Re: NEC-LIST: CFA and the Hately advertisement

At 04:02 PM 5/21/99 EDT, you wrote:
>In a message dated 5/21/99 3:53:45 PM Eastern Daylight Time,
>dbaker_at_postino.up.ac.za writes:
>
><< memory serves me right, some operators at HF prefer to use a
> so-called 190 degree mast (little over 0.5 lambda), in order to reduce
> the skywave component and to increase the ground range before one gets
> interference between sky- and groundwave at night. I do recall an
> article where the authors described a means of using toploading with
> stay wires (insulated) to get comparable results to the 190 degree
> tower with a 90 degree (lambda over 4) tower. The details are lost in
> my filing cabinets somewhere.
> >>
>
>That is, indeed, confusing. IF you can get a 1/2 wave height, then WHY
>use a CFA?
>
>Anyone know?
>
>73 Chip N1IR
>

Greetings Chip:

By now you will have seen the replies mentioning the 5/8 wavelength
(wl) antenna, and surely in your radio-amateur experience you will
have seen them used on mobile VHF/UHF installations: "sky-wave
radiation" represents wasted power, and the 5/8 wl "squashes" the
radiation into a horizon-hugging coverage, with "gain" over a 1/4 wl
whip.

As for the CFA mounted atop a tower, it may function as nothing more
useful than an elaborate form of top-loading, akin to the esoteric
contraptions adorning (USA) "Citizen Band" antennas, designed
primarily to extract money from the unsophisticated consumer. As
top-loading, the CFA would increase the tower's electrical length,
"proving" that a CFA has gain compared to a tower! ... :-^)

Notwithstanding that, there is the "wishful thinking" school of
antenna design, that is best avoided by going "back to basics", to see
what the antenna is really doing (or what is really the antenna). In
the case of "something" (e.g. a whip) atop a tower, the feedpoint
impedance is the sum of the impedances looking in both directions
(upwards to the whip, downwards via the tower to "ground"). That's why
a whip (not on an "infinite groundplane") has either a 1/4 wl set of
radials, or a 1/4 wl coaxial sleeve below it, to stop the rest of the
tower becoming part of the antenna.

This makes prediction of the CFA performance atop a tower interesting,
from both the resultant feedpoint impedance, and radiation pattern
aspects.

The tutorials by the learned persons here are most interesting, and
educational, but sometimes induce a Poynting-twist to my neck as I try
to follow the mathematics! I look forward to more developments,
NEC-models of it, just in case the Admiral gets a visit from a CFA
salesman, and we have to come up with an assessment of its performance
on a warship ... Shiver me timbers !!!

 David G. Robinson, P. Eng.
Ships Radio Systems Engineer
     DGMEPM/DMSS 8-3-4
      (819) 994-8487
(reply to drobinso_at_dmcs.dnd.ca)
    (not to the firewall!)
Received on Tue May 25 1999 - 19:37:11 EDT