Working portable, I have been using my old and trusty commercially made W3DZZ. Indeed, it has been serving me nicely over the years. Being basically a dipole it has been performing as one could expect a dipole would perform. No more, no less, no problems, no complaints. For portable activity itīs, however, a bit heavy and cumbersome. The traps are heavy as is the balun, the wire is meant for permanent istallation and thus sturdy and it needs a coax (RG-58) which add weight to what you have to carry around. Something more nimble and lighter with about the same performance would be nice.
Activating nature reserves within the SMFF Program and some hills within the SOTA Program has been the main aim of my portable activity during the last year. In both I need to be able to quickly switch between two bands; in the first case between 80 and 40 meter and in the latter case between 40 and 30 meters. A trapped end-fed half-wave dipole seems to be the ideal solution. Such an antenna/wire can be made very light and any coax is of course not needed.
The first task was therefore to construct two traps; one for 30 meter and one for 40 meters. As I will be working at qrp-levels and as everything has to be as light as possible I made them of PVC-pipes 40 millimeter in diameter and from RG-174 coax. Instructions how to do you can find here. You can also use "Coaxtrap" by G4FGQ. You will find it here. Both calculataror give about the same results. After quite a few attempts I ended up with 7,4 turns for the 30-meter trap (10.120 MHz) and with 10,25 turns for the 40 meter trap (7.020 MHz) using RG-174.
Of course you can use a 42 meter wire to work both 80 and 40 meters as a double half-wave and as a single half-wave respectivly, but using a trap will shorten the wire with about 10 meters. I have not noticed any significant decrese in performance on 80 meters because of this. The main advantage is that with 10 meters less to worry about the wire is a lot easier to put up in the field.
Each trap has to be tuned to the frequency you are aiming at. One canīt rely on some instructions specifying the number of turns. Even if you use RG-174, as I did, you canīt be certain itīs the same stock that I used. You have to tune them yourself! This is my way of implementing Marcīs suggestion how to tune a trap.
We are dealing with an end-fed half-wave dipole so you of course also need a tuner to match the high impedance at the end of the dipole to the 50 ohms of your tranceiver. As I need to quickly switch between two bands I have made a tuner with two variable capacitors and a switch. The tuner is based on the ideas of AA5TB.
The trap how it looks when "in operation"
The antenna in invertet-vee configuration supported by a 12 meter SpiderBeam Mast. Itīs quick and easy to raise the mast with the wire attached. A whole lot easier than hoisting the old W3DZZ!
Of course you donīt have to buy a SpiderBeam Mast. The antenna is so light you can use any kind of resonably long fishing pole you can buy in your local shop for sport-fishing equipment.
Setup for adjusting the length of the wires
On the lower band 80 meters on the 40/80 meter antenna and 40 meters on the 30/40 meter antenna the trap will act as an inductor and thus shorten the antenna/wire. A fraction of the transmitted power will be lost as heat in the trap, but the effect will hardly be noticeable. Itīs just a matter of parts of an S-unite at the receiving stationīs end. The advantage of having a shorter and easier to erect antenna far outweighs the disadvantages
Donīt get confused!
The whole length of the wire, starting at the connecter at the tuner,
is part of the antenna and thus in the length of wire indicated below.
Twenty 1/2 watt 100 kiloohm resistors in parallell makes five kiloohm. Just what you need to adjust
the tuner for minimum SWR before you connect the antenna. After thatīs done itīs just a matter of
connecting the antenna and trimming the wires to minimum SWR!
End Fed Half-Wave Antenna with some gain
Having used the end fed Half-Wave antenna, with and without traps, for some time the next step was to see if you could construct something that would give a bit of gain. As itīs an antenna for portable use Iīm still limited to one support only.
After having fiddled around with the 4nec2 software for a while I came up with what basically is a vertical two element beam for the 40 and 20 meter bands. Even if itīs far from ideal itīs anyhow an improvement over the simple End Fed Half-Wave and may theoretically give close to 5 dBi of gain.
Both antennas below was tested in the field during the IARU Region 1 Fieldday in the beginning of June 2010 with very good results.
40 meter band
20 meter band
Above is what I have come up with for the 30 meter band. Itīs supported by a fiber glass pole. It can be used both with and without the reflector. The difference is about 3 dB, as can be seen in the gain diagram below. Of course one needs a tuner at the feedpoint of the antenna.
Let us assume you use an End Fed Half-Wave Antenna on 40 meters. The antenna is fed at one end via a tuner at ground level and supported at the other end by a fiber glas mast. What you get is a sloping dipole. The question is then how tall the mast should be to get a satisfactory result. Of course the ultimate would be to have a 20 mast so that you would get a half-wave vertical antenna. That would give you an excellent DX-antenna and a very good solution for your home QTH. For portable operations it would, however, not be very practical. A mast of that length is too difficult to erect for one person and to heavy to carry.
Playing around with 4nec2 antenna modeler you will find that with a mast shorter than 10 meters the performance will degrade rapidly. Above 10 meter the performance will improve significantly only above the 15 meter level. This means that the the end of a sloping Half-Wave should be at least 10 meters up in the air, but if you want significantly better performance you must aim for a mast capable of bringing the end more than 15 meters up.
These calculations are made in the theoretical world. In the real world they are probably totally irrelevant. Out in the bush you will be surrounded by trees, bushes and features of the geography. One setup will not be the same as an other. Even if i can not be shown in theory, every meter above 10 meters probably make sense in practice as it will raise the antenna above close by objects.
Another consideration is of course how heavy a mast you want to carry and how tall a mast you can afford...
Results after a lot of field use
As of time of writing (December 2011) I have been using an End Fed Half-Wave antenna and a simple tuner during more than 60 SOTA activations on 30 meters and 89 SMFF:s on 40 and 80 meters. Most of the time I have had a fiber glass pole as support. The end/apex of the wire has been 10-11 meters over ground in both cases. I have found this antenna to be up to the task generating good signal reports all over Europe and around Sweden with four to five watts output.
I have understood that the feed point impedance of an End Fed Half-Wave antenna can vary between 2500 to 5000 ohm depending on the thickness of the wire, the configuration, height over ground and environment factors. I have also noticed that the length of the wire is not very critical as long as it is close to a half-wave. One or two meters shorter or longer does not seem to matter very much.
If you forget the counterpose wire at home, do not panic. You will be heard anyhow.
The Hendricks SLT+ is an excellent tuner for tuning half-wave antennas.
The best location for the antenna is in the clear and just a couple of meters from salt water. You will be completely blown off by the results! The next best location is on a hilltop and in the clear with no trees around. However, you will be heard even in the woods among high trees. The only thing you have to watch is the wire tuching any thick brances. If it does it will degrade performance significantly!
I do not say that an End Fed Half-Wave antenna is some kind of miracle antenna. It performs just like any other type of dipole. However, if you are looking for a simple and very portable antenna that is easy to put up and get operating it is hard to beat. With a telescopic mast it takes less than five minutes to erect in almost any condition and in any place.