When I first became involved in SOTA all my activations were on 2 metres (initially fm but now mostly ssb) but due to chasing mainly hf portable stations I quickly became interested in the possibility of using those frequencies myself while out and about.
The main problem I came across was finding practical antennas that would be suitable. There are many commercially available but they were not quite what I wanted. There are also many designs on the web but very few had proper write ups about the practicalities of their use and performance.
The only option was to build something myself. I have written a document about it which I would like to share:
This has been the antenna that I’ve carried to all my hf activations since October 2008.
Good stuff Carolyn. Its a good design, it probably saves a lot of time having two bands available with no manual switching.
I have used the flexweave wire for various antennas, but find it to be too heavy for portable work. I dont think it would make much difference to efficiency if you changed it for say 16/0.2mm pvc covered wire.
Your signal was very good yesterday, so it obviously works
The weight saving probably won’t be that great to be honest and I had a drum of flexweave cable to use up. I’ll model (actually Helen will :o))the antenna with the thinner cable; I would expect the bandwidth to drop slighty.
I’ve found the antenna with the larger diameter cable to be very easy to set up and take down with quite thick gloves; they have been needed on some of my recent activations :o)
Personally I’ve found dipoles a lot of hassle to setup (I tend to activate solo). I’ve used a 20m vertical and that worked alright. But I’ve found VHF better for myself anyway, seeing as how I tend to go for the high stuff.
Hi Carolyn, thanks for an excellent read. Thought the analysis work was well put together, with some good ideas for improving the robustness of the design.
Personally I’ve found dipoles a lot of hassle to setup (I tend to
activate solo). I’ve used a 20m vertical and that worked alright. But
I’ve found VHF better for myself anyway, seeing as how I tend to go
for the high stuff.
Thank you for all the replies.
When I first built the antenna I thought it was going to be too big and too unwieldly to take on the hills but after a few pratice runs on a local playing field, to make sure it actually worked, that trepidation soon subsided.
On the smaller “pimples” I will operate solo (and if the weather is good bigger ones) and now have no problems setting up the antenna on my own in everything except the very worse conditions.
I’ve now done 23 activations with this antenna all on 60m, all but a few with 80m as well and occasionally 40m. 40m with QRP levels of 5W is very hit and miss depending on propagation. For example a couple of days ago while on Fan Gyhriych I had been told Peter ON3WAB could hear me on 5MHz so I QSYed upto 40m where I will usually find him waiting. Conditions seemed to be very strange; where I would usually get calls from near Europe nothing was heard and the band was very noisy. After a couple of calls both Christine (GM4YMM) and GM7UAU gave me 5/9 signal reports. Then the skip seemed to go very long and I had two contacts from Slovenia and one from Czech Republic before the band became very noisy with QRM.
I have found that going for the obscure and out of the way hills I have been doing lately it is the most reliable way of activating and qualifying the summits at QRP levels.
Carlolyn, nice article and thanks for the inspiration! My version of this antenna will be lightweight and have 30/40 m in one dipole and 20 meters on the other. We dont have 60 meter here, and I dont have 80 m in my rig. Now, where is all my wire…
In reply to G6WRW:
Superb article, Carolyn. Well done and just the tonic for a new M6 whose about to buy an FT817ND and use that SOTA Pole for HF.
I was looking at the piece by Iain M1OOO at http://www.qrz.com/callsign/M1OOO and this left a lot of unanswered questions on construction details [for which he’s agreed to send pictures]
I can’t have 5MHz yet so would build the second dipole for 14.285 MHz SSB. I hope the 9.58 metres of wire will act as a good guy line. On page 7 you wrote ‘Using the basic design it would be quite easy to add other higher bands to the antenna but care would be required in minimising interaction between the two dipoles.’ Please explain what this care is and how my 20m band dipole would interact.
Any antenna is affected by its surroundings. Many of the formulas you see published are for ideal situations. Sadly most of us cannot set up an aerial in an ideal situation so the actual antenna performance is less than the ideal performance.
In the case of a link dipole the links are used to alter the physical length of the wire. It’s easy to assume that when you remove a link and shorten the aerial that disconnected piece has no effect. But to varying degrees the disconnected length may effect the tuning of the piece still connected due to its physical proximity to the antenna. And the effect of the disconnected piece may be greater when its length is much greater than the piece remaining.
You normally have to do a fair bit of suck it and see engineering. It’s a classic example of the humerous saying that the difference between theory and practice in practice is greater than the difference between theory and practice in theory!
There are zillions of articles about these antennas on the 'net. Google is your friend. Or you can search the reflector for old articles we have written about our antennas.
David, I would recommend experimenting with antenna modelling. You can get a limited version of EZNEC with the Antenna Handbook from ARRL (which is also a good book). MMANA is free I think.
Its then easy to experiment with antenna designs before you build them.
I tend to agree with Andy that practice doesn’t necessarily follow theory, especially when it comes to antenna systems because of the many variables that can affect the accuracy of the model. For example, because ground is an integral part of the antenna system it must be included in the model and does affect performance.
If the apex angle is too acute, relative to the vertical (which would be the case if the ends of the 20m dipoles legs are close to the ground relative to its feed point), the radiation pattern would still be similar to the 40m pattern in my article. On the surface all well and good, but the antenna would not appear to work very well; the mode of propagation needs to be taken into account.
On the 3 lower bands (80, 40, and especially 60 metres) we are exploiting near vertical incident skywave (NVIS) propagation where our signals are reflected by layers in the atmosphere almost directly above the antennas location, hence my antennas near vertical pattern (no side lobs). 20 metres is not propagated in this way and the usual practice is to try to get signals as far as possible; with an antenna close to the ground this can be a problem. Horizontal dipoles only a few metres above ground, as is the case with most portable installations, will have little or no radiation towards the horizon.
For 20 metres (and above) the antennas pattern needs to have components that send signals out at lower angles relative to the horizon. The minimum height above ground which begins to provide these angles is approximately 5 metres at the feed and 4 metres at the ends of the dipoles for a 20m dipole (modelled with 4nec2); in effect a shallow apex angle. In practice that means the either the ends of the dipole have to be supported in some way (not many trees on most of the highest summits) or the guy lines have to be very long or (and this is why linked dipoles work) for the 20m dipole to be a “link” in a 80m dipole.
So what I meant when I said other higher bands could be added I should’ve correlated that to say “in the 80m legs not as additional or separate dipoles“. The interactions mentioned are more to do with harmonics and the very complex relationships between the dipoles.
Basically what I’m saying is that if you have 40m as one dipole and 20m as the other the 20 metre dipole won’t be efficient as a radiator unless its feed and ends are high above the ground (difficult to do on most summits). If my design is changed too radically it would be better to model any new arrangement before lengths of wire are wasted and any fruitless trips are undertaken. If you want bands above 40 metres and are looking for DX contacts then I would recommend either the normal linked dipole arraignment or a vertical like the Super antennas MP1. My antenna was designed to specifically take advantage of a relatively short range propagation mode.
Carolyn (G6WRW)
Happy to answer any questions here or by e-mail
BTW the vast majority of my portable operating is @ 5W
In reply to G6WRW:
What comprehensive answers! Thanks. I’ve bought the materials today to construct the antenna. I’ll build the 40m and 80m legs with 2mm dia flexweave that was on sale in Radioworld today and use a pair of plain guylines to replace the 60m leg till say 2011-ish when I’ll be able to use 5MHz.
I appreciate now how heavy all that 4mm flexweave must be to carry as there’s 2kg in 50m of 2mm dia flexweave. Does SOTA = Sherpa on the Air? {;¬)
As it’s my first HF antenna [and the FT817 will be in the post on Monday] that’ll be great for a start. 14.2MHz SSB can wait. I’ll have 10m of flexweave left over with which to consider what antenna changes to do next.
That may be a 20m band section from the apex to a new link to the 40 metre band section.
No rush. I’m going to make an “ATU” per the MFJ902 clone on M0JHA’s website as my 2E0 / 2E1 project first. http://www.m0jha.com/about1.html
Cheers
David [M6WOW]
I use bog standard 7/32 PVC covered hookup wire. It certainly has the resilience as mine has been set up 108 times now without issue. The whole caboodle comprising 60m/40m antenna, 80m extensions, winders, coax-feeder, connectors, guys and the box it all fits in weighs 590g.
Be wary of buying wire from radio dealers. The put a significant markup on it. You can get miles of wire for the same price from your local electrical factors.
