Posted by: GW7AAV | March 19, 2011

Linked Dipole for Portable Operations Such as SOTA

One of the things those of us involved in amateur radio for a number of years tend to forget is that things we have learned over time are not necessarily common knowledge even among our peers. So when I am operating HF portable and I tell people I am running a linked dipole I expect them to know what I mean. The truth is rather sad in that more often than not they do not have a clue. For the majority of UK hams at least the G5RV is as adventurous as it gets. Try to tell them their antenna is a poor choice and you might as well be an atheist preaching from the pulpit to a church full of Southern Baptists. For home use even something as simple as an Off Centre fed dipole could offer a better bet than the a G5RV in a similar space, but that is a discussion for another time.

For now I want to talk about the linked dipole as used by numerous SOTA operators for two reasons. One: several people have asked me to write something and Two: an Internet search for linked dipoles did not come up with anything useful. The main advantage of a linked dipole is there is no requirement for an ATU (antenna tuning unit) so we have one less heavy item to carry in our back-packs. Because on SOTA we as the activators choose the frequencies we work on we can optimise our antennas for the very frequencies we operate on. So on 10 metres I can normally be found around 28.5MHz and so I started my linked dipole by cutting a half wave dipole for about 28MHz and then using an antenna analyser or the Yaesu FT-817 and an SWR bridge I fold back the ends until the lowest dip in SWR (or max field strength if you have facility to measure it) is at 28.5MHz.

Note: Fold back. Do not trim until you have determined where to cut. A lot of amateurs trim a 1/4 inch a time until the SWR is acceptable, but that way you end up with an antenna that has had either too much or too little lobbed off.

Next I add an insulator as seen above and a link. I have Used Anderson Power Pole and automotive type bullet connectors. I would also recommend the small gold bullet connectors used by the radio control model community and bought cheap in bulk on eBay. Power Poles have the advantage that they are easy to deal with with cold fingers or in gloves. Then I add the section of wire required to make the total length suitable for 20 metres and tune in the same way and so on.
The pictures are of a linked dipole made by John GW4BVE and available on Flickr. I made my linked dipole independently of John and when I did a joint activation with him was surprised at the similarities of his design and mine. The differences are I use a commercial dipole centre, I don’t bother with a balun and I use RG-58 coax. I have actually never seen RG174 coax or plugs for sale other than mail order, which is the main reason I don’t use it. It would be much lighter to carry of course. The main surprise I got was when I measured my dipole elements they matched John’s sizes in the above picture exactly. However if you are using a thicker wire than we used or tuning for the CW end of the bands your lengths could vary a little from the sizes shown. I have since constructed a second dipole blind using John’s measurements and when I checked later with an antenna analyser it needed no adjustment.

For deployment in the field the antenna can be strung from handy trees, but as on most hills trees are not an option then a telescopic 6 to 10metre fishing pole (also known as a Roach Pole) is used as a mast. The antenna is deployed in an inverted vee formation.

The fishing pole mast is held up by guys. I use a collar made from PVC which fits just above the bottom section of the fishing pole with three short guys and use heavy duty rock pegs, that look like huge masonry nails to hold the ends. Other activators guy the mast near the top, while some use a single guy and let the antenna act as the other two guys. The choice is yours.

The reason I guy at the bottom is that it keeps the guys short and easier to manage. I do not like the single guy method as it adds additional strain on the antenna. Easy of management is the reason for the next image. Winders can be made from ply as John shows here or made from plastic card (available from model shops) like mine or purchased from kite suppliers, eBay or from Richard G3CWI’s SOTApole website.

The ends of the dipole as shown in John’s images have an insulator and then a length of thin guy rope. When deploying the dipole it is a good idea to get the ends as high off the ground as possible. Tests have shown that the difference between having the ends of the dipole low to the ground and at a mere three feet (1m) off the ground can be considerable. As a result of these tests I now raise the ends of the dipole by using my walking poles, extended to their full length, to hold the guys on the dipole ends just before they pegged. To simplify this a Y shaped double guy can be fitted to simplify the support. Because I rarely activate alone I use a single guy and a pair of walking poles on a cross formation to do the same thing.

This last image shows John’s dipole centre made from a disc of perspex with a hole for using the single guy method of supporting the dipole and a choke balun. As I said I use a commercial dipole centre and although the choke balun is a good idea I have never noticed any RF feedback running even 100 watts without one.

My thanks to John GW4BVE for use of his images. John says:

I have been asked to publish some photos of my /P HF antenna. It is a dipole with the element lengths adjustable by plugging/unplugging connectors. The antenna weighs 580grams including feeder and winders. The wire is surplus thin connecting wire and the feeder is RG316 (PTFE RG174). Insulators are small pieces of scrap plastic. It is normally erected inverted V fashion with the centre supported by a 7 metre fibreglass fishing pole and the ends by my walking poles. This antenna covers 80m, 60m, 40m, 20 & 10m, but it can be made for any combination you chose. No ATU is required. The winders are made from Lightply, which is sold by good model shops.

The drawing shows approximate dimensions of my HF dipole. The balun has been omitted for simplicity and I now use RG316 feeder which I find is higher quality and of course has less loss, although as the feeder is so short the loss at HF is negligible. Please note that you should tune the antenna at the height you are going to use it on the hills. Make each section slightly longer and cut to minimum SWR starting at the highest frequency and working to the lowest. You will not get full coverage on 80m so it it is best cut for your preferred operational frequency.

The balun could be omitted to make the antenna lighter. The balun core is 3.5cm diameter and is made of type 77 ferrite with about 20 turns of coax.

The insulators ensure that there is no strain on the connector itself. The connectors are Powerpoles and are available from CPC and Farnell in the UK or can be ordered from the USA. Of course any reliable single pole connector could be substituted.

Note: Click on the images to take you to John’s images on Flickr where they can be seen full size.

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