Re: NEC-LIST: cross field antenna

Chuck Counselman wrote:

> I was curious so I wrote to the NEC-LIST:
> >I'd never heard of a "cross field antenna" (CFA) before reading about
> >it recently on this list.... Does anyone on the NEC-LIST have...a sketch
> >or drawing of a CFA...?
>
> Paul Moody <paulmoody_at_onaustralia.com.au> was kind enough to e-mail a
> dimensioned sketch to me.
>
> When I saw it I checked my computer's calendar to see whether I'd been
> asleep for a few weeks and the date was April 1st. I feel like it
> must be. The structure depicted in Paul's drawing is just a (very)
> short, capacitively end-loaded, dipole. No way or means is provided
> to alter the phase relation between the current in the wire and the
> charge on the end plates. (I'd call the end plate a "top hat" if this
> were a monopole fed against ground; perhaps I should call the end
> plates top and bottom hats.) Unless I missed something or something
> was missing in Paul's drawing, you don't need NEC-4 to understand this
> antenna.
>
> -Chuck.

Well I was going to post a photo of the actual antenna since of course
the diagram is too simple however this was not possible ( long story
). I have built a number of these and have better things to do if they
did not work. There is a phase relationship ... only a resistor
exhibits in-phase voltage and current ... the variable cap does indeed
alter the phase of the current in the wire ( H ) and the voltage
across the plates ( E ). I will be posting a photo of the antenna (
shortly I hope ) and if I can get my FS gear sorted out an indication
of Field Strength performance, BW etc. There is a reason for the
antenna dimensions ( for operation at 28 Mhz ) that may not be
clear. ( calculating the reactance of the two plates at 28.5 Mhz might
give you some clues). The matching circuit is more critical
(mechanical ) than the simplicity of the diagram indicates ... however
the electrical schematic is correct. I dont know about NEC but the
design for this antenna was derived from the original CFA and an
elementary Smith chart exercise. I dont know about the validity of CFA
claims yet but the Smith chart indicates that the BW will be wide
... the trick is in getting the terminating impedance value ... hence
the particular dimensioning.

This is my personal opinion and of course the arguments you put
forward are very compelling. I am unsure exactly how the antenna works
... it should not .

--Paul.
Received on Thu Feb 25 1999 - 07:17:25 EST