Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users

Dashboard
Notifications
Mark all as read
Q&A

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

+3
−0

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?

Why does this post require moderator attention?
You might want to add some details to your flag.
Why should this post be closed?

4 comments

EMI could do that. Maybe make an antenna that works in a close but quiet part of the spectrum and test? Andy aka‭ 22 days ago

@Andy aka‭ The purpose here is to measure pre-made antennas from various manufacturers to see if they can be used for the specific frequency. Still, if EMI is the cause, then moving the fundamental to some quiet band won't be reliable either, because I'd still get harmonics from TV/radio broadcasts etc. And misc radiated noise from whatever electronics that happen to be in the room. Spectrum analyzer, voltage supply, computers etc. Lundin‭ 21 days ago

Spectrum analyser + tracking generator sounds more complicated than a (vector) network analyser, which is especially designed for this task (and other). Note that there are now cheap so called "nano-vna" on the market that give decent results for antennas up to 1GHz. coquelicot‭ 12 days ago

@coquelicot‭ It's not complicated at all assuming you got the stuff already :) Though a high quality directional coupler costs a bit. Tracking generators are very easy to use, just attach two coax to input & output on the spec, then click on the button that says tracking generator, and that's pretty much it. Lundin‭ 11 days ago

2 answers

+1
−0

It sounds like you are doing the measurements right. However, I expect the problem is in the space around the antenna.

At 434 MHz, the wavelength is 690 mm or 27 inches. Everything out to about a meter or 1½ m should be considered near field, and can effect the antenna directly. This includes whatever table it is sitting on. Even with a table that's transparent to RF, it needs to be a meter or more above the floor. Usually dry light wood is a good material for such a table.

Even then the surroundings can still matter. If there is something near by, like a flat piece of metal, that reflects the radio waves, it can still matter even if it's out of the near field. This is because the reflection has a fixed phase offset from the transmitted signal.

If you have access to an anechoic chamber, try your tests there to see if they become more repeatable. If so, then you know your environment is the problem.

If you don't have access to an anechoic chamber, try an open field test. See if you can suspend the antenna several meters above the ground. If possible, put some RF-absorbing material on the ground below the antenna. Failing that, move the antenna up higher so that the reflection off the ground is more attenuated.

Local transmission, especially at ISM frequencies, are probably not much of an issue. The signal you are feeding the antenna should be many many times larger than anything the antenna might pick up from elsewhere.

Why does this post require moderator attention?
You might want to add some details to your flag.

1 comment

Yeah maybe nearby walls could be an issue. When I did this I had the antenna mounted vertically against a metal plate surface approx 300x300mm but the fixture was placed around 1-2 meters from a wall. Consisting of plaster, not reinforced concrete, but still. Maybe if I use one that's 1x1m and keep it in an open environment. Lundin‭ 21 days ago

+0
−0

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.

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

Why does this post require moderator attention?
You might want to add some details to your flag.

4 comments

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. Lundin‭ 15 days 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. Olin Lathrop‭ 15 days 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. Lundin‭ 14 days 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. Olin Lathrop‭ 13 days ago

Sign up to answer this question »