If we are going on a holliday we want, and for some of us a need, to take HF equipment with us. I don't have the need, but I like it (HI). And I am married with a loving wife who has no problem when I take some equipment with me. There is only one exception; there must be room for her books and toys for the qrp. If that's packed I can fill the rest with ham stuff. Wow, now I know why I am so lucky. But I have a small car so I have to figure out what I will take with me on holiday. I decided to take my FT817 and a simple wire antenna who's working on several bands. Wtih..... or without a tuner? Yeah that's the question. Let's see what's possible. You never know what kind of space there is to hang out a full-size antenna. So we have to look to buy a short antenna...... Ouch buying is a dirty word for Hams, especially the Dutch ones. In my Dutch dictionary the word buying is not present (HI). But I like to make these tings myself. The several options there are I wrote down below;
Foldback dipole fed with coax (single band)
An easy way what makes it possible to hangout a long antenna on a smaller place is to bend back the ends of the antenna. Remark that the mechanical length of the dipole should be a little bit longer when we bend back than a half wave dipole. How much longer I don't know, but experimenting or with simulating it's easy to find out. There is also a difference in the distance of the fold back wire and the dipole. With my Cobra antenna the distance of the wires is about 8cm, but in some articles they speak about 20 and sometimes even about 50cm distance.
The disadvantage of feeding a (fold back) dipole is that it's in fact a monoband antenna. Maybe is folding back not a bad idea but must we change from coax to open line. With a twin feed line and a symmetrical tuner we can use the antenna as a multibander. The rule for such an antenna is that the antenna length has to be a halve wave of the lowest usable frequency. For working on the bands 10-40m the antenna should be minimum 2x10m long.
Extreme foldback dipole
Maybe we need more wire than a half wave dipole. At home I use 3x7,75m for each side of the dipole. I feed this dipole with an open feeder and than via a 1:4 balun with a couple of meters coax to the set. My tuner takes care of the rest. This type of antenna is called the Cobra- antenna. But to take this with me on a holiday no thanks, to complex.
Inductive shortend dipole
There is a lot to find about on the internet and the books. A couple of things must be kept in mind when we design an inductive shortened antenna:
1. Never go shorter than 50% from the original size otherwise the efficiency drops dramatically.
2. When you move the coil to the feed point, the induction decreases, but the efficiency drops also.
3. We don't want a very low efficient antenna, but also don't want extreme large inductions, so that practically the best place to put an inductor is in the middle from each dipole half.
But with my design I am not placing the coils in the middle, which has an other reason. I want to try to design a dualband coax fed dipole.
In several books are graphical presentations visualizing the relation between the overall length of the shortened dipole and the distance between the coils. You can readout the impedance of the coil and with the frequency and the impedance is easy the inductance to be calculated. Some investigation on the internet brought me at the local department of our national radio club called VERON Afdeling Friese Wouden (www.veron.a63.org/cqfrwden/jrg16/sep-01/spoel.htm). They calculate the inductance for the shorter dipole antenna. I don't want to give you this formula, because it doesn't fit on a single line. But these guys have split the formula in easy to understand formulas and than it make a lot of sense. I wrote a program in Visual Basic 6.0 that shows the calculation of the
inductance and also the practical way to design the inductor.
As I wrote before, I changed the place of the inductor, that's because I want a dualband antenna for 20 and 40m.
As we look at the drawing than the 2x490cm is a dipole for 20m. The size is a little bit shorter than normally, but that's because the coil and the rest of the wire has a little influence on 20m. The program screenshot below shows that we have to use a 25,5uH inductance. IF you look at down-left on the screen you see that with a diameter of 40mm de coil has a length 40mm if we use wire with a diameter of 1.33mm the number of turns are 31. I made this coil and the length was 42mm and the inductance with 31 turns was 25,4uH. So I had to ad a turn. If you don't want to do that it's not so bad, because then you should take the210cm length a little bit longer to compensate the smaller inductance.
After the coil there will be a piece of wire (210cm) to make it work on 40m. The coil has an impedance on 20m form about 2300 Ohm. That will block the signal on 20m for the most of it. I made this and in the feed point I made a 1:1 balun and the results where not so bad. On both bands the SWR is less than 2. Adjusting the length of the wire was necessary to bring the SWR on both bands below 1:1.5. But also the place to hangout the antenna has influence on the SWR. It's easy to hang the antenna 2 meters above the ground and adjust. But hanging the antenna on its final place the SWR is gone. Always adjust an antenna on the place where it should be hanging permanently. Thats why adjusting the antenna takes time and some patience. Take that time and you will be able to build a very usable antenna for 20 and 40m. I use this antenna on portable operation, so I don't want to take a tuner with me. And my FT817 gives just 5W so I want to use its power optimal.
The Halfwave antenna
A complete different approach is the half wave antenna. This antenna has in the feed point an impedance from 1-5kOhm. A famous one to transformate this impedance to 50 Ohm is what the German called the Fuchskreis. Named after the designer Herr Fuchs. It's a LC combination that transforms the high antenna impedance to the so wanted 50 Ohms from the transceiver. From Frans PE0F I got an article about impedance transformation with coaxial stubs. Easy to design and easy to make. The drawing below shows how it's made. Remember this is absolute a monobander.
It's great! Works very fine. Easy to make, a couple of PL259 and some coax and you are ready. The dimensions are on the drawing, but remember that its 0.216 lambda multiplied the reduction factor of the coax. (0.66 for RG58). For 20m that's 2.98m. The short ended stub is about 48cm long. The big advantage of this coaxial stub is that it does not matter how you lay it on the ground. You can even role it up. Almost no influence on the SWR. And to change the length of the half wave wire you can exactly adjust the SWR to 1:1. I tried this in my backyard and it works fantastic. Even when the coax was laying on the ground. But remember this is really a monoband antenna. For backpacking this is a very small and easy antenna. You can even change the RG58 for RG174. Piece of rope to the one end, drop it in a tree and QSO-ing right away. Absolute a recommendation for the holiday. I will take him with me.
Download Portable with wire as a pdf : Portable with wire.pdf