[BC] Isotron AM Antennas

[Cowboy]

On Friday 20 February 2009 09:42 pm, Tom wrote:
>  So, a six-inch by 10 ft PVC pipe wound with that same 100 ft of #10 (I 
>  have a 500' spool of #14 available) insulated wire will self-resonate 
>  higher in the band than the 2 inch x 20 ft. 

 Not necessarily.

>  Then adding in a whip antenna at the far end of the coil will add 
>  capacity, right?  Moves the resonance DOWN, if I'm remembering correctly. 

 Yes, and no.
 That whip is there primarily to reduce corona and burning of the high
 voltage end of the coil. ( a short whip )
 Similar affect is achieved by spreading the last several turns spacing
 at the high voltage end, as is frequently seen on high voltage chokes.
 It *could* then be considered as a base loaded whip, but that's a
 rather grey area. At what point is it a loaded whip, and not simply
 a corona reducing voltage distributing device, or end loading
 capacitance ?

 The "rubber ducky" antenna is a type of incon, self resonant at some
 frequency. They work surprisingly well, but it's still a compromise
 antenna.

>  Same 100' wire, but a short fat coil instead of a tall skinny one. 
>  
>  I've run a couple of different programs trying to get a handle on what's 
>  going on - and either I'm missing something, or the programs are, 
>  because neither one seems to match either each other or what I got 
>  empirically.....

 They won't.
 You need to consider the coil as a coil, or at least as an incon.
 Even if the inductance was the same ( it isn't ) the self capacity is a
 function of physical space occupied.
 ( as is the radiation resistance at any particular frequency )
 ((not to be confused with feed point impedance ))

>  Actual (first try) was actually 175' of #14 on 1" PVC, close-wound.  
>  couldn't find ANY resonances with that one...
>  
>  Started over with 2" PVC... (arm ached for a week after rewinding all 
>  that copper.....).  Works, but not as well as the power line CC 
>  connection...

 As Richard points out, the device does have a self-resonant point, modified
 by ground, and anything else in proximity.
 This will be exceedingly difficult to model accurately.
 Of the various types I've built, the easiest way to identify resonance, is
 with a dip meter. You could bridge it, but it's not the easiest way.

-- 
Cowboy