El 13-09-11 20:32, Wimpie escribió:
El 13-09-11 17:30, Antonio Vernucci escribió:
Hi Wim,

despite my wire is only 4mm in diameter, SWR does not vary at all
applying a steady full-power carrier (not even some small oscillations
of the reflected power meter needle). I then presume that no big
corona effect takes place (also because I see no corona light at
dark). I am not sure whether a light corona effect (i.e neither
visible nor influencing SWR) could take place, but if so I presume
loss would anyway be very low.

I will check again under rain conditions (test not done so far), but I
am not sure whether rain increases susceptibility to corona, with
respect to hot summer days (with umidity close to 80% sometimes). Any
idea at that regard?

73

Tony I0JX

Hello Tony,

It is not the voltage that will introduce loss (as the field lines
will go into air and frequency is low), but it is (corona)
discharge. Whether this occurs, depends on presence of sharp edges
and especially sharp (double curved) surfaces with small radius,
think of bolts, threaded rods, etc that point away from the tower
into the air.

A thin end of an antenne element can also cause breakdown as it
points far into the air (away from other solid structures) and the
voltage maximum will be at the elements of your HF antenna.

In case of your 8 kVrms (that is 12 kVp), an antenna element with
10mm diameter and spherical (smooth) end may result in 2.4kVp/mm at
the end. If the end of the element is just straight (just cut
aluminium), you will exceed 3kVp/mm and corona will occur.

If (corona) discharge occurs, you will notice a sudden change in
VSWR with increasing power.


--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM


Hello Tony,

As long as your wire with D=4mm is not close to other structures (say
0.25...0.5m), you will not exceed 1 kVp/mm when the wire itself is
carrying 12kVp. You still have some margin before getting close to 3
kV/mm.

At ground level, influence of HF frequency on dielectric breakdown
voltage of air is negligible. If the wire is coated, the diameter
increases, hence reducing the E-field at the plastic-air interface.

When VSWR doesn't, change (with increasing power), the loss due to
corona discharge will be minimal. Of course when you lose 10W from
2kW, you will not notice this probably. However if you get a discharge
between metal and plastic insulation, the plastic will fail in the end
(due to spark erosion).

conducting Droplets (rain) do reduce the breakdown voltage as it
distorts the field. This introduces 'hot spots'. Unfortunately I don't
have data on this.

I could imagine that moisture/water layer on the wire would introduces
some loss (as you get a current through the moisture layer). But this
is not due to corona effect.

It can be calculated, as you know the voltage and you can calculate
the capacitance to ground (so you know the capacitive current that
"leaves" the wire.



Short note on change in VSWR due to corona.

Many reflectometers use diode rectifiers with relative heavy load. If
there is no bias (or the bias is wrong), you will not notice small
variations in VSWR, when VSWR = (almost) 1.

If you have a directional coupler you may use your oscilloscope or
spectrum analyser as detector device. The oscilloscope (50 Ohms input
impedance) you can use to verify if the residual VSWR is because of
higher harmonics or real reflection. Of course the spectrum analyzer,
oscilloscope with FFT or a receiver (with heavy attenuation) will show
you the desired signal directly.

If you match to very low return signal, even a few Watts loss due to
corona you can detect at full power. you may need to do this during
very quiet weather as small mechanical changes due to elasticity of
the construction materials will lead to small changes in reflection
coefficient.

--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM