Saturday, 19 March 2011
Linked Dipole For Portable (SOTA) Operation
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.
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.