Subj : Re: 9 el Tonna v 12 el ZL special To : alt.ham-radio.vhf-uhf From : Doc Savage. Date : Tue Oct 11 2005 06:27:23 From Newsgroup: alt.ham-radio.vhf-uhf Steve wrote in news:434972AF.1040603 @blueyonder.co.uk: > Simon wrote: >> Hi all, >> Has anyone any experience of these 2 antennas, I am currently using the 9 >> element Tonna which is around 10.95 db gain, the 12 el ZL special on >> Moonrakers website quotes a gain of 14dbd. This seems very high for such a >> short boom. Any info would be helpfull as Im having a hard time believing >> the claims. >> >> Thank you >> Simon >> >> > Simon > > I would stay with the 9ele Tonna. > > The 9ele Tonna is a respected antenna in the VHFDX world and many > portable DX stations, and fixed, use them. Not optimised like the Msq or > Vargarda but still capable of good performance. The Tonna aerials also, > despite their fragile appearance, are very well made and last well. > > Gain is proportional more or less to boomlength, in a reasonably linear > fashion and extra elements/fancy driven elements in that boomlength do > not add to gain. > > Given the above, ask yourself whose adverts you believe most! > > HTH Steve G8IZY > > Hi Steve and Simon. Whilst I agree with Steve in principle regarding his description and the quality of the Tonna. Some years ago, a friend and I built the 12 ele ZL described by Fred Judd G2BCX in the "Out of Thin Air" booklet published by Praccy Wireless in the 80's, reprinted from a January 1978 article in the magazine. The extra gain comes from the dual radiating 'end fire' 1/8y elements which are fed 180 degrees out of phase, but this is increased forward gain, and not necessarily an increase in receive gain, though I see no reason why the gains should not be closely symetrical. Judds Smith chart plot for the field intensity pattern of the 12 element ZL shows an extremely favourable f/b ratio, but the radiation pattern Smith Chart plot shows unequal sidelobes on the vertical plot which may or may not be due to the antenna being offset mounted part way up the rotator stub during measurements. However, the VSWR plot shows the ZL to be flatter over the band than a comparable commercially available yagi, even with a 20M feed coax. (1.1:1 LF to 1.6:1 HF is quoted on the plot). As Steve has already said, gain is to some degree dependent on boom length and the number of elements, but gain is also dependent on beamwidth. The Smith Chart plot shows the 12 ele ZL to have a beamwidth of around 30 degrees which is pretty narrow for an antenna with such a short boom. I haven't been able to find a Smith Chart plot for the Tonna or another similar antenna to compare the two, and I won't quote manufacturers or importers figures, which are inevitably gained under laboratory conditions rather than rooftop conditions. All that said, there's little point in going to the additional expense of buying a new antenna for the sake of 4bB gain, especially if you are happy with the Tonna in all other respects, but there is some knowledge and hands on experience to be gained from reading the article and perhaps making the ZL he describes as a spare for portable qrp contesting. I once worked into YU from a site 220ft asl in Lancashire on an FT290R with the home brew ZL described in the booklet strapped to a layby signpost with bungee cords and steered by hand. I made mine with a detachable front half for transport on a bicycle (I was only 15 at the time), and being only 3.2 metres long, there was no need of an additional 'trombone' support If the booklet (u1.25 1978 price) or reprints of the original 1978 article are no longer available from PW Publications, drop me a line and perhaps I can loan you my copy or something. spamfighter at freeuk dot com .