[TowerTalk] QST switchable antenna

[Gene Fuller]

P.S. With my feed systam I don't worry about exact matching and tune the 450 
ohm line in the station with an old Johnson Matchbox.  Gene / W2LU

----- Original Message ----- 
From: "Gene Fuller" <w2lu at rochester.rr.com>
To: "K8RI" <K8RI-on-TowerTalk at tm.net>
Cc: <towertalk at contesting.com>
Sent: Sunday, November 27, 2011 11:05 AM
Subject: Re: [TowerTalk] QST switchable antenna


> 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 their resonant frequency,  thus 
> turning them into areflectors. My guess is that it 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 degrees 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
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>