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UHF mobile receiving issue

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hakwye8518

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Sep 1, 2010
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118
Location
Town of Islip, NY
Current setup is as follows. motorola xpr 4550 450-512 band split mobile radio. Tram 3/4 NMO permanent mount with 17ft of RG58\U with a mini uhf connector. Antenna is a laird 5/8th wave 450-470 nmo antenna. Antenna is mounted 8 inches behind the panoramic sunroof, 8 inches in front of the am/fm sirius antenna, the antenna is centerline of the roof with a 760-870 5/8th wave nmo 14 inches to the right and a 450-470 5/8th wave 14 inches to the left those 2 antennas are for receive only.
My problem is when i am receiving local fire departments the rssi varies by 15-20 dbm whether actually driving or stationary. The rssi measurment is via the radio by pressing the left arrow 3 times then the right arrow 3 times. What is considered normal for the rssi to vary. and what are possible causes. I plan on shortening the cable to aprox 10ft as soon as i receive the proper crimp die. When transmitting into the local 70cm dmr repeater aprox 7 miles away the repeater receives my signal at -90 dbm and i receive the repeater at -88 to -95 dbm while I'm stationary.
 

freddaniel

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May 6, 2011
Messages
96
Location
Newport Beach, CA
With the limited information provided, let me take a shot at your concern. You seem concerned the RSSI varies 10 to 15 dB as you listen, at a 7 mile distance from the repeater. Being you are in New York state, I would first assume there are many trees in the 7 miles between you and the repeater. Signals in the 450 band are affected by foliage, and particularly damp foliage. There may also be some building or hills in between.

Lets convert everything into dBm to compare. Let's say you mobile and repeater operates at 40 watts [+46 dBm] and both receivers at 0.5 uV [-113 dbm] for 20 dB quieting. This means your "link budget" is 159 dB for satisfactory communications. If you subtract the path loss of 107 dB for a line-of-sight path, then you would still have a 52 dB fade margin. [sorry for the microwave link budget approach]

Another way to look at this is taking the 40 watt [+43 dBm] mobile and subtracting line-of-sight path loss of 107 dB, will yield a maximum signal at the repeater of -64 dBm. If your -80 dBm repeater receive signal is correct, you are losing 16 dB somewhere along the way. This could be trees, buildings, hills, etc. If the signal varies while you are setting still, then it is likely you are not receiving the signal line-of-sight and therefore the signal will vary due to reflections, refractions, polarization changes, and phase cancellation.

This is why cellular base stations use dual receivers with dual antennas, that are often times different polarization. This yields an improvement of at least 15 to 20 dB.

Anyway, get use to the signal varying when you are not line-of-sight, and sometime when you are.

Last, to determine if your other antennas are affecting your numbers, simply go to a large open parking lot and drive in a circle, with and without the other antennas, to see the difference.

Good luck
 

mmckenna

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Variations in received signal strength is normal. There can be a lot of causes. An incomplete list:
Multipath
Atmospheric conditions.
Problems at the transmit site.
Local interference
Receiver desense

Probably a lot more.
But, it's not uncommon, even when you are not moving. There is no standard about how much is acceptable because there are so many variables.
 
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