On 9/24/19 1:14 PM, Wes wrote:
Why all of the complications? The preamps are unity gain.
Aha.. I was hunting around for a circuit description. Yeah, it looks
there's a noninverting buffer followed by a 1:1 transformer.
So it's more of an impedance buffer. What Z does it present to the
element?
(I do a lot of work with HF receivers that use short (2-3 meter)
antennas feeding high Z buffers, so they're more "E-field" probes)
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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