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on each side) at top and bottom, resonated on 14.175 by a 7.3 uH inductor with a Q of 100. (This Q might be a bit unfair to the Sigma, but it is in the ballpark and won't affect the relative field strengths at different heights above ground, only the absolute values.) The material was 20mm diameter aluminium. I investigated the gain on 20m at various base heights above Real/High Accuracy ground (Medium). With the base from 1 to 5m above ground, the pattern is a single lobe, with elevation of maximum radiation decreasing from 26 degrees (1m up) to 18 degrees (5m up) and the max gain of the lobe increasing slightly from -2.48 dBi to -2.07 dBi. From 6m to 10m the elevation angle of the main lobe continues to decrease down to 13 degrees at 10m up, while a secondary lobe develops at high angle (55 degrees or so). The strength of the low-angle lobe weakens slightly to -2.16 dBi at 10m. With the base 10m up and higher, the high-angle lobe is stronger than the low-angle lobe. However the low-angle lobe increaeses in strength fairly rapidly, from -2.16 dbi at 10m to -0.68 dBi at 20m up. The angle of this lobe continues to decrease, down to -0.68 dBi at 20m up. In the meantime the high-angle lobe is also getting stronger, up to 0.24 dBi at 20m, while its elevation decreases from 46 degrees at 11m, down to 28 degrees at 20m. Looking at a fixed low elevation angle of 6 degrees, the radiated power increases continuously from -7.9 dBi at 1m up, to -2.2 dBi with the base 20m above ground. I conclude that the reduction in ground losses with increasing antenna height is sufficient to provide power for the high-angle lobe, without detracting significantly from low-angle radiation. (There is a 0.09 dB reduction in the maximum strength of the low-angle lobe from 5m up to 10m up, but over the same range of heights the radiation at 6 degrees increases by 1.2 dB as the elevation angle of the low-angle lobe is reduced). Although there is a dip in the radiation pattern between the high-angle and low-angle lobes, I would not go so far as to call it a null as it is generally less that 3 dB. My conclusion is that if you want a single lobe (on 20m) then a base height of 5m is about right. If you want to maximise radiation at low angles (in the 5 - 10 degree range) then get it up as high as possible. You pays your money and takes your choice :) 73, Andrew ZS1AN > -----Original Message----- > From: KEN SILVERMAN [mailto:k2kw@prodigy.net] > Sent: Thursday, April 24, 2003 5:21 AM > To: Rich Holoch; Force12Talk@qth.com > Subject: Re: [Force 12 Talk] RE: Sigma 5 Height > > > >Cebik has an article > > on short hatted verticles and he seems to agree with the 6' > height . . . . > . > > I haven't read Cebik's article, but the issue is not that 6' is the magic > number, but the goal is to raise the feedpoint to 1/2 to 5/8 wavelength > high. With the feedpoint at 1/2 wavelength, you compress the > lobe and get a > slight gain at the lower angles. This is of course at the expense of the > higher angles. > > One of the real pluses by going no higher than 1/2 wavelength is that you > keep one big fat lobe over all the useful takeoff angles. Once you start > raising the feedpoint above 1/2 wavelength, your lobes start to split, > leaving nulls in typical arrival angles. Not good! Just remember that > feedpoint height is dependant on frequency. If you optimize for 20m, you > will have a big null at useful takeoff angles on 10/12 etc. > > Kenny K2KW > > > > > -------------------------------------------- > Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. > Force12 Web Site: http://www.force12inc.com > > To Submit Message to the List: Force12Talk@qth.com > To unsubscribe and view the Message Archive: see http://qth.com/force12/list For problems with the list: contact n4zr@qth.com -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force12 Web Site: http://www.force12inc.com To Submit Message to the List: Force12Talk@qth.com To unsubscribe and view the Message Archive: see http://qth.com/force12/list For problems with the list: contact n4zr@qth.com |
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