FT-7900R ZUM Repeater Build - Part 3

Repeater Build Part 1 - Repeater Build Part 2 - Repeater Build Part 3

Over the last two weeks, I have been experimenting with various settings on the FT-7900R ZUM Radio Pi-Star repeater. After going through frequency calibrations, TX delay and TX/RX offset settings, I'm still having an issue with one particular thing.

The Problem

I have been unable to mitigate what seems to be a delay in the digital processing. This occurs after releasing the PTT of a transmitting radio. A short <1 second clip of the last transmission is heard coming back over the transmitting radio from the repeater. If you have ever heard a short-path long-path echo, you know what I'm trying to describe. It's more annoying than anything else, but something I'd rather not have happening, if at all possible.

Having gone through everything I could think of with the Pi-Star software settings (a short list for me), I began looking at hardware. Everything appeared to be working as it should, but then I had a light-bulb moment: "Maybe it really is a signal processing-induced delay."

Modification

The repeater is built around a Raspberry Pi 3 B+ that I once used as a desktop computer, even though it was pretty slow. When the Raspberry Pi 4 came out, I made the 3B to 4B change, and Wow! What a difference in performance! So if it made such a huge difference in a desktop environment, I wonder what it will do in a repeater controller environment. There's one way to find out.

A few minutes later, I had the Rpi 3 B+ disconnected from the ZUM Radio controller and replaced with a Raspberry Pi 4 B 2GB fitted with a full heat sink case and Argon programmable fan. I swapped out the Raspberry Pi OS card with a back-up copy of the Pi-Star repeater image and in under five minutes, the repeater was up and running.

Result

Almost any reasonable thing is worth trying when you are experimenting. For this repeater project, it has been a little of this. a little of that, old parts, spare parts, and a lot of fun. As for the Raspberry Pi 3 B+ to 4 B computer swap, I hear no perceptible difference with the original delay echo problem.

Hey, if nothing else, it looks like the blinking LED on the USB/Serial converter for the Nextion display is blinking a little faster, but that could just be me. So for now, I'll leave it with the Rpi 4 B and monitor the CPU temperature and Argon fan activation cycle frequency for a while.

New Hypothesis

Maybe the delay echo is caused by the collinear antenna setup feeding back, even running at very low power. I guess I'll find out when I get my frequency allocations from the Wisconsin Association of Repeaters and get a duplexer ordered, tuned, and installed.

Repeater Build Part 1 - Repeater Build Part 2 - Repeater Build Part 3

FT-7900R ZUM Repeater Build - Part 2

Repeater Build Part 1 - Repeater Build Part 2 - