Topband: Shunt fed tower

[Joe Subich, 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.

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
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