On Sun, 09 Jul 2006 13:33:43 -0700, in alt.internet.wireless , Jeff
Liebermann wrote:

[misc.consumers.frugal-living dropped from distribution list.]

Roy Lewallen hath wroth:

John is correct. A receiving antenna, when matched, reradiates half the
power it receives.

If a receiving antenna did absorb all the impinging power, it would be a
lot easier to make a shield or a stealth aircraft.


I'm not so sure. I couldn't find any specific references to this
effect in several books I skimmed.

I trust your judgement in such matters and you have far more expience
than me, but something seems wrong or I'm missing something. Can you
point me to any books or refernences? I just skimmed Chapter 2
(Fundamentals of Antennas) in "Antenna Engineering Handbook" by Jasik
(1961) and found no obvious mention of this effect.

Scroggie's "Foundations of Wireless", the book I cut my teeth on (and
my father before me...) mentions this in the chapter on Radiation and
Aerials where he describes the reratiation as a fact of great
importance in recieving aerial design.

I think however Rod's final remark is perhaps the most telling - if an
antenna really did absorb all the energy landing on it, there would be
highly curious side effects.
--
Mark McIntyre

Jeff Liebermann wrote:

I'm not so sure. I couldn't find any specific references to this
effect in several books I skimmed. Same with internet searches. If
true, then the concept of converting solar power in an orbital
satellite, converting it to microwaves, beaming it down to an antenna
array in the middle of the desert, and converting it back to
electricity, isn't going to work if the array re-radiates half the
power. That's going to ruin quite a few nifty science fiction stories
and innovative business plans.

I also note that the common microwave path analysis calculations don't
take re-radiation into account. For example, if I start with an EIRP
of perhaps XX dBm from a transmit antenna, -YY dB of path loss, and ZZ
dB receive antenna gain, the power delivered to the receiver (ignoring
coax losses) is calculated at (XX - YY + ZZ) dBm without any mention
of the -3dB that would need to be subtracted if half the receive power
is re-radiated from the rx antenna. It would seem that the common
formula and web forms for link calculations are -3dB off.

I trust your judgement in such matters and you have far more expience
than me, but something seems wrong or I'm missing something. Can you
point me to any books or refernences? I just skimmed Chapter 2
(Fundamentals of Antennas) in "Antenna Engineering Handbook" by Jasik
(1961) and found no obvious mention of this effect.

I could add more references to the ones already mentioned, but you
should be able to find it in most antenna texts. Look in the index under
aperture and scattering cross section. When dealing with path loss
calculations, the effective aperture is used, and this has the
reradiation already accounted for. In fact, the reradiated power has its
own descriptive unit, the scattering aperture. A good and brief
description of these can be found in Kraus' _Antennas_, p. 29ff, and
many other texts.

You're right that the antennas used to receive beamed power will catch
only half of it at best. But many, many business plans have been
developed and billions in stock sold for schemes which are much less
plausible. For starters, how about the current idea of hydrogen "fuel",
"made from water"? (For those not acquainted with the harsh reality of
thermodynamics, it takes more energy to extract hydrogen from water than
you'll get back when you burn it. Charlatans notwithstanding, there's
just flat no way around this little fact.) Then there's SDI. . .

Roy Lewallen, W7EL

On Sun, 09 Jul 2006 22:15:00 -0700, Roy Lewallen wrote
in :

You're right that the antennas used to receive beamed power will catch
only half of it at best. But many, many business plans have been
developed and billions in stock sold for schemes which are much less
plausible. For starters, how about the current idea of hydrogen "fuel",
"made from water"? (For those not acquainted with the harsh reality of
thermodynamics, it takes more energy to extract hydrogen from water than
you'll get back when you burn it. Charlatans notwithstanding, there's
just flat no way around this little fact.) Then there's SDI. . .

The point of hydrogen is a means of energy storage and transmission, not
power generation. Typically the hydrogen will be released from water by
solar or nuclear power. See:
http://en.wikipedia.org/wiki/Hydrogen_economy
http://auto.howstuffworks.com/hydrogen-economy.htm
http://www.aip.org/pt/vol-57/iss-12/p39.html

--
Best regards, FAQ for Wireless Internet: http://Wireless.wikia.com
John Navas FAQ for Wi-Fi: http://wireless.wikia.com/wiki/Wi-Fi
Wi-Fi How To: http://wireless.wikia.com/wiki/Wi-Fi_How_To
Fixes to Wi-Fi Problems: http://wireless.wikia.com/wiki/Wi-Fi_Fixes

On Sat, 08 Jul 2006 16:08:23 GMT, John - KD5YI
wrote:

Rod Speed wrote:
Geoffrey S. Mendelson wrote

Rod Speed wrote


Is one transmit and the other receive?
Or are they both transmit and receive?


