It’s not the smartest idea really is it? A quarter wavelength vertical whip for the 20 meter band should be around 5 meters (17ft) tall. Why build a vertically challenged whip that is less than a sixteenth of a wavelength long and expect it to work? And … with a QRP transceiver too!
Yes, indeed it’s not the smartest idea in the world, but Elecraft and others sell such an antenna and many hams use them. So is there another angle to this idea? Am I missing something? I watched another of G3OJV’s videos recently in which Peter Waters (of the UK’s Waters and Stanton amateur radio store) put a different perspective on the idea, so I decided to try it.
G3OJV’s idea is to force the counterpoise to do most of the radiating. Huh? If the counterpoise is raised above ground it will radiate – in fact, if the radiating element is only 48 inches (about 1.2 meters) tall, the counterpoise will do most of the radiating. I decided to model the idea using EZNEC to find out how to configure such an antenna so that it would actually have a chance of working. The model yielded some very interesting surprises.
But there’s a hidden gotcha!
The radiation pattern looks fantastic, and the gain is equally impressive at 4 dBi. It’s no wonder these little critters sell so well, but wait, let’s take a closer look. The NEC-2 engine I use for antenna modeling is notoriously unreliable for antennas that are close to the ground. I had set the Ground Type to Real | MININEC. Now suppose we take another look using Ground Type: Real | High Accuracy. Whoa, now we lose 6.4 dB; the actual gain figure is now predicted to be minus 2.4 dBi.
There are two ways to look at this result. First, the optimistic view: -2.4dB is less than half an S-unit at the receiving end – the receiving station probably won’t even notice the difference. Second, the pessimistic view: if our transmitter is pumping out a mighty five watts of RF, the amount of RF actually leaving the antenna drops to less than 3 watts (ignoring any losses in the feedline). On a good day when the propagation is favorable we are still going to make contacts. But, if the propagation is unfavorable, or we are trying to break a pile-up, our self-imposed handicap is going to be a problem. And, all of this assumes that NEC-2 is not being optimistic in its calculations.
But wait, there’s more bad news
I only modeled the far field propagation of an unloaded 48 inch whip worked against a 17 ft (5 m) single raised counterpoise. Of course, a base-loading coil is also required to match the impedance of the short whip. We’ll get back to that in a moment.
Let’s go ahead and waste more of our radiated power
Typically, very short whips are used for portable operations, and the temptation is to simply throw the counterpoise wire on the ground. Now EZNEC Pro/2+ set for high accuracy ground shows a gain of -3.31 dBi and our actual radiated power drops even further to a little over 2 watts. We are on our way to QRPp! (QRPp refers to radiated power of 1 watt or less). Remember, these little baby whips work best when we force the counterpoise to do most of the radiating. If we lay the counterpoise down on the ground the whole strategy is defeated (but the earthworms will be grateful for the warmth).
About that loading coil
The usual method of constructing a loading coil is to wind wire around an air-core former, or some other non-conducting material. Sometimes the wire used is copper, sometimes it is stainless steel. Now we have to take into account the DC resistance of the wire. Copper is good, silver or gold is better; stainless steel is ungood. You don’t need to be a rich ham to use silver or gold (although it might a good hedge against inflation if you wind all your coils with solid gold wire); due to the skin effect gold-plated wire would work very well. Even copper-plated stainless steel would be better than unplated stainless steel.
The website 66pacific.com calculates that a loading coil of 9.1 microhenries is required to match a 48 inch whip at 14.150 MHz. That would require 30 turns on a 1 inch diameter air-core former. The length of wire required will be about 95 inches (2.4 m). The same coil can be wound around a powdered iron toroidal core with considerably less wire. Less wire means less resistance loss. Are we nit-picking here? Yes, the resistance losses are fairly small, but remember, we are already down to 2 watts out of our original 5 so every watt counts.
The long and winding coax that leads to your transceiver
Our best bet is to use a very short whip mounted directly on the antenna connector of our transceiver, otherwise coax loss can also become an issue. But make sure your antenna connector is mechanically capable of supporting the weight of a loading coil and whip. Some radios have a BNC antenna connector directly mounted on the transceiver’s circuit board which is an obvious weak point. Further stress is added if the counterpoise directly connects to the shell of the BNC.
I built a 48 inch whip antenna; how well did it work?
I own a collection of old ham stick antennas. Hamsticks have two parts, a helically wound loading coil section and a stainless steel whip. I grabbed one of the whips which happened to be the magic length of 48 inches and mounted it on a tripod with the feedpoint about 4 feet above ground. I then added a loading coil wound with 16 gauge enameled copper wire around a small type 2 toroidal core. My counterpoise was a single radial wire, 17 ft long sloping down from the feedpoint to a few inches above ground at the far end. EZNEC predicted that mounting the far end close to the ground improves the performance.
I fired RF at the antenna, through a few short feet of RG-58 coax, from my mighty mini QMX transceiver at nominally five watts and monitored the SWR on the QMX – all was good. Time to go hunting.
I considered it risky to attempt a POTA activation with an experimental antenna so the trial was to be a backyard hunting session. Well, knock me down with a feather, I actually made a contact with my microwhip. Propagation conditions that day were forecast to be good on 20m and I could hear several POTA stations on the air. I selected a station in Florida which is over 2000 km from my QTH in Ontario, Canada. I could hear the Florida station fairly well so, with a good deal of trepidation, I called him on CW with 5 watts into my test antenna. He came back to me right away, and after a couple of attempts to copy my callsign, we signed off and I put the contact in the log with his RST report to me of 229.
Wow, is this antenna a “keeper”?
No.
There is a popular expression in the QRP world: “QRP – when you want to send the very least”. My interpretation is: when you want to send the very least – get a 48 inch whip!
What is your experience?
If you are still wondering (really?) what my opinion of these vertically challenged antennas could possibly be, then let me say there are better options for ultra-portable lightweight antennas. Even a slightly longer whip would be better. I also built a version with a 9 ft whip – the extra length makes a lot of difference. Adding a top (capacitance) hat improves performance even further.
But, I am sure there are Ham Radio Outside the Box readers with entirely different experiences. For example, if you are an FT8 operator, the lower signal to noise performance – even over CW – may help to squeeze contacts out of a highly compromised antenna. SOTA operators working from the top of a mountain out in the Rockies may experience very short whips very differently. Perhaps if I were to take my 48 inch whip out to one of Ontario’s soaring peaks which sometimes scrape the clouds at 500 m (1640 ft) above sea level I might have a different experience. But then again I might go to the same high point with a real antenna!
Let me know what you think in the comments.
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