On 9/24/19 11:09 AM, VE6WZ_Steve wrote:
Yesterday I decided to measure the actual voltage at base of my short HI-Z
verticals while I transmitted on the TX array.
Since I first posted about the coupling capacitor failures in these units, I
have received private emails form 4 others with the same problem.
My question is what has caused these failures? Is it a lightning event, is it
high-potential wind static on the vertical elements or is it from TX RF
overload.
TX overload seemed possible at my location because my array is less that 100’
from the TX antenna. I built a diode detector probe and in the field measured the
actual voltage when TX. The results indicate at most I see 50 v at the pre-amp. The
coupling caps are 63v rated units. This would indicate that RF overload is not the
cause, but I would really like thoughts and opinions from others.
This is the 0.1 uF input capacitor? What kind of capacitor is it
(there's hundreds of 0.1 uF, 63V caps out there)
I'm not sure that your measurement probe (shunt diode, 0.1 DC block,
100k load) is telling you the right values -> since it's just wires
hanging out in space, could there be parasitic C (or L) that's fouling
things up?
If I were going to measure the voltage at the antennas, I'd put a known
voltage into the preamp (say, 1mV) and measure the output voltage, so I
know the gain of the amp. Then I'd fire up the TX and look at the
voltage you're seeing at the output of the preamp.
FWIW, 50V is huge - I'd be using 100V capacitors there, but I'd worry
more about the opamp frying. Although, maybe the current is limited by
the series 470 ohms and the opamp clamps it to the supply rails.
I have redesigned the amps to include a relay to short out the antenna when not
powered up, so I feel I am protected from any cause.
Here is a link to a YouTube video that shows the test set-up and the actual
measurements I made in the field. I also talk about using gas discharge tubes
as a remedy.
https://youtu.be/TmM_YnLob68 <https://youtu.be/TmM_YnLob68>
73, de steve ve6wz
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