Joel Koltner wrote:
If I take an antenna that's resonant at, say, a couple GHz, and operate
it well below that frequency (say, some hundreds of MHz), it's clear
that for something simple like a dipole, its radiation pattern is the
usual "bagel" shape that an "elemental" (infinitesimally short) dipole
would give you.

But say I use something like a patch antenna that's designed for 2.4GHz
and build enough of a matching network that it presents a 50ohm
impedance to, say, a 70cm transmitter. Does the radiation pattern
change much? Will it become so lossy (radiation resistance rapidly
heading towards zero) that this isn't really a good idea in the first
place? (I wouldn't be surprised if a patch antenna actually doesn't
radiate much at all outside of the antenna's own resonances...) Or
perhaps it's not possible to say, in general, what happens and one needs
to perform simulations on a case-by-case basis?

I'm asking based on the thought that there are a lot of pretty nice,
off-the-shelf antennas out there that were designed to be resonant
(using, e.g., quarter-wave dimensions) at some pretty high frequency
(2.4GHz being a common one, of course), and I'm interested in how viable
it is to use these antenna for 2m/70cm amateur radio use.

Thanks for the input,
---Joel

The rule for any antenna is small - broadband - efficient: pick any two.
You've chosen small, leaving you your choice, within reason, of one of
the other two -- although as it gets really small, the efficiency choice
gets less achievable. The lower the frequency, the more the patch will
look like a capacitor, with lots of energy stored between the plates
each cycle, and a small fraction of that being radiated. The large
energy storage means high current, which means high I^2 * R loss and/or
high loss in dielectrics due to very high E fields. This is the same
basic problem you have with all electrically small antennas. You'll end
up losing more and more in the matching network as frequency drops, too.
If you do manage to minimize loss you'll end up with an exceedingly
small bandwidth.

Expect the same pattern from the far-below-resonance patch as you get
from a small loop or dipole.

Sorry, as Robert Heinlein (and others) said, TANSTAAFL (There Ain't No
Such Thing As A Free Lunch).

But you'll probably be able to make some QSOs with it and, with the help
of some mystical mumbling about equilibrium, photons, critical coupling,
and reflected power waves, you'd surely be able to collect a gaggle of
true believers.

Roy Lewallen, W7EL