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
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