Re: [PSES] Antenna tilting or bore sighting for radiated emissions

[Paolo Roncone]

  Hi Larry,

Thanks a lot for your insightful comments. Yes I'm familiar with the setup
and with the antenna 3dB beamwidth.
My question aims to help me estimate how much is it worth the investment in
a mast with tilting or boresighting capability for our 3m semi-anechoic
chamber.
Most of the tests we perform are to CISPR standards. In CISPR 16-2-3
antenna directivity and beamwidth are covered, but there is no explicit
requirement for antenna alignment with the EUT as in ANSI C63.4.

We use a Schaffner CBL 6141A bilog antenna in the 30M-1G range and a
Schwarzbeck BBHA9120D horn antenna in the 1 - 18 GHz range.
Most of the EUTs we test are table-top, with maximum height of about 2m.
Semi-anechoic 3m setup below 1GHz, fully-anechoic (absorbers added on the
floor between antenna and EUT) above 1GHz.

As you mentioned, this subject is more relevant in the > 1GHz range, where
antenna and source beamwidths are narrower. However, the analysis and
numbers I found in standards and literature so far are mainly related to
the below 1 GHz frequency range.

Antenna directivity: the uncertainty budget contribution is per CISPR
16-4-2 (Annex A) with our setup (3m SAC, Hybrid antenna) as shown below:

Vertical polarization, 30 - 1000 MHz range: 1.37dB max improvement in
measurement uncertainty with tilting for > 200MHz.
Total expanded (2-sigma or 95% confidence level) expanded uncertainty
improvement of 1.06 dB with the tilting.
Horizontal polarization, 30 - 100 MHz range: 0.29dB max improvement in
measurement uncertainty with tilting for > 100MHz, no difference below 100
MHz.
Total expanded (2-sigma or 95% confidence level) expanded uncertainty
improvement of 0.10 dB with the tilting.
1 - 18 GHz range (both polarizations): 0.87 dB directivity difference
contribution to measurement uncertainty. No mention of tilting effects.

There is a work by Austrian Research Center a few years back. See abstract
at the link below:
https://ieeexplore.ieee.org/abstract/document/4652052

"For a test distance of 3 m the measurement uncertainty can be reduced from
5.62 dB to 5.43 dB with antenna tilting. With a bore sight antenna tower a
reduction to 5.31 dB is feasible. The bias and uncertainty given by CISPR
16-4-2 are not adequate."

This work shows a best-case improvement of just about 0.3 dB of measurement
uncertainty in the 30M-1G range with a 3m measurement distance.

My feeling so far is that investing in a tilting/boresighting mast is not
so worth it, although someone above me already decided to buy it...

Thanks again and best regards,
Paolo


On Wed, Mar 29, 2023 at 7:25 PM Larry K. Stillings <
la...@complianceworldwide.com> wrote:

> Paolo,
>
>
>
> I’m going to go on some assumptions here that you are familiar with the
> measurement setup and process in Part 15 and ANSI C63.4:2014. If you look
> in section 8.2.4 there is a discussion on antenna beamwidth
>
>
>
> You need to keep the antennas within their 3 dB of radiation or reception
> also known as its half power beamwidth. This typically does not come into
> play from 30 MHz to 1 GHz since most if not all standard EMC antennas are
> very wideband receivers in multiple directions. Although you should look at
> the datasheets of your measurement antennas to be sure.
>
>
>
> Where this does come into play (mainly) is above 1 GHz. You will find when
> you study the datasheets of various antennas used above 1 GHz, that their
> reception gets narrow.
>
>
>
> Let’s take for example the EMCO / ETS Lindgren 3115 antenna and there are
> many other similar antennas from ARA, Com-Power, etc. that follow this
> design which ironically was only designed for usage from 1 to 12 GHz. Here
> is a copy of the 3115’s ½ power beamwidth, you will observe that it has
> good E plane reception until about 14 GHz, and then drops of significantly
> from 15.5 to 17.5 GHz to 15 degrees or so.
>
>
>
> By doing some math you can figure out at a given measurement distance (say
> 3 meters) that a 15 degree half power beamwidth will only stay in the
> antennas reception area at some given height above the floor. You have a
> product on an 80 cm table, and then you are raising and lowering the
> antenna from 1 to 4 meters. At some point up the mast you are going to get
> out of the receiving beamwidth of the antenna and technically won’t be
> measuring any signals from the product at a given distance.
>
>
>
>
>
> This is really what became termed as bore-sighting although I believe that
> language has been removed from the standards at this point, as staying
> within the cone of reception or ½ power beamwidth is a much more accurate
> description of what needs to be done. An antenna mast that supports
> bore-sighting will fulfill the requirements as it will “point” the antenna
> towards an 80 cm high turntable at a 3 meter distance so it is within its ½
> power beamwidth at all times as it travels up and down the antenna mast.
>
>
>
> Larry K. Stillings
> Compliance Worldwide, Inc.
> *Test Locally, Sell 

