Re: NEC-LIST: CFA in short

In a message dated 2/23/99 6:36:09 AM Eastern Standard Time,
paulmoody_at_onaustralia.com.au writes:

<< Hmmm, Thinking of the CFA as a dipole is IMHO not correct.
 Convention seems to be that an antenna must be resonant before it
 will radiate ( efficiently ) .... note I said efficiently. >>

Nope; I have a fractal dipole (Koch) which is very efficient--AND
LARGE electrically -- over a 20:1 range of frequency. However it is
not resonant --yet has a VSWR less 2:1 over that frequency range . It
is certainly true that many small antennas have rapid changes in
reactance around the resonant frequency. That has nothing to do with
efficiency, save for the ohmic loss induced to make the antenna
resonant.

<< I operate a 6 foot loop antenna on 3.5 MHz. This antenna is well
 below the 'normal' size of antennas for this band. Is this loop a
 dipole ?>>

Yes. It is a form of folded dipole, with length far less than a
wavelength. Not all 'dipoles' are 1/2 wave in length electrically. I
understand the confusion in the term and am happy to be further
enlightened by others here.

<< Bandwidth conditions aside the antenna radiates because of the
field conditions that exist at resonance .... not because it is
resonant. ( after all I could show you plenty of resonant systems
that do not radiate ). Resonance is a means to an end. >>

It radiates because it has a radiation resistance which (presumably )
is of order or larger than the ohmic loss. It's resonant because it
has been loaded (somewhow) to cancel out the reactance.

<<As far as radiation of EM goes you only have to look to the
sky above ... there are plenty of EM sources in the galaxy that radiate
across the full EM spectrum ... as far as I know there is no 'antenna'
involved at all>>

THIS is a different radiation mechansism. It is called 'blackbody
radiation'. And indeed, ALL antennas radiate at ALL EM
wavelengths. But that statement, although true, is basically
irrelevant to your point.

Look at an antenna a a tuned circuit. Your dipole antenna is tuned at
discrete and hi Q frequencies which are harmonically related. If you
EITHER fractalize your dipole or make it self complementary it will
become very broad band. Otherwise it's impedance response is like a
comb.

<< ... so how does this radiation / power conversion occur .>>
 
I'm going to bow out; not because I want to be rude, but because those
kind of questions strike me the wrong way. For example; I (good
naturedly) tell my students to look it up in a book; and if asked I
recommend one. But the flavor of the internet is that most are
offended when you say this. As I cannot give a 'pointer' (or rather
won't) and don't want to repeat what is found in many books, I'll let
someone else try .

Or if you want a reference I will suggest one or two.

73
Chip N1IR

> 73 de VK3PGM

Paul--

You quote me but your comments are in Jos's direction:-) But I have answered
your comments anyway.

73
Chip N1IR
Received on Thu Feb 25 1999 - 06:57:00 EST