John wrote:
Whilst trying to source a "digital" TV antenna I came across some with all
external surfaces plastic. One was a small yagi with all external surfaces
plastic, hopefully with metal elements embedded. Another a "T" shape made
out of plastic conduit with elements inside conduit.
My question is how do they work?. If they are detecting electrical fields
how does increasing source impedance by 100,s of megohms improve things?.
Capacitive coupling, I suppose at the frequencies involved there would be
some.
If it works as well as all metal why doesn,t every one use it and stop
corrosion?
Hope this is not too off topic.

I'll bite.

Look up Yagi. Mr Yagi was an English speaking Japanese graduate student
who happened to study antennas in the 1930's under Professor Uda. Unfortunatly
for him and history Prof. Uda did not write English, so he left documenting
to the world his discoveries, and they have since been know as Yagi antennas.

His discovery was that if you take an antenna, say a dipole, and place another
element in the correct position, it adds directionality to the antenna. This
reduces its ability to receive signals in all directions and increases its
ability to receive them in others.

(simplifed explanation follows)

Research has since shown that a slightly larger element, not connected to the
antenna, acts as a reflector and increases directionality in the opposite
direction, i.e. it becomes the back of the antenna.

A slightly smaller element, also not connected in the right place acts as
a director, causing more restriction and more gain in its direction.

You get better results by adding directors than reflectors.

While the size of the actual antenna element effects the frequency response
of the element, (wider elememnts, wider bandwidth), in most cases, it does
not matter how big the reflectors and directors are, only their length and
position matter.

TV antennas are unusal in that they require a very wide bandwith and other
designs such as log-periodic antennas are common, but Yagi designs are used
for single channel, or single band antennas.

So for example, if you wanted to receive all the stations in one direction,
you would use a wideband antenna, if you wanted to receive one partictular
station, a Yagi for that frequency would be smaller and cheaper.

One could make an antenna out of a piece of paper, drawing the elements on it
with a conductive pen and it would work. You could figure that out, that a
UHF TV signal may fit on an 8 1/2 x 11 paper, but a lower fequency one
would need more space and bigger elements.

For recepetion, one only has to build an antenna that would survive its
location, so for example a UHF antenna made out of a sheet of plastic,
or embedded in one would make a good TV antenna near the sea shore.

It would at as a flag in the wind, so it may not be as good a choice as
you think. Putting it in your attic, might be a good choice.

Lots of ham radio antennas have been made from a stick of wood as the beam
(the center piece), copper tubing for the driven element (the real antenna
part) and coat hanger wire for the passive elements.

Geoff.


--
Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379
To put it in terms everyone understands, the US debt is over 150 Facebooks.