Hi Pete and Dick, modeling closely spaced conductors has always been
problematic.
My experience with shunt fed verticals has shown that a reasonably sized
shunt feed device ends up with an impedance of much less than R=50 and
includes a series L of a few hundred ohms at the feed point.
The included series inductance provides the key to matching without having
to add an extra inductor. One basically adds a series C to allow
cancellation of just enough L to arrive at a parallel circuit with R=50 and a a
shunt value of inductance that is then canceled with a parallel C. Also known
as series to parallel conversion.
Sounds a lot like an Omega match doesn't it? Once adjusted, one can run up
and down the band by small adjustments to series C. The shunt part doesn't
change very much so usually it can be left alone.
One can use small fixed and/or variable capacitors to perfect the design
and then replace with the bigger high power parts.
73,
Gerald K5GW
In a message dated 6/23/2015 1:03:34 P.M. Central Daylight Time,
n4zr@contesting.com writes:
>From my own experience, this is not an easy thing to model with
precision. I've been doing NEC-2 modeling for a long time, and had a
comprehensive model of my complete towerand all antennas when I set out
to model a 160-meter shunt feed. Even so, and despite a number of
climbs to try to find what the software suggested might be a 50-ohm tap
point, no joy. I finally wound up using a 45-foot or so tap point and
an Omega match. No inductor required.
73, Pete N4ZR
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On 6/22/2015 2:43 PM, RLVZ--- via TowerTalk wrote:
>
> Hi Guys,
>
> I'm looking for someone to model a simple Shunt-Fed tower for use on
> 80-meters. I have no experience using modeling software and don't have
time to
> learn until after I retire, so I'm hoping someone is willing to model
this
> for me.
>
> I'd like to model this tower to help determine the aprx. location of
the
> 50 ohm resistive feed point tap at a frequency of 3.550 mhz. I realize
> that I could feed the tower at almost any height and make it work using
a LC
> Network, but I'd like to keep it simple as W8JI once wrote, by using: "a
> simple gamma capacitor and shunt feed conductor with the tap point
adjusted to
> find 50 ohms resistive". (no inductor required with this simple feed
> method) Since the tower is short at 46', it will be relatively easy to
climb
> it in order to find the exact 50 ohm resistive tap location. However,
with
> my bad back, modeling it should get me in the general area of the 50 ohm
> tap location and save me several trips up and down the tower which
would be
> much appreciated.
>
> Here's the tower specs:
>
> 1) Rohn 45 tower with 18" face width, tower height is 46'.
>
> 2) Non-Conductive guy wires. (no steel guy wires)
>
> 3) Top mounted Bencher Skyhawk Tribander. (24' boom)
>
> 4) 10' aluminum DC grounded VHF antenna top mounted, extends the overall
> tower/antenna height to 56'.
>
> 5) Proposed Shunt Feed system: 10 gauge copper wire run parallel to the
> tower, from the base of the tower upward to the 50 ohm resistive tap
point.
> I'm flexible with the spacing distance of the wire from the tower, but
am
> leaning towards an 18" separation unless modeling recommends otherwise.
>
> In addition to determining the estimated 50 ohm resistive tap height, it
> would also be very helpful if the modeling software would estimate the
value
> of the needed capacitance. (such as 150pf)
>
> Thanks & 73!
>
> Dick- K9OM
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>
>
>
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