Terry:
I think you have your numbers correct. I measure about 17-jX ohms for my 60
foot, top loaded vertical on 160. My ground system has about 40 radials but
they are shorter than 1/2 wavelength on 160.
My suggestion is to put a matching network at the base of the tower and then
use the tuner in the shack to get the final adjustments. The idea is to
reduce the VSWR on the feed line to the point that the losses and voltage
peaks are of no concern. For my particular situation I use a 3 uH series
inductor into 2.95 uH shunt inductor with the shunt L on the feed line to
the shack side.
Using hand wound inductors for the two you should be able to come up with
prototype inductor values that reduce the SWR going down the line to
something better than 4:1 over the entire band of interest on 160 meters.
The final inductors should be designed to able to handle 5 amps of current
if you run 1500 watts into them. I think mine are about #10 wire, but that
is probably more than is required. The loss in the network should be less
than 5% with good #12 wire wound on PVC forms. The coils could be even air
wound if you use #10 or #8 copper wire. Reasonable Q can be achieved with
length about 1.5 to 2 times the diameter.
The series inductor can be thought of as cancelling the -jX and the shunt
inductor as a up transformer to get the resistance to about 50 ohms.
This method is sort of the reverse of the system in the recent NCJ where
Jay, WX0B, made his vertical longer than 1/4 wave so he could cancel the
reactance with a series capacitor. Here you deal with the capacitive
reactance by adding a series inductor and then step up the resistance.
Tod, K0TO
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Terry Conboy
> Sent: Friday, February 06, 2009 2:18 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] 43' Vertical - Feed Point Tuner or
> Shack Tuner?
>
> It would appear that the SWR curve that is still on the Array
> Solutions web site was measured with a misapplied 4:1 voltage
> balun (which puts half of the antenna RF voltage on the
> outside of the coax shield).
> http://www.arraysolutions.com/Products/zero_five.htm
>
> This could probably explain how an SWR of under 2:1 is
> obtained in the 160m band, where a 43 foot antenna alone has
> a modeled impedance of about 7.5 - j 570 ohms over a good
> ground system with a 5 ohm equivalent loss resistance. This
> should give an SWR over 100:1 in either a 50 ohm or a 200 ohm system.
>
> 73, Terry N6RY
>
> On 2009-02-01 8:09 AM, K4SAV wrote:
> > Here is an article that should answer your question:
> > http://www.vk1od.net/multibandunloadedvertical/index.htm
> > Also notice the paragraphs about the balun used in some of
> these antennas.
> >
> > Here is a link to a long discussion on this balun, which eventually
> > determined that the manufacturer was including the wrong type balun
> > for use in this system.
> > http://www.eham.net/forums/Elmers/201882
> >
> > Jerry, K4SAV
> >
> > Richard Thorne wrote:
> >
> >> I have a vertical in my backyard that I currently feed at the base
> >> with an SG-239 200 watt tuner. I want to increase my
> power from 100
> >> watts to 400 watts. In order to accomplish this I either need a
> >> tuner at the base like an SG-235 or CG-5000 both expensive.
> >>
> >> The other less expensive solution is a tuner in the shack.
> >>
> >> So how much loss will I have if I tune a 125' length of
> coax? I know
> >> it works as I bypassed the base tuner and used my rigs tuner. No
> >> problem tuning, I'm just curious how much loss I'll have
> compared to
> >> the tuner located at the base of the antenna.
> >>
> >> Rich - N5ZC
> >> _______________________________________________
> >>
>
> _______________________________________________
>
>
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