I’ve been setting up a small trailer for use as a mobile, independently powered radio station for use at HF and VHF. To that end, it has its own deployable solar panels, internal power storage, and dedicated power conversion electronics. There are quite a few 12.6 VDC VHF and HF radios out there, and I own several, so the radios are less of a problem than a situation where I have to make some kind of fun decision between multiple good options.
VHF antennas are no problem. But the HF antenna… There’s the rub. If it’s to be truly portable, that means it cannot be large, and it will not be high in the air. Those are very significant drawbacks for most antenna systems; they directly impact both receive and transmit performace. So I went hunting. Eventually, I happened upon the Monogap 20 meter antenna, which is a vertical dipole; easily portable within the frame of reference of what can be safely transported in the bed of my pickup, light, easily mounted and unmounted, inexpensive, and — as it turns out — a surprisingly good performer.
The end result was very pleasing; I got the antenna performance I wanted, portable enough to make me happy. The price was good too; just over $100 USD. So you can take that as a recommendation if you don’t care to read any further. However…
There were a few assembly problems. The manual isn’t very good. The instructions are all jammed together, there are no photographs or detail drawings in the context of the instructions (there are two pages, unfortunately back to back, that show parts and a crude drawing of the final assembly which is of little to no use in seeing if you’ve built the thing correctly) and there were some actual mechanical issues as follows:
The center insulator was pre-drilled incorrectly. There are pairs of holes in one of the aluminum elements which are supposed to line up with pre-drilled holes in the insulator intended for self-tapping screws. They don’t actually line up:
So, you end up drilling into the element, offset just a few fractions of an inch from where the pre-drilled holes actually are (so structural integrity is less than it would otherwise be, although I don’t think it’s a serious compromise.):
Finally, there’s a design issue. near the base of the bottom element, the supplied coaxial cable exits the assembly through a manufactured notch in the element. This is intended to prevent the coaxial cable from being injured — crimped or severed — by a “guillotine” effect against the secondary aluminum tube that goes inside the PVC mount; without this notch, the cable would be cut where the cable exits the lower element.
However, as you’ll observe in the image below, the antenna is assembled using sharp, self-threading screws. These extend a considerable length into the interior of the aluminum antenna elements. There is one right by the coaxial cable exit point, as well as others further up the assembly (not shown here.) The coaxial cable will rub against the points of these screws, and it is my estimation that the end result of that will eventually result in damage to the coaxial cable. There is an identical screw on the opposite side of the element here, and at all the other locations as well.
So there are many opportunities for erosion of the insulation and (eventually) the cable conductors. I’m not sure what to suggest here other than mount the antenna so it doesn’t move, and so the cable does not move. It would have been far better if the cable ran down the outside of the assembly so it could be zip-tied or otherwise firmly affixed in such a way as to not put the coaxial cable at risk:
Because the antenna has three wire radials integrated with it as part of the basic assembly, I suggest you grab yourself a package of lawn staples in order to get the radials spread out to their full length and equally separated from one another. Those radials matter; don’t just lay them randomly on the ground, or you’re giving up a lot of performance for no good reason.
The antenna can be made (somewhat) easily portable by unscrewing the four screws at the upper portion of the center PVC insulator. This splits the antenna into two more-or-less equal sections which can in turn be easily transported via pickup or trailer, and then trivially re-assembled upon arrival at the Awesome Field Location.
So overall, I give the antenna a 7/10 based on the assembly problems and lousy instructions; but as an easily portable vertical radio antenna, I give it a 9.5/10. Basically, once you manage to get it built correctly, I expect you will be very happy with its performance, portability, structural integrity, and general affordability, just as I am.
It took myself and a friend about two hours to build it. Would have been about half that with better instructions and no dealing with manufacturing errors. The result, however, was well worth the candle — even if it had taken twice as long. Assembly only happens once. Once built, the 20m Monogap is pretty fabulous, really.