Subject: [NEXTEL-1] Passive Antennas

From: [email-masked]
Date: Thu, 04 Sep 2003 00:00:54 -0400

-------- Original Message --------
From: Eierman David-CFED01 <[email-masked]>
Subject:Passive Antennas

Several different path loss issues at play here.
Passive repeater imposes two sets of path losses (Base to passive and
passive to portable), so inbound path loss from portable to passive, passive
antenna gains & cables losses, path loss from passive to base site, and base
receive network gain/losses must be added.  

Passives work OK if the distance from the base site is very short, the
coupling loss inside the building is low (fairly open structure), and the
major path loss issue is building penetration loss (not other path
obstructions).  

Example talkback path:
+37 dBm 5 watt VHF portable
-10	antenna gain
-50	assumed coupling loss inside building
+6	passive #1 antenna gain
-2	cable loss
+6	passive #2 antenna loss
-97	10 mile free space path loss
+6	base receive antenna gain
-3	base receive line/filter loss
==============================
-107	dBm signal level into base receiver

Note that mounting the outdoor passive antenna at roof level (let's say 30
ft) provides 6 to 12 dB improvement in path loss due to antenna height
relative to a portable located 6 ft off the ground at same location.  This
makes the passive to base path look more like a point-to-point path (closer
to free space).

Assuming that noise floor at base receiver is below -124 dBm, then for -107
dBm level, there is 17 dB of margin above receiver noise level, sufficient
for a faded path.  I would consider this level about 50% reliable at worst
case 50 dB coupling loss inside building.  If there is an ambient noise
floor, the margin is reduced.  

A few more dB increase in passive antenna gain would make significant
improvement.  

In this particular case it looks like it is worth the effort to try it.  

Regards, 

David Eierman  
Principal Staff Engineer 
Motorola, CGISS, Spectrum Strategy 
Phone:       410-712-6242 
Email:        [email-masked] 
Fax:            410-712-6208 
Cellular:       443-421-0659 

At 09:49 PM 9/2/2003 -0700, you wrote:
>I have used passive repeaters in numerous situations where metal 
>buildings (i.e., hangars) effectively shielded the users from the 
>repeater sites.  In most cases I have used beams on the roof, a short 
>run of LMR-400 and an inverted 1/4 wave omni inside the building 
>("upward radiation angle facing down).  This arrangement has worked 
>well for me on high band, UHF and 900 MHz as long a the repeater is 
>close enough for the system losses to be tolerable.
>
>The best way to learn if it will work for you is to try it!  The 
>experiment is not particularly expensive or time consuming to install.
>
>
>Wayne Maynard,  WB6BFN
>Communications & Frequency Management
>Lockheed Martin Aeronautics
>home email:  <[email-masked]>

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