Cheap radiated EMC lab made with four separate quarter wave dipole antennae?

[treez]

Hi,
We wish to do rough radiated EMC scans in our open-field , outdoor, radiated emissions site. We will simply get four antennas……50MHz, 100MHz, 150MHz and 200MHz… each a quarter wave dipole. We will then put their outputs into an RF combiner, and feed this to a spectrum analyser.
What will be our main problem?

 

[Easy peasy]

Better to cover the various spectrums one aerial at a time - be sure the dipole length is parallel to any long slots in the case - and/or arrange the input / output wires in the same way parallel to the antenna - you are seeking worst case ...

 

[betwixt]

open-field , outdoor, radiated emissions site.

Not a great idea - even here is a very rural location, not even street lighting, the RF spectrum is packed with signals. You really need a screened chamber. I have been tracking a strange emission here for some time and finally isolated it as the pressure monitor inside next doors car wheels. There are so many sources around you will have great difficulty isolating your EMC from everyone else's.

Brian.

 

[treez]

Thanks, i agree but we'll just have to calibrate it out best we can..official radiated emc testing in a proper chamber is utterly phenominally expensive.

 

[FvM]

Pre-compliance measurements of radiated emissions can identify design flaws and verify the effect of EMI related designs modifications. To distinguish relevant emissions from external radio sources, you can e.g. refer to near field measurements.

 

[mtwieg]

There are numerous ways to do pre-compliance testing at low cost. The main limitation is that your measurements will be relative only. So, for example, if you know your device fails at 200MHz by 10dBm, then you can use your own lab setup to try and knock down that emission by 10dB before bringing it back for real testing.

We will simply get four antennas……50MHz, 100MHz, 150MHz and 200MHz… each a quarter wave dipole. We will then put their outputs into an RF combiner, and feed this to a spectrum analyser.

I wouldn't try it this way. If the antennas are close together, they will couple to each other, thus changing their characteristics. If you space them out a lot, then they're no longer measuring at the same location. Either way, the setup will not approximate a single antenna with a broader bandwidth (even if your combiner has perfect isolation....).

Swapping out individual antennas is the way to go. I doubt you'll need to see more than an octave of bandwidth at any given time anyways.

If there happens to be strong ambient emissions in the same band you're troubleshooting, then you are basically SOL without a shield. Depends on the characteristics of the source.

 

[treez]

Yes , thankyou , we are now seeking the most broadband , cheap antennas for 100MHz, 200MHz and 50MHz. Plus a cheap spectrum analyser than can do the same windowing function as the radiated approvals labs do.

 

[dick_freebird]

I would not combine antenna signals, there
could be nulls based on position and wavelength
that give you a bogus summation. Better to
measure each antenna's signal and then do
the analysis of total radiated power.

 

[FvM]

For the frequency range of 30 to 300 MHz, biconical wideband antennas are preferred due to there relative handy size.

 

[treez]

Thanks, i have sent out a quote for such an antenna...

Dear Sir/Madam of Schwarzbeck.de
Guten Tag,
Ich hoffe dies erreicht Sie bei guter Gesundheit.
Please quote for your BBA 9106 biconical elements in combination with the VHA 9103B antenna holder/balun. (Antenna and Balun).
We wish to use it to do Radiated Emissions testing for Class B EN55032 products.
Its just for pre-compliance.

...And i suppose we will also need to purchase pre-amplifier, spectrum analyser, and EN55032 windowing software for this also? Not to mention some kind of stand for the antenna which doesnt interfere with its reception?