Thanks Dave for all the info. I will give it ago with the original spacing.
I have also ordered the MFJ-259B to help tune it. Just have to work out how
to use it properly. Hi.

73


"Dave Platt" wrote in message
...
In article ,
Barrett wrote:

What I meant to say, is there any benefits in performance if I changed all
the gaps to 3 inch's rather than keep them to the original Cobwebb spec?

Changing the end spacing in this sort of antenna alters the
capacitance between the ends. This has a very significant effect on
the impedance at the feedpoint, and thus on the SWR. If the original
design had a different spacing and had a good SWR match, and you
change the spacing to 3" and don't make any compensating change to the
wire length, I suspect that you'll find that the antenna is no longer
as well matched and won't work as well with a 50-ohm feed source.

If the changes in tip spacing / capacitance aren't too serious, you
could probably compensate with an antenna tuner/transmatch. There
would be some amount of excess loss in the feedline due to the higher
SWR, but it might not be enough to affect performance significantly.

On the other hand, if the CobWebb with the 3" spacing has had its
radiator lengths adjusted to compensate for the change in tip-to-tip
capacitance, it might match up just fine. If that's the case, then
I'd expect that its performance would probably be indistinguishable
from an equally-well-matched Cobbweb with different tip spacings.

When I built a 2-meter halo antenna, I found that a change of 1/16" in
the tip spacing made a big difference in the SWR. After several
attempts to construct a tip spacer that was neither too wide nor too
narrow (but was *just* right, like a good bowl of porridge) I gave up.
I just used one of the "too wide" spacers, and then added a bit of
aluminum tape at the end of one tip and trimmed it until the SWR was
as good-as-it-got.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!