Use of field strength meter

[@rt]

Hello,
I understand a cheap passive field strength meter can be made of a couple of diodes, a capacitor, meter, and antenna, or better still, a multimeter with added small board.
also that this is good for relative measurements ie. it isn’t calibrated to tell you Watts or something, but may compare one antenna with another.

I aim to measure insertion loss of adapters added between a handheld transceiver and it’s antenna. Say when you have SMA on the handheld, but want to use a BNC antenna.
My understanding is that the insertion loss for such an adapter is negligible.

It occurs to me that a field strength meter could also measure any ill effect on VSWR presented to the handheld by adding adapters, as this would also ultimately reduce radiated power.

By all of the above logic, my understanding is that a field strength meter would expose any ill effect of using one or more of these adapters, or even a small coax between a HT and antenna.


To perform such tests, I plan to come up with jigs to clamp the meter and the DUT stationary so their antennas don’t move closer together between tests, etc.,
and also I believe the devices should be some distance apart so the meter’s antenna is not in the H field of the radiator, but other than that, they should be in the same room.

...

I’ve been accused of "devising tests to prove my own correctness instead of truly trying to question my own judgements”.
So am I leaving anything out here?

My predictions are that:
A) Adding an SMA to BNC adapter insertion loss would be <1dB, and not noticeable in any practical way for 2 meter band,
B) Adding an SMA to BNC adapter will not adversely affect VSWR presented to the HT.


Cheers
VK4JJY

[radiolistener]

if you want to test insertion loss for adapters, you can use NanoVNA

[@rt]

I only have a single port VNA... the Mini 600, but if you have one and time to test an adapter like that, I’d be interested in the plots around the 2m band.

[radiolistener]

For 1-300 MHz these SMA-BNC adapters have insertion loss much below 1 dB.

For example two SMA-BNC adapters + 1 meter RG58 BNC cable leads to about 0.25 dB loss at 150 MHz and about 0.5 dB at 300 MHz.

I'm too lazy to perform more precise measurements, it's obvious that insertion loss is less than 0.1 dB per SMA-BNC adapter.

[@rt]

For 1-300 MHz these SMA-BNC adapters have insertion loss much below 1 dB.

For example two SMA-BNC adapters + 1 meter RG58 BNC cable leads to about 0.25 dB loss at 150 MHz and about 0.5 dB at 300 MHz.

I'm too lazy to perform more precise measurements, it's obvious that insertion loss is less than 0.1 dB per SMA-BNC adapter.

Thanks Screenshots and/or pics of the setup would be better, but it’s a start. I assume someone else with a Nano VNA will disagree if you are wrong.

[cdev]

You have to keep them clean and screw them on carefully to not damage them.

[@rt]

It just occurred to me you could also measure insertion loss with a power meter and dummy load, but unfortunately I only have access to the dummy load at the moment.
Funny the person in disagreement with me has a NanoVNA, dummy load and power meter good for up to UHF, and could have measured that in the time spend talking about it
That was only one small part of it though.

[radiolistener]

has a NanoVNA, dummy load and power meter good for up to UHF, and could have measured that in the time spend talking about it

okay, here is test setup and measurement.
First S21 chart is a direct SMA cable connection.
And second S21 chart is through 2 x SMA-BNC and 2 x BNC-SMA adapters.

Direct cable measurement already looks pretty flat, so I didn't performed calibration.
As you can see, for 148 MHz we have:
- 0.0002 dB loss for cable only
- 0.0344 dB loss for cable and 4 SMA-BNC adapters in series

So, 4 x SMA-BNC adapters add loss about 0.0344 - 0.0002 = 0.0342 dB.

And each SMA-BNC adapter has about 0.0342 / 4 = 0.00855 dB insertion loss.

It means, if you pass 100 W power through it, power loss on SMA-BNC adapter will be about 0.2 W
Long coax cable will add much more insertion loss.
For comparison, 10 meters of RG316 cable has insertion loss about 3.17 dB at 148 MHz. This is like 370 SMA-BNC adapters in series


PS: cable is 20cm RG405 with soldered SMA and SMA-BNC adapters bought on aliexpress.
Of course quality of SMA-BNC adapter may vary from seller to seller, but I think the difference will be not significant.
Also you're needs to note that insertion loss may be worse if you use dirty connectors, so clean it with ethanol before use.

[@rt]

Thanks
Wow. Does that software come with NanoVNA hardware? and is the hardware as accurate as that graph and figures are precise?
Maybe for the $60AU or whatever it is I should get one. My Mini600 is really just an STM discovery (development board) I could use for something else.

[radiolistener]

and is the hardware as accurate as that graph and figures are precise?

I don't know, because I don't have very precise reference to check it.

For sanity test, here is Chinese 6 GHz 1 dB attenuator from aliexpress.
NWT7 linear detector measure this attenuator as 0.94 dB at 50 MHz.
At DC I measure it's value with DMM as 0.88 dB.
I don't know what is the real value of this attenuator, but I assume it is close to 0.9 dB.

And here is how this 0.9 dB is measured with NanoVNA:

[@rt]

The thought crossed my mind to buy a cheap attenuator for use with the field strength tests,
but then it’s not really needed if such overwhelming evidence already presented is going to be denied.
I linked this thread to the person in question, who is not a member, but will be able to read the thread.

I might have a desire to make a fixed freq filter one day, for which dual port VNA would be essential I guess.