Building a small 2.4 GHz detector:

http://www.atvinderby.co.uk/2.4Ghz_SWR_Meter.htm

Regarding the dimensions of the wire loops:

ant: 28 mm square
reflector: 30 mm square
separation: 10mm
wi 14ga solid (1.6mm dia)

Are these dimensions center-to-center of the conductors? Or are these
dimensions the space between the conductors? With a conductor diam of 1.6mm,
this is more than 10 percent of one dimension (separation), so I think it's
important to get it right.

Also, is the reflector to be grounded or floating?

Thanks.

http://www.atvinderby.co.uk/2.4Ghz_SWR_Meter.htm

Regarding the dimensions of the wire loops:

ant: 28 mm square
reflector: 30 mm square
separation: 10mm
wi 14ga solid (1.6mm dia)

Are these dimensions center-to-center of the conductors? Or are these
dimensions the space between the conductors? With a conductor diam of 1.6mm,
this is more than 10 percent of one dimension (separation), so I think it's
important to get it right.

It's probably not actually all that important. Quads are not highly
in their dimensioning. Altering the positioning between the driven
(active) element, and the reflector, will change the antenna's pattern
(gain and front-to-back radio) slightly, and will alter the feedpoint
impedance slightly. Neither of these changes is going to be
particularly significant (and quite possibly not easy to detect) in
this situation. The fact that there's a lot of additional metal in
the neighborhood of the antenna (the BNC connector, etc.) is probably
going to alter the pattern more than the spacing different will
(compared to a theoretically perfect quad in free space).

The pattern of a two-element quad is fairly broad, and the
front-to-back ratio isn't terribly high. It's not the best design for
pinpoint aiming / location of a microwave source... but that isn't
really what this device was intended for. As a field-strength meter,
it'll work fine (and probably just about equally well) with a 10mm
separation edge-to-edge or center-to-center.

Also, is the reflector to be grounded or floating?

In a quad, it is usual for all of the elements other than the driven
one to be floating. Since the article says that the reflector is
fastened with a dollop of glue, I think it's safe to assume that it's
intended to be floating.

That detector is a slick little design - thanks for posting the link!

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

It's probably not actually all that important.
dplatt

- - -

Thanks for your comments and evaluation of the project. They are helpful to
someone not skilled in the black art of wireless :-)

I plan to modify the design a bit, eliminating all coax connectors &
adapters, using just 2 banana plugs (and supporting plastic block) as the
connection to the meter.

Is it any more efficient to use copper foil or PCB material as the reflector
rather than wire?

Thanks.

In message .net,
SparkyGuy writes
It's probably not actually all that important.
dplatt

- - -

Thanks for your comments and evaluation of the project. They are helpful to
someone not skilled in the black art of wireless :-)

I plan to modify the design a bit, eliminating all coax connectors &
adapters, using just 2 banana plugs (and supporting plastic block) as the
connection to the meter.

Is it any more efficient to use copper foil or PCB material as the reflector
rather than wire?

Thanks.


A novel bit of kit, looking at the photos it appears to be being used
with a DVM, depending on how you want to use it you may find an analogue
meter easier. It will show changes better than a DVM. How about buying
a movement and a box and building a totally dedicated unit? I'm tempted
to make something similar.
--
Bill
May God defend me from my friends; I can defend myself against my enemies.

Bill wrote:
looking at the photos it appears to be being used
with a DVM, depending on how you want to use it you may find an analogue
meter easier. It will show changes better than a DVM.

The author says he tried that and it didn't work as well:
"I tried connecting the antenna directly to a micro amp moving
coil meter, however there was very little meter deflection from
a Wireless LAN card."

Impedance too low, perhaps?

Clifford Heath.

On Mon, 08 Dec 2008 08:37:34 +1100, Clifford Heath
wrote:

Bill wrote:
looking at the photos it appears to be being used
with a DVM, depending on how you want to use it you may find an analogue
meter easier. It will show changes better than a DVM.

The author says he tried that and it didn't work as well:
"I tried connecting the antenna directly to a micro amp moving
coil meter, however there was very little meter deflection from
a Wireless LAN card."

Impedance too low, perhaps?

Clifford Heath.

Maybe, but more likely, not enough RF to do anything useful with an
un-amplified meter.

What's missing from everyone's posting is what they plan to do with
this field strength meter.

Let's play with the numbers. The average wi-fi access point belches
about +12dBm (32 mw) RF into an antenna with perhaps 2dBi gain. The
FSM has a gain of perhaps 0dB. At a distance of about 3 meters (10ft)
the path loss at 2.4GHz is about 50dB which will deliver about -30dBm
to the diode. On the most sensitive voltage range, the DVM has an
input impedance of at least 1Mohm and can be treated like an open
circuit load to the detector.

-30dBm into 50 ohms is 0.224 vrms. Transforming linearly from 50 ohm
reference to perhaps 150 ohms loop antenna impedance yields about
600mv rms. (Note: I know this is a bad guess but it's midnight and
I'm tired). Resonating the loop probably raises the impedance even
higher, thus resulting in even more rectified voltage.

