Subj : Re: 9 el Tonna v 12 el ZL special To : alt.ham-radio.vhf-uhf From : Doc Savage. Date : Wed Oct 12 2005 06:35:18 From Newsgroup: alt.ham-radio.vhf-uhf Steve wrote in news:LZP2f.10730$U9.10476@fe3.news.blueyonder.co.uk: > Doc Savage. wrote: >> >> 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. I'm happy to be corrected. I only glanced at the plots and saw them transcribed onto what appeared to be a Smith Chart. I also need to correct another figure which I included. The driven elements are 135 degrees out of phase, not 180 degrees. >> >> 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! Oh dear, long forgotten memories of the RAE evening classes are creeping back. >> >> 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. Re-reading the article again, I note that Judd did make a distiction with the gain figures and quotes only forward measured gain of 13.5dB at the 3dB down points. In fact, he makes that point twice in the first page and a cursory glance over the construction and 'tuning' details did not appear to turn up any receive gain figure or estimation. He also reminds us that a simple 2 element ZL [endfire] (described elsewhere in the booklet) has a forward gain of approx 6dB over a dipole with a single driven element, so perhaps the end fire arrangement does not compute precisely to the single driven element yagi formula. The same booklet also describes constuction of a 16 element ZL with a 4 element plane reflector which has a stated gain of 16dBd for a boom length of 4.26 metres, but again, the text concentrates on the advantages of having greater erp and only mentions the receive charateristics in passing. > >> >> 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. If the extra gain is all forward, the only advantage is that one could interfere with stations further away and not be able to hear their cussing. If the gain is the same in both directions, there's the risk of increasing the qrn and qrm to the detriment of the required signal. Although I am happy to accept Fred Judds forward gain measurements, I find it curious that he makes no mention of the receive gain in either of the three articles on ZL antennas (articles cover 2, 3, 12 and 16 ele ZL's) unless we are supposed to assume the antenna gain is symetrical in forward and receive. Having only just thought about checking the price, I've changed my mind and ordered one for myself. I saw "Moonraker" and "Tonna" and immediately though of HF Yagi prices reminiscent of telephone numbers and thin walled 18 foot aluminium 'wands'. I'm not particularly active on 2 or 70cm from home apart from a scheduled net, because only 2 other voices ever reply on 2 and I can't hear anybody on 70cm through the anti social collinear, but u125 might be a good investment. I can see why manufacturers don't 'push' sales of ZL antennas. Single driven element antennas are cheaper and easier to mass produce, and easier for the customer to assemble, whilst the ZL has more components and needs some expertise and precision to assemble from the manufacturers space saving package kit of parts [shudders at the thought of reading Japglish instructions on the roof in October]. > > 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. Agreed, but the location is quite wierd where VHF was concerned. It was 'discovered' by covert multimode CB DX'ers who regularly wiped out the UK's first TV re-diffusion service whose antennas were also located on the same hillside overlooking Burnley, Lancs. I doubt many CD'ers ever cycled over the hill with the rig slide mounted to the handlebars and powered by a motorcyle battery in a tupperware box in the saddlebag looking for a 'sweet' spot. Athough it is a hilltop, there are hills on 3 sides. In one location, I could regularly work into Cheshire and Cumbria and GD and occasionally into NI using only the FT290R handbag and home brewed quarter wave screw-in antenna from my chosen spot. 200 yards further east, those paths were gone and new ones opened into Yorks and Notts with several repeaters available of needed. FWIW, the YU contact was made during either the first or second Practical Wireless 2M QRP (3W) contest. From memory, I believe the conditions were relatively flat, but I can't rule out Es or 'Ducting'. It was the pile-up during our exchange of details which made me swear never to contest again. >> >> 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 > You have my email addy if you want to look at the booklet Pse excuse me not including my full call in public. I have a stalker and I'm qthr with 4 young children. 73 G4 Doc .