Published at 10:46 on 5 August 2018
I’ve spent much of the summer building an HF portable ham radio station. I chose the µBITX kit from India, with a case from Amateur Radio Kits (also based in India). This was mainly done on the basis of:
- Low current draw (particularly on receive), and
- Low cost, because money is scarce right now.
The µBITX seemed to satisfy both requirements and still had fairly good reviews. Its biggest annoyance is that the receiver doesn’t have an AGC (so volume varies greatly depending on signal strength) or an AM mode (which limits its usefulness for shortwave listening). All in all, though, it was still a good deal and I am not disappointed with it. For under $200 and some assembly work, I got a decent HF portable rig.
That leaves the antenna. I’ve fought the portable antenna struggle a bit before. My first such purchase, several years ago, was the Buddistick. I was not impressed, mainly because the loading coils are extremely awkward and fiddly to adjust. You must hook one of these special banana sockets onto the coil at exactly the right point, and the tiny hooks that engage with the coil are a pain to both engage and disengage. Worse, you virtually never get the right spot to tap on first try, so you have to fiddle with the darned things multiple times to get the antenna tuned up. So scratch that idea.
I came up with the idea of using a doublet fed by window line and tuned with a balanced-line tuner (the latter item purchased used for about $30 at a hamfest). That idea ended up being dumped (after building and using the antenna a couple times) for a variety of reasons:
- Window line is a pain to deal with. It’s stiff, plus its conductors are brittle and fracture easily, plus there’s no good ready-made connectors for them that screw together and resist spontaneous disconnection (a must if the height and distance of the antenna varies due to portable use).
- Putting an antenna into a tree is a pain. Antenna launchers are prone to tangle line and get stuck in the tree. Do this twice, and odds are you’ll have at least one headache. Spending hours putting a doublet or a dipole into the trees is a justifiable time expense if the antenna is permanent and going to be used for years. Not so for a portable one that will be used for days or hours.
- A doublet cut for the 60 meter band is just under 100 feet (30 meters) long. That’s more demanding of real estate than your typical campsite can offer. And you still can’t use it on the 75 meter band, which is a big minus during a sunspot minimum (and if you’re interested in regional communications).
So enter the random wire. I did end up with an MFJ-971 tuner as a result of my doublet fiasco. Turns out it does an excellent job of matching most random wires as well. And I had the wire from the doublet I could salvage and use for the random wire.
But “random wire” is a misnomer: you won’t always get acceptable results from tuning up any old random length of wire. If your tuner is at the feed point, and your random wire is not even ¼ wavelength long, you will get no current peaks on the wire. This will seriously compromise its ability to radiate. If the random wire ends up being ½ wavelength or a multiple thereof, you will have a voltage peak at the feed point, which will frustrate your ability to feed energy into the antenna, which in turn compromise your ability to make the antenna radiate. One must choose a length which avoids these pitfalls at any frequency of interest. If one also wants to maximize portability, that means you must choose the shortest such length. The exact answer varies, but comes out to somewhere in the neighborhood of 72 feet, depending on who does the calculating, if you want to use all the ham HF bands from 80 to 10 meters.
That leaves the issue of feeding the antenna. The simplest thing is to just use a single lead-in wire. No coax (and no associated losses due to a very poor match between coax and random wire), no awkward, stiff, breakage-prone ladder line, just a nice, simple, inexpensive, easy-to-use wire. But “lead-in” and “antenna” are big misnomers. There is no difference between the two in this case; the “lead-in” will radiate just like the “antenna” does! Connect a lead-in wire to an antenna at a carefully-chosen length, and the length is no longer carefully chosen, and you might be at a multiple of ½ wavelength again.
There’s a simple, elegant solution to that problem: cut a piece of 72′ wire, design an insulator that can hold the near end of it using friction alone, use that for the near-side anchor, and connect the tuner directly to the single piece of 72′ wire. Place the near-end insulator wherever is needed to get the required bit of slack wire to serve as a lead-in. But that begs the issue of designing such an insulator.
This is what I came up with:
It’s a 1/2″ PVC tee connector. One of the straight ends has been drilled with a 5/16″ hole for passing an anchoring rope through. The wire passes in through the 90° end and out the end opposite the drilled one. Where it enters, it is secured by a No. 4 rubber stopper; as the tension of the wire pulls on the stopper, it will merely tend to pull the stopper in tighter, thus the stopper holds the wire securely.
A random wire requires a counterpoise, of course: there is in reality no such thing as a “monopole” antenna; all antennas are two-terminal devices. What I am doing is trailing a 60-foot wire along the ground, plus using an alligator-clip lead to ground the tuner to my truck chassis.
How does it work? Quite well, based on my initial results. It seems to get out much better on 75 meters than my previous random wire (which was too short to have a current peak). It’s twice as easy to set up as the doublet (only one end must be launched into a tree, not two). It’s shorter than than the doublet, and installs at an angle, further reducing its real-estate demands. It can fit in a standard campsite. Yet despite the smaller size, unlike the doublet it allows me to operate on 75 meters. I noticed no slippage at the near-end insulator, which behaved exactly as intended.
There is a drawback to this design, and it is RF exposure. Because it doesn’t use transmission line to feed the antenna, this design will radiate in the immediate vicinity of the operator and radio equipment. That’s not so big a deal at QRP power levels, but I wouldn’t want to use this design at higher power. Overall, I consider the freedom to use a simpler antenna design to be yet another advantage of QRP for portable use.