For most of us a precise model is not possible. EZNEC will give you an
approximation. The tower is represented as a cylinder equal to the cross
section of the real tower. You could measure the existing R and J and
then in EZNEC just keep adding loading until EZNEC agrees with your
measurements. Then move the shunt wire around in EZNEC until you hit 50
ohms. The result will give you a ballpark but it saves a lot of climbing
and rearranged real wires. A yes, to be clear that 50 ohms is at the
bottom of the shunt feed.
On 12/15/11 4:14 PM, Larry wrote:
> I haven't done much modeling in the past.I have a KT36XA which would be very
> ugly if I had to model it precisely. I also have a linearly loaded 2 el 40M
> yagi.
> I suspect that the loading wires probably are negligible in the overall
> scheme
> of things at 160M. So I would guess that there some approximation that would
> give reasonable results as a place to start on the tower. Suggestions?
>
> 73, Larry W6NWS
> ----- Original Message -----
> From: "W2XJ"<w2xj@nyc.rr.com>
> To:<topband@contesting.com>
> Sent: Thursday, December 15, 2011 10:34 AM
> Subject: Re: Topband: Shunt fed tower
>
>
> 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
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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