The main thing to keep in mind with any passive repeater scheme is that
path loss isn't additive--it's much worse than that.
To illustrate: in free space, if the pathloss from point A to point B is
100dB, then the path loss from point A to point C (midway between A and B),
and *then* separately on from C to B, is 188dB! (each 1/2 path would be
6dB less, or 94dB, but then twice).
If instead of midway, you break the path into 90%/10% segments, then the
pathloss of the two segments is 99dB and 80dB, for a total of 179dB. A bit
better.
This fact leads to a couple of guidelines:
1. The passive repeater needs to be very near one end of the total
path. One reply mentioned the old AT&T Long Lines microwave relay system
as having used passive repeaters. Their use is not at all uncommon in
microwave point-to-point links. But you'll nearly always see them very
close to one end or the other of the active stations.
2. In the eventual deployment, the "loss" from breaking the path into two
segments (in the initial example, that loss would have been 88dB), less the
antenna gains involved, has to be less than the excess pathloss that is
introduced by the obstacle that is preventing the direct path from working.
An example illustrates best. Say there is a repeater with an EIRP of 1kW
(+60dBm). Off to a ridge the pathloss is 100dB, so the received
(isotropic) power on top of the ridge would be -40dBm. But down in the
valley the signal is -110dBm, too weak for an HT to fully quiet.
Let's say it's a 90/10 split in distance. Then, with no antenna gains in
the passive repeater, the signal in the valley from the passive repeater
would be -40dBm-80dB (the 10% pathloss) or -120dBm. If we had total
antenna gains on only 10dB on the ridge, we'd break even. With 2x 10dB
antennas (not too hard to do) we'd be 10dB better in the valley, or
-100dBm, which might serve an HT with a decent antenna. If you could
really go for it and have 2x 15dB antennas, you'd be up to -90dBm in the
valley--not bad at all.
Note the delta--the excess path loss from the ridge was 70dB. The loss
from breaking up the path into two segments was 80dB. So 10dB total
repeater gain breaks even, more improves the situation.
So yes, they can work--with sufficient attention to detail.
At 12:31 2005-05-13, David Jordan wrote:
>Hi Folks,
>
>Does anyone remember the old days of two to three TV channel reception?
>
>Does anyone remember when the local town geek who would go up to the top
>of the mountain and install two VHF antennas, one high gain antenna
>pointed toward the big city where the TV transmitter towers were located
>and one low gain antenna pointed downward toward the small town in the
>valley? He just connected the two antennas with twin lead. No power
>amplifiers (no electricity up there). This was popular back in the
>days...did it work?
>
>Our repeater club is thinking about trying a similar approach to expand
>the coverage area of our repeater. We've got some great locations for
>placing a passive satellite receiver relay set-up based on this concept
>described above. We've got a few ridges around us that block our primary
>repeater antenna signals.
>
>The question is does this approach really work?
>
>If yes, is there any magic to the length of coax which connects the two
>antennas?
>
>73,
>dave
>wa3gin
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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