Maximum legal gain?
www.tonez.co.uk
external antenna? I cant seem to find any details of the power output of this
device, but the normal distance is quoted as being 500m outdoors.
Thanks.
Mark
up recently, it still does. The maximum permitted gain would depend on what
type of operation you intend, point-to-point or point-to-mulitpoint.
www.tonez.co.uk
campfire when Mark interjected with:
Point to point.
Don Widders
<please.reply.to.n...@website.at.tonez.co.uk> wrote in message
news:Ef_m9.2311$Fv2.219456@wards...
easier to use dBm for the power -- that would be 12.5dBm for the 900AP.
For fixed, point-to-point operation you probably can't find an antenna with
high enough gain to put you over the limit!
The "don't cross this line ever" absolute max is 30dBm. For every 3dBi of
antenna gain over 6dBi you must reduce power (from 30 dBm) by 1 dBm. Since
your power is already reduced to 12.5 dBm, this is 30 - 17.5 and since
you'll probably lose at least a half dBm in connectors and coax, call it 18
dBm. Multiply that by 3 and you get 54dBi and you get to add the 6dBi
gratis, so if you can find a 60dBi antenna, you can use it with your 900 AP.
There is another catch, and that is to be completely legal, the antenna and
coax must have been approved WITH the 900AP. Using such low power this
shouldn't be a problem. If the FCC ever DID knock on your door and you
showed them the above calculation, at worst they would tell you to get an
antenna approved for use with the 900AP. In reality, they'll never knock on
your door and if they did, they would probably see what you're doing and
just leave you alone.
Just for fun, if you found a HUGE 60dBi parabolic antenna, your Effective
Isotropic Radiated Power would be a whopping 31.6 MEGAWATTS!!! In other
words, from a mile or two away it would take more than 30 MW into an antenna
that radiates equally in 3 dimensions to create the same field intensity as
a 60dBi antenna will create from your AP. The flipside of that is trying to
aim an antenna whose beam width is so incredibly small.
Don W.
John Roland Elliott
Lessee ...
- suppose you can get 25dB from a 3 foot dish. For 60dB you need 35dB more
than a 3 foot dish. That's about eleven doublings in power, or five and a
half multiplications by four.
- Since doubling the diameter of a dish multiplies its aperture by four,
maybe gain increases that way too.
- 2 raised to the 5.5 is about 45 and 3 times 45 is 135.
- does it take a 135 foot diameter dish to get 60dB?
How 'bout some (order of magnitude) corroboration?
- http://www.csgnetwork.com/antennaparaboliceffcalc.html says a 39 meter
(126 feet) 60 dB 2437 MHz dish corresponds to 100% efficiency.
- http://www.csgnetwork.com/antennaparaboliceffcalc.html says a 53 meter
(172 feet) 60 dB 2437 MHz dish corresponds to 55% efficiency.
- the gain formula (Eq 1, page 18-13, ARRL Antenna Book 19th edition) says
173 feet, 55% efficiency, 2437 MHz dish is 60dBi gain.
The Statue of Liberty is 151 feet tall.
Incidentally, as long as I've go the bible out and Excel open (according to
Eq 2, page 18-14, ARRL Antenna Book 19th edition), the half-power (-3dB)
beamwidth is about 0.16 degrees. At a distance of a mile, the half-power
beam would be 15 feet in diameter, if I still understand trigonometry (not a
foregone conclusion at my age).
"Don Widders" <wid...@talkwithoutdifficulty.org> wrote in message
news:fm%m9.108922$q42.3...@bin3.nnrp.aus1.giganews.com...
johnekus
antenna would be diffraction limited so the actual power concentration in
terms of watts per meter squared would never be close to the previously
calculated value.
If this kind of power concentration were possible, imagine the effect on an
airplane or bird accidentally intercepting the signal. Zorch!!! Kind of like
an ant under a magnifying glass in the sun.
Presenting power gain in terms of DB represents an exponential function. It
doesn't take a lot of DB's before you are in the realm of science fiction.
JK
http://www.crak.com home of Gulpit(tm) the wireless packet sniffer for the
masses.
"John Roland Elliott" <JohnRolandE...@m-attbi.com> wrote in message
news:VL0n9.29682$Pz.2...@rwcrnsc51.ops.asp.att.net...
