At a prior QTH I had a 90' Rohn 25 with 16' of mast above and stacked 4el W2PV design yagis. The 1/4 wave resonance was 1585 KHz if I remember and the 1" CATV hardline used for a gamma was space 3'. Using an Omega match the 2:1 BW was about 30KHz so I modified the amp to load higher in the band without having to resort to switching relays or motor drive. Voltages were very high at 1200W which required wide spaced bread slicers of 9KV from hamfests and 15KV fixed 857 series caps as padders so I could use low C variables.
It worked good enough to win a 160M CW contest, CQ if I remember in 87 or 88. Carl KM1H ----- Original Message ----- From: "W2RU - Bud Hippisley" <w...@frontiernet.net> To: "John Harden" <jh...@bellsouth.net> Cc: <topband@contesting.com> Sent: Thursday, December 15, 2011 7:27 AM Subject: Re: Topband: Shunt fed tower Many shunt-fed, loaded towers on 160 exhibit narrow bandwidth and are difficult to match with a single series capacitor for one simple reason: The gamma rod (shunt wire) is TOO CLOSE to the tower. A few years ago, after struggling with Omega matches in conjunction with MANY trips up my tower, I modeled my system with EZNEC. For me, the "sweet spot" was to position the gamma rod SEVEN (7) FEET from the tower! For my tower (92 feet of Rohn 45, 8 feet of mast above it, shorty 40 at 97 feet and 4-el. 20-m monobander at 92 feet), the tap point is 57 feet up. My minimum SWR (in a 50-ohm system) at my center frequency is around 1.4:1, but my 2.0:1 SWR bandwidth increased (with no change in my skimpy radial field) to over 75 kHz as a result of my modeling efforts. Having struggled with Omega matches for years before that, the present setup is a joy. One way to get in the ballpark without doing any serious modeling is to think about the gamma matches you've probably seen (and maybe even used) on your 20-meter beams. Very roughly, since 160 meters is 1/8 the frequency of 20 meters, all things being equal, the gamma rod spacing on 160 should be eight times what it is on 20. If your 20-meter gamma rod is 7 or 8 inches from your driven element, that's equivalent to 5 or 6 feet on 160. Of course, a grounded, shunt-fed, top-loaded tower isn't exactly the same as a full-size half-wavelength Yagi driven element, but the comparison is at least a good starting point. Construction: My local ACE hardware store stocks 8-foot lengths of angle aluminum, which is what I used for my horizontal tap rod. Their heaviest-duty stock is more than strong enough to support itself plus the top of my gamma rod. I don't support the weight of the entire rod -- which consists of stepped diameters of plumbing tubing -- that way — I simply "steady" the top portion while making electrical connection to the tower at the tap point. (The nearest Lowe's has even heavier aluminum stock, but if you're using wire instead of heavy tubing, the ACE stock is plenty strong enough.) The bottom of my gamma rod sits on a single piece of 2x8 pressure-treated lumber from the scrap bin. I use a couple of scrap lengths of 1x2 furring strips between one face of the tower and the gamma rod to maintain spacing along the length of the rod. It ain't pretty, but it works...I apologize to no one about my signal on 160! Bud, W2RU _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK ----- No virus found in this message. Checked by AVG - www.avg.com Version: 10.0.1415 / Virus Database: 2102/4081 - Release Date: 12/14/11 _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK