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There have been references to pickup coils to use the Nanovna as a GDO. Since I have zero experience with a GDO and have a need to measure some inductors, could someone provide some specifics on constructing the pickup coil (or coils)? What size wire for the coil? How many turns? Is the coil connected from the center pin to the shield of the connector?

Thanks.
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Bob AF9W

On Fri, Jun 30, 2023 at 06:28 PM, Bob AF9W wrote:

There have been references to pickup coils to use the Nanovna as a GDO. Since
I have zero experience with a GDO and have a need to measure some inductors,
could someone provide some specifics on constructing the pickup coil (or
coils)? What size wire for the coil? How many turns? Is the coil connected
from the center pin to the shield of the connector?

Thanks.
--
Bob AF9W

Hi Bob

If you look at /g/nanovna-users/message/33324 you will see a picture of a pickup coil I made. It's 5 turns of 0.9mm enamelled copper wire on a 20mm diameter PVC former. This worked well at around 20MHz.
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Mike G8GYW

I hope this is not a silly question, but where do you need GDO, if you have a VNA? That is, you can measure coils and capacitors with VNA. Measuring resonance frequency is bit harder, because Q gets easily too low, and it depends on firmware how you can find the resonance.

I am measuring traps in a bandpass filter in circuit for tuning the filter. I have created some pickup coils that "sort of" work but I am looking for an improved solution. I'm using the Nanovna to find the resonant frequency of the traps in a Cauer filter.

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Bob AF9W

Thanks,, Mike.

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Bob AF9W

On 6/30/23 2:07 PM, Leif M wrote:

I hope this is not a silly question, but where do you need GDO, if you have a VNA? That is, you can measure coils and capacitors with VNA. Measuring resonance frequency is bit harder, because Q gets easily too low, and it depends on firmware how you can find the resonance.

It's hard to measure an assembled trap (or measure self resonance of a coil) with traditional 1 or 2 port fixtures, because the fixture is part of the system. I suppose you could develop some appropriate calibration loads (equiv to Short, Open, Load) to "dembed" the fixture.

But if what you're interested is resonant frequency, then a "loosely coupled" coil hooked to the VNA to measure S11 works pretty well. The loose coupling reduces the loading from the measurement system, and you can support the unit under test well away from other "stuff".

We've used a similar technique at JPL to measure the resonance and Q of a microwave cavity at cryogenic temperatures. You can fill the cavity with, say, liquid methane, or ethane, or mixtures of them, and measure the EM properties. The cavity has two loop probes and we make multiple measurements as the probe orientation changes, and from that can calculate the effect of the probe.

(Why? Because oceans on Titan are likely liquid methane, ethane, etc)

On Fri, Jun 30, 2023 at 01:28 PM, Bob AF9W wrote:

need to measure some inductors

The nano can measure inductance all by itself, too. Or, at least give you the impedance at the frequency you are measuring with.
And the recent -H firmware versions have a specific "find resonance" function/feature/menu choice as well. But, as mentioned above, the connections between nano and coil will limit the absolute accuracy of the reading. Some users build special connection jigs for particular measurements.
Connecting your trap directly to the nano may give you the info you need too. You can see the shape of the curve as well as get numerical values.
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Doug, K8RFT

Ok.

I understand. If you cant remove coil from its surrounding circuit, a GDO is usefull.

"(Why? Because oceans on Titan are likely liquid methane, ethane, etc)," interesting. Good luck with that probe. i follow Spacex to see when people get to the moon again.

Is there a good a GDO or schema somewhere. Mine is from Heathkit from stone age.

I occasionally see them for sale at hamfests. I have gotten a few that way, but I shouldn't have - I haven't used any yet. Not even the big, (relatively) heavy old U.S. military one.
If you look for one that way, be sure that any that you find still has its set of plug-in coils.
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Doug, K8RFT

On Monday 03 July 2023 08:20:00 am DougVL wrote:

I occasionally see them for sale at hamfests. I have gotten a few that way, but I shouldn't have - I haven't used any yet. Not even the big, (relatively) heavy old U.S. military one.
If you look for one that way, be sure that any that you find still has its set of plug-in coils.

I had the thought to get one some time back, and started looking on ebay. It's amazing how many of these units do NOT have any of the coils with them! I know of one guy who took on one of those and proceeded to try and make his own coils, but I'm not feeling that ambitious these days...

I wonder where all of those coils go?

--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
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Information is more dangerous than cannon to a society ruled by lies. --James
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