Thursday, April 13, 2017

Antenna Alchemy - End Fed Half Wave Antenna and The Way of the Goat

drawing de DL3TU

Sota Mountain Goat is an award for accumulating 1000 activator points from mountain summits (see post). At least in Colorado, many of the winter activations are harrowing experiences in cold weather, deep snow, and high wind. Since it's not always practical to "hang around" at the summit the speed to set up and tear down can be critically important. No one wants to invest extreme energy and time climbing a mountain for an activation a descending with nothing to show for it. So, quick deployment and effectiveness are the watchwords for the kit.

When I first started SOTA activations, I looked around at what other, wiser, Goats were using in their kit and tried to copy them. I favored the lighter weight alternatives - no heavy radios, batteries or antennas for me. Aside from the transceiver, and mountaineering gear, the antenna stands out as the most important element of the system. I noticed that most of the Mountain Goats were using the End Fed Half Wave, or EFHW for short. While I had never previously used one, I thought I knew just about everything I needed to know about them. After all, isn't it just another flavor of vertical...? I was wrong. What I discovered shocked and intrigued me, as I think it will you, too.

At first, I was a bit confused about the difference between a random length end-fed, and the EFHW. The terms seem to be used interchangeably. Some of the articles I read had charts that showed lengths to avoid when cutting so-called "random length" wires. Turns out that if you randomly cut your random length wire to one of the "forbidden" lengths, it won't match, even with a tuner, and the SWR losses will be high. So, I initially avoided them. Only later did I learn that those forbidden lengths happen to be the half wavelengths or HW part of the EFHW, and these lengths are where some magic happens.

The Sota Goats don't want to dilly dally at the summit in dicey weather. They often need to set up, work their quota and get down. So, they don't have time for the one thing that we are all told we absolutely need in order to make a vertical work efficiently - radials. They need to extend a pole, string a wire, attach it to the radio and get on the air. The fastest way to do that, and put out a decent signal too, is with the EFHW - an antenna with a forbidden length.

The problem with the EFHW, the reason that it is "forbidden", is that it has an outrageously high feedpoint resistance at resonance - on the order of 3000 ohms. This is not a number that is friendly to any radio and it is outside of the range of typical matching devices. However, it is the high feedpoint resistance that is the secret of its success. Tame it and it will serve you well.

Why is the high impedance that we've been told to avoid actually an advantage? To answer that, let's examine the characteristics of a cousin of the EFHW that we are all familiar with, the quarter wave vertical. The radiation resistance of the quarter wave vertical is about 36 ohms. In a typical installation (4 or so radials) the ground resistance can be substantial - let's say 42 ohms for the sake of illustration. So the antenna presents a decent match to 50 ohm coax of 36 plus 42 ohms = 78 ohms, a good match to 50 ohm coax. The efficiency of this antenna is calculated by dividing the radiation resistance by the radiation resistance plus the ground loss... Rr/(Rr + Rg) = 59%. About 40% of the power is lost in the ground system. The situation gets much worse, however, if only one or two radials are used. The ground loss will rise to perhaps 80 ohms or greater and the efficiency will drop to 25% or so. In other words, the 5 watt QRP radio lugged to the top of the mountain will radiate around 1 watt with one radial attached to a quarter wave vertical.

In a fixed station installation, reducing ground loss is simply a matter of putting down a lot of radials. In the field, this is not so practical. Instead of putting down more radials, one solution is to raise the feedpoint impedance, the reducing the current flow at the base of the vertical and the corresponding ground loss. So, compared to a quater waver vertical with a poor ground systems, the situation improves somewhat with the so-called end fed random wire when it is longer than 1/4 wave. The typical impedance of this antenna is around 500 ohms. The radiation resistance isn't typically any higher than that of a quarter wave vertical but there is less current at the base of the antenna and you can get by with fewer radials. Avoid the "forbidden" lengths (43 feet is a popular length) and it works well with a 9:1 unun. A matching device (tuner) is generally required.

Now let's look at the EFHW. At resonance, the feedpoint resistance is approximately 3500 ohms. Consequently, there's even less current at the base of the antenna, obviating the need for radials. So, with just a short 3 foot counterpoise this antenna should be as efficient as a half wave dipole, making it ideal for quick deployment. Used with traps or jumpers a tuner is not required. SWR of less than 2:1 at resonance is typical with a 64:1 unun (16:2 winding ratio). Refer to the posting for instruction on how to build one.

But there has to be a catch, right? Otherwise, everyone would be using these cheap and simple antennas instead of quarter wave verticals. Well, yes, but most of the limitations just aren't very relevant for QRP in the field. Here are a few of the characteristics and limitations of the EFHW. There aren't any deal breakers on this list for SOTA work. The reason that this antenna isn't more popular in fixed stations has to do more with impedance matching issues than anything else - the popularity of coaxial cable for transmission lines and the lack of good high-efficiency unun's made it impractical, until recently. Broadband unun's with high transformation ratios are still largely the stuff of experimenters - but the experiments have been promising. So, have at it.
  • The EFHW can be built for multiple bands by installing traps or jumpers. I use an EFHW built for 40m with a jumper to shorten it for 30 meters. It works well on 40, 20, 10 with the jumper inserted, and on 30, 17/15 with the jumper open. SWR is 1:1 on the half wave frequencies, and matchable with the KX2 internal tuner on the multiples. I have recently built a trapped antenna with good results.  
  • Ground losses increase off the resonant frequency and efficiency suffers- cut it precisely for your SOTA freq.
  • While the feedpoint resistance of the EFHW is quite high, the radiation resistance is approximately the same as a half wave dipole, 73 ohms (the EFHW is essentially a half wave dipole fed at the end instead of the center). Though radials aren't required, ground proximity and other ohmic losses will adversely affect efficiency, depending on the average height of the wire above ground, and the efficiency of the matching transformer, traps, etc., in the system.
  • the impedance of the EFHW is too high for most matching devices (tuners). A 4:1 or 9:1 unun can bring it into range for most tuners. 
  • for a low SWR without a matching device, the antenna requires approximately a 49:1 unun impedance transformer (14:2 winding ratio) to match 50 ohm sources. It may be necessary to add a capacitor across the source for a match on the higher bands. I used a 64:1 transformer with success as described in another posting. 
  • A counterpoise with an ideal length of .05 wavelengths is recommended. Without a counterpoise, the coax and rig will create the counterpoise. This might be adequate, but if you can affect the SWR by touching the rig, experiment with the counterpoise. 
  • High impedance transformations can be tricky and inefficient. Don't assume your unun is lossless. Care should be taken in the selection of the toroid to avoid losses greater than 10dB. If possible, measure the power into and out of the unun to confirm the efficiency. 
  • Be aware of power limitations and do not overdrive unun's to saturation. Ferrite unun's can be permanently damaged by excessive power. In other words, QRP unun's are for QRP!
  • Commercial 50:1 unun's in various power ratings are only available from a few suppliers such as QRPGuys, and Pactenna (and high power MyAntenna unun's for base station use). 
In summary, a well designed EFHW is an ideal antenna for QRP situations where a fast deployment is required and center fed dipoles or verticals with radials are not practical. It has been analyzed theoretically and field tested extensively. It is so effective that it has won over many Mountain Goats who started out thinking otherwise. When you absolutely, positively have to set up very quickly and make at least four QSO's after hiking 5 miles to the top of a mountain, the EFHW is a good way to go.