On Wed, 22 Oct 2014 21:33:03 +0000, Brian Reay wrote:

"Wayne" wrote:
"Steve" wrote in message ...

On Wed, 22 Oct 2014 17:36:31 +0100, FranK Turner-Smith G3VKI wrote:

"Wayne" wrote in message
...
"gareth" wrote in message ...
Try this ...

http://farside.ph.utexas.edu/teachin...es/node94.html

This is one of a series of lectures by a prof at Texas Uni.

In fact, if you go right back to the home page of
http://farside.ph.utexas.edu/teaching,
this leads to a most excellent revision of the necessary EM
theories, and,
briefly glancing thereto, the post grad stuff even exceeds my
current interest and knowledge.

I'm fairly sure now that this area is where I came across the
governing formula that I alluded to recently in this NG when doing
my own revision previously in 2005, although the URLs and lecture
node numbers have changed since then.

When I get time, I'll browse through the links.

However, back to your original assertion that your theory has short
antennas as being inefficient compared with longer antennas (I'm
assuming you are talking half wave dipoles and such).

If 10 watts is delivered to a short antenna, where does it go if it
is not radiated just as well as 10 watts delivered to a long antenna?

Dissipated as heat?

# Probably proportionately more will be lost as heat as a very short #
antenna will be a low impedance, therefore current, driven job and I
sq*R # losses within the antenna will play their part. Apart from those
# additional losses, it should radiate all that is left, ... I think.

Actually no. The loss resistance tends to be dwarfed by the radiation
resistance, so losses in the antenna are not the problem.

I think you're missing the point I was making. That is; as antennas
become shorter and shorter, an ever increasing amount is lost as I^2 R.
In normal antennas you are correct that RR swamps I^2 R but as antennas
get shorter and shorter I^ R becomes a much larger factor as the
intrinsic antenna impedance drops and drops whilst the current rises and
rises.

The problem is matching. A small antenna has a narrow BW so you tend to
need a matching system. That is where the losses will be, plus in any
feeder.

Of course, if you only need a narrow BW and can arrange a low loss
feeder plus load the pa correctly, then pa is happy, low feeder loss,
the RF gets to the antenna.

The antenna RrRL so antenna loss is low.

Yes, matching becomes a serious issue but that is not what we are talking
about.

RF has only one place left to go, to be radiated.

Agreed.