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Re: RF Active Probe

 

That is 0.75pf in parallel with 10M¦¸ resistor.

Like a scope probe but lower capacitance. Not sure why if you are going
to the trouble to build that

why you couldn't go with a higher resistance and more gain.

?????????????????????????????????? Mikek

On 8/18/2021 1:59 PM, Donald S Brant Jr wrote:
That 0.75pF is 4K¦¸ at 50MHz, way down from 10M¦¸....
73, Don N2VGU



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Re: Will ch1 show an antenna signal?

 

On 8/18/21 11:51 AM, Mikek wrote:
Can I connect an antenna to ch1 and expect to see any signals?
I wouldn't think so. It's *sort* of like a spectrum analyzer - except it's got a few kHz measurement bandwidth, and if you're scanning 200-620, you've got steps every 4 MHz. The odds of you actually landing on a live signal are kind of low.


I did a scan, 200MHz to 620MHz of a TV antenna and had no trace.
Is the signal just to small? Would a 20 db amp be enough to put it on the screen?
Nope - it's more a "probability of intercept" thing. Aside from VNAs not really being designed for this, it's the bandwidth vs step size problem.

This is a difference where people who are used to "sweeper + spectrum analyzer" sorts of measurements get fouled up - the VNA *sort of* does the same thing, but on a sweeper+SA you can set the measurement bandwidth really wide on the SA (and all it does is add noise).? You don't get that sort of choice on a VNA.? They tend to be very narrow band (narrower the better, really).


I want to compare two antenna to see frequencies (channels) they are weak on.
It seem like it would show a signal, but probably something I don't understand about the operation, this will help me learn.
You want the $50 spectrum analyzer - then you can set the resolution bandwidth really wide, and you'll see the signals.? If you're looking for TV, digital TV is a sort of flat pedestal about 4-5 MHz wide.? So a resolution bandwidth of 5 MHz would be a good choice.? Then it's all about noise figure.? Most spectrum analyzers don't have great NF, but then, neither do TVs.
Re: RF Active Probe

 

That 0.75pF is 4K¦¸ at 50MHz, way down from 10M¦¸....
73, Don N2VGU
Re: Will ch1 show an antenna signal?

 

No? ?The Nano looks at the amplitude and phase of the signal it generates.
You might see some minor amplitude variations of a strong external signal, but the Nano would nave no idea what frequency it was on.?
Wrong tool for that job.? ?Hey, I currently own 7 Spectrum Analyzers and consider that TinySA very cute, and cheap!? And unlike the others, fits in my pocket!? ? Kent On Wednesday, August 18, 2021, 01:51:38 PM CDT, Mikek <amdx@...> wrote:

Can I connect an antenna to ch1 and expect to see any signals?
I did a scan, 200MHz to 620MHz of a TV antenna and had no? trace.
Is the signal just to small? Would a 20 db amp be enough to put it on the screen?
I want to compare two antenna to see frequencies (channels) they are weak on.
? It seem like it would show a signal, but probably something I don't understand about the operation, this will help me learn.
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Mikek
Re: Will ch1 show an antenna signal?

 

The NANOVNAs are not suited for this application. A spectrum analyzer is.
Take a visit to the following site:



Dave - W?LEV

On Wed, Aug 18, 2021 at 6:51 PM Mikek <amdx@...> wrote:

Can I connect an antenna to ch1 and expect to see any signals?
I did a scan, 200MHz to 620MHz of a TV antenna and had no trace.
Is the signal just to small? Would a 20 db amp be enough to put it on the
screen?
I want to compare two antenna to see frequencies (channels) they are weak
on.
It seem like it would show a signal, but probably something I don't
understand about the operation, this will help me learn.
Mikek





--
*Dave - W?LEV*
*Just Let Darwin Work*
Will ch1 show an antenna signal?