Glenn Elmore
johnekus wrote:
> Of course you have to consider that the wavefront produced by such an
> antenna would be diffraction limited so the actual power concentration in
> terms of watts per meter squared would never be close to the previously
> calculated value.
>
> If this kind of power concentration were possible, imagine the effect on an
> airplane or bird accidentally intercepting the signal. Zorch!!! Kind of like
> an ant under a magnifying glass in the sun.
>
> Presenting power gain in terms of DB represents an exponential function. It
> doesn't take a lot of DB's before you are in the realm of science fiction.
However, 60 dB above 30 milliwatts is only 30 kW ERP, not 30 MW. Don W was
calculating ERP for a 90 dBi antenna.
In any case, the energy is spread out over the aperture of an antenna and
the density is smaller not larger than that from a lower gain antenna.
Gain is only the far-field value and is a comparison with the field that
an isotropic antenna would produce at the same place. Nowhere is the
actual flux level more dangerous, or even as dangerous/intense as it is
near a lowgain monopole or dipole radiator.
To cook a bird it takes real cooking power. All that the best antenna
or coupling system can do with 30 mw is deliver 30 milliwatts. As mentioned,
this is also limited by the wavelength and cannot be focussed to a point.
The practical consequence is that one should be a lot more concerned about
leaks in a microwave oven (~650 watts into a crack or slot radiator which is
relatively low gain and has small aperture) than from ANYTHING you can do with
your 802.11b device (except maybe poking yourself in the eye with the antenna or
something (:>)
Glenn Elmore
>
>>>Isotropic Radiated Power would be a whopping 31.6 MEGAWATTS!!! In other
>>>words, from a mile or two away it would take more than 30 MW into an
>>
>>>Don W.
>>>
>>>
>>
>>
>
>
Don Widders
10 years ago to Puerto Rico. I chose that place because I really wanted to
see first-hand the radio observatory at Arecibo. It's magnificent! A 1 km
dish with the most powerful transmitter in the world and the world's most
sensitive, liquid nitrogen-cooled receiver!!!
Don W.
"John Roland Elliott" <JohnRolandE...@m-attbi.com> wrote in message
news:VL0n9.29682$Pz.2...@rwcrnsc51.ops.asp.att.net...
Don Widders
news:3D9CAC32...@corridor.biz...
OK, guys, what's 3 orders of magnitude among friends? The error in my
calculation was only in units. I said 31.6 million Watts when I really
meant 31.6 million milliwatts or 31.6 kW. And yes, I do realize the energy
is spread out over the entire (pretty big) aperture.
It was a fun exercise that dramatically demonstrates what a gift the fixed
point to point rule really is. I don't expect the DLink 900AP has ever been
approved with any 60 dBi antenna because it would make no sense for
terrestrial applications. Still, this does show how (for fixed point to
point operation) one can get a lot more out of a good antenna than an
amplifier.
Don W.
Michael Erskine
Not to mention anyone in the beam for the first mile would not need a
microwave oven... :)
-m-
David Taylor
> antenna gain over 6dBi you must reduce power (from 30 dBm) by 1 dBm. Since
> your power is already reduced to 12.5 dBm, this is 30 - 17.5 and since
He's in the UK, the FCC rules don't apply.
David.
Don Widders
news:MPG.1806e0358...@news.cis.dfn.de...
Yeah, that makes sense. It's hard enough to enforce the rules within U.S.
border -- enforcing them overseas would be a MAJOR headache! :-)
Don W.
Don Widders
Yeah, that makes sense. It's hard enough to enforce the rules within U.S.
borders -- enforcing them overseas would be a MAJOR headache! :-)
Don W.
David Taylor
> border -- enforcing them overseas would be a MAJOR headache! :-)
My understanding is that the ETSI limit applying to the UK is 100mW EIRP
or 20dBm. So if the the AP is transmitting at 13dBm then I make that
7dBi antenna max gain.
I'm happy to be corrected on this.
(My links are *almost* legal!) ;-)
David.
www.tonez.co.uk
around the campfire when Don Widders interjected with:
> OK, guys, what's 3 orders of magnitude among friends? The error in my
> calculation was only in units. I said 31.6 million Watts when I
> really meant 31.6 million milliwatts or 31.6 kW. And yes, I do
> realize the energy is spread out over the entire (pretty big)
> aperture.
