NG4RF/R 927.0250 MHz

NG4RF/R is located on Mount Oglethorpe in Pickens County, Georgia, east of Jasper. The SERA-coordinated repeater frequencies are 902.0250 MHz input and 927.0250 MHz output with DPL 073.

Hardware

The repeater hardware is a Motorola Quantar commercial repeater with a ARR preamp added on the receive. The TX power is 100w and the RX sensitivity is -125 dBm for 12 SINAD.  Hear Clear is enabled.  Non-Hear Clear radios work fine too, just sound a little different.  Here is a picture of the installed hardware:

A Scala 11 dBi vertical antenna is installed 165 ft up the tower and fed via 1-5/8 Heliax. The antenna is approximately 3300 ft ASL, more than 2000 ft above most of the metro Atlanta area!

Coverage

The repeater has excellent coverage on the north side of metro Atlanta, most of which is line-of-site to the repeater antenna.  The picture below (click to super-size) shows the view looking south from the site.  The 900 MHz antenna is on the left.  In the super-size picture, looking left to right, on the horizon you will see Sawnee Mountain, Stone Mountain (behind the tower), Atlanta skyline, Sweat Mountain, and Kennesaw Mountain.

Here is a Radio Mobile coverage prediction for North Georgia.  This prediction is for mobile station with roof-mounted antenna.  Green areas should be solid coverage, and yellow areas will be marginal (popcorn noise). Hear Clear helps with the noise in marginal areas when you are moving.  Again, click to super-size.  Of course, actual coverage is +/- your specific station's performance.   e.g., HT coverage will be less, while fixed (base) station coverage will be more.  Fortunately, we already have reports of good HT coverage, both stationary and mobile, for much of the North Side!

Noise Survey

A big challenge on 900 MHz is interference from Part 15 consumer devices. SERA normally coordinates 900 MHz repeaters with inputs in the 902.5125 MHz to 902.7250 MHz range. However, SERA will coordinate from an "additional pairs" pool with inputs in the 902.0125 MHz to 902.0750 MHz range if Part 15 device proves to be a problem.  

Suspecting noise would be a problem, I performed a noise survey of 902 - 903 MHz using KE5FX's Spectrum Surveillance software, a spectrum analyzer, and a preamp.  In the picture below, the upper, dotted line is the "max hold" line for a 10-minute capture on the actual repeater antenna.  The maximum noise at 902.150 MHz and above is approximately -90 dBm, while in the "additional pairs" range, the maximum noise is approximately -120 dBm, an improvement of 30 dB!  This was the data needed to convince SERA to coordinate from the lower frequency range!

Here is a time-lapse video over a ~ 1 minute period showing the nature of the noise in the 902 - 903 MHz range.

Credits

Many thanks to the team who helped get this system on the air:

• • Frank K4SJR

  • Zyg AF4MP

  • Pete K4PRA

  • Bill N4TIC

Their generous help over multiple weekends made it all possible!