One problem with using RG-8 or RG-213 coax as a
capacitor and lowers the voltage rating is the shield.
When it's cut you have many sharp copper wire ends
that greatly increases the probability that you get an
arc over at either ends of the peace of coax used as a
capacitor. You will also have problems with the change
of resonance as that peace of coax changes positions
in relation to the loop element. The fix is to simply
remove the shield and replace it with a suitable
length of 3/8 copper tubing. This copper tubing should
be clamped rigidly to the element and at least 1 inch,
preferably 2 inches of the center wire and insulation
sticking out of each end of the copper tubing. Also
make sure there are no sharp points or burs on each
end of the copper tubing. The copper tubing is clamped
on one side of the element gap and the center wire and
insulation soldered to the other side of the element
gap. This will increase the voltage rating and make
for a steady resonance/frequency point.
--- aa0ni@... wrote:
I read at this link...
... that the voltage rating for RG-8 and RG-213 is
4000V and 3700V. I
don't know if anyone else can verify these facts.
If this is the case... then one might be able to
load this antenna to
50-60 watts safely (using the magnetic loop
calculator in the groups
files section).
I based this on the following information inputted
into the magloop
calculator:
20 ft circumference
0.5 in diameter tubing
7 MHz operating frequency
25 Watts ... 2.0 kV (probably the highest 'safe'
power to use)
50 Watts ... 2.8 kV
60 Watts ... 3.1 kV
87 Watts ... 3.7 kV
100 Watts ... 4.0 kV (that would be too close to the
edge for my comfort)
Interestingly enough - a loop of this size is
calculated at 43%
efficiency when using this magloop calculator. I
guess I was using a
different program (DOS based) to derive the
efficiency values I
previously posted (10-15% efficiency from memory).
When I rerun it
using the DOS program, it takes into account height
above ground, and
it suggests more like 24% efficiency when placed .71
meters above
ground at the lowest point... and at 50m, it is
44.5% (very comparable
to the magloop calculator). However, I also notice a
discrepency
between the two calculators when it comes to
calculating capacitor
voltages ... it suggests that if I use 20W of power
with a similar
antenna, that the voltage will be about 3.7kV.
I will be posting the alternate DOS based calculator
that I found
somewhere on the web a few months ago.
- Daniel
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