Agreed, my bad semantics. That said I was thinking more of a sloping shunt similar to was was used in broadcast. I don't think many will have a "too tall" problem on 160 M unless some lucky person has around 300 feet or more.
On 12/15/11 10:59 AM, Joe Subich, W4TV wrote: > On 12/15/2011 10:34 AM, W2XJ wrote: >> Vertical antennas have been shunt fed for over 70 years. There is no >> magic involved. Very few MW verticals are ever resonant and resonance >> is irrelevant. > All true but one does not want a tower that is "too tall" that the > pattern deteriorates. > > > The easiest way to deal with matching is to first model on EZNEC which > > will give an approximation of where the shunt should be connected and > > then physically moving the shunt to find the 50 ohm point which should > > be determined by measurement. > > The shunt is *not* connected to the 50 Ohm point. It is connected to > the point that is *transformed to 50 Ohms* by the effects of the gamma. > The key is to find a combination of tap point, gamma spacing and rod > diameter that result is 50-jx Ohms at the bottom of the rod with a > usable bandwidth so the -jx can be cancelled by a single capacitor. > > 73, > > ... Joe, W4TV > > > On 12/15/2011 10:34 AM, W2XJ wrote: >> Vertical antennas have been shunt fed for over 70 years. There is no >> magic involved. Very few MW verticals are ever resonant and resonance is >> irrelevant. The only important thing is to match the TX so it is happy. >> The easiest way to deal with matching is to first model on EZNEC which >> will give an approximation of where the shunt should be connected and >> then physically moving the shunt to find the 50 ohm point which should >> be determined by measurement. Once that is accomplished, measure the J >> and calculate the necessary C to cancel it. >> >> On 12/15/11 10:17 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 >> > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK