Joe, W4TV, Thanks for the info on the insulators. ! I just ordered some myself. These are great for making a fat gamma rod as you indicated. (They are a little pricey though, a whopping 35 cents apiece! hihihi) Here is the webpage on R&L for these insulators if anybody wants to check it out. http://store.rlham.com/shop/catalog/product_info.php?products_id=66844
73, Bob K6UJ On Dec 15, 2011, at 7:17 AM, Joe Subich, W4TV wrote: > > > On 12/15/2011 7:27 AM, W2RU - Bud Hippisley wrote: > >> 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. > > That is true but a 100 foot tower with decent sized 20M monobander or > 24 foot boom tribander with the front/back elements grounded and a > short 40 meter yagi will most certainly have a natural resonant point > below 1.8 MHz. Additional side mounted yagis will further lower the > resonant point. A tower with resonant point below 1.8 MHz will have > a higher impedance which will transform badly in a "gamma" with high > "element to rod ratio" and narrow spacing. > >> 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. > > R and L Electronics (www.randl.com) has insulators for "cage" dipoles. > They are about 3.5" OD with 12 1/4" holes on a roughly 3" diameter and > make excellent insulators for a "fat" gamma rod. One can use 3, 4, or > 6 wires in the cage and achieve effective diameters between 2 and 3 inches. > > 73, > > ... Joe, W4TV > > > On 12/15/2011 7:27 AM, W2RU - Bud Hippisley wrote: >> 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 >> > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
