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@sdellington.us
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