 

Can I connect an antenna to ch1 and expect to see any signals?
I did a scan, 200MHz to 620MHz of a TV antenna and had no trace.
Is the signal just to small? Would a 20 db amp be enough to put it on the screen?
I want to compare two antenna to see frequencies (channels) they are weak on.
It seem like it would show a signal, but probably something I don't understand about the operation, this will help me learn.
Mikek
Re: RF Active Probe

 

The specs say 0.75 pf and 10M¦¸.

On 8/18/2021 6:35 AM, roncraig1@... wrote:
This probe is just a capacitor input coupled FET source follower with a voltage regulator to run the FET. Input capacitance loading in in 10-15 pF range.

Over a limited freq range it provides a high impedance input.



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Re: danger measuring antenna

 

If you look at the typical pi-network configuration of these vintage
transmitters, there is nothing for the final plate blocking capacitor to
work against. Therefore, it will never charge to the DC potential as a
dipole or other balanced antenna is a DC open circuit. Heaven forbid
should a bird light across the center insulator of such an antenna fed by
such a transmitter. That choke does two things: 1) gives something the
plate blocking capacitor can work against to charge so there is no DC
potential on the feedline, and 2) serves as a 'fuse' as Jim Lux has
described. Of course, the DX-40 has no fuse ! The unit I just received to
resurrect - my novice transmitter - has been modified with the addition of
a fuse - good work. My original from Heath some 61 years ago did not
contain a fuse on the chassis. Oh........., I forgot, it did, but it was a
bit dangerous. The AC plug contained cartridge fuses in both sides of the
AC plug. They were exposed to contact at the line cord end of the AC plug
- open to a rather "awakening" experience.

Dave - W?LEV

On Wed, Aug 18, 2021 at 3:48 PM Jim Lux <jim@...> wrote:

On 8/18/21 8:25 AM, Victor 4X6GP wrote:
Those chokes were rated at 125 ma or 300 ma at the most. If the plate
blocking capacitor were to short, they would burn out before the primary
fuse blew. Whatever the function is supposed to be, it couldn't protect
against a shorted blocking capacitor.

I'm not so sure - if you're building a 3kV, 0.5 A supply (1.5 kW DC
power) then you'd be putting in something that can suck up half an amp
for at least long enough to trigger the overcurrent protection. A 300mA
choke can probably take twice that for a short time. And remember, it
doesn't have to be a very good choke while the fault exists - it can
melt the form, it can saturate (if it has an iron core), etc. It just
has to last longer than it takes the fuse or breaker to trip.


I would think, for instance, that the choke could be comparable to the
plate choke from the power supply - both have to be able to potentially
stand off the entire HV power supply (if the tube or surroundings arcs
to ground).









--
*Dave - W?LEV*
*Just Let Darwin Work*
Re: danger measuring antenna

 

On 8/18/21 8:25 AM, Victor 4X6GP wrote:
Those chokes were rated at 125 ma or 300 ma at the most. If the plate blocking capacitor were to short, they would burn out before the primary fuse blew. Whatever the function is supposed to be, it couldn't protect against a shorted blocking capacitor.
I'm not so sure - if you're building a 3kV, 0.5 A supply (1.5 kW DC power) then you'd be putting in something that can suck up half an amp for at least long enough to trigger the overcurrent protection.? A 300mA choke can probably take twice that for a short time.? And remember, it doesn't have to be a very good choke while the fault exists - it can melt the form, it can saturate (if it has an iron core), etc. It just has to last longer than it takes the fuse or breaker to trip.


I would think, for instance, that the choke could be comparable to the plate choke from the power supply - both have to be able to potentially stand off the entire HV power supply (if the tube or surroundings arcs to ground).
Re: danger measuring antenna

 

Those chokes were rated at 125 ma or 300 ma at the most. If the plate blocking capacitor were to short, they would burn out before the primary fuse blew. Whatever the function is supposed to be, it couldn't protect against a shorted blocking capacitor.

