The oak pollen numbers are way down, have had some good air-washing
rains and I've started going outside again. So am starting to get onto
specific measured experiments to explain WHY (not if) Q changes only
in a minor way, but witnessed and verified experiments at N1LN showed
a huge reduction in elevated radial's ground involvement (80 to 20
ohms) going from (old) THHN to bare wire.
First experiment: remove the 24 feet of insulation on K2AV inverted L.
See if NEC4 predicts results, or the removal does something else.
Story:
First L over FCP at K2AV was an up 90', out 105', at the south end of
skinny N-S property over creek with horizontal ending a bare 30 feet
from the service road in a big loblolly pine. Power company ran a new
13 kV primary line on my side of the service road. Would never be safe
to attempt anything in the loblolly now only 20' from the 13 kV line.
And that's plus any noise from the line.
So erected new L up by house in only available remaining free air
space long enough for 160. It was a smaller L with some trees inside
the bend. These trees were from some sprouted acorns that were allowed
to grow up after we bought the property in 1988. After some storm wind
snaggles in the now almost 30 year old oaks, and too many fault
causing shorts into one tree at the far end of horizontal, I slid 24
feet of 19 mil teflon sleeve over the end of the wire. #11 sleeve, #12
bare wire. It did stop the faults but also moved the SWR center down
the band. Given the sequence of events, getting complete set of
before/after on adding the insulation, based on a single controlled
factor was impossible. The wire was very difficult to get up again.
So last fall I began negotiations about the trees with my tree-lover
wife. The trees were now over the roof in places, dropping dead
branches on the roof and on cars parked on the concrete garage pad,
besides making the last 160m L site on the property increasingly
unusable. Happily this resulted in calling the tree guy for outright
removal of three trees and severe trimming of three others.
This area is now a sunny spot and the oaks are being replaced by crepe
myrtles which love the sun. Crepe myrtles will never get high, and
will be absolutely gorgeous four months of the year. Ah, the value of
careful negotiation. I had been thinking I might have lost 160 again.
As it stands now from RBN reports the new, smaller, really ordinary,
run-of-the-mill sized L is a better performer than the old L down by
the creek. The baby crepes are growing gang-busters, and when we
trimmed the overgrown azaleas after bloom this spring, we found a new
dogwood in a place we were going to plant another crepe. A good omen.
All is well.
But when the tree guy was all done, the L still had the 24' of teflon
insulation on the far end of the horizontal.
Yesterday afternoon after two days of sunny, drying wind, we had a
predicted and actually occurring three hours of zero wind. Quickly
measure parameters, including AIM4170 scan 1.75 to 1.9 MHz. Drop the
wire, very easy to do now with trees gone, remove the 24' of
insulation with dull pocket knife using "the method", quickly pull
back up and remeasure. No wind to vary height or longitudinal
position.
NEC4 predicted less than one ohm change in feed R at X=0, and about 1
kHz increase in X=0 frequency for the removal of 24 feet of 19 mil
teflon insulation at the far end of the horizontal. Thickness of
insulation verified with micrometer.
MEASURED about half ohm decrease in R at X=0 (I consider this "in the
noise" and not significant), and a ** 6 kHz ** increase in X=0
frequency (from 1.819 up to 1.825). The latter is a clear change,
still in there this morning.
If all that was going on was that the wire looked a little longer with
the insulation, the NEC4 prediction of +1 kHz should have held.
The current working hypothesis for the N1LN results is that in
addition to standard dielectric lengthening caused by insulation of
wires in space, is that the dielectric of the wire's insulation
increases capacity to the accumulation of conductive or partially
conductive objects in the vicinity of the wire, specifically to the
dirt underneath.
There is of course no way to bookkeep trees and such in NEC4. In the
case of radials, it seems pretty probable that the effect of ground is
predicted in the ground approximation methods. NEC4 does NOT predict
the N1LN measured results. This certainly would not be the first time
that a model's necessarily finite coding scheme could be defeated by a
situation not tested for and specifically calculated in the program
code. There is ALWAYS a list of non-calculable gotcha's to avoid. This
is just one more on a list. Get in line.
Further experiments are planned for more extensive to determine if
and what kind of loss figure should be assigned, and some kind of
formula for it.
Here's the history of the X=0 points, since the new L went up. Lengths
unchanged all along, FCP at 11 feet to clear tractor shed, vertical
wire length is 55', horizontal is 88 feet:
Original, oak trees inside bend of L, X=0 at 1819-20
Add 24 feet insulation, X=0 at 1814-16
Trees removed, trimmed, X=0 at 1819
Insulation removed, X=0 at 1825
The horizontal was left at 88 feet all along to keep the current max
high on the vertical wire.
Today's 1.825 center (which wanders up and down a little in the wind)
is obviously ideal. At this writing since yesterdays removal of
insulation, seeing raw feedpoint R at 35-39 depending on ground
conditions, which is the first time I've had an L up that approached
the NEC4 predicted R (32 ohms) for my exact dimensions in a model.
That speaks to a low level of incidental loss.
The Alpha 8410 will now operate up to 1.865 without fault at full QRO,
so the experiment has providentially centered the antenna perfectly
for contests.
Have to build somethings for the next set of experiments.
73, Guy K2AV
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