In article ,
John Doty wrote:

Telamon wrote:
In article ,
John Doty wrote:


Frank Dresser wrote:


I recognized Telemon's antenna formula as something very much like
the transmission line formula. I'm not sure how it applies to
resonant receiving/transmitting end fed wires. If it does, I'd
like to learn something.

For a wire antenna, the field configuration near the wire is very
similar to the field inside a coaxial cable. Unsurprisingly, it has
similar behavior: the bulk of the energy tends to propagate along
the wire and not radiate. This leads to Schelkunoff's
approximation: you calculate the current distribution along the
antenna as if it was a transmission line, and then calculate the
radiation due to that current distribution. You can get the antenna
impedance by calculating the impedance of a lossy transmission line
(with loss equal to the radiation) with the assumed current
distribution. You get the reception properties by reciprocity.


It boils down to this, smaller RF current loops radiate less
effectively.

Yes, but what does that have to do with the discussion above?

Well the original question asked what the characteristic impedance of
the antenna was and its connection to the radio. Since this is a SWL
news group the antenna questions here tend to be "what is the most
effective way to string a long wire" so I can get good reception so I
attempted to relate the antenna height to the discussion pointing out
yet again in another way why higher is better.

The wire will become a better antenna the higher it is off the
ground.

Often true. Nevertheless, a low Beverage can be an extremely
effective antenna.

Yes, but a Beverage antenna to be effective has to be much longer than
a random wire in general. The Beverage is a good antenna if you have
the real estate in the needed direction of the desired station. Most
people do not want a directional antenna for SWL listening.

If the wire was vertical instead of horizontal then it would not
look like transmission line where the inductance and capacitance
are evenly distributed over its length.


Actually, the Schelkunoff approximation works quite well in that
case. The field near the wire is not strongly affected by its
orientation. The characteristic impedance varies only logarithmically
with distance from ground, so that except for a modest bump in the
immediate vicinity of ground, the inductance and capacitance per unit
length are nearly constant. If this was not true, the Schelkunoff
approximation would be nearly useless.

I was just trying to illustrate the reason why a formula for the
characteristic impedance of a horizontal random or long wire "looks"
like a formula for a transmission line with two parallel elements and
how the same wire vertically does not look the same because you no
longer have the constant or even distribution of the inductance of the
wire over its length along with the same capacitance to ground over its
length.

If the wire is vertical the capacitance to ground to a point on the
wire becomes smaller as you move up the wire toward the top. You no
longer have the equal distribution of capacitance and inductance over
its length.

--
Telamon
Ventura, California