On Wed, 27 Aug 2008 09:34:38 -0700, Jeff Liebermann
wrote:

I
can probably conjur my bad guess of a model of the Sinclair antenna,
but I'm playing vacation and would rather be doing something else.

No, I can't. Neither EZNEC or 4NEC2 will model dielectrics.

Also, this might be useful:
http://www.antennex.com/shack/Feb06/lpcad.html
LPCAD 3.0 will generate an NEC file that can be analyzed by your
favorite modeling program.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Wed, 27 Aug 2008 10:22:19 -0700, Roy Lewallen
wrote:

I routinely use EZNEC to design antennas printed on a substrate. I do
the basic design then apply a fudge factor to the conductor width and
length derived from comparisons between model results and measurements
of real antennas. Final designs usually take some adjustment even at
that. However, I haven't tried this for any antenna that depends
strongly on coupling between elements, such as a Yagi, because the
mutual coupling will also be affected by the substrate and won't be so
easy to approximate.

I once tried scaling a similar antenna for a PCB dielectric and ran
into that problem. I had G10 on one side of the elements and air on
the other, each with its own dielectric constant. Another version
used glass as a substrate. I made a simple breadboard for testing and
found that a good first guess was the geometric average (sqrt of the
sum of the squares) of the dielectric constants. However, when I
actually built the antenna, I had to do considerable tweaking and
guesswork. It's been many years, but I don't think my models and
reality ever agreed.

I also tried to use MSTRIP40 for modeling a dipole array on a PCB.
http://rze-falbala.rz.e-technik.fh-kiel.de/~splitt/html/mstrip.htm
It's intended to do patch and panel antennas, and works well for
those. It can also do a dielectric sandwitch, which might be handy.
However, when I tried to do a simple dipole array, I got bizarre
results, probably due to some mistakes on my part. A single dipole
works fine (it's supplied as a sample file), but I just couldn't get
arrays to work. I just noticed there's a later version available, so
I may try again. However, I'm playing vacation this week so it will
have to wait.

In the case of a log periodic, I'd adjust the model
transmission line to try and get the same Z0 and velocity factor as the
line on the substrate, which you might have to determine by measurement.
That would help, but you'd still be lacking accurate mutual coupling
information. At best, I think, a model would get you in the ballpark,
with a final design requiring some tweaking.

Did you notice the odd patches at the ends of the elements in the
photographs?
http://11junk.com/jeffl/antennas/Sinclair%20SRL441-2P/index.html
Methinks Sincair may have also done some tweaking.

Thanks for the hints.

Roy Lewallen, W7EL

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Thanks folks Got me curious about adapting this to 1296, or 2404
MHz for quick and dirty antenna to use in contests! tho its an
interesting design, by the time you create this, feed this, and with
the loss's envolved, probably NOT worth the effort! An interesting
use of PC material, non -the-less ! Maybe the Log Periodiotic is less
critical to Board Xc, and Spaceing, then other antenna types? But,
then the (stripline) feedline loss's alone would probably cancel any
gain from the antenna. Again folks , thanks!
When I start thinking- it gets Dangerous out there!

Jeff Liebermann wrote:
On Wed, 27 Aug 2008 10:22:19 -0700, Roy Lewallen
wrote:

I routinely use EZNEC to design antennas printed on a substrate. I do
the basic design then apply a fudge factor to the conductor width and
length derived from comparisons between model results and measurements
of real antennas. Final designs usually take some adjustment even at
that. However, I haven't tried this for any antenna that depends
strongly on coupling between elements, such as a Yagi, because the
mutual coupling will also be affected by the substrate and won't be so
easy to approximate.