Canvey Island, Essex, England Justin@g0ksc.co.uk
Twin boom quad
144MHz LFA Yagis
144MHz LFA Yagis

Low Noise LFA Yagis designed by G0KSC free to build for personal use.

144MHz LFA Yagis
70cms LFA Yagis
70cms LFA Yagis
Twin-Boom G0KSC Quads
G0KSC Twin-Boom Quads
Twin-Boom G0KSC Quads
G0KSC Custom Dish feeds - Above installation @ HB9Q
Custom low-noise dish feeds
Custom low-noise dish feeds
G0KSC Custom Dish Feeds

Above installation @ HB9Q

G0KSC Custom Dish feeds - Above installation @ HB9Q
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Read Time: 1 - 2 minutes

In this months RadCom (October/November 2009), the monthly RSGB magazine,  Peter Dodd, G3LDO reviews two commercial LFA Yagis provided by Vine antennas. Whilst he reported 'The best patterns he had seen' the real world polar plots showed pattern distortion yet he failed to mention the reasons why. Furthermore, I am surprised he did not contact me and question this problem at which time I could have given him the answers he should have reported.

A part completed Pawsey Stub Balun before weather proofing

The patterns for both a 4 element 6M version and a 9 element 2M version showed a small amount of distortion around the base of the antenna and particularly within the rearward lobes. This is the result of the antenna being fed WITHOUT a true 1:1 balun. This fact can be proven by switching the coax from side to side( reversing connections) which will swap the distortion from one side of the antenna to the other.

Within the article Peter experiments with both the Vine supplied Choke balun (coiled coax) and a toroid variant of the same. However, while these are called baluns, they do nothing to rectify the mis-match between an unbalanced feedline (coax) and a balanced antenna (the Yagi). The Choke balun mearly helps prevent the resulting common mode currents from travelling back down the coax, radiating and de-tuning the Yagi. This said, one or the other  (baluns) being used in the article (perhaps both) where not effective in this task as the antennas bandwidth moved when switching between the two clearly indicating the coax WAS becoming apart of the antenna and as a result, de-tuning the array.

By not using a true 1:1 balun at the feed point, pattern distortion will be seen. The resulting pattern is typical of that found within Yagis fed with a gamma match which is an unbalanced matching arrangement. A true 1:1 balun connects to the unbalanced coax with an unbalanced feedpoint one side and provides a balanced output at the antenna side of the balun which in turn, keeps the radiation pattern symetical. I have several examples of these upon my 'creating a balun page', my favourite being the Grounded Pawsey Stub.

Be sure not to make the same mistake with your next Yagi and ensure a true 1:1 balun is used at the feedpoint for best results!