Richard Fry wrote:
Following below are some thoughts of mine in response to an email sent
to me.

There are some good engineers reading this NG, and I am asking for
comments on what I wrote -- corrections where seen necessary, and any
other thoughts.

Thanks.

RF

+ + + + +

----- Original Message -----

The term "accept power" is interesting. To me, it's just a measure
of the input impedance of the antenna. If the resistance (radiation
plus loss resistance) is zero, you're not going to get it to absorb
power no matter what you do.

____________

Yes. And the input conditions depend on the ability of the radiator to
generate EM fields. No current can enter and "flow through" a radiator if
it doesn't have some place to go. If a radiator is not electrically long
enough to allow differential current to exist along its length, it cannot
generate EM fields. It is the di/dt along the radiator length that
generates those fields.


Actually, you can charge up anything to produce a field. If the field is
static, though, it won't radiate. Also, short radiators work just fine
if you excite them with frequencies whose wavelengths are on the same
order of magnitude. Finally, if you can keep the losses down, radiators
that are small compared to a wavelength of the frequency you want to
radiate, work well, too, though you may have to be satisfied
with narrow bandwidths and make heroic efforts to feed them. Finally,
you might be more accurate if you substituted 'dq/dt' with 'di/dt',
but even that won't guarantee radiation: transmission lines have
plenty of charge acceleration, but are designed specifically to
not radiate. You might want to rethink your ideas. Try not to
be so reductionist. You don't want to end up like Cecil.
73,
Tom Donaly, KA6RUH





Adding a matching network at the antenna input doesn't change the
instrinsic
ability of an antenna to radiate. That is determined by the factors
described in the paragraph above. A matching network can permit the tx to
increase its rated, safe output power while driving that poor antenna, but
any extra power available at the antenna input because of that will be
subject to the same poor radiation efficiency as if the matcher wasn't
used.
And much of any added power from the tx may get dissipated in lossy output
system components other than the antenna, rather than being radiated. The
antenna itself will still have the same directivity/gain that it had before
the matching network was added.

Improving the ability of a poor antenna to generate EM fields per unit of
source power is possible in a limited way only by increasing its electrical
length. "Capacity hats" and inductances incorporated into the radiating
structure can be used, as examples. This also raises the antenna radiation
resistance and reduces the reactance at the antenna input -- making it
easier to match into, and reducing system losses.

Someone gave an example of a pager antenna or something that had a
rated gain that was below isotropic. It'd be interesting to see where the
losses were.


This is covered in my comments above, I believe.

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