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'[EE] Antenna recomendations for EMF detector'
2008\01\20@155308 by Peter Todd

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As part of a larger project I've been developing ways to detect mobile
devices that emit radio waves, cellphones, bluetooth, wifi etc. I've got
one design that seems to work, based on a RF log detector chip. More
info, as well as test results detecting GSM cellphones and other
wireless devices, is on my website:

http://petertodd.org/tech/emf-detector


What I don't have is an antenna.  I'd like it to be a small omni
directional pcb antenna, that'll pick up frequencies above ~800mhz. (GSM
in Canada uses a 850mhz band and a 1900mhz band) My RF log detector chip
is rated from 50mhz to 3000mhz, although I'm assuming that detecting
lower frequencies would be a bad thing as stuff like CB radio's probably
put out relatively large amounts of power. With no antenna the system
seems to work decently well, at least for GSM phones, but I'm assuming
better design would let me detect those phones from farther away.

Any suggestions on antenna design?

- --
http://petertodd.org
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2008\01\20@214029 by Sean Breheny

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Hi Peter,

I'd suggest looking up the following:

log spiral
log periodic
broadband antennas
discone antenna
heillical antenna
coil antenna

There are a general class of antennas which have no "characteristic
dimension" like a dipole would (length). Instead, they have a
"characteristic ratio", which, from a fundamental point of view, means
that you can argue that they cannot possibly prefer one frequency to
another (within some range). Some of these fit into that class (log
spiral and log periodic at least - not sure about discone).

In reality, they work by having a current distribution which expands
and contracts to the right length to resonate at whatever frequency
you are using.

I think the disadvantage is that they are larger than a single-band
antenna for any of the bands in their frequency range would be.

If you don't need very good sensitivity, then a simple ground plane
antenna set up for the lower end of your frequency range might work
very well. It will be less efficient, attenuate to varying degrees,
and exhibit more complex antenna patterns as you go up in frequency,
but is probably OK for your application. A small coil antenna could
also work well. I think you might be able to get that to maintain less
reactance change over the frequency range, at the expense of lower
radiation resistance (hence, lower efficiency).

Sean




On Jan 20, 2008 3:48 PM, Peter Todd <spam_OUTpeteTakeThisOuTspampetertodd.ca> wrote:
{Quote hidden}

> -

2008\01\20@231138 by Debbie

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> On Jan 20, 2008 3:48 PM, Peter Todd <.....peteKILLspamspam@spam@petertodd.ca> wrote:

> > As part of a larger project I've been developing ways to detect mobile
> > devices that emit radio waves, cellphones, bluetooth, wifi etc.....
> >
> > What I don't have is an ...
> >
> > Any suggestions on antenna design?

For PCB-etched antennae, maybe do a Google for fractal aerials (or antennae, in
US English). Small size, broadband and quite elegant looking.

eg.

http://en.wikipedia.org/wiki/Fractal_antenna

There's a Spanish site around - can't find it right now - that had some PDFs on
fractal aerial design.

best - Debbie.


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2008\01\21@170148 by Brooke Clarke

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Hi Peter:

The simplest antenna would be a quarter wave whip above a ground plane.  These
are great for frequency ranges that are much less than an octave.

The next simplest broad band antennas for this frequency range might be a disk
cone or inverted disk cone.  These cover a number of octaves of frequency.  One
element, the disk, is just some flat metal that doesn't need to be a disk.  The
other element is a cone with it's pointed end toward the ground plane.  The low
frequency limit is set by the cone point to base length and the high frequency
by the geometry where the cone point meets the ground plane.

A standard disk cone has the cone as the ground pointing up with a disk sitting
on top.  These are commonly used with police scanner radios or WiFi hacking.
www.ramseyelectronics.com/downloads/manuals/DA25.pdf
http://www.ramseyelectronics.com/site/default.asp?search=antenna&page=search

The inverted design often uses the metal on a PC board as the disk ground and
adds an inverted cone above it.

The Biconical is just a discone where the disk is removed and replaced by a
second cone (tip to tip), See: http://www.prc68.com/I/OE254.shtml
This one is driven from a balun transformer contained in the hub and so all the
elements are at DC ground.  A very good thing to limit noise and signals that
are below the frequency of operation.

You may want to add a transformer or balun at the input to your RF log amp to
high pass filter the input.  I.e. eliminate lower frequencies like FM
broadcast, taxi, police, pager base stations (probably the strongest signals)
etc. from getting into the amp.  Better a filter structure.

Similar to the biconical is the bow tie.  Commonly used in corner reflector TV
antennas.

More on various antennas at:
http://www.prc68.com/I/Ant.shtml

--
Have Fun,

Brooke Clarke
http://www.prc68.com/Alpha.shtml  All my web pages listed based on html name
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam

2008\01\21@180843 by Peter P.

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Peter Todd <pete <at> petertodd.ca> writes:
> Any suggestions on antenna design?

Most anechoic chamber measurements for RFI EMF etc use a NIST dipole for field
strength and a log periodic antenna for almost everything else. The problems
begin if you need to cover several bands (like 800MHz to 2.4GHz). The wideband
antennas suggested by others may work, or not. A custom made PCB antenna will
likely work best (one that resonates near 900, 1800 and 2400 MHz). Some people
on this list may be able to help with this (I can't).

Peter P.



2008\01\24@165828 by Peter Todd

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On Sun, Jan 20, 2008 at 09:39:36PM -0500, Sean Breheny wrote:
{Quote hidden}

Yup, I think discone fits in there as well, at least, if I understand
wikipedia's entry on it.

> In reality, they work by having a current distribution which expands
> and contracts to the right length to resonate at whatever frequency
> you are using.

!

Sounds like black magic to me. :)

> I think the disadvantage is that they are larger than a single-band
> antenna for any of the bands in their frequency range would be.

More difficult fabrication too. I think I can rule out the discone for
instance due to the cone shape, same goes for the log spiral antennas.
Log periodic would be doable in a pcb, but it's very directional from
what I read on it.


I couldn't find much info on coil antennas, at least for higher
frequencies. Looks like the ham world uses lots of loop antennas, for
say the 10mhz band, but I didn't find any info on using them for higher
frquencies.

> If you don't need very good sensitivity, then a simple ground plane
> antenna set up for the lower end of your frequency range might work
> very well. It will be less efficient, attenuate to varying degrees,
> and exhibit more complex antenna patterns as you go up in frequency,
> but is probably OK for your application. A small coil antenna could
> also work well. I think you might be able to get that to maintain less
> reactance change over the frequency range, at the expense of lower
> radiation resistance (hence, lower efficiency).

My current pcb isn't an ideal ground plane by any means due to it's
small size, though I did try simply attaching a length of wire to the
pcb vertically. It seemed to work as well, once I trimmed the length a
bit to about 1.5", longer and the noise floor gets higher, shorter and
there is less signal.

This seems to be a very simple solution, so I might just use it. I'll be
doing some more experimentation, as well as making up a larger board
with a bigger ground plane to test that out.

- --
http://petertodd.org
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