Some JKantennas designs do not have insulated elements, eg the reflector
on my 2L 80m yagi and the 40m. The heavily interlaced designs (eg
Navassa) are insulated. All are NEC4 designs according to discussions
I've had with Ken Garg.
I agree modeling is harder with boom-element plates that electrically
connect to the elements. It's the plate width discontinuity, not the
grounding that causes problems. My own NEC4.2 modeling of a 5L 17m + 3L
30m single boom+feed used insulated elements for that reason. The plate
approximations are probably good enough for HF monoband designs. My 5L
monobanders are custom OWAs with grounded elements.
There are some guidelines for modeling closely spaced wires which have
worked for me.
I am convinced from experience that W8JI is correct that P-static is
really St Elmos fire discharge from element tips due to high atmospheric
voltage gradients in storms. To prevent arc tracking on insulated
element insulators perhaps a high value non-inductive resistor to boom
might be warranted. WX protection makes it complicated IMO.
Grant KZ1W
On 10/28/2019 12:40 PM, K9MA wrote:
Does anyone know of a modern NEC-designed yagi that does NOT use
isolated elements? Perhaps there is some software that can accurately
model the effect of the boom, but I suspect it's way out of the price
range of the designers of amateur antennas. I know NEC4 is better than
NEC3, but still has problems with very closely spaced wires.
I'd expect the water to pretty effectively bleed off rain static, but I
don't know about snow. This will be my first winter with isolated elements.
73,
Scott K9MA
On 10/28/2019 13:58, john@kk9a.com wrote:
Like most modern day contest stations, I built my Yagis with insulated
elements and direct feed. According to W8JI's site, grounding the
elements makes little difference in precipitation static.
https://www.w8ji.com/pecipitation_static.htm?? I have not seen any
issues using them and as a plus it makes modeling easier.
John KK9A
K9MA wrote:
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
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