[TowerTalk] how does one accomplish a 'stub' for destroying resonance on yagi elements?

[jimlux]

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.
>
> If I were to try this on the LL it would of course have to be on the far side of the LL... I’d suspect placement isn’t an issue since wherever it is placed it will destroy resonance on 15 ... with the flip side being not to introduce resonance, at least for me, on 20 since I have 20’s nearby the 40...
>
> or... I could maybe just do a trap from the center close point of the LL wires back to the upright ... just not sure if that would be high enough impedance (when using it on 40) not to destroy performance on 40.
>
> or... a series trap shorting the LL lines ... again just not sure if it would be high enough impedance to not effect 40...
>
> I suspect both of these latter two wouldn’t be high enough impedance and it would noticeably effect 40.
>
> xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> ok – sorry – stub.
>
> Specifically adding something to a 4 element 40 to eliminate 15 meter interaction? I believe Optibeam does this – not sure how they do it. Is this just using the element with a parallel line tuned through a cap for max current on 15? If so any suggestions on how long to make it and where specifically to put it on each element 1/2?
>
> Or is there a way to use a relay, open normally, closed when on 15 that would add some impedance to the antenna causing it to be detuned at 15? Alternatively I can rotate the antenna 90 degrees but boy it would be so much better not to have to do that to avoid interaction on 15.
>
> thanks in advance,Gary K9RX
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk