Joe Subich, W4TV wrote:
> A common mode choke does not solve the issues of grounding the
> shields of the coax in the K1WA design. The K1WA design relies
> on the 3/8 wave open coax acting like an inductor to tune the
> unused slopers as reflectors. Unfortunately, when the shield
> is connected to ground, the ground connection adds capacitance
> in parallel with the inductance and DETUNES the reflector.
>
> I know of no commercial switch that opens both sides of the coax
> and floats the shields. Someone building an antenna that uses
> this principle needs to: 1 )build their own switch, 2) work with
> a length of feedline (5/8 wave ?) that will yield the proper XL
> when SHORTED or 3) use a half wave of feedline and coils. In
> all three case you will still want an effective common mode choke
> at the feedpoint.
>
>
And around here you want every line grounded when not in use.
73
Roger (K8RI)
> 73,
>
> ... Joe, W4TV
>
>
>
>
>
>> -----Original Message-----
>> From: towertalk-bounces@contesting.com
>> [mailto:towertalk-bounces@contesting.com] On Behalf Of Roger (K8RI)
>> Sent: Monday, April 20, 2009 8:37 PM
>> To: jim Jarvis
>> Cc: k4fj@k4fj.com; towertalk@contesting.com
>> Subject: Re: [TowerTalk] sloping dipole arrays
>>
>>
>>
>>
>> jim Jarvis wrote:
>>
>>> I have seen K1WA's excellent article in the Antenna
>>>
>> Handbook for years
>>
>>> regarding sloping half-wave dipole arrays. His design
>>>
>> switches both
>>
>>> the center conductor and braid for each feedline.
>>>
>>> Is it really important to switch the braid?
>>> 73, Steve, K4FJ
>>> -0-
>>>
>>> YES.
>>>
>>> The K8UR sloper array design, and the variants upon that, require
>>> that the
>>> reflectors not be connected to the driven element. If you use a
>>> switch which
>>> ties all shields together, you'll have half of your
>>>
>> reflectors and
>>
>>> your driven element
>>> fed together.
>>>
>> That is why you install a choke/current balun on the feedline
>> the proper
>> distance from the antenna. Then the shields can remain at DC ground
>> (for safety)
>>
>> 73
>>
>> Roger (K8RI)
>>
>>> God knows what pattern will result. Or, knowing
>>> this audience,
>>> only an adept NEC user with a lot of time will know what the
>>> resulting pattern
>>> will be.
>>>
>>> The technique of feeding each dipole via 3/8 lambda coax from the
>>> switch is a particularly neat way to assure that the
>>>
>> undriven elements
>>
>>> function as
>>> reflectors.
>>>
>>> N2EA
>>>
>>>
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
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>>> TowerTalk@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>>
>> _______________________________________________
>>
>>
>>
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>
>
>
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