146.985 K4OGB Repeater - Albemarle NC
Topographic map with marker at exact tower location around 670 feet AMSL
PL tone: 100.0 Hz used when necessary. Repeater also transmits this tone.
Override the PL restriction temporarily by pressing * 8 5 on your keypad
Equipment: GE Mastr Exec II converted mobile, approximately 35 watts out of the power amplifier
Built By W4DEX and N4DBM
Antenna: Decibel Products DB-224 four element dipole array 150-160 MHz model
Antenna Position: 140 feet above ground on south-east side of tower
Feedline: Andrews 7/8 inch Heliax with approximately 0.8 dB loss
Controller: Micro Computer Concepts RC-1000
Duplexer: Wacom four cavity band-pass band-reject
Power Supply: Astron RS-12M
Emergency Power: Generator provided by the county
Repeater TX power out of duplexer: ~ 25 watts
System ERP (reference 1/2 wave dipole) is 85.1 watts
Receive Sensitivity: 0.5 microvolts for 20 dB SINAD through duplexer. No desense from TX
Lightning Protection: Polyphaser N-type bulkheads mounted on copper ground buss bar common with
AC power primary, tower ground, telephone line ground, and building ground. MOV protection on AC line.
THE 146.985 REPEATER is located near the center of Stanly county in Albemarle, NC. The callsign is
K4OGB which belongs to the Stanly County Amateur Radio Club.
The repeater has been located at several previous sites, but Stanly Community College seems to
work the best for covering the county as it is centrally located at a good site. Thanks to the Stanly County
911 director for permission to place the repeater on the county communications tower.
Shown below is the under-side of the repeater which contains the squelch-audio-system board, the RC-1000
controller board, tone board, and thermostats under the heat-sink.
Shown below is the top side of the repeater which contains the RF sections. At the power amplifier, there are
several transistors which makes a total power rating of 110 watts (as the manufacturer suggests) However, for
continuous duty, it is dangerous to run this kind of power even with fans. You may do one of two things. First,
you may crank the power down to the 40 or 50 watt level and be fairly safe. But the transmitter draws 13 amps
at this level and still gets pretty warm.. What I did was bypass the last two transistors by removing a trace
connector and running the actual coax to the duplexer straight into the collector of the 40 watt transistor through
a DC isolation capacitor. I also added a variable resistor to the base of the power control transistor so I could
vary the collector voltage manually and reduce transmit power accordingly. I run the power amp now at around 35
watts and it only draws about 6 amps. Now that's much more efficient!
...and the base-plate mounted to the 19 inch rack panel with 12 volt fans to cool the power amp..
Shown below is the building and tower base..
At the time, we have the repeater sitting on top of a cabinet which contains digital relay equipment for the county.
In the near future, all of this equipment will be placed in a large cabinet provided by county.
The top half of the tower and antennas..
Looking down from our antenna mount..
THE ANTENNA PATTERN is obviously going to be affected by the tower, especially since the tower is
approximately 4 feet wide at the antenna level. On the plot below, note that the red circle is 6 dB of antenna
gain. Each dotted green line represents 5 dB increments. To the south-east, there is obviously a few dB more
gain than to the north-west. However, the certain spacing of the antenna from the tower eliminates deep nulls
in any direction.
THE VIEW of down-town Albemarle from our antenna mount on the tower. Look closely and you will see key land markers such as First Baptist
Church and the courthouse. On the right side is the water tank at Eastgate. Of course, the reason the repeater
doesn't work so well to the due east is in the background, Morrow Mountain and two other hilltops.
A full (304 KB) picture of this can be downloaded HERE