The resultant DC voltage will be about the same at about 600mv DC.
However, for wi-fi use, the xmit duty cycle is quite low, sending just
beacons, when there is no traffic moving. The 1000pf cap will need to
be paralleled with a much larger cap in order to form a usable peak
detector.

So, with a high imput impedance DVM the antenna and detector is quite
efficient and can generate quite a bit of voltage. However, when the
DVM is replaced with a non-amplified meter, the relatively low
impedance of the meter shorts out the diode and signifigantly reduces
the Q of the resonant loop antenna, resulting in much lower detected
voltage.

I've built several transmitter hunt "sniffers" using almost exactly
the same circuit, except that I use a hot carrier Schottky diode for
the RF detector, and the cheapest Harbor Freight DVM (because it
always gets destroyed or lost during the hunt). For 2.4GHz, I use a
dish antenna, RF amplifier, step-up xformer (coax balun), diode
detector, and DVM.

I also made a really ugly hack that uses a satellite TV signal meter.
http://www.sadoun.com/Sat/Products/Accessories/Meters/SF95L-DSS-FTA-Satellite-Signal-meter.htm
These work from about 900 to 1800MHz with a minimum detectable meter
sensitivity of about -70dBm. They are comatose at 860 (cellular) and
2.4GHz but are great for 900 and 1900MHz (PCS) sniffing. The input
port is connected to a 900MHz or 1.2GHz antenna of any sorts. The
output goes to a 75 ohm load and a 12V battery (gel cell) through an
RF choke.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Thanks for your comments and evaluation of the project. They are helpful to
someone not skilled in the black art of wireless :-)

I plan to modify the design a bit, eliminating all coax connectors &
adapters, using just 2 banana plugs (and supporting plastic block) as the
connection to the meter.

That should certainly work!

Is it any more efficient to use copper foil or PCB material as the reflector
rather than wire?

I believe not.

PCB material will probably be less efficient - standard FR-4
fiberglass-and-epoxy has significant losses at 2.4 GHz. Also, the
presence of the FR-4 will change the effective length/diameter of the
reflector loop... you might have to change the loop diameter to
compensate.

Foil would probably be bit less efficient than wire, assuming that the
width of the foil was the same as the diameter of the wire - less
surface area for the current to flow through, and perhaps more current
crowding to the edges of the foil due to skin effect. It'd also be
significantly less rugged.

Once again, neither of these losses of efficiency is likely to be
terribly important in this application, but why run the risk?

I'd just stick with solid copper wire of a reasonable diameter/gauge,
as it'll be electrically efficient and mechanically strong and rigid,
easy to get, and inexpensive.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:

Is it any more efficient to use copper foil or PCB material as the reflector
rather than wire?

I believe not.

PCB material will probably be less efficient - standard FR-4
fiberglass-and-epoxy has significant losses at 2.4 GHz. Also, the
presence of the FR-4 will change the effective length/diameter of the
reflector loop... you might have to change the loop diameter to
compensate.


I would think so, at 2.4Ghz there is, no doubt, "only skin effect."
Although, I doubt you would notice a great change in efficiency, it
should be prove-able, at least on paper. The biquad uses a solid
reflector plate for good reason, wire is cheaper/easier to obtain. If
the foil side faces the element, I should doubt any rf gets past it to
the fiberglass-epoxy to be "lost" in efficiency.

Foil would probably be bit less efficient than wire, assuming that the
width of the foil was the same as the diameter of the wire - less
surface area for the current to flow through, and perhaps more current
crowding to the edges of the foil due to skin effect. It'd also be
significantly less rugged.


My statement above applies here ... all the wire element can "see" is
reflector, with foil.

Once again, neither of these losses of efficiency is likely to be
terribly important in this application, but why run the risk?

I'd just stick with solid copper wire of a reasonable diameter/gauge,
as it'll be electrically efficient and mechanically strong and rigid,
easy to get, and inexpensive.


Tear apart some LNA/satellite feeds to the dish, you will not see them
using wire reflectors there; I think there is good reason.

Just my opinions drawn from my readings/study, looking at commercial
equipment and hands-on ...

One thing I do totally agree with, it WILL work well with just a wire
reflector.

Regards,
JS

John Smith wrote:

...
One thing I do totally agree with, it WILL work well with just a wire
reflector.

Regards,
JS


I built this, actually, a while back. Toss in a director (what is one
more wire?); I found that to be a worthwhile mod ...

Regards,
JS

An interesting design...

Keep in mind that you are building a relative, not absolute measurement
device. ie you can calibrate (or not) it anyway you like. Any piece of
wire will have a voltage induced into it by a passing EM field!

The only real concern is that you may want to ensure that the
directivity of the quad is reasonably unidirectional. Some quad designs
have pretty bad front to back performance. This may give you erronous
results if you start seeing reflections or other TX's of the back of
your antenna. The easy way to overcome this is to calibrate the pattern
by rotating the unit in an active 2.4GHz field and noting some values.
You can then use these in later tests.

Cheers Bob

SparkyGuy wrote:

Regarding the dimensions of the wire loops: