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Losses in shunt-fed towers
Yes.. but that's usually a LONG way from where there's detectable RFI or noise
for instance. When you can see visible corona from some meters away, you're probably in the 10s or maybe low 100s of kV range. My antenna voltage gets up to about 12 kV peak. I will try listening at a short wave receiver placed very close to the antenna and tuned to a vacant frequency slot. Increasing the transmitted carrier power, at the power level at which the corona effect has instated, I should note an abrupt received noise increase. . 73 Tony I0JX |
#22
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Losses in shunt-fed towers
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 |
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