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