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On Sun, Jan 20, 2008 at 09:39:36PM -0500, Sean Breheny wrote:
> Hi Peter,
> 
> I'd suggest looking up the following:
> 
> log spiral
> log periodic
> broadband antennas
> discone antenna
> heillical antenna
> coil antenna
> 
> There are a general class of antennas which have no "characteristic
> dimension" like a dipole would (length). Instead, they have a
> "characteristic ratio", which, from a fundamental point of view, means
> that you can argue that they cannot possibly prefer one frequency to
> another (within some range). Some of these fit into that class (log
> spiral and log periodic at least - not sure about discone).

Yup, I think discone fits in there as well, at least, if I understand
wikipedia's entry on it.

> In reality, they work by having a current distribution which expands
> and contracts to the right length to resonate at whatever frequency
> you are using.

!

Sounds like black magic to me. :)

> I think the disadvantage is that they are larger than a single-band
> antenna for any of the bands in their frequency range would be.

More difficult fabrication too. I think I can rule out the discone for
instance due to the cone shape, same goes for the log spiral antennas.
Log periodic would be doable in a pcb, but it's very directional from
what I read on it.


I couldn't find much info on coil antennas, at least for higher
frequencies. Looks like the ham world uses lots of loop antennas, for
say the 10mhz band, but I didn't find any info on using them for higher
frquencies.

> If you don't need very good sensitivity, then a simple ground plane
> antenna set up for the lower end of your frequency range might work
> very well. It will be less efficient, attenuate to varying degrees,
> and exhibit more complex antenna patterns as you go up in frequency,
> but is probably OK for your application. A small coil antenna could
> also work well. I think you might be able to get that to maintain less
> reactance change over the frequency range, at the expense of lower
> radiation resistance (hence, lower efficiency).

My current pcb isn't an ideal ground plane by any means due to it's
small size, though I did try simply attaching a length of wire to the
pcb vertically. It seemed to work as well, once I trimmed the length a
bit to about 1.5", longer and the noise floor gets higher, shorter and
there is less signal.

This seems to be a very simple solution, so I might just use it. I'll be
doing some more experimentation, as well as making up a larger board
with a bigger ground plane to test that out.

- -- 
http://petertodd.org
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