Topband: Shunt fed tower

[W2XJ]

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
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