Hi Roger -
The feedline from the dipole center back to the relay box, in my opinion,
needs to "float". The dipoles that are not being driven depend on the
reactance from the open feedline to lower the resonant frequency of the
dipole, thus turning it into a reflector. My guess is that is would mess
things up to put all of the shields on a common ground. I use a three dipole
system on 80 and feed each dipole with a quarterwave of 72 ohm KW twinline
followed by a quarterwave of 300 ohm KW twinline, then the relay box. I then
use three DPDT relays to put one dipole at a time onto 450 ohm openwire
feeder back to the station. If you want a real eye opener the next time
you're on your PC with a modeling program, note what happens whem you fold
the bottom half of the dipoles back to the foot of the tower.With a 100 foot
tower it brings the takeoff angle down to about 18 degrees! What is
happening is that the horizontal radiation is being canceled, leaving you
with resonant vertical radiators,with the effective spacing and height
being determined by the tower height and not requiring a ground radial
system. I find my system with three dipoles and a 100 foot tower to be VERY
effective. 18 degree takeoff angle, around 3-4 dbd gain, and, depending on
conditions, 10-20 db down at 120 degree off azzimuth. Any particular system
should be modeled to account for the number of dipoles being used, tower
hieght, and feedline characteristics, to determine the optimum feeder length
back to the relay box.
Gene / W2LU
----- Original Message -----
From: "K8RI" <K8RI-on-TowerTalk@tm.net>
To: <towertalk@contesting.com>
Sent: Friday, November 18, 2011 8:36 PM
Subject: Re: [TowerTalk] QST switchable antenna
> On 11/18/2011 4:50 PM, Jim Lux wrote:
>> Well, I've got part of the model in. Copied the dimensions from the QST
>> article and the developer's MA presentation..
>>
>> Not getting a good match yet, nor the F/B... but it's probably some
>> little fiddling needed with dimensions, etc.
>>
> Check the antenna handbook. There's been one of these in there for
> years although I don't know if it was in the last one or not. I'll have
> to check.
>
> IIRC they bring the individual feed lines back to a remote antenna
> switch mounted on the mast. The feedlines are a quarter wave with an RF
> Choke just before the antenna switch. I don't remember if they ground
> the connections other than the active one or not.
>
>
> 73
>
> Roger (K8RI)
>>
>> CM QST switchable
>> CM Ashraf Abuelhaija design
>> CM dimensions taken from slides Vortrag_MA_Abuelhaija_Ashraf.pdf
>> CE 20m
>> SY L = 5.3'wire length
>> SY DR = .00021 ' wire radiu (0.14 mm2)
>> SY Hc = 7 ' height of center
>> SY HE = 4.35 ' height of wire ends
>> SY Y=4 'distance to side of antenna middle elements (fore aft
>> Y=0)
>> SY X1=4.62/2 'distance fore/aft for middle elements
>> SY X=4
>> SY CL = 120 ' pf Load
>> GW 999 1 0.05 -0.05 HC 0.05 0.05 HC DR 'feedpoint
>> GW 10 50 0.10 0 HC X 0 HE DR 'wire #1
>> GW 950 1 -0.05 -0.05 HC -0.05 0.05 HC DR 'loaded element feed
>> GW 20 50 0.05 -0.05 HC X1 -Y HE DR
>> GW 30 50 -0.05 -0.05 HC -x1 -y HE DR
>> GW 40 50 -0.10 0 HC -x 0 HE DR 'wire #4, directly aft
>> GW 50 50 -0.05 0.05 HC -x1 y HE DR
>> GW 60 50 0.05 0.05 HC x1 y HE DR
>> GE 1
>> LD 5 0 0 0 58000000
>> LD 0 950 1 1 0 0 CL*1e-12
>> EX 0 999 1 0 1.0 0.0
>> GN 2 0 0 0 13.0 .005 0.0 0.0
>> FR 0 1 0 0 14.15 0.0 0.0 0.0
>> EN
>> _______________________________________________
>>
>>
>>
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>
>
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