[Elecraft] PAR endz-fed

[Geoffrey Mackenzie-Kennedy]

Jim Brown

On Tuesday, March 03, 2009 at 4:34 PM, Jim Brown wrote.


> On Tue, 3 Mar 2009 01:53:19 -0000, Geoffrey Mackenzie-Kennedy wrote:
>
>>Yes, I take your point. It is the fine tweaking of phase that I find 
>>easier
>>to do with a capacitor!!
>
> Please tell me how you would do that with this antenna! :)   The 
> "tweaking"
> involved requires nothing more than looking at my measured data and 
> winding the
> choke to put the resonance where you want it. It isn't very "tweaky" --  
> the Q
> of a choke like this is around 0.5.

The inductance of the coil formed by the outer braid of the coax feeder in 
the L-C traps that I used is less than that of a choke, and when space wound 
and tuned to resonance by the trap's capacitor the Q of this tuned circuit 
is quite high, but not high enough to compromise the operating bandwidth of 
the driven elements on 40m.

I am not absolutely certain as to why a small change in the resonant 
frequency of the traps in a two element vertical array affects their 
phasing, albeit slightly. SWR measurements taken on the individual elements 
did show a small change in their drive point impedances as one of the trap's 
resonant frequency was changed by a small amount, which would affect the 
phasing between elements of course.

Unfortunately I had to change from using vertical arrays on 40m to 
horizontal arrays, in some directions our large oak and beech appeared to 
distort the radiation pattern of the verticals.

73,

Geoff
GM4ESD