Use of field strength meter - Page 1
Forum section: electronics / rf-microwave

author: @rt
author_rank: Super Contributor
timestamp: January 26, 2021, 05:43:15 pm
content: 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.CheersVK4JJY

author: radiolistener
author_rank: Super Contributor
timestamp: January 26, 2021, 06:12:31 pm
content: if you want to test insertion loss for adapters, you can use NanoVNA

author: @rt
author_rank: Super Contributor
timestamp: January 26, 2021, 06:19:22 pm
content: 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.

author: radiolistener
author_rank: Super Contributor
timestamp: January 26, 2021, 06:34:40 pm
content: 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.

author: @rt
author_rank: Super Contributor
timestamp: January 26, 2021, 06:46:14 pm
content: Quote from: radiolistener on January 26, 2021, 06:34:40 pmThanksScreenshots 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.

author: cdev
author_rank: Super Contributor
timestamp: January 26, 2021, 07:28:23 pm
content: You have to keep them clean and screw them on carefully to not damage them.

author: @rt
author_rank: Super Contributor
timestamp: January 26, 2021, 08:39:28 pm
content: 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 itThat was only one small part of it though.

author: radiolistener
author_rank: Super Contributor
timestamp: January 26, 2021, 10:13:28 pm
content: Quote from: @rt on January 26, 2021, 08:39:28 pmokay, 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 seriesSo, 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 WLong coax cable will add much more insertion loss.For comparison,10 meters of RG316 cablehas insertion loss about 3.17 dB at 148 MHz. This is like 370 SMA-BNC adapters in seriesPS: cable is20cm RG405 with soldered SMAandSMA-BNC adaptersbought 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.

author: @rt
author_rank: Super Contributor
timestamp: January 26, 2021, 10:46:14 pm
content: ThanksWow. 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.

author: radiolistener
author_rank: Super Contributor
timestamp: January 26, 2021, 11:21:36 pm
content: Quote from: @rt on January 26, 2021, 10:46:14 pmI don't know, because I don't have very precise reference to check it.For sanity test, here is Chinese6 GHz 1 dB attenuatorfrom 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:

author: @rt
author_rank: Super Contributor
timestamp: January 26, 2021, 11:57:09 pm
content: 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.