I think I have a problem on the 28MHz band with the homebrew Cobwebb that I
have made.
The SWR is 1.1 and is tuned at 28.500MHz.
If I listen to this band it receives strongest at 28.500 and drops off
sharply after about 300KHz each side along with a rise in SWR It can reduce
a S9 signal down to S1 on both sides.
Is this how the Cobwebb performs or is there some thing wrong with it?
I like to listen to a few people on 27MHz that I know. They are only 5 miles
away. I have no problem hearing them on a G5RV but I can only just here them
on the Cobwebb.
Many thanks
"Barrett" wrote in message
k...
I have some questions about the choke and feed line on the Cobwebb.
Does the co-axial choke balun work better if the cable is wound on top of
each other into a tube shape rather than just winding it round and zip
tying it together in a round bundle?
Can I attach a SO239 Chassis socket to the end of the box where the coax
enters and then plug a PL259 into that for the feeder or will the socket
allow current to bypass the choke balun and enter the feed line?
Thanks
"Barrett" wrote in message
o.uk...
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
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