"John" wrote in message
news
Here is the 'T" antenna I referred to. It is totally plastic.

http://www.happywanderer.net.au/page...9&parent2id=24

If link doesnt work the website is www.happywanderer.net.au and the image
is on first screen. The totally plastic covered yagi I saw was somewhere
in ebay. I,ll see if I can find it again.
Regards
John



"John" wrote in message
. 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.
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

It is probably what is called a "folded dipole."

We used to make them out of common TV twinlead. They have a characterisstic
impedance around 300 ohms, same as the twinlead, so the black block at the
hub is likely to be a 300-75-ohm balun (trannsformer) to match the coaxial
cable lead-in.

You determine the frequency of interest and cut it to size, accordingly.
They're not too great for wide-band coverage, but you might get lucky. (The
wide-band issue is why big, expensive antennas always have elements of many
different sizes. "One Size Fits All" definitely does NOT apply to antennas.

"Sal"