[Milsurplus] BC-230 questions and observation

[Mike Hanz]

Ken was just using a Thevenin equivalent to simplify the model, Hue.  It 
does make some approximations, such as assuming (r*2*pi*f*C)^2 is much 
greater than 1, which on my fingers and toes doesn't seem to compute 
very well but it's still early in the morning yet.  Another assumption 
is that you are always operating at somewhat less than the natural 
frequency of the antenna, thus making the antenna capacitive.  Using the 
3MHz point that Ken chose for estimation, Sandretto's plot for the DC-3 
show a radiation resistance of 0.8 ohms and a capacitive reactance of 
250 ohms at that frequency.  However, the plot also points up the fact 
that aircraft antennas were obviously used at other frequencies, and 
that their radiation resistance, even at their natural frequency of 1/4 
wavelength, was significantly lower than a vertical antenna, for example 
- the DC-3 antenna had a radiation resistance of only 3.8 ohms at its 
5150kHz natural frequency.  Above the 1/4 wave point the antenna changes 
to have an inductive reactance.

Kd6b@aol.com wrote:
 > I suspect we need a little better numbers
 > for the antenna series resistance but you get the point.

Something I sent to the ARC-5 group a while back might be useful for the 
number crunchers to have some fun on:

"If you're interested in some of the more arcane aspects of the antennas 
the command transmitters were designed to feed, I put an old 1942 
['Electronics' Magazine] article up at
http://members.cox.net/mymhh/Aircraft%20antenna%20design%201.GIF
http://members.cox.net/mymhh/Aircraft%20antenna%20design%202.GIF
http://members.cox.net/mymhh/Aircraft%20antenna%20design%203.GIF.
(If you have a browser with variable magnification like Opera, set the 
zoom value to 20% - the pages are fairly large to allow high resolution.)"

The nomograph was developed using empirical data, so it should reflect 
very close to reality, at least for WWII aircraft hardware and geometries.

Best 73,
Mike

Hue Miller wrote:
> ----- 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
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