Problem matching real Log Periodic to NEC
 

On 01/10/10 08:03, John Passaneau wrote:
.... are the boom is one of the trickiest parts. As an example if you
used a
1" square boom and modeled it as a 1" diameter wire there would be a
problem. They are not equal.

Here is a little tool for designing practical square section spines:
http://vk1od.net/calc/tstl.htm .

Owen

Problem matching real Log Periodic to NEC
 

On Wed, 29 Sep 2010 16:36:06 -0700 (PDT), Dave D
wrote:

Just thought I'd throw this out there to see if anybody had any
ideas... I was recently playing with an 800 to 1000 MHz Log design. It
came out quite well in the model. It was showing that it should do
about 1 to 1.5 average SWR, with a couple little spikes of 2.0 or so.
So I went ahead and built it.

Now I know from past experience, that testing logs is very tricky.
Unless you have a phase stable cable right up to the feed point, you
can get all kinds of crazy readings. But still, you can usually get
the gist of what's going on. So I go to test the actual finished
antenna, and across the intended range, it's showing SWR as high as 7
or 8, and the best it goes down to is maybe 3. Totally different than
the model. As a test, I went to the NEC model and took off all the UHF
elements, and just added one VHF element to the low end... It showed
around 155 MHz in the model. I did the same thing on the actual
antenna and got 1.1 around 153 MHz. So at that range, it's right on.
But I am baffled as to why it is so terrible on the UHF. I tried
shorting the back end and not shorting. No real difference.

Any ideas?

Thanks,
Would love to see fotos of the antenna you built.
Is there a stub at the end or is there a termination resistor?

w.

Problem matching real Log Periodic to NEC
 

I'll try to get a shot Helmut...

Well, I tried the 61 material ferrites, and it lowered the SWR
significantly. No matter where/how the cable comes off, it appears to
work best right behind the feed point. Only one of the large 1" ones
seems to be needed. I tried adding more, but it didn't really make a
difference. It's still not perfect, but it's way better than it was
before.