Cutting My Losses

I experienced a very rare event this past weekend. I had set up my QRP field portable station at a very busy Lake Huron beach on a hot, sunny July afternoon. The scene was idyllic; blue sky, calm water, thousands of happy people. Deep beneath our feet lay the huge underground caverns of the world’s largest underwater salt mine stretching for kilometers out under the waters of the lake. Huge “Great Laker” ships lie in the harbor loading their holds. Meanwhile I was pounding brass trying to activate the Great Lakes Waterfront Trail, POTA entity CA-6003.

My radio was my new companion, the QRP Labs QMX (low band) 5-watt transceiver that had already proved its mettle on previous activations. But today came that rare event – a busted activation. Propagation conditions were not good and my antenna choices were limited by the need to operate in a busy beach environment. The activation started well, with 4 QSOs in the log in the first few minutes but, despite numerous CQs, the responses from hunters dried up completely. I resorted to hunting other activators and added a couple more QSOs to the log but that was it. My last QSO brought a signal report of 519 then nothing, nada, zippo.

Put on a Happy Face!

I told my wife that this happens to everybody once in a while but I still had fun playing radio outdoors on a beautiful day. Inside my head though … Grrr

Well, it wouldn’t be a learning experience if we didn’t fail from to time. It has been fun playing with lots of different types of antennas, but now it’s time to get serious and sort out the wires that work on a bad day from the fancy pants experimental ones.

Maybe it’s time to get back to basics methinks. Basics like the common or garden dipole or any antenna that is resonant, that doesn’t need a loading coil, transformer or transmatch. When you only have peanut power to work with and the Sun is pounding out flares and coronal mass ejections you can’t afford lossy antenna system components. Let’s take a look at loading coils for example.

For years I have been using base loaded vertical antennas, with a great deal of success. But, base-loaded vertical antennas have a big disadvantage. They use a coil at the bottom of a vertical radiating element to electrically lengthen a short physical antenna. If the radiating element is a quarter wave long and fed at it’s base, then the feedpoint is also the point of maximum current – and hence maximum radiation. If we then place a loading coil at that same point the losses will be at their highest. Losses can be lessened by placing the coil higher up the radiating element. The following chart illustrates the point.

As we can see from the table, the 90 degree point (feedpoint) is where current is at its maximum. The 0 degree point is the tip of the radiating element a quarter wave away from the feedpoint. The current declines on a sinusoidal basis.

If a loading coil is placed at the base of the radiating element the current flowing through it will be at 100% and i^2*R losses will be at a maximum.

If the radiating element is center-loaded (45 degrees), the current is down to 70% of its maximum and losses will be lower.

Alternatively, lose the loading coil and that source of power loss will be eliminated.

I have been talking about the antenna I call “Long Tall Sally” quite a lot recently. It is a stretched ground plane with an elongated radiating element and correspondingly shortened radials. It has been a solid performer – until that day on the beach when it wasn’t. Long Tall Sally has a feedpoint impedance of around 200 ohms and requires a 4:1 transformer – at the feedpoint. A 4:1 transformer introduces the possibility of loss. So maybe it’s time to give Long Tall Sally a vacation and go back to using a plain vanilla quarter wave ground plane antenna until propagation conditions improve.

That which is old is new again

Several years ago I built a simple, QRP ground plane antenna that comprises a 17ft radiating wire and two 17ft radials soldered to a female BNC connector. It can be suspended from a pole, or a tree limb and if erected sufficiently high such that the feedpoint is about 12 feet above ground, the radials can be sloped away at a 45 degree angle, which will yield a very low SWR. That antenna may be seeing action again very soon.

An alternative Ground Plane Antenna using a telescoping whip

There are times when it is more convenient to erect a ground-mounted antenna. After doing battle with the ancient rock of the Canadian Shield by trying and failing to drive a pointed stake into the ground to support a whip, I have resorted to another old bit of gear – a highly modified photo tripod.

The tripod has been adapted so that it lies almost flat on the ground. The legs can be spread to cover a wide area of several feet to provide a stable base for vertical poles. A 17ft whip is screwed into the mirror mount attachment at the top of the support pole.

The feedpoint is only a little over 6 feet high so it isn’t possible to spread radials out at the optimum 45 degrees but that isn’t a problem; the antenna still has an SWR of 1.04:1. The low feedpoint will still be less lossy than having the feedpoint on the ground as is common with even expensive commercial antennas.

A Simple Dipole – and a slightly better alternative

The humble dipole has saved an activation once before; maybe its time to give it another chance. I did what many QRPers do and improvised with available materials – hence the center insulator is a plastic bottle cap (orange object in the picture). Hey, it works and cost me nothing. Each leg of the dipole is about 16.5 feet long, adjusted for an SWR of 1.01:1. The pole is only 14 feet high so it may not yield the absolute best radiation pattern, but its very light – constructed from two crappie poles. One of the crappie poles sits on top of the base section of the other. The base section sits inside an aluminum tube supported by the tripod. To make this work I had to remove the end plug from the pole. It is glued in place but was easily taken off by gently heating it with a heat gun.

So it’s back to the field sometime very soon to see if these two simple wire antennas can get better results in the face of adverse propagation conditions.

Unlike other recent antenna builds (e.g. Long Tall Sally) they require a bigger footprint on the ground. That means I will have to choose my operating sites more carefully so that other park users are not negatively affected. Heck the ends of these wires are only at around 100 volts or so at QRP power levels. A little light electro-therapy never hurt anyone did it?

Oh, one more thing. There is another choice available in my antenna bag. It uses twice as much real estate as the dipole, but it has some gain and it also doesn’t require any lossy elements like transformers, ununs, baluns and tuners. It has been discussed before on this blog; it is very simple to build and has proved its worth with more than one successful POTA activation. It is known as the VP2E (Vertically Polarized Two Element). Follow the link to read all about.

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