On Sep 30, 12:38*pm, Owen wrote:
On 01/10/10 05:06, Dave D wrote:



On Sep 30, 12:28 am, *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.

Does that disturb the characteristic impedance of the spine transmission
line? If so, does that contribute to the problems you observe?

If this LP is mounted to a mast somewhere forward of the rear element, I
would consider dressing the feedline away from the feed point at right
angles to the spine, installing a common mode choke near the feedpoint,
and bringing the feedline to the mast some distance below the spine.

If you were to perform a test where the LP is mounted a wavelength or
more above ground, pointing to the zenith, feedline falling downwards
from the feedpoint, common mode choke near the feedpoint, it would
reveal whether the array is working as intended. If your actual mounting
causes it to work differently, you know where the problem is.

Owen

Well, it does different things based on how the cable is routed, but
none are really better than the other... just different. I tried
coming off the back, and also at the mid point... It changed, but not
better or worse overall... The points of good and bad just changed
position across the range.

I'm going to try the ferrites. What do you think about these?
http://www.mouser.com/ProductDetail/...PQp4FXwkz9g%3d

On 9/30/2010 4:56 PM, Dave D wrote:
On Sep 30, 12:38 pm, wrote:
On 01/10/10 05:06, Dave D wrote:



On Sep 30, 12:28 am, 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.

Does that disturb the characteristic impedance of the spine transmission
line? If so, does that contribute to the problems you observe?

If this LP is mounted to a mast somewhere forward of the rear element, I
would consider dressing the feedline away from the feed point at right
angles to the spine, installing a common mode choke near the feedpoint,
and bringing the feedline to the mast some distance below the spine.

If you were to perform a test where the LP is mounted a wavelength or
more above ground, pointing to the zenith, feedline falling downwards
from the feedpoint, common mode choke near the feedpoint, it would
reveal whether the array is working as intended. If your actual mounting
causes it to work differently, you know where the problem is.

Owen

Well, it does different things based on how the cable is routed, but
none are really better than the other... just different. I tried
coming off the back, and also at the mid point... It changed, but not
better or worse overall... The points of good and bad just changed
position across the range.

I'm going to try the ferrites. What do you think about these?
http://www.mouser.com/ProductDetail/...PQp4FXwkz9g%3d


The one question I've not seen asked is how close to what you built is
your model? Everything is critical at those frequencies and as little as
1/32" can make a difference. If your model doesn't match exactly the
antenna then it's no wonder it's not working as expected. To me modeling
the phasing lines between the elements be they separate from the boom or
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.


John W3JXP

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