Subj : Re: 9 el Tonna v 12 el ZL special To : alt.ham-radio.vhf-uhf From : Steve Date : Tue Oct 11 2005 15:27:55 From Newsgroup: alt.ham-radio.vhf-uhf Doc Savage. wrote: > 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. Hi Doc Tx and Rx gain are identical, if the reciprocality theory is to be believed. As far as I know this theory has not been disproved. > 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. I think that you actually meant Polar plot rather than Smith chart. Polar plots can be drawn with dB scale or voltage scale. The same antenna will look very different with the voltage scale looking to the eye much more complimentary regarding F/B and sidelobes. I do not know what method Judd used so one has to be careful here. Often there is asymmetry in the plot of antennas which, presumably, is due to the mounting arrangement as you say. > > 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). VSWR bandwidth, is not normally of great concern as the antenna is usually operated in only the DX part of the band. 20m of any feeder is going to make the VSWR look better than it really is, in this case the lossier the feeder the better! > > 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. Er no. I said: 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. The Moonraker 12 ele ZL has a boomlength of 3.2m, or 1.54 wavelengths at 144MHz. Gain and beamwidth go hand in hand. Approx. 30 degree beamwidth is found in antennas in the 3.2 w/l range such as the 17ele Tonna and CC 19ele. Both these antennas have approx. 13dBd gain... still 1dB short of the 14dBd claimed my Moonraker for an antenna less than half the length! I calculate a gain of 10.37dBd for the Moonraker. Using the same formula for the Tonna with a boom of 1.65w/l gives a gain of 10.47dBd, not too far from their claimed 10.95dBd. The formula comes from chapter 7 of the "VHF/UHF DX Book", now sadly out of print, and is: Gain (dBd)= 7.8log*Boomlength in wavelengths+9. > > 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 Well if there was 3 odd dB extra, it would be worthwhile. Better than stacking 2 of the Tonnas in fact! This amazing performance would not have been missed by the weak-signal DXers, who strive for fractions of a dB improvement in their systems. The antenna designers would, surely by now, have ditched the conventional dipole driven element in favour of the ZL system. I do not know of any 2m DXer using one or anything developed from it. I would imagine that the QSO with YU was by Sporadic E (Es). Quite often signals via Es are extremely strong and contacts are possible with just about any antenna and power providing one is lucky enough to be in the right place at the right time. Many mobile stations have been surprised to have made these sort of contacts with a whip or halo antenna and modest power. > > 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. It would be very interesting if somebody with the ability and knowledge could run this design through NEC or a similar computer program and see what it comes up with... Best wishes Steve G8IZY .