EMC pre-compliance antennas in Australia

[liteyear]

In a similar vein to this thread, I'm building a very basic EMC pre-compliance setup to use in a professional setting. I have an SSA3021X Plus, a set of H/E probes and a current probe on the way. Based on the advice from copious guides out there, a LISN and maybe a TEM have to be next on the shopping list.

In the meantime, I'm trying to get my head around a basic antenna setup for DUT that won't fit in a TEM. I'm certainly not looking to replicate a test house. Instead I'm looking right down the end of the spectrum towards "good enough for a first pass".

I've seen the Aaronia antennas, which look fantastic, but from what I can see are an unnecessarily large step up in price from the "rabbit ears" style that Ken Wyatt and co tend to recommend. And here's where the endless rabbit warren search starts! These style of antennas are very regional specific, given the economies of scale that have driven them. After a billion laps around the Google, I've come up with this combo:

• For 47-862MHz: Antsig Flat Panel VHF/UHF Antenna: https://www.bunnings.com.au/antsig-flat-panel-indoor-vhf-uhf-antenna-with-external-amplifier_p4360387
• For 700-3000MHz: RFShop RFS0727-DR11 LPDA https://rfshop.com.au/product/11-dbi-wideband-directional-antenna-nf/

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Both options suffer and benefit from commoditisation. They're cheap, go by various names, and specifications are difficult to be sure about.

Can you see any reason why these would not be suitable? Are bigger antennas, like a Tecsun Discone Q3071 and an RFI LPDA7030-11 combo better?

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[Pitrsek]

Cheapest I could find was the tekbox antenna. If you can spare the low end extension, there is a small version of AAronia bicolog, not much more expensive then tekbox. Which current probe did you ordered?

[julianhigginson]

Can you walk us through what exactly you expect to be able to do with this antenna?  Like, where you will use it and what kind of signal you'll see with it exactly?

I have a fair bit of tekbox EMC gear now that I use for precompliance testing, because they're good bang for buck - but I don't have their antenna because even though I've considered it a few times, it's not going to do much that I need.

At the end of the day, unless you have a good and complete setup for measuring with an antenna, anything you catch with even a calibrated antenna is unknown, because:
1) Add in unknown reflections and resonances in the space you're working in, and then the calibration of the antenna becomes irrelevant... so you need a calibrated anechoic chamber or an OATS. not a regular electronics lab.
2) With no isolation from the rest of the world, you'll spend an awful lot of time turning stuff on and off in order to try and work out what's even your DUT, and what's just crap in the environment. (actually that happens in anechoic chambers I've been in, too.. just not so much)

And that's why the internet EMC experts don't tell you to buy thousands of $ in antennas for a basic DIY test setup.


Neither of those antennas from bunnings or rfshop look that great to me for EMC purposes.. you don't know what their operating range is even meant to be

OK you can guess, 700MHz to up past 2.45G for the rfshop one, based on what they say it's goot to use for, but can you be sure it has no bad blind spots where it's deliberately filtering out bands that aren't of interest to its purpose as a radio signal antenna in order to make it more interference proof? at least the tekbox antenna basically gives you that whole range it's specced for.

I'm not even sure what to expect from a digital TV antenna with an amp like the bunnings one...

Does your spectrum analyser have a tracking generator? if it does, you could buy two of these rfshop antennas and set them up maybe 5m apart in a football field, pointing right at each other, polarised the same way... maybe with an rf amp to drive the TX antenna for better noise immunity (watch out for breaking rf regulations!)  then you could try to run sweeps and measure the frequency response of them as a pair... assuming they are each responding the same way (Tx response same as RX response) then you might infer a frequency response of those antennas?

But if you want to do that, I'd probably start instead with a handful of doubles of the log periodic antennas from here...  https://www.wa5vjb.com/ they are probably close to what's actually inside the aaronia LP units... but with no calibration or nice housings or anything else.

[liteyear]

Thanks for the response and I’m sorry I didn’t see it until now!

The current probe is the Tekbox one.

The anticipated usage (subject to lots of change from a whole heap of learning to come) was to find a space (possibly OATS style) with a relatively stable, reasonably low level baseline RF spectrum, as measured by each antenna. Then put the DUT at a sensible distance from the antenna, turn it on, and measure the delta in the spectrum, with one antenna at a time. If anything looks alarming, decide whether it might threaten an EMC test result, and if so do a round of investigation and mitigation before heading to the testhouse. If nothing looks alarming, then nothing lost, but it’ll still need a trip to the testhouse for a proper look. Other use cases are: compare results from the cheap antennas to results from the testhouse and start to get a picture for how to use the former to predict aspects of the latter; and secondly, to quickly assess the impact of design changes on the RF signature of the DUT.

From what I’ve read, your (1) concern can be mitigated by doing a baseline measurement first and just looking at the delta. Notwithstanding the fluctuation dramas, is that not true? The (2) concern is still valid, but I gather that’s just the nature of EMC work. If there’s something well above background noise when the DUT is on and not when it’s off, then it might be worth a closer look. If it’s hidden in the background noise, then I’m probably less interested anyway.

The amplitude over frequency of the antennas is indeed hard to pin down, so yes, I was planning on generating a reference sweep and mapping it out. I expect there’ll be blindspots but beggars can’t be choosers. The SA does have a tracking gen. The Bunnings jobby is indeed a bit of a wildcard, but try as I might I couldn’t find a superior option. 30 odd bucks is a cheap learning exercise!

What makes you think the inside of the RFShop jobby isn’t like the wa5vjb things?

[JoeyG]

In your own EMC test lab/OTS - the question remains  how do you know if the DUT is failing the EMC limits?   

One way is to have a  comb generator with certified results from a fully qualified EMC test lab/OTS. 
On your own test lab/OTS, you can  relative compare any DUT measurements  against the absolute measurements of the comb generator at similar frequencies.

While it isn't an absolute measurement... it can provided you with a worry factory or no worry factory  (and if you need to mitigate any DUT emissions).
Before the real  EMC tests

On another note  pointing the antennas to the sky  has can reduce ground emissions on antenna testing.

[tautech]

Some more ideas in this thread:
https://www.eevblog.com/forum/testgear/howto-calibrate-diy-emc-pre-compliance-antenna/

[julianhigginson]

Like joeyg said, if you are doing emissions precompliance without knowing exactly what your dut's levels are, you run the risk of spending a whole bunch of time and effort squashing things that aren't actually a problem. Or over engineering your solution taking it way below where it could happily be... Incurring extra bom and development cost that ends up in the sale price.... Or maybe worse, spend time working on something, even over engineering due to a perceived issue in one area, only to have the product end up 3db over a limit in another spot you didn't see...

Once you have a real measurement then you immediately know that any problem you are working on is real, and you have a real target to hit. Your work goes from something wishy washy and undefined, to something very specific and achievable.

[Elasia]

I like having them just for a quick pre scan and see if anything absurd stands out, go in to an emc shop and get the process rolling and then if  fail, come back and look specifically at those select frequencies and probe around till hitting on whatever might be transmitting it... sometimes can be something as simple as an ungrounded / unconnected micro controller pin