Hi Bob.... the parasitic elements are all DC grounded... so although I could do
something possibly with the driven element it would probably not really gain me
a great deal.
Jim,
Thanks for the comment... see above – so no way to do networks at the center.
Before I bought the antenna I was reassured it would not be a problem – by Mike
himself. It took LOTS of asking to finally get that to happen (a response to
any of my questions related to actual use) ... and to be honest I had modeled
the 15’s and added in a 2 element full size 40 model and found that it did not
interact... I mentioned that to Mike and didn’t hear anything back on that (no
“you idiot, of course there will be interaction”) ... so apparently the
modeling didn’t work or more than likely I did something wrong. it wasn’t until
I asked around and found someone to actually check the SWR of the 40 on 15,
thanks ti Richard, N5ZC who said it was really low – that is when I got
concerned about it.
Some will say just live with it – but I’m one of those anal retentive ones that
believes when you invest a bit of $$ (not a small one at that) you’d rather
have most things ‘right’ ... I’m always in the camp of “a 1/2 db won’t matter
... but a 1/2 here and a 1/2 there and 1/3 here begin to add up to real losses
(or savings)”.
Finally: VE4XT, Kelly, wrote to me direct and had suggested an open sleeve stub
with the ‘open’ end near the boom.... it turns out that is what Optibeam do on
theirs only the open end is out on the element. Thanks to Kelly for suggesting
this. I’m going to see if I can figure something out for this. If the stub
needs to be 1/4 wl for 15 then there isn’t enough room to place it before the
LL attachment point as it is only 10’ out ... however from what I can see on
the Optibeam antenna theirs is just under 9’ long...
Gary
Gary,
How about if you "open" the driven element and the directors with relays and
then the 40M el's will be invisible
on 15. ?
Bob
K6UJ
On 1/3/16 9:55 AM, jimlux wrote:
On 1/3/16 6:18 AM, StellarCAT wrote:
How’s this for a solution... specifically for the antenna I have ... the M2
40M4LLDD which has linear loading wires that loop back towards and come close
to the vertical uprights that support them... what if I installed a vacuum
relay with one side connected to the center of the LL end ... and the other
side to the upright thus when activated it would short the LL! This would most
definitely destroy any resonances on 15. I could have the relays all connected
together such that when 15 is the active band it automatically closes the
relays... by using ferrites on the power lead to the relay it would isolate any
other effects of the arrangement. Granted it is 8 relays ... but I believe it
would be quite effective... thoughts anyone?
I found a picture on how Optibeam does it and indeed I was right – they
just use a parallel line to the element with a cap in series with this line to
create a parallel tuned trap. But they have coils – and thus can do this
readily ... the LLDD doesn’t and I don’t know that I could implement this
outside the LL wires... but I’ll see... I understood, possibly incorrectly,
that NEC2 can’t handle close wires – if it is spaced say
3” would that model ok?
is there a gap in the middle of the elements (all elements?) You could put a LC
network there that is low impedance at 40m (i.e. resonant) and some non-zero Z
at 15m. It should be possible to make something that is pretty low loss with
lumped elements.
NEC is not all that good at modeling close conductors.
Or, if you have existing loading components (the LL wires?) can you add a tuned
network/trap to them. Have you called M2? They might have some ideas on where
that trap could be.
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