Measure radio output with clamp/coil EXTERNALLY?

[xntrik]

I’m looking for ways to measure a GMRS walkie’s output that DOES NOT have a removable antenna.

I have not opened it yet, but also seriously doubt that what I immagine is just a little springy coil for antenna has any type of connector. Most likely directly soldered to the board, making any INLINE adapters unusable.

I currently have a TINYSA, RTLSDR, as well as an oscilloscope. Can anything be done, open to any/ALL hairbrained ideas.

Thank you!

 

[ecps92]

have you checked the FCC ID and/or gone to the manual ?
most that have no removable antenna are mostly FRS and under 1 watt

 

[KevinC]

Back when I was on the bench I used a relative signal strength meter as my quick check for power out of a portable. I had eyeball calibrated it to guesstimate how much was leaving the antenna. Not perfect by any means, but better than nothing,

 

[K4EET]

If you are willing to open up the case and disconnect the non-removable antenna at the circuit board, you can use the following IF your oscilloscope has a 50 ohm termination capability.

Using RG-174/U coax, solder it to the circuit board where the antenna was connected, use an in-line 40 dB attenuator that can handle 5 watts at the transmit frequency, and using another piece of RG-174/U coax, connect that directly to a 2 GHz oscilloscope (not using a scope probe) with the termination impedance set to 50 ohms. Then calculate the RF power from the resulting peak-to-peak voltage taking into account the 40 dB attenuator.

If your oscilloscope does not have a 50 ohm termination capability, you can also measure the HT’s output power per the following video.

Note that it is recommended that the bandwidth of the oscilloscope be three to five times the frequency under test. Hence at about 500 MHz, the minimum bandwidth of the oscilloscope should be about 1500 MHz.

Why do you want to know the actual RF power being sent to the non-removable antenna? The manufacturer’s specifications for that radio model are not good enough or are you trying to verify that the transmitter is meeting the published spec for power out? Also, what make and model of the HT are you testing? Just curious…

 

[prcguy]

If you are willing to open up the case and disconnect the non-removable antenna at the circuit board, you can use the following IF your oscilloscope has a 50 ohm termination capability.

Using RG-174/U coax, solder it to the circuit board where the antenna was connected, use an in-line 40 dB attenuator that can handle 5 watts at the transmit frequency, and using another piece of RG-174/U coax, connect that directly to a 2 GHz oscilloscope (not using a scope probe) with the termination impedance set to 50 ohms. Then calculate the RF power from the resulting peak-to-peak voltage taking into account the 40 dB attenuator.

If your oscilloscope does not have a 50 ohm termination capability, you can also measure the HT’s output power per the following video.

Note that it is recommended that the bandwidth of the oscilloscope be three to five times the frequency under test. Hence at about 500 MHz, the minimum bandwidth of the oscilloscope should be about 1500 MHz.

Why do you want to know the actual RF power being sent to the non-removable antenna? The manufacturer’s specifications for that radio model are not good enough or are you trying to verify that the transmitter is meeting the published spec for power out? Also, what make and model of the HT are you testing? Just curious…

The procedure mentioned here might work for very low frequency RF like the lower HF bands but not at CB or above due to the long lead lengths shown in the video which are grossly too long. There is no accurate way to measure RF power by externally coupling to a UHF walki talki with a fixed antenna. However you should be able to get a good idea using an oscilloscope and probe as described above both with several times the rated frequency range of the radio being tested.

Open the radio and probe the antenna feedpoint on the circuit board inside with the high frequency probe and oscilloscope. Assuming the fixed antenna is close to 50 ohms you can measure the P to P voltage at the feedpoint, convert to RMS then calculate power into 50 ohms. The measurement will be skewed by the accuracy of the probe and ground connections and the impedance of the antenna which you have no control over. But I think this would be the most accurate measurement you can make without disconnecting the antenna and soldering in a feedline to an actual wattmeter and load.

An RF millivolt meter rated for the frequency can also be connected to the antenna feedpoint for very low power measurements like 180mw or less as they usually have a limit of about 3 volts of RF if my memory is working.

 

[xntrik]

I’m trying to compare two HT’s output (IDEALLY) against what their “advertised” ratings are, and what they REALLY are. I just did a very dirty side by side from 2ft away, on my tinysa with NO antennas on it. One (we’ll call it 001) measured -72.4dBm, the other (we’ll call it 002) -81.9dbm. Sooooo 🤷‍♂️ is 1 more powerful than 2? I bought the tinysa to aim an ota antenna and it worked splendidly, but honestly, I gave VERY limited idea how to read this thing… as probably obvious.

 

[xntrik]

Im attempting to modify/build two, possibly three radios that are as small as possible, and as high powered as possible, which I’ll most likely end up modding for temporary placement in the smallest waterproof Plano box I could find, with a (already purchased) waterproof kpin mic/speaker, and probably mod the ht’s case as well with an SMA connector to swap a REAL antenna onto it, but also have the same removable antenna thread onto a bulkhead on the aforementioned Plano case. These will be for use while kayaking (quite possibly a good distance between units), and around our property.

 

[nd5y]

You could use a field strength meter but you probably need to be more than a couple feet away and realize that you will never get the same reading twice becaus you can't control the environment and keep anything in the exact same position.

Where I used to work they had test fixtures for measuring the receive sensitivity of pagers with internal antennas and no connectors. The fixture kept the pager in the same position and from moving. They had a chart for the output level of the service monitor for the pager sensitivity pass/fail point. I never took one apart so I don't know what was on the other end of the coax. Probably a small loop oriented to match the position of the antenna in the pager. I don't know if it presented a 50 ohm load to the service monitor. They had a bunch for different pager models. They had to be used in a screen room.

You might be able to do something like that in reverse.

 

[K4EET]

The procedure mentioned here might work for very low frequency RF like the lower HF bands but not at CB or above due to the long lead lengths shown in the video which are grossly too long.

@prcguy is correct that you want cables as short as possible on any test bench measurement when dealing with RF. I agree that the video violates that rule. As for the scenario of making that measurement at or above 27 MHz, I have not had any major issues with results basically being where I expected them to be ballpark-wise in the testing of VHF/UHF HT guts. Granted, the results are for the hobbyist and are not expected to be critically accurate. As “they” say, close enough for government work. 🤣 @prcguy does provide good alternatives that are less invasive. Thanks for the suggestions @prcguy!

 

[prcguy]

I’m trying to compare two HT’s output (IDEALLY) against what their “advertised” ratings are, and what they REALLY are. I just did a very dirty side by side from 2ft away, on my tinysa with NO antennas on it. One (we’ll call it 001) measured -72.4dBm, the other (we’ll call it 002) -81.9dbm. Sooooo 🤷‍♂️ is 1 more powerful than 2? I bought the tinysa to aim an ota antenna and it worked splendidly, but honestly, I gave VERY limited idea how to read this thing… as probably obvious.

What kind of radios were they? What kind of antennas on the HTs and the analyzer? Lets take the higher power radio and say its a 5W VHF model, and with 9.5dB difference between them the other radio would be .56W if antennas are identical.

I suspect your radios were closer matched than that and there is just a huge amount of inaccuracy in your measurement. If the antennas are not absolutely identical that will seriously skew the measurement as will being too close to the TinySA where it can overload easily and give bogus readings. Doing the measurement you described indoors near anything metal will cause all sorts of reflections and moving an inch any direction will change the results. Even experts comparing two different HTs on an antenna range would have to perform many measurements to make sure everything is playing nice and they are getting useful results.

 

[mmckenna]

Im attempting to modify/build two, possibly three radios that are as small as possible, and as high powered as possible, which I’ll most likely end up modding for temporary placement in the smallest waterproof Plano box I could find, with a (already purchased) waterproof kpin mic/speaker, and probably mod the ht’s case as well with an SMA connector to swap a REAL antenna onto it, but also have the same removable antenna thread onto a bulkhead on the aforementioned Plano case. These will be for use while kayaking (quite possibly a good distance between units), and around our property.

Modifying the radios in any way (even adding an external antenna) voids the FCC type acceptance.
That's your choice.
But there may be easier ways to do this. What's the reason for picking these bubble pack radios for the modification?
There may be better/easier/more robust ways to do this without relying on a toy.

 

[merlin]

Use a calibrated field strength meter. It works comparing a known radio to x number of unknowns.
To calibrate a field strength meter take a signal generator that has a calibrated output.
You would need a clear area to test a radio(s)

 

[merlin]

The procedure mentioned here might work for very low frequency RF like the lower HF bands but not at CB or above due to the long lead lengths shown in the video which are grossly too long. There is no accurate way to measure RF power by externally coupling to a UHF walki talki with a fixed antenna. However you should be able to get a good idea using an oscilloscope and probe as described above both with several times the rated frequency range of the radio being tested.

Open the radio and probe the antenna feedpoint on the circuit board inside with the high frequency probe and oscilloscope. Assuming the fixed antenna is close to 50 ohms you can measure the P to P voltage at the feedpoint, convert to RMS then calculate power into 50 ohms. The measurement will be skewed by the accuracy of the probe and ground connections and the impedance of the antenna which you have no control over. But I think this would be the most accurate measurement you can make without disconnecting the antenna and soldering in a feedline to an actual wattmeter and load.

An RF millivolt meter rated for the frequency can also be connected to the antenna feedpoint for very low power measurements like 180mw or less as they usually have a limit of about 3 volts of RF if my memory is working.

I have had this work accurately up to 950MHz, but UHF and above, coax, even solder on solder connections is critical.