Re: [PSES] Antenna tilting or bore sighting for radiated emissions

[Larry K. Stillings]

Paolo,

I’m going to go on some assumptions here that you are familiar with the 
measurement setup and process in Part 15 and ANSI C63.4:2014. If you look in 
section 8.2.4 there is a discussion on antenna beamwidth

You need to keep the antennas within their 3 dB of radiation or reception also 
known as its half power beamwidth. This typically does not come into play from 
30 MHz to 1 GHz since most if not all standard EMC antennas are very wideband 
receivers in multiple directions. Although you should look at the datasheets of 
your measurement antennas to be sure.

Where this does come into play (mainly) is above 1 GHz. You will find when you 
study the datasheets of various antennas used above 1 GHz, that their reception 
gets narrow.

Let’s take for example the EMCO / ETS Lindgren 3115 antenna and there are many 
other similar antennas from ARA, Com-Power, etc. that follow this design which 
ironically was only designed for usage from 1 to 12 GHz. Here is a copy of the 
3115’s ½ power beamwidth, you will observe that it has good E plane reception 
until about 14 GHz, and then drops of significantly from 15.5 to 17.5 GHz to 15 
degrees or so.

By doing some math you can figure out at a given measurement distance (say 3 
meters) that a 15 degree half power beamwidth will only stay in the antennas 
reception area at some given height above the floor. You have a product on an 
80 cm table, and then you are raising and lowering the antenna from 1 to 4 
meters. At some point up the mast you are going to get out of the receiving 
beamwidth of the antenna and technically won’t be measuring any signals from 
the product at a given distance.

[cid:image003.jpg@01D96240.E2F5D150]

This is really what became termed as bore-sighting although I believe that 
language has been removed from the standards at this point, as staying within 
the cone of reception or ½ power beamwidth is a much more accurate description 
of what needs to be done. An antenna mast that supports bore-sighting will 
fulfill the requirements as it will “point” the antenna towards an 80 cm high 
turntable at a 3 meter distance so it is within its ½ power beamwidth at all 
times as it travels up and down the antenna mast.

Larry K. Stillings
Compliance Worldwide, Inc.
Test Locally, Sell Globally and Launch Your Products Around the World!
FCC - Wireless - Telecom - CE Marking - International Approvals - Product Safety
357 Main Street
Sandown, NH 03873
(603) 887 3903 Fax 887-6445
complianceworldwide.com
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From: Paolo Roncone
Sent: Wednesday, March 29, 2023 1:06 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Antenna tilting or boresigjting for radiated emissions

Hi all,

Is there an FCC / ANSI requirement, or  maybe just recommendation, for antenna 
tilting or boresighting wrt radiated emissions in a semi-anechoic chamber ?

If yes, can you please get me the reference section in FCC Part15 or ANSI std 
or somewhere else?


Thanks a lot in advance!
Paolo
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