Passive Repeater
 

Hi Bryan

It appears as though we have woken some major discussion... Someone may
even do the calculation for you!

The parabolic dish/grid "focuses" whatever power you start with. This
means more power or a higher gain antenna will mean more signal at the
far end. The result is additive. The power limitation is a legal one. I
dont remember whether it is 1W (30dBm) or 4W (36dBm) EIRP for
802.11/2.4GHz. The term EIRP refers to the power of the focused beam as
if it was the same level that would be obtained from a theoretical point
source antenna. In simple terms if you take the 4W EIRP and subtract the
antenna gain you will get the output maximum allowed at the transmitter.
In your case this would be;

36dBm(EIRP) - 24dB(antgain) = 12dBm or about 8mW

Keep in mind that you also have to allow for cable loss between the
transmitter and antenna. Numbers in the order of 6-12dB are not uncommon
so you would have to subtract this as a system loss. A 6dB loss would
bring your max transmitter power up to 18dBm or 32mW. Keep in mind that
advertising can be confusing. A bigger number on an antenna brochure
always seems better than a smaller one, but may be misleading. I dont
quite understand you statement about the grid antenna being adjustable
in power. (Also note cable loss affects the receiver too so you should
try to minimise it)

These are pretty low levels and are unlikely to upset any passing birds!
I wouldnt however sit in front of the dish for any extended length of
time. I'm just paranoid though!

re joining the antennas, yes just join the pigtails exactly as you
mentioned. Note others comments on the greater effectiveness of a
billboard reflector though. I didnt mention this thinking you wanted to
keep the visual effect small. I guess you could always disguise it as
something else though!

The higher the gain of the passive devices the larger the signal that
will be relayed. Higher gain means narrower or sharper directivity. The
problem is to work out how much you actually need without just trying it
and maybe failing. You could equally as well go for a couple of your
24dB grid antennas joined.

I should also mention that one of other major importances of a directive
antenna is to reduce multipathing or the signal bouncing off objects to
the side of the main path and getting to the antenna a little later than
the main signal. This can cause problems and slow your link down, may
even make it unusable.

Oh and speaking of which did you try bouncing the signal off the house?

I dont see the antenna mounting (separation) on both sides of the
utility pole as being a problem.

Also checking. There is a max distance parameter that will need tweaking
in a 802.11 setup. Make sure that is set to 2km or so.

Cheers Bob


Bryan Martin wrote:
The building location is maybe 500 feet away from where the repeater would
be. The parabolic grid is maybe 3000-3500 feet away from the repeater
location.

Passive Repeater
 

Hal Rosser wrote:
"(one acts like a receiver, and the other acts like a transmitter.)"

Back-to-back antennas both act like transmitters. Neither cares if its
energy came from radiation sweeping it or fed to it from its feedline.

The most energy any antenna can extract from a passing wave is 50%. That
is only when the receiving antenna is perfectly matched to its load. In
this case the load is an identical antenna, so the chances for a match
are good. But, at least 50% of the energy captured by the receiving
antenna is going to be reradiated by that antenna right back in the
direction it came from.

Another problem is the rapid decline with distance in the first
wavelength from the antenna. It losses 22 dB in field strength in the
first wavelength from the transmitting antenna. After traveling through
a second wavelength from the antenna (distance doubled from the antenna)
the field strength declines by another 6 dB. It loses 6 dB every time
the distance doubles. So, after 4 wavelengths, total loss will be 34 dB.

At great distances form the transmitting antenna, double the distance is
such a great distance that the field strength hardly varies at all even
when traveling directly toward or away from the transmitter.

It is obvious that path loss is a function of frequency and distance if
only from continuous expansion of the sphere of the electromagnetic
wavefront. There are less watts per square meter when the watts are the
same but the number of square meters is growing. Our initial loss
distance was determined by wavelength which is inversely related to the
frequency.

Best regards, Richard Harrison, KB5WZI

Passive Repeater
 

Richard Harrison wrote:
. . .
The most energy any antenna can extract from a passing wave is 50%. That
is only when the receiving antenna is perfectly matched to its load. In
this case the load is an identical antenna, so the chances for a match
are good. But, at least 50% of the energy captured by the receiving
antenna is going to be reradiated by that antenna right back in the
direction it came from.
. . .

This is a bit misleading. Typically, an antenna extracts only a tiny
fraction of the energy from a passing wave. The actual amount of energy
it does extract is expressed as its "capture area" or "effective
aperture". A recent posting I made includes a way to calculate this if
you know the antenna's gain. Because the "capture area" is usually a
tiny fraction of the wave's total area, only a tiny fraction of the
wave's energy is extracted.

Roy Lewallen, W7EL

Passive Repeater
 

An example of what Richard is saying is found when listening to a
none-too-strong FM broadcast station: if a big airplane flies into the path
near either end of the path, one will notice significant vector addition of
signals (heard as flutter).

Billboard (reflector) type passive reflectors have several applications.
One is to place the reflector on a high edge of a ridge that is not
accessible part of the year (or that does not have electric power) so as to
facilitate propagation along a valley.

"Passive Repeater" is used in about three ways: (1) large reflector;
(2) back to back connected, gain antennas; and (3) back to back connected
antennas with filter and gain blocks in the connection (sometimes,
bi-directional).

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Richard Harrison" wrote in message
...
Bryan Martin wrote:
"Was this supposed to help or was it just a slam?"

No, my response was factual, neumeric, and specific. It was not meant as
a slam. Reflectors are less lossy than back-to-back antennas. If you are
using microwaves, the size of the reflector may be practical.

The peflector in the path is most effective when placed near either end
of the path. The reflector can also produce gain over that produced by
the antennas at the path ends. It must be slightly concave to do so.
Commercial reflectors often come equipped with an adjustment to suck in
the center of the reflector for maximum gain once alignment of the
dishes is accomplished. This isn`t speculation. I`ve designed the paths
and performed the adjustments with my own hands.

Best regatds, Richard Harrison, KB5WZI