configuration and re-assembly is a snap (literally.) No de-soldering of connecting wires was necessary. I simply raised the upper circuit board off the pin/jacks and rotated it to the right, like opening a book. This exposed both the ferrite bar antenna and the place on the front half of the radio case that needed to be drilled for the new antenna jack. (see photos.) I drilled the side of the case with a Dremel tool, though it should be possible to use an electric drill.... carefully. I installed the new jack, with two wire pig-tails already attached and wrapped the ferrite bar with 8 turns of that same 28 ga. wire wrap wire. I love that stuff, since it is "self stripping" using a hot iron. I then simply cut the wires to length and made staggered solder connections. I did not insulate the joints, since they were staggered and would be in a protected area of the plastic case.

Re-assembly went well and quickly and I soldered together a RG-174-based patch cord (BNC on one end, the 2.5 mm phone jack on the other) and plugged into the Wellbrook Phased Array. It worked like a charm, the first time!

John B.
Stillwater, Oklahoma, USA
Rcvrs: WiNRADiO 313e, Eton e1, NRD-535(kiwa-mods) + Flocka Ultralights
Antennas: 700' NE/SW mini-Bev, Wellbrook Phased Array (pre-production version)

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