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

[Roger (K8RI) on TT]

Are the parasitic elements insulated from the boom?

73

Roger (K8RI)

On 1/3/2016 Sunday 1:46 PM, Robert Harmon wrote:
> 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.
>>>
>>> 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
>>> _______________________________________________


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