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,

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

Any ideas?

Inadequate decoupling of the transmission line at the feedpoint.

73's
Richard Clark, KB7QHC

Dear Dave D. (no call sign):

I have found it highly desirable with large and smaller HF LPDAs either to
decouple at the feed-point or to choke common mode right as the coax leaves
the boom to go down the mast. Of course, one could do both. Once you have
cleaned the transmission line, you need to attend to the termination at the
end of the boom. Your BW is small for a LPDA so design should be straight
forward.

Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:

"Richard Clark" wrote in message
...
On Wed, 29 Sep 2010 16:36:06 -0700 (PDT), Dave D
wrote:

Any ideas?

Inadequate decoupling of the transmission line at the feedpoint.

73's
Richard Clark, KB7QHC

Thanks guys... I've never had that problem before. Any tips for what
clamp-on ferrites might work best, or how many? Or the best way to
configure them?

On 30/09/10 15:23, Dave D wrote:
Thanks guys... I've never had that problem before. Any tips for what
clamp-on ferrites might work best, or how many? Or the best way to
configure them?

Distributed capacitance is the enemy of any multi turn balun, whether in
solenoid style (eg Ugly Balun) or toroidal winding. You could however
try to exploit the self resonance by placing it in the band you wish to
use, then the issue is whether the resonance peak in Zcm is sufficiently
wide.

The W2DU style ought have sufficiently low self capacitance to operate
below resonance at 800 to 1000MHz. I would look to #61 material
suppression sleeves, but it may be that a lossier material works for
your application.

Owen

On 30/09/10 15:23, Dave D wrote:
Thanks guys... I've never had that problem before. Any tips for what
clamp-on ferrites might work best, or how many? Or the best way to
configure them?

I meant also to ask how you dressed the coax away from the feedpoint.

I often see pics of coax tie wrapped to one leg of the transmission line
spine of the LP. Guaranteed to create common mode current.

Owen

On Sep 30, 12:28*am, Owen wrote:
On 30/09/10 15:23, Dave D wrote:

Thanks guys... I've never had that problem before. Any tips for what
clamp-on ferrites might work best, or how many? Or the best way to
configure them?

I meant also to ask how you dressed the coax away from the feedpoint.

I often see pics of coax tie wrapped to one leg of the transmission line
spine of the LP. Guaranteed to create common mode current.


Thanks Owen,

In this case I had it down the middle between the haves, through clips.

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.

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.