Larry,
Because of the complexity of modeling without going crazy, although in
simple situations it will get you in the ball park, I would highly
recommend a 3 or 4 wire cage space at least 24 inches from the area near
the rotor plate on a, let's say, 70 to 80 foot tower. The drop wires
should be #8 or #6 copper and tied together in a ring supported by
porcelain insulators (PVC not recommended in some circles) around at
the base with one wire connected from the ring going to your proposed
ATU. With a MFJ bridge measure the feed wire's reactance and impedance
against ground. With one climb have your tower climb buddy work his way
from the top in 2 foot increments jumpering the cage to the tower with
large alligator clips (nothing fancy for this purpose) and tell him or
her to keep coming down until you get close to 50 ohms. (It can be 40 to
60 ohms as that is sweet point enough you me) Then back a better
connection using split copper bolts with three jumpers to the tower.
Whatever the reactance is you can tune out that inductive reactance with
an equal value of capacitance. As Guy said forget about the tower being
resonant anywhere since in such circumstances you may never get that. A
tap coil to ground will get you with a simple L network and series cap
should get your SWR to 1:1 even if the sweet point is a bit off. Again
the components should be, flat wound coil with correct tap connections,
a vac of at least 750pf with broadcast mica paders if required for more C.
I have found that single wire shunt feeds are the most problematic to
work with, especially when the beams are on multiple levels. A larger
diameter cable, if you must only use a single wire shunt can be obtained
from using a length of CATV .750 which is 3.5 inch in diameter. But a
big shunt doesn't look all that hot and a three wire cage is beautiful,
looks like it will work, and in fact does.
Good luck,
Herb Schoenbohm, KV4FZ
On 12/15/2011 5:14 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:
>>
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