[Milsurplus] BC-230 questions and observation

[Hue Miller]

----- Original Message ----- 
From: <Kd6b@aol.com>

> However, there are two ways of 
> representing an equivalent circuit for an antenna, a series circuit, as you 
> pointed out, or as a parallel equivalent circuit.

Ken, however, the actual radiation resistance is a low
resistance, in real life, so the "correct" way to represent
this is as a 50pF cap (for example) and the Rr, which
may be as low as a fraction of an ohm for an aircraft
antenna (short, horizontal, low to ground= airframe.)
BTW, a book by Sandretto, 1942, sez the reason that
this simple matching circuit was used, was to minimize
losses when dealing with low Rr antennas. 

HOWEVER- it did occurr to me: i don't think a harmonic
radiation analysis can be done except by field strength.
Consider: the harmonic sees a different antenna than
the fundamental. Thus hanging a spectrum analyzer
across the antenna gives an untrue reading for harmonics.
Imagine an antenna approaching 1/4 wave. It is capacitive
reactive, with Rr approaching a peak ( still a low resistance,
altho a peak for its neighborhood.)  Now, the 2nd harmonic,
however, the antenna looks like it's approaching 1/2 wave,
inductive reactance, and approaching a high impedance of
several thousands of ohms. This makes it real hard, i think,
to predict how strong the harmonic will be, or even to use
a spectrum analyzer and expect that to give trustworthy
figures. 
What about if the antenna is resonant, like a 1/4 wave. Well,
it's still not so simple. At the fundamental, say 3105 kcs, 
the antenna is very low to its "ground" and Rr is very low.
At maybe the 3rd harmonic, the antenna is 3/4 wave, okay,
but the effective height above ground is a larger part of a
wavelength, and the Rr is higher, so it's not the same antenna
as at 3105.
How's that sound?

Now, if i modeled that at, say, 1/10 the wavelength, 10x the
frequency, and actually field-strength tested that, would that
pretty much be a valid test, do you think? I say this frequency,
around 30 MHz, because it's not yet quite in VHF land, with
vhf peculiarities, and yet it's maybe still small enuff to practically
model. Opinion?
Tnx, Hue Miller  KA7LXY