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

On Tue, 30 Mar 2010 15:43:05 -0700, "Joel Koltner"
wrote:

Does the radiation pattern change much?

Hi Joel,

Depends on your meaning of "much." Given the ad-hoc nature of your
goal (and the resolution of any S-Meter in those services): you
wouldn't be able to tell any difference at all.

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?

Loss will be largely dictated by match, and the matching components'
and the radiator's Ohmic loss.

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.

Because they look cool? That will change fast when you add in the
matching accessories (and that doesn't mean fashion coordination).

73's
Richard Clark, KB7QHC

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

Hi Richard,

Thanks for your help...

"Richard Clark" wrote in message
...
Because they look cool?

More because they don't take up a lot of space. I realize that any reasonably
small antenna for 2m (meaning: "fits in the palm of your hand") is going to be
a compromise anyway, but if you can have a nice-looking antenna that performs
as well as a rubber ducky, heck, I'll have it look nice too...

---Joel

Thanks Roy, that's a great summary.

If I started from the vantage point of wanting to design a flat panel-style
(patch-like) antenna for 2m (and let's assume narrow band is fine for the
moment), are there any obvious starting points besides a big patch that's
being operated well below resonance (let's assume I want my panel to be no
bigger than 6"x6")? Just meandered lines are probably as good (and can be
readily simulated in NEC :-) )? (I've read Randy Bancroft's book on patch
antennas and have a reasonably good feel for how they operate... although he
makes it clear there's often plenty of empirical design involved too,
particularly when you want to get fancy and achieve circular polarization by
exciting multiple modes at once.)

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.

It's a shame than Nathan Cohen has already largely cornered the market on
overselling the potential of fractal antennas. :-)

---Joel

On Tue, 30 Mar 2010 19:32:52 -0700, "Joel Koltner"
wrote:

Because they look cool?


Hi Joel,

I noticed your response to Roy you made reference to Chip and
fractals.

A fractal will work, but back then we learned the lesson of Chip's
carefully crafted problems he posed to us such that only his antenna
would fit into the box he specified:
"Show me your best antenna that fits into
0.1WL by 0.005WL by 0.05WL
with a CCW dogleg twist in the longest
dimension that is elevated
22.5 deg above the horizon
and my fractal will beat it."

So, in commemoration of that sort of criteria, what do you mean by
...don't take up a lot of space.

73's
Richard Clark, KB7QHC

Joel Koltner wrote:
Hi Richard,

Thanks for your help...

"Richard Clark" wrote in message
...
Because they look cool?

More because they don't take up a lot of space. I realize that any
reasonably small antenna for 2m (meaning: "fits in the palm of your
hand") is going to be a compromise anyway, but if you can have a
nice-looking antenna that performs as well as a rubber ducky, heck, I'll
have it look nice too...


Stated that way, it should be possible to create a patch antenna that
has comparable performance.

Or a "patch looking" antenna.

There's a couple ways to go about it, none of which involve starting
with a 2.4 GHz patch operated at 1/20th the design frequency.

1) use something as a dielectric that has a high epsilon, so the
wavelength is shorter. For cellphones and the like, various ceramics
like alumina are used. It's pretty easy to get to epsilon=10, but that
only gets you to 1/3 the size.

2) build something like a meander line on a suitable substrate. This is
sort of the squashed flat version of a rubber ducky loaded vertical.
Rather than making a 3d spiral which is a "radiating inductor" you do it
on a flat surface. It's a bit tricky because a simple back and forth in
a single plane won't radiate very well (the field from one meander
cancels the adjacent one). But a spiral might work.

3) low loss lumped loading components.

But, to return to your original 2.4GHz patch.. it's going to look like a
capacitor of some sort at 144 MHz. The feedline would probably do most
of the radiation.

"Richard Clark" wrote in message
news
So, in commemoration of that sort of criteria, what do you mean by
...don't take up a lot of space.

I'm liking panel-type antennas that are on the order of, oh, say, 0.2\lambda
by 0.2\lamba by perhaps .05\lambda or less. So electrically small, but not
super-electrically small. :-)

Thanks Jim, that gives me some good ideas to try out...

Joel Koltner wrote:
"Richard Clark" wrote in message
news
So, in commemoration of that sort of criteria, what do you mean by
...don't take up a lot of space.

I'm liking panel-type antennas that are on the order of, oh, say,
0.2\lambda by 0.2\lamba by perhaps .05\lambda or less. So electrically
small, but not super-electrically small. :-)


Considering a dipole is 0.5 lambda by 0.01 lambda, getting a factor of
2.5 smaller is easy. (that is, you want 40cm by 40cm or so)

A simple short dipole with capacity hats and inductive loading will get
you there quite nicely.

Imagine a "I" with the feedpoint at the middle, on a piece of FR-4 or
G-10 (epsilon about 2). make it, say, 30x30 cm. You could also make the
dipole diagonal across the square, and have the "capacity hats" be along
the edge.

You could fool with NEC for a bit and get pretty close.

Or, do what I'd do.. get yourself a piece of suitable insulating
substrate (a plastic cutting board or piece of acrylic window pane) and
some copper foil tape. Lay it down, measure it with an antenna analyzer
or similar, and start adjusting.

I've built more than one antenna like this using copper foil tape on
paper or plastic sheets.