They're normally both transmit and receive.


That's a shame. Here in Israel we are limited to 100mw EIRP, which severly
limits the transmit antenna. There is NO limitation on the receive antenna.


A receive antenna has no EIRP, it doesnt radiate any real power.




Actually, when properly matched, it radiates half the received power.

True, but it's such a low level as to normally be considered
insignificant. Unfortunately some take that to mean there is none.

If there were none, a Yagi antenna which used all passive elements
save for the driven element would not work. On receive that driven
element plays an active (pardon the pun) roll.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Roger

On Tue, 4 Jul 2006 21:31:06 +0000 (UTC), (Geoffrey
S. Mendelson) wrote:

Beverly Erlebacher wrote:
I'm confused about this "access point" thing.

An access point is a device with an ethernet port on one side and a wifi
port on the other. In technical terms it would be called a "bridge". It
"bridges" two seperate network segments, although in this case they use
different topologies (100Base-T and WiFi).

Is that the same as a "router"?


The usual WiFi router consists of a four port ethernet hub (LAN ports),
a seperate single ethernet port (WAN port) and a WiFi access point.
It is set up to "route" between the single (aka WAN) port and the other
two ports, the LAN and WiFi. Most of what it does for routing is NAT (network
address translation) and some sort of IP tunneling.

If you ignore the WAN port and just use the LAN ports, you have a four port
hub and an access point.


Is it as simple as buying a second router (routers are familiar to me) and
just hooking that second router to the first router by cable and that would
extend my range by the distance of the cable connecting the two routers?

Yes. Just make sure to use the LAN ports. It would be best to use different
channels. Most WiFi clients are smart enough to use the channel that is the
strongest if they have access points on more than one with the same SSID.

Make sure to use encryption. Encryption is NOT to keep your data safe,
nothing can do that. If someone is intent on accessing your network, WEP
encryption will not keep them out.

True, but the ones that use the dynamic encryption can do a reasonably
good job along with a firewall and router. The golden rule is
"Nothing is bullet proof"


What it is for is to convince the guy driving down the street looking for an
open network to send out SPAM, or "share" kiddie porn, to drive on.
Unfortunately, most users don't even change the SSID of their network, let
alone set an encryption key.


One of the locals mentioned doing a bit of "war driving" around town
just for curiosity. About 80 to 90% of the networks heard were
unencrypted AND over half of those _still_used_
the_default_name_and_PW.
I use hard wired Cat5e in a gigabit network as with the amount of
traffic wireless is just too slow even if it is full duplex.


NOTE I only receive the two amateur radio news groups.
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com


Having tuned in late, if you want to have an open network, look up PublicIP.
It's a "live cd" that runs on a PII or better (x86) computer and provides
all the functions you need to offer a secure and safe open network.

Geoff.
Roger

wrote
John - KD5YI wrote
Rod Speed wrote
Geoffrey S. Mendelson wrote
Rod Speed wrote

Is one transmit and the other receive?
Or are they both transmit and receive?

They're normally both transmit and receive.

That's a shame. Here in Israel we are limited to
100mw EIRP, which severly limits the transmit
antenna. There is NO limitation on the receive antenna.

A receive antenna has no EIRP, it doesnt radiate any real power.

Actually, when properly matched, it radiates half the received power.

True, but it's such a low level as to normally be considered insignificant.

Which is what I said in different words.

Unfortunately some take that to mean there is none.

The word REAL was used for a reason.

If there were none, a Yagi antenna which used all passive
elements save for the driven element would not work. On
receive that driven element plays an active (pardon the pun) roll.

Having fun thrashing that straw man are you ?

On Tue, 11 Jul 2006 08:34:24 +1000, "Rod Speed"
wrote in :

wrote
John - KD5YI wrote
Rod Speed wrote
Geoffrey S. Mendelson wrote
Rod Speed wrote

Is one transmit and the other receive?
Or are they both transmit and receive?

They're normally both transmit and receive.

