REPEATER IS TEMPORARILY UNAVAILABLE
This home-brew repeater project has been a good learning experience, and rewarding because it contributes to the hobby and to the enjoyment of other area hams. So if you have a couple of old packet-capable analog FM radios laying around, maybe you will find a project like this as rewarding as I have.
With the ZUM Radio MMDVM controller, the repeater can operate in YSF, DMR, D-Star, P25, or NXDN modes. In a near-future controller firmware and software update, analog FM will also be available. The end goal is to run YSF digital with FM analog for backup.
Components
- Yaesu FT-7800R transmitter
- Yaesu FT-7900R receiver
- Yaesu SMB-201 cooling fan
- ZUM Radio MMDVM repeater controller board
- Raspberry Pi model 3 B plus
- Geekworm heat sink armor case
- 16 GB or larger SD card
- Pi-Star MMDVM software image, run in expanded file mode
- Home-brew wire harness, two mini DIN-6 to RJ45
- Home-brew wire harness, ZUM Radio board to RJ45
- Network patch cable of desired length for connecting the two wire harnesses
- Nextion NX4024K032_011R (optional) and USB/serial converter (easier screen layout updates)
- Bridgecom Systems BCD-440U duplexer
- Diamond MX-72H duplexer (the APRS digipeater iGate shares the repeater antenna)
- LMR-400 coax
- Comet GP-6 antenna, 9.0 dB gain, 57 feet AGL
Controller
Assuming your Raspberry Pi is already encased in the heat sink, the Argon programmable fan is working, and the ZUM board is attached with a set of male/male GPIO pins, lets move on.
First, conduct a simple ohms test on your mini-DIN cable pins and wires to determine what color goes to what pin, just to be sure. In the Yaesu FT-7900R user manual, the pins are identified by both number and function. That, combined with the pin information from the ZUM Radio, the connections in the chart below will work properly for a duplex repeater and two radios, or a simplex hotspot and one radio. These are identified by TX (transmitter), RX (receiver), and TRX (transmitter/receiver)
Pin 2: top right - GND
Pin 3: middle left - PTT
Pin 4: middle right - RX9600
Pin 5: bottom left - RX1200
Pin 6: bottom right - PKS (SQL)
The Pi-Star MMDVM software image is easy to write with Etcher, then configure for your setup. Not much to explain here. Once Pi-Star is up and running, just fill out the fields with your data and click Apply Changes. Open a SSH terminal window to add the Argon fan software, and configure it to your liking.
Software
I'm not covering the basic software setup, so if you need more information regarding this, comment below and I will add in whatever you ask for.
Frequency calibration adjustments are not needed in the Pi-Star software because no transmitter is directly tied in with the software, unlike hotspot transmitters such as the ZUMspot, and others. This is handled by the radios, and they were tuned at the factory to be FCC compliant.
One extra thing I added is a line in the "system cron" file to have the system reboot itself every Monday at 05:15. Below is a screenshot of the cron file with the added line,
15 5 * * 1 root reboot >/dev/null 2>&1
This file is accessed by the Configuration/Expert/System Cron menu selections in the Pi-Star software. It's very easy to do, but be careful doing so, as everything in the file means something, even spaces.
Using the highlighted text above, and the picture below as a reference for the cron file in Pi-Star,
- Select all the text highlighted in blue above, and COPY
- Place the cursor after the word "command" in the line "# m h dom mon dow user command"
- Hit ENTER to make a new line
- PASTE the text in, as in the picture below
- Click "Apply Changes" and it should now look like this
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| Edited Pi-Star system cron file |
Transceivers
Setting up the radios is probably the easiest part. In this case, the Yaesu FT-7800R and FT-7900R radios are set to VFO simplex with packet mode enabled at 9600 bps so it is recognized by the MMDVM as a digital signal. The mode the MMDVM will recognize and repeat is determined by the mode selected in the controller's Pi-Star software, so any of the digital modes supported by the MMDVM board can be used. The K9KMS repeater is set to YSF mode. When users' radios are in the "DN" setting, other data like DG-ID, distance, bearing, and their call sign are sent with each transmission.
Antennas
I learned a lot about antennas and radiation patterns working on this project. While waiting for frequency coordination, testing in the real world can be done using a collinear setup, on shared non-protected repeater (SNPR) frequencies, at very low power. This is done with two antennas, one for RX and one for TX, placed in the same vertical axis, separated by height as much as possible. Note, the proper use of a collinear antenna system requires far more antenna separation than what is described here.
In my case, the tower mounted Comet GP-6 antenna is temporarily used for RX only. A second temporary TX antenna is placed 6 feet off the ground, in the same vertical axis as the RX antenna (almost), separated vertically by about 30 feet. In this configuration, the antennas are most in the null of their respective radiation patterns, so interference is reduced to a minimum. Transmit power must be kept as low as possible or the system will experience significant interference. At 5 watts output, the above described setup works, but with minor noise in the transmission. At 10 watts output, however, the system experiences bigger problems. The repeater hears itself and desensitizes substantially, resulting in a signal output quieter than the 5 watt transmission.
With the repeater system complete, a Bridgecom mobile duplexer and Comet GP-6 antenna serve in balance with the rest of the repeater's components. This low budget home-brew repeater should have generally good coverage in a 12.5 mile radius. Further testing will find if this holds true, with the area's terrain and heavily wooded environment where the repeater is located.
Cooling
Using this setup in simplex or duplex mode will cause the transmitting radio to transmit far more often than normal; the duty cycle will increase significantly. Limit the transmitter power to 20 watts or less, to extend the life of the transmitter.
Even so, the transmitter will get too hot, so a good cooling system such as the Yaesu SMB-201 or similar home-brew is recommended. A low-power relay placed in the SMB-201's switch wiring, and connected with the transmitter's fan wiring as a trigger, can be used to activate the SMB-201 fan when the transmitter's fan cycles on.
That's it. Need more information? Just ask.
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| A 10 mile radius is shown here, however, coverage may vary due to the repeater's antenna height and the surrounding terrain. The repeater's output is 20 watts / ERP 26 watts / EIRP 43 watts. |
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