Re: NEC-LIST: cross field antennas

In a message dated 2/12/99 8:07:44 AM Eastern Standard Time,
paulmoody_at_onaustralia.com.au writes:

<< Chip,
 
         The elevation testing was to determine if the ground was
 influencing the antenna operation. My premise was the following
 ... take a dummy load and test the input impedance at various
 elevations. The impedance should not change ( of course ) unless the
 coax was radiating etc etc. I am treating the antenna as a dummy
 load. If the ground was involved I would expect to see a change in
 impedance ( my equipment detects phase and / or amplitude variation at
 the feed point ) with varying elevation. My understanding of the CFA
 is that it should operate like a light bulb ie you pump RF into it and
 it radiates. ( ie no groundplanes etc )
 
 I was using the term isotropic radiator to indicate that the antenna
 does seem to operate like a light bulb ( ie ground independant ) and
 point source radiation in horizontal. ( well I have not done vertical
 FS testing ).
 
 I have not made formalised field strength tests as such though I do
 use a remote sensing FSM to determine if anything is being radiated. I
 have used the antenna on @ 28 Mhz and made successful contacts but
 have not revealed that I was using it to the other party. I tried
 briefly to compare FS with a 5/8 groundplane at elevation of 8 meters
 but was inconclusive.( radiation patterns are not comparable I
 suspect )
 
 However I really need a 50:377 ohm broadband transformer the impedance
 ratio is critical.
 
 regards & 73>>

Hi Paul--

I understood your point. I am saying--without hesitation and with full
conviction--that it's wrong.

Your analogy to a light bulb is inappropriate as this is black body
radiation from a radiator spaced many, many of orders of magnitude
above any ground.

I am saying--without hestation or doubt--that the radiative component
of any impedance (radiation resistance) will produce an anisotropic
power pattern in a ground environment such as you describe. There are
NO isotropic radiators in such an environment.

If the ground wasn't affecting the power pattern then the impedance
you measure is dominated by the ohmic loss. Indeed, it must be
swamping the radiation resistance.
 
Chip
Received on Fri Feb 12 1999 - 19:48:39 EST