Topband: Shunt Feed - Insulated Elements on Yagi

[Guy Olinger K2AV]

Concerning voltage breakdowns using yagi-loaded towers on 160 meters...  I
have heard stories of arcing to beat the band, that included a fair amount
of destruction. I have heard stories of nothing needed for good operation,
even at QRO. All these stories have to exist in the same logical space.

All that you need to lower across-the-insulator voltage stress to a lot of
insulated elements is to have just one fairly moderate impedance 160m path
from the coax shield to the end of a single half-of-a-driven-element. 500
ohm 160m path through a high band balun to a 20 or 40m driven element would
be quite low enough to put the high voltage at the tip of that 1/2 element.
You would then have far less voltage across the insulation to the other
elements, not enough to arc even at QRO. The controlling length at 160
would be tower plus coax length from tower top to balun plus 1/2 length of
that driven element.

If the feed arrangement blocks access from the coax to the driven element,
and the elements are insulated, for sure there can be arcing. And there are
quite a few scenarios for arcing.

The mounting plate/u-bolts plus insulation plus insulated element IS a
capacitor from the element to the boom. This will constitute a certain
amount top loading. My self standing 76 foot tower plus an
insulated-element C31XR on top was resonant at 1740. That's a lot of added
electrical length for a completely insulated yagi. That's 14 insulated
elements to add extra electrical length, which includes three halves of
driven elements that are only separated by a high band bead balun from the
feed coax.

All the entire-lot models for my place have to assume a resonant-on-160
tower back there, which does have a bearing on RX antenna ideas in that
part of the lot.

73, Guy K2AV

On Thu, 18 Dec 2014 10:05:16 -0600, Joel Harrison <w5zn at w5zn.org> wrote:
>>  > Stan - For about 8 years, up until this past spring, my shunt fed tower
>>  > for 160 had a KT34XA (insulated elements, later upgraded to a KT36XA
>> same
>>  > insulated elements) at the top and I never experienced any issue at all
>>  > running 1500 watts, even during a few contests where, as you know, you
>> are
>>  > running "continuous" for an extended period. I've read discussion about
>>  > whether the coax should run inside -vs- outside the tower, mine ran
>>  > outside.
>
>