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.
Many thanks
John

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.
Many thanks
John

I think we'll leave this one to mr Bialek to answer...

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.

"John" wrote in message
. au...
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?.

Pardon? The coating or conduit shouldn't affect the impedance of the
antenna. The radio signal should pass through the plastic and hit the metal
antenna element.

Are these antennas intended for indoor use where the plastic coating is for
decorative effect?
If the antennas are intended for outdoor use then substantial coating or
conduiting will increase the wind loading of the antenna It also needs to be
u/v stable.

Regards, Ian.

Ian wrote:
"John" wrote in message
. au...
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?.

Pardon? The coating or conduit shouldn't affect the impedance of the
antenna. The radio signal should pass through the plastic and hit the metal
antenna element.

Apparently you have missed the interesting discussions with our Polish
friend.

On Mon, 28 May 2012 16:36:56 +1000, "John"
wrote:

Whilst trying to source a "digital" TV antenna

There's no such thing as a digital antenna or HDTV antenna. There's
nothing specific in the construction or design of the antenna that
will make it better or worse for receiving digital or HDTV signals, as
opposed to analog or digital signals. Modulation methods do not
affect antenna performance (unless you're using very wide band
modulation).

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.

URL? Picture? Vendor name? Clues? Try not to be so vague.

My question is how do they work?.

Badly. A "small" antenna always involves some manner of compromise.
It can be bandwidth, gain, efficiency, or all of these.

If they are detecting electrical fields
how does increasing source impedance by 100,s of megohms improve things?.

It doesn't improve anything. Antennas much be matched to their loads.
If you're plugging this contrivance into a 75 ohm TV receiver, then
you'll need some means of matching the 100's of Mohms to 75 ohms.
Otherwise, you will have a bad case of mismatch loss. In many cases,
such mismatch loss can be tolerated, but not for VHF/UHF TV.

Capacitive coupling, I suppose at the frequencies involved there would be
some.

Oh? There's magic involved. No coax or twinlead cable? It just
capacitively couples the antenna to the receiver. Truly amazing.

If it works as well as all metal why doesn,t every one use it and stop
corrosion?

Because I have not clue what "it" is. It "it" a piece of wire shoved
into a soda straw size pipe, an aluminum tubing antenna spray painted
with plastic, or a PCV schedule 40 contrivance with a wire inside?
Please try to be more specific in your descriptions.

Hope this is not too off topic.

It sucks. No numbers, clue clues, no specifics, nothing new, no
interest, and very little entertainment value.

Many thanks

One would be sufficient.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

"John" napisa³ w wiadomo¶ci
. au...
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.

Yes. The frequency do the work.

Rob wrote: "Apparently you have missed the interesting discussions with our
Polish
friend."

It was not the discussion. I only citate the Giants:
"In 1867 Lorenz wrote: " Ludvig Valentin Lorenz, "On the identity of the
vibrations of light with
electrical currents," Philosophical Magazine, Vol. 34, 1867, p. 287-301"

http://books.google.pl/books?id=caJd...page&q&f=false

On p. 301 he wrote:
"The present general opinion regards light as consisting of backward and
forward motions of particles of aether."
If this were the case the electrical current would be the progressive motion
of the aether in the direction of the electrical current."

In today's words: "Light is the oscillatory flow of electrons".

So no problem for electrons to flow through the plastic if it is matched to
the frequencies.
For example,You must use the different type of glass for different wave
length.

The ice is O.K. for the RF but the water not.

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.

It is too off the teaching programs.
S*

Szczepan Bialek wrote:


It was not the discussion. I only citate the Giants:

You only regurgitate very old writtings, many on which have been shown to
be either incomplete or outright incorrect in light of modern research.

This is usually followed by an out of context snippet from Wiki that is
totally misinterpreted.

Go babble your nonsense somewhere else.

On Mon, 28 May 2012 19:06:03 +0200, "Szczepan Bialek"
wrote:

Whilst trying to source a "digital" TV antenna I came across some with all
external surfaces plastic.

In today's words: "Light is the oscillatory flow of electrons".

So no problem for electrons to flow through the plastic if it is matched to
the frequencies.
For example,You must use the different type of glass for different wave
length.

The ice is O.K. for the RF but the water not.

I believe he said plastic, not glass or ice.

Is your theory that if you repeat the same garbage over and over,
eventually someone will believe it?

I might as well be part of the problem, instead of the solution.
Adding to your electron belching antenna theory, such antennas should
gain and lose mass as they transmit and receive electrons. If we have
(for example), an antenna with 1A of RF current, that's equal to 1
coulomb/second. 1 coulomb is:
6.24x10^18 electrons
which should be belching:
9.11*10^-29 g/electron * 6.25*10^18 coulombs/sec
= 5.69*10^-9 grams/sec
If your bogus theory is correct, you should be able to weigh your
antenna and see it loses some mass in transmit, and gains some in
receive.

Garbage in, Science out.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Many thanks for your time Gents, The antennas were intended for outdoor use
and constant handling. being encased in hard plastic obviously enhances
those functions. My experience has been at the other end of the spectrum so
to speak and I (incorrectly) assumed an antenna picks up an electrical
signal. Putting a layer of insulating plastic on it seemed contradictory. If
antenna is detecting magnetic signals obviously a different story.
When I mentioned impedance I mislead you. Wasnt referring to antenna
impedance but the impedance looking back form the recieving antenna to the
signal source ( thats what us old analogue designers do, Norton/Thevenin
equivalent circuits etc !!!!) and the effect on that a layer of plastic
has.
Your combined efforts have answered a lot of my questions, thanks for your
time.
Cheers
John
"Rob" wrote in message
...
Ian wrote:
"John" wrote in message
. au...
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?.

Pardon? The coating or conduit shouldn't affect the impedance of the
antenna. The radio signal should pass through the plastic and hit the
metal
antenna element.

Apparently you have missed the interesting discussions with our Polish
friend.