[TowerTalk] QST switchable antenna

[Gene Fuller]

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 at tm.net>
To: <towertalk at 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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