>
> It was a fun exercise that dramatically demonstrates what a gift the
> fixed point to point rule really is. I don't expect the DLink 900AP
> has ever been approved with any 60 dBi antenna because it would make
> no sense for terrestrial applications. Still, this does show how
> (for fixed point to point operation) one can get a lot more out of a
> good antenna than an amplifier.
>
> Don W.
I never knew it would be so complex, but I am reassured to learn that a 10dB
antenna will be OK.
David Taylor
> antenna will be OK.
By whom? As I understand it that's true if you have 3dB cable and
connector loss to stay within 20dBm EIRP.
I'd love to be told otherwise for UK regs.
David.
Glenn Elmore
>>
>>Just for fun, if you found a HUGE 60dBi parabolic antenna, your Effective
>>Isotropic Radiated Power would be a whopping 31.6 MEGAWATTS!!! In other
>>words, from a mile or two away it would take more than 30 MW into an antenna
>>that radiates equally in 3 dimensions to create the same field intensity as
>>a 60dBi antenna will create from your AP. The flipside of that is trying to
>>aim an antenna whose beam width is so incredibly small.
>>
>>Don
>
>
> Not to mention anyone in the beam for the first mile would not need a
> microwave oven... :)
>
Well, if they wanted to quickly heat their food, I'm afraid they would STILL
need a microwave oven.
A 90 dBi antenna at 2.4 GHz would be roughly one mile in diameter. While 30 mW
into it would create a *far field* intensity the same as 30 MW (megawatts)
into an isotropic antenna at the same distance,(~18 MW into a dipole), that
intensity is in the FAR FIELD of the bigger antenna. The far field distance
for an antenna is about 2*D^2/lambda, where D is the physical dimension of
the antenna and lambda is the wavelength. For an antenna of that size this
is about 24,000 miles!
An antenna doesn't reach its ability to focus until you get into the far field.
In the near field, the first mile for the 90 dBi antenna example, the 30
milliwatts is spread out over the entire antenna area of about 3/4 of a
square mile. This means that if you hold a dipole (or a coffee cup wanting
to be heated) out in front of it at about a mile distance, you see only
that portion of 3/4 of a square mile spread 30 milliwatts that the
coffee cup/dipole intercepts. This would on the order of -90 dB below 30
milliwatts or -105 dBm. You wouldn't even be able to maintain an 802.11b
link across the 1 mile without a directional antenna on the other end as
well. It isn't until you get to roughly the distance of a geostationary
satellite that the signal would appear as though it had emanated from a
30 megawatt ERP source. At that distance, the antenna would be working
effectively. With the path loss of 191 dB, the +105 dBm ERP radiator would
produce about -86 dBm at an 802.11b card input. Probably almost enough to
maintain an 11 Mbps link.
Turning things around to transmit back to earth from geostationary orbit, it
would take about 90 dB of combined satellite/earth-based 802.11 antenna gain to
get things to work. A 20 dB (~ 2') directional antenna at the ground end and a
500' antenna at the satellite. Or perhaps more practically, a 16' antenna and 30
watts at the satellite. As you may notice, this is in the general realm of
transmitter and antenna sizes for satellite TV broadcasting which uses
information rates and bandwidth comparable to 802.11b.
Directional antennas don't create energy. They only redirect energy that an
isotropic antenna would have radiated in every direction into a single or
a few directions. And they only do this in their far field.
Glenn Elmore
K Bloch
address. The maximum level allowed in the UK is significantly lower
then what is allowed in the USA. The legal limits here do not apply
anywhere else. Kind of like US law, don't go to the UK and tell them
the constitution gives you rights cause once you leave the US you are
not protected by our laws.
"Don Widders" <wid...@talkwithoutdifficulty.org> wrote in message news:<fm%m9.108922$q42.3...@bin3.nnrp.aus1.giganews.com>...
www.tonez.co.uk
enlightened us with these immortal words:
Don Widders.
David Taylor
> >
> > David.
>
> Don Widders.
Don was quoting FCC regs.
David.