That's a shame. Here in Israel we are limited to
100mw EIRP, which severly limits the transmit
antenna. There is NO limitation on the receive antenna.

A receive antenna has no EIRP, it doesnt radiate any real power.

Actually, when properly matched, it radiates half the received power.

True, but it's such a low level as to normally be considered insignificant.

Which is what I said in different words.

Unfortunately some take that to mean there is none.

The word REAL was used for a reason.

If there were none, a Yagi antenna which used all passive
elements save for the driven element would not work. On
receive that driven element plays an active (pardon the pun) roll.

Having fun thrashing that straw man are you ?

Take a deep breath. He was more or less on your side.
Not nice to lash out at him too (or anyone for that matter).

--
Best regards, FAQ for Wireless Internet: http://Wireless.wikia.com
John Navas FAQ for Wi-Fi: http://wireless.wikia.com/wiki/Wi-Fi
Wi-Fi How To: http://wireless.wikia.com/wiki/Wi-Fi_How_To
Fixes to Wi-Fi Problems: http://wireless.wikia.com/wiki/Wi-Fi_Fixes

wrote in message ...
On Sat, 08 Jul 2006 16:08:23 GMT, John - KD5YI
wrote:

Actually, when properly matched, it radiates half the received power.

True, but it's such a low level as to normally be considered
insignificant. Unfortunately some take that to mean there is none.

If there were none, a Yagi antenna which used all passive elements
save for the driven element would not work. On receive that driven
element plays an active (pardon the pun) roll.


Still, that's a special case.

I can choose a different special case with a different result.
For a large, high-gain aperture-type antenna such as a big horn or dish,
virtually the received energy orthogonally incident on its cross-section
will be sucked up and absorbed.

Therefore you can't really say that an antenna always radiates at least
half the received power.

Don

On Mon, 10 Jul 2006 18:24:27 -0400, wrote in
:

On Tue, 4 Jul 2006 21:31:06 +0000 (UTC), (Geoffrey
S. Mendelson) wrote:

Make sure to use encryption. Encryption is NOT to keep your data safe,
nothing can do that. If someone is intent on accessing your network, WEP
encryption will not keep them out.

True, but the ones that use the dynamic encryption can do a reasonably
good job along with a firewall and router. The golden rule is
"Nothing is bullet proof"

Not even in the same universe -- WEP is easily cracked in minutes,
little more than the Emperor's new clothes. Use WPA with a strong
passphrase for any real security.

One of the locals mentioned doing a bit of "war driving" around town
just for curiosity. About 80 to 90% of the networks heard were
unencrypted AND over half of those _still_used_
the_default_name_and_PW.

Yep ... really bad ... shame on the wireless hardware companies!

I use hard wired Cat5e in a gigabit network as with the amount of
traffic wireless is just too slow even if it is full duplex.

Wi-Fi is half duplex.

--
Best regards, FAQ for Wireless Internet: http://Wireless.wikia.com
John Navas FAQ for Wi-Fi: http://wireless.wikia.com/wiki/Wi-Fi
Wi-Fi How To: http://wireless.wikia.com/wiki/Wi-Fi_How_To
Fixes to Wi-Fi Problems: http://wireless.wikia.com/wiki/Wi-Fi_Fixes

John Navas wrote
Rod Speed wrote
wrote
John - KD5YI wrote
Rod Speed wrote
Geoffrey S. Mendelson wrote
Rod Speed wrote

Is one transmit and the other receive?
Or are they both transmit and receive?

They're normally both transmit and receive.

That's a shame. Here in Israel we are limited to
100mw EIRP, which severly limits the transmit
antenna. There is NO limitation on the receive antenna.

A receive antenna has no EIRP, it doesnt radiate any real power.

Actually, when properly matched, it radiates half the received power.

True, but it's such a low level as to normally be considered insignificant.

Which is what I said in different words.

Unfortunately some take that to mean there is none.

The word REAL was used for a reason.

If there were none, a Yagi antenna which used all passive
elements save for the driven element would not work. On
receive that driven element plays an active (pardon the pun) roll.

Having fun thrashing that straw man are you ?

Take a deep breath.

Get stuffed.

He was more or less on your side.

Duh.

Not nice to lash out at him too

You wouldnt know what a lash was if it was applied to your lard arse.

(or anyone for that matter).

You get no say what so ever on whose arse gets lashed either.