[TowerTalk] Passive Repeater Antenna

Terry Conboy n6ry at arrl.net
Fri Jun 30 11:54:35 EDT 2006

At 07:56 AM 2006-06-30, WA3GIN wrote:
>Looking for comments from those that have used passive repeater 
>antenna systems. Our local repeater has excellent coverage in the 
>DC/NOVA area but the area is rimmed by ridges to the west which 
>shadow large areas of NOVA.  We thought we'd take a page from the 
>cellular folks and install a passive repeater antenna at the ridge 
>with 5 ele pointed back at the repeater antenna and a 2 or 4 element 
>beam pointing toward the weak reception area.
>We're looking for recommendations as to the gain of the antenna 
>pointing to the weak signal area.  Our thinking is low gain fat 
>pattern rather than higher gain and less coverage.


You need to so a simple analysis of the free space losses for the two 
paths (repeater-to-passive, passive-to-mobile).  When you add those 
losses, and offset them with the antenna gain, it's very likely that 
you will find that the signal is under the thermal noise floor.

Free space loss at 2m is 80 dB for one mile, then another 6 dB for 
each doubling of distance.
See http://www.comsearch.com/microwave/tools_fsl.jsp for a nice free 
space loss calculator.)

I worked in cellular for 15 years and I never saw a passive repeater 
(other than that silly thing that Larsen sold for personal 
vehicles).  All of the repeaters we used were active, with lots of 
gain, and it was frequently hard to keep them from oscillating!  For 
example, here's an ADC unit for 800 MHz, 85 dB gain and a 50 watt 
amp: http://www.adc.com/Library/Literature/100591.pdf

Microwave systems sometimes use billboard passive repeaters, but they 
have very high effective gain to overcome the free space loss of the 
added path, which is usually fairly short.

73, Terry N6RY

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