Why all of the complications? The preamps are unity gain. Doesn't the output
voltage tell you something about the input voltage? If they saturate during a
measurement, turn the power down until they don't and then calculate the worst case.
Wes N7WS
On 9/24/2019 12:07 PM, jimlux wrote:
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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