[TowerTalk] DC grounding of parasitic eles with insulators ?

[K9MA]

The reason the parasitic elements are isolated from the boom is that the 
boom would detune them. AFAIK, no one has figured out a way to model 
elements shorted to a boom, and experimentally tuning an antenna is 
expensive.

I can think of 3 reasons to connect the elements to the boom:

1. Bleed rain/snow static
2. Protect the insulators in case of a nearby lightning strike
3. Protect the DE balun when the tower is shunt fed

A 1 megohm resistor would take care of number 1. An RF choke might take 
care of number 2 without detuning the parasitics.  If you shunt feed the 
tower and don't have something like a hairpin match on the DE, a choke 
ala N9NB's recent QST article is required. (This is sort of the opposite 
of a common mode choke.)

That said, i recently put up a JK Mid-Tri, which has all the elements 
isolated. Since I shunt feed the tower, I put the N9NB choke on the 
driven elements. (And the same on the D40 above the beam.) I didn't 
think of putting the bleed resistors on the parasitics. I don't think 
rain will be a problem, but we'll see about snow static. If a lightning 
strike blows up some insulators, I'll probably have bigger problems.

73,
Scott K9MA



On 10/28/2019 11:51, Jim Thomson wrote:
> Ok,   the  ref  +  dir  on  my  modified F12 340N...3  el  40m  yagi  are  insulated  from  the  boom.
> Whats  the  easisest way  to  dc  grnd the eles.....  and  also  leave  the  insulator between ele  halves intact???
> Resistor,  choke,  strap  ?
>
> I  also  have a nema  box,  with  2 x vac relays  +  strap  coil... to  allow  for  4 x segments  of  REF.....and  also  DIR  tuning.
>
>
> DE  is  not an  issue,  hairpin across  the  feedpoint.    CM  choke  1st....  then  Braid /  shield  is  bonded  to  top and  bottom  of  tower.
> Another  nema box  at  DE....  but  with  2  strap  coils,  and  each  coil  tapped  with 2 x vac relays.   4  relays  in  total  for  DE.  That
> provides  for   7 x DE  tuning  segments.
>
> I  want the  ref +  dir  dc  grounded  at  all  times.
>
> Jim  VE7RF
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-- 
Scott  K9MA

k9ma at sdellington.us