73,
Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO

On 18/08/2021 2:40, Brian D wrote:


"David Eckhardt" <davearea51a@...> ( davearea51a@... ) wrote:


I
remember my Novice transmitter, the Heathkit DX-40. It had a
large-valued
RF choke from the back of the PL-259 to chassis. The sole
purpose of that
RF choke was to keep voltages from building up on the
feedline - same as
the resistor in question.
Manly to prevent dangerous voltages on the feedline due to breakdown in
the
plate capacitor leaking HT onto the feedline, essential on high power
valve
PAs.
Re: Mosley traps measurements question

 

Thanks for the info.
Luckily I figured out the capicator value and was able to derive the frequencies of the trap. Also the Nano VNA in Smith format will work. I probably will post Mosley info on the trap group. It applies to many multi band antennas.
Re: RF Active Probe

 

I should have added that a dead or alive function using the vna as a troubleshooting aid is a reasonable application.
Assume the probe is essentially ideal as far as the circuit under test is concerned. That is to say, zero loading. An
active stage input,output could be monitored as a s21 measure. Hence a signal trace analyzer. That does have utility.

Alan
Re: SMA connector pulled off of cable.

 

Short link to that Amazon cable assembly would be the URL to get to Amazon then add /dp/B07YH9NNDQ
I always use the smile.amazon since I donate to a good cause.
--
73 de Phil KE3FL
Re: RF Active Probe

 

Thinking out loud, it's a unidirectional device, which maybe useful on port 2 in some way, but not useful on port 1 in any way that I can think of. Seems more appropriate for a scope, SA, or RF voltmeter.
Re: RF Active Probe

 

I build one of my own a few years back for trouble shooting.
You hold it near the oscillator, you hold it near the amp, you hold it near thedriver, you hold it near the final amp.? In a few seconds you know what area ofa transmitter is not doing it's job.? ? Like Cliff said, great for trouble shooting, butnot precision measurements.? ? Tiny SA recommended for that job vs the NanoVNA.Kent 2E0VAA/W5

On Wednesday, August 18, 2021, 06:17:50 AM CDT, davebb123456 <davidbrowne76@...> wrote:

Hi Clif
Please let me know how you get on with it
Maybe I can use it with my TS2000 ?thanks Dave
2E0DMB

On Wed, 18 Aug 2021, 10:30 avvidclif, <clif@...> wrote:

Nobody said anything about calibrating it. It's a probe looking for
signals with some measure of safety. Once found, then if needed,
calibration could be done. This a troubleshooting tool not a lab
measurement/design.
Re: RF Active Probe

 

This probe is just a capacitor input coupled FET source follower with a voltage regulator to run the FET. Input capacitance loading in in 10-15 pF range.

Over a limited freq range it provides a high impedance input.
Re: RF Active Probe

 

Hi Clif
Please let me know how you get on with it
Maybe I can use it with my TS2000 ?thanks Dave
2E0DMB

On Wed, 18 Aug 2021, 10:30 avvidclif, <clif@...> wrote:

Nobody said anything about calibrating it. It's a probe looking for
signals with some measure of safety. Once found, then if needed,
calibration could be done. This a troubleshooting tool not a lab
measurement/design.
Re: RF Active Probe

 

Nobody said anything about calibrating it. It's a probe looking for signals with some measure of safety. Once found, then if needed, calibration could be done. This a troubleshooting tool not a lab measurement/design.
Re: RF Active Probe

 

On 18/8/21 11:15 am, Ray Anderson wrote:

That probe might be useful for use with the spectrum analyzer to provide a means to probe some high-z circuits without loading them down with the 50 ohms presented by the SA input, but I think it would be fairly useless with the VNA.
I'm not sure that I agree with this. For typical simple use of a VNA with the ports of a 2-port DUT, sure, but if you're wanting for example to examine standing waves in a multi-pole filter or a DUT with more than 2 ports, RF switches and active probes are very useful tools to have.


- Roland 9V1RT (whose fingers are still sensitive after hundreds of N-connector disconnect-connect cycles on the weekend while tuning a duplexer without the aid of RF switches or an active probe)
Re: RF Active Probe

 

Active RF probing with the VNA is useful.



However, you would have to figure out how to apply this probe to a cal and measurement routine
that would provide useful results. For example, an in-circuit S21 response as outlined in the note above.

Alan
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