On 19 Nov, 22:45, Rich Grise wrote:
My current UHF antenna is a 14" Radio Shack clip lead clipped from the
center of my coax to the venetian blind. I have to hand-adjust it
for the channel, weather conditions, phase of the moon, etc. I can
do this because I can _see_ an indication of signal strength.

Well, with this new Fascist "No More Free TV" crap, I'm gonna need
a real UHF antenna. My budget is exceedingly limited, but I have
a supply of materials (GTAW filler rod, with some coppery-colored
coating, so it solders like a dream, and is as stiff as piano wire)
to build an antenna with.

But I've been searching the web for some weeks now, and I can't
seem to find any kind of formula, except there was this program
I downloaded - LPDA.EXE, which runs on DOS. Unfortunately, it's
in Russian or Polish or Uzbekistani - one of those East Yurp
languages. Here's a screen snap:http://mysite.verizon.net/richgrise/...rog-Output.gif

Which I went through pretty much by-guess-and-by-gosh - can
anybody read that stuff?

There are a lot of factors I don't know about, like "Tau", and
all of the specific designs on the web are flat - something is
telling me I want one of those pyramid-shaped ones, but I really
don't know the difference (between that and flat) - it's probably
something to do with bandwidth or F/B ratio or whatever.

My local library has no ARRL Antenna Book (!), and did I mention
I have a seriously limited budget?

So, how do I pursue this? It'd be nice to have a program that will
calculate the whole thing for me, but am I dreaming? If I want to
send myself to Log-Periodic School, where should I start?

Or, does anyone have a UHF-TV log-periodic design that they'd share? :-)

Thanks,
Rich

I have used the procedure in the following paper:
http://www.urel.feec.vutbr.cz/ra2008...tracts/116.pdf

I slightly modified the design (e.g. I used several different
diameters for the elements), but basically I used the same formulae.
I did not use their formula for deciding the right spacing between the
support tubes to get the right characteristic impedance, because I
suspect that maybe it is only valid over a limited range of spacing.
Instead, I measured the characteristic impedance of just the tubes
with no elements attached on a VNA that could plot impedance vs
distance in a sort of TDR mode, and I adjusted the spacing until it
was right. I epoxied some very small (roughly 1mm cube) spacers
between the support tubes to keep the distance right, once it was
adjusted properly. It is important that the space between the support
tubes is mostly filled with air, not epoxy or any other dielectric,
because the wave needs to propagate at the right speed between the
tubes so that the elements are fed in the right relative phase, and
this won't happen if there is something with a higher dielectric
constant in there. I used those brass tubes that you can get in hobby
shops, but unfortunately brass is fairly resistive and I did not
silver plate it. I would have used aluminium if it were easier to
solder in a reliable way.

The finished antenna had a good return loss over the desired frequency
range (sorry, can't remember the numbers) and it did the job for which
it was intended although I don't have a measured gain value etc.
because I don't have an antenna range.

I have a spreadsheet for calculating the element lengths (in
Openoffice.org format) if you are interested.

Chris