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
>
>
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