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Comments on Is there a way to reliably measure antenna return loss outside a lab?

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Is there a way to reliably measure antenna return loss outside a lab?

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Assume I'm a complete beginner at RF.

Is there a way to measure return loss of an antenna, in such a manner that I can reliably reproduce the measurement later on?

What I'm talking about is producing the antenna characteristic graph to show what frequency it was adapted to and how wide it is. That is, the classic frequency vs dBm graph with a dip at the expected center frequency.

I've never quite managed to do this in a satisfying manner. I can do it in two ways, either the manual way which involves using a spectrum analyser with tracking generator and a 50ohm directional coupler. I connect the tracking generator to the input of the directional coupler, then measure how much energy that bounces back. I also have access to an antenna measurement instrument that does all of this automatically.

Using either method, I get a graph that looks somewhat correct. The antennas are typically either 433MHz or 902MHz 0 gain omnidirectional with a width of +-/50MHz from center at most.

However, if I nudge the setup and place the antenna slightly differently, or just leave it and do it again another day, the energy dip can move some +/-30MHz. I've tried to use a fixture so that the antenna sits mounted & grounded the same way every time, but still there's considerable variations.

I'm not using any signal damper, could that be a problem? Am I wrong thinking the spec should be able to deal with its own tracking generator?

Or am I naive to think I can do this accurately outside a lab? Will EMI really affect measurements that much?

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3 comment threads

I expect your measurements are accurate enough, but now you realize how reflections within 10 wavelen... (1 comment)
What have you learnt? (1 comment)
General comments (4 comments)
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The RSSI detector in the receiver is the best field tool. In Windows I had a tool (Wifi Radar?) that read the Broadcom IC RSSI and displayed a time plot of the results as I changed laptop orientation a couple of degrees and the results might change from -74 to -84 dBm and result in occasional errors at 54 Mbps.

The 2nd best tool is a 3dB splitter to measure the return peak voltage with a diode and DMM but requires calibration.

  • When I was doing the same in ‘77 I found that the return loss was depending on people walking into the lab and thought I had just invented a remote intrusion alarm from the reflected waves at multiple wavelengths. This was after tuning the antenna for RL>20 dB so the directional coupler was very sensitive. It simply used a Schottky diode on the return path to a DMM to measure the reflected wave after calibration with lab equipment. You could use a stripline directional coupler or a hybrid 3 dB splitter or anything direction to measure return loss for tuning to a minimum.

You can also have a thermal sensitivity problem as well as a sensitivity to the shift in effective wavelength from reflections.

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1 comment thread

General comments (6 comments)
General comments
Lundin‭ wrote almost 3 years ago

One crazy idea I have: what if I mount the antenna inside a large metal box? So it is grounded but inside a Faraday cage. Then the environment would be the same and no reflections from walls or people.

Olin Lathrop‭ wrote almost 3 years ago

@Lundin: No, a "metal box" around an antenna would have a major effect on the antenna if in the near field, and would reflect the signal from the antenna back on itself if in the far field. If that metal box were lined with ferrite tiles, then it can work at some frequencies. You have to carefully check how much attenuation you get from the specific thickness of ferrite at your frequency of interest.

Lundin‭ wrote almost 3 years ago

@Olin Lathrop‭ So essentially the box would have to become a "mini EMC lab" :) Though large enough to not be in the near field, so not so "mini" any longer either. Maybe the easiest solution here might actually be to do the measurement outdoors in the open.

Olin Lathrop‭ wrote almost 3 years ago

@Lundin: Actually the right kind of box doesn't need to be large. The walls can be in the near field as long as they appear as more space to the antenna. There are actually RF boxes built on this principle. You need just the right ferrite tiles so that the walls have the same 377 Ω impedance as free space does. Once you achieve that, the walls are indistiguishable from free space, so near field versus far field doesn't matter.

TonyStewart‭ wrote over 1 year ago

Lundin‭ The reality is your results of variations are real and certainly affected up to 10 or more wavelengths from reflections.

The other reality is s21 is also affected by these reflections with multipath, so the net effect is to measure RSSI where possible in your receiver and test the variations due to antennae location, orientation and antenna tuning. One antenna ought to be well-known or broadband omni-direction to isolate the differences. Thus Ricean Losses from multipath become dominant factors at fringe levels along with antenna mismatch.

A sweep generator or a network analyzer is the best lab tool, while the RSSI detector in the receiver is the best field tool. In Windows I had a tool that read the Broadcom IC RSSI and displayed a time plot of the results as I changed laptop orientation a couple degrees and the results might change from -74 to -84 dBm and result in occasional errors at 54 Mbps.

Andy aka‭ wrote over 1 year ago

TonyStewart‭ welcome to the punishment planet