[BC] Isotron AM Antennas

[RichardBJohnson at comcast.net]

Of course, but the resistive part of the input impedance
will get lower, the shorter the overall length. This will
seriously hurt the performance.

When trying to analyze things like this, it is helpful to
look at the limits. That's look at something that is extremely
short, simply an small coil with an open end. The capacity
between turns and the capacity off the open end resonates
with the coil's inductance. There will be two major resonances,
the parallel resonance, which represents a high impedance which
we don't want, and the series resonance which represents the
low impedance. With a small coil, the entire resistive component
has virtually no radiation resistance, the stuff than launches
a radio wave. Therefore its efficiency as an antenna would be
near zero.

At the other extreme, we have large uncoiled inductor the length
of a typical series-fed antenna, perhaps 150 feet or more. Such
a device will have its radiation resistance dominate, therefore
it will have a high efficiency, perhaps 85% or more. If the
device losses remain the same (they don't, but we just want to
see a picture in the minds eye), and you have 85% efficiency
if it's 150 feet in length, and 0% efficiency if it's 0 feet
in length, then a 20 foot section should be about 20/150 * 85 =
 11% efficiency. If you go to 10 feet, it will be 10/150 * 85 =
5.6 % efficiency --not too good.

Now, in principle, the inductance of a solenoid varies as the
square of the number of turns, so that there should be some
square-root function used in the above "linear" mathematics.

However, the coefficient of coupling between turns is not
unity so the inductance, and hence its electrical length,
varies almost directly with length.

When I lived in Bolder Colorado, I made one of these things for
my neighbor's kid so he could have a "radio station" until, of
course he started to recognize girls, lost interest. The radio
transmitter was at the high-end of the band, and the counterpoise
was a cyclone fence that went around the property. I could be
heard on a car radio in Denver.

Cheers,
Richard B. Johnson
Website http://AbominableFirebug.com

----- Original Message -----
From: "Tom" <Radiofreetom at gmail.com>

Hmm...

That explains what I did wrong when I tried a BCB "rubber duckie"  
Thanks, Richard!

Quick question -

If the pipe were double the diameter, could it be built on a 10-foot 
section?  Without doing the math or loading EZNEC - I'm on short time 
right now - seems like it would.

RichardBJohnson at comcast.net wrote:
>
> If you have no tower standing, get a 20 ft section of 2-1/2 in plastic sewer pipe
> (two 10-foot sections). Support it over your ground-plane with three guys
> made of plastic rope. Run coax to its bottom, and connect the shield to your
> ground-plane (dig a hole and locate one radial). Wind the plastic pole with
> #10 insulated wire over its entire length before you erect it You will need
> a 100 ft roll. This will be the "tower." Find its resonant frequency by connecting
> the end near ground to ground and bringing a real portable radio (the kind with
> a continuously- tuning dial) near it. A sharp peak in noise (or distant stations) will occur
> at resonance. Cut off some wire to bring the resonant frequency up to your
> station's frequency. Do the final tuning by spreading out turns. 

-- 
Tom Spencer
PG-18-25453 (nee' P1-18-48841)
http://radioxtz.com/