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2.4GHz absorption by plastics

J

joseph2k

Jan 1, 1970
0
martin said:
none of my neighbours have microwave ovens, neither do I :( or maybe
:(


martin

Maybe, maybe not. Microwave absortion is very frequency specific (molecular
resonances). Microwave oven characteristics (typically 2160 Mhz) may not
translate to WiFi bands, just a few hundred Megahertz away.
 
V

Vladimir Vassilevsky

Jan 1, 1970
0
joseph2k said:
martin griffith wrote:
Maybe, maybe not. Microwave absortion is very frequency specific (molecular
resonances).

Yes, the absorption is frequency specific. However this specifics shows
up at the frequencies of tens of GHz. It is irrelevant to microwave ovens.
Microwave oven characteristics (typically 2160 Mhz) may not
translate to WiFi bands, just a few hundred Megahertz away.

Huh?
Microwave ovens have nominal frequency of 2450MHz (actually they
generate a pretty wideband splash). This is the same ISM band that used
by WiFi, Bluetooth and such.


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com
 
J

joseph2k

Jan 1, 1970
0
Vladimir said:
Yes, the absorption is frequency specific. However this specifics shows
up at the frequencies of tens of GHz. It is irrelevant to microwave ovens.


Huh?
Microwave ovens have nominal frequency of 2450MHz (actually they
generate a pretty wideband splash). This is the same ISM band that used
by WiFi, Bluetooth and such.


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Hmmm. There is no frequency data on the nameplate of my kitchen unit, so i
did some searching. And there they are at 2450 MHz right on top of 802.11g
channel 9. The 2160 came from the nameplate data of the last microwave
oven that i saw that had a frequency specified.
 
I've tried Mr Google, but I cant seem to find any tables about how bad
plastics are at absorbing RF.

For fun, I'm just about to make Jason Heckers' helix antennahttp://www.wlan.org.uk/jhecker.html

Any links?

martin

FYI, alt.internet.wireless discusses this topic often.

I prefer the biquad antenna, which you can augment with a dish. I have
a short-cut method to build this antenna. With a combination of these
photographs and this link, you should be able to figure it out.

http://martybugs.net/wireless/biquad/
http://www.lazygranch.com/images/wifi/wifi_bq_1.jpg
http://www.lazygranch.com/images/wifi/wifi_bq_2.jpg

Note you don't need to make the loop a square. Use a circle of the
same circumference.

There is a disadvantage to using the helix. It will receive both
horizontal and vertical polarization. Most sites just send in one
polarization. In busy areas, the same channel will be used in
different sites with different polarity. Now if you use the helix to
illuminate a dish, then the circular polarization is fine and perhaps
desirable. That is, you could sniff out signals without the
attenuation associated with having the wrong polarization.
 
C

Chris Jones

Jan 1, 1970
0
joseph2k said:
Hmmm. There is no frequency data on the nameplate of my kitchen unit, so
i
did some searching. And there they are at 2450 MHz right on top of
802.11g
channel 9. The 2160 came from the nameplate data of the last microwave
oven that i saw that had a frequency specified.

That's in the 2110 - 2170 range that is allocated to the downlink signal of
UMTS (3G) mobile phones in many countries. Those frequencies were sold for
a price that people found surprisingly expensive at the time.

Chris
 
M

martin griffith

Jan 1, 1970
0
On Sat, 21 Jul 2007 11:41:41 -0700, in sci.electronics.design
FYI, alt.internet.wireless discusses this topic often.

I prefer the biquad antenna, which you can augment with a dish. I have
a short-cut method to build this antenna. With a combination of these
photographs and this link, you should be able to figure it out.

http://martybugs.net/wireless/biquad/
http://www.lazygranch.com/images/wifi/wifi_bq_1.jpg
http://www.lazygranch.com/images/wifi/wifi_bq_2.jpg

Note you don't need to make the loop a square. Use a circle of the
same circumference.

There is a disadvantage to using the helix. It will receive both
horizontal and vertical polarization. Most sites just send in one
polarization. In busy areas, the same channel will be used in
different sites with different polarity. Now if you use the helix to
illuminate a dish, then the circular polarization is fine and perhaps
desirable. That is, you could sniff out signals without the
attenuation associated with having the wrong polarization.

Thanks Miso, I'll start lurking there.


martin
 
R

Rich Grise

Jan 1, 1970
0
That's in the 2110 - 2170 range that is allocated to the downlink signal of
UMTS (3G) mobile phones in many countries. Those frequencies were sold for
a price that people found surprisingly expensive at the time.

It's also plop in the middle of S-band, which is loaded with military
radars, and there's an ionospheric window there so there's a lot of
demand for uplink/downlink channels.

Cheers!
Rich
 
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