On Thu, 31 May 2012 13:21:50 +1000, "John"
wrote:

Here is the 'T" antenna I referred to. It is totally plastic.
http://www.happywanderer.net.au/page...9&parent2id=24

Not quite totally plastic. Inside the ABS or PVC tubing are some
wires. Most likely, a length of twinlead forming a folded dipole.
While not the most sophisticated antenna available, it's probably
sufficient for campers and caravans.

However, there's a small problem. If you cram an antenna into a PVC
tube, the resonant frequency goes down. There's no set value for the
velocity factor for PVC or ABS, so the resonance change will vary with
manufacturer, doping, and the position of the moon. It's not a big
deal for a low-Q minimal gain and wide band antenna such as this
folded dipole. However, as the gain goes up, the tolerances for
element lengths become more critical. This means that changes in
effective length and resonance caused by PVC or ABS pipe becomes an
important consideration.

"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?

The distance between the metal elements and the dielectric (PVC or
ABS) is also a consideration. Obviously, if the pipe diameter were
fairly large, the effect of the pipe would be minimal. Similarly, if
the pipe were molded around the conductor, the effect of the pipe
would be maximum.

There's nothing magic about coating an antenna to prevent corrosion.
This is an important consideration for trailers, campers, and
caravans. The antenna has to survive freeway speeds and be fairly
aerodynamic. Coating the antenna with smooth plastic does this.

If you feel ambitious, and happen to have either a grid dip meter or
an antenna analyzer, just build any resonant antenna (probably at VHF
frequencies) and watch the resonant frequency change as a PVC or ABS
pipe is slid over the antenna elements. I build a collinear antenna
out of alternating pieces of coax cable. It tuned perfectly, until I
slid the antenna into a PVC pipe. Tuning changed from 146MHz to a
useless 135MHz. Filling the pipe with urethane foam (fence post
compound), lowered the frequency a few more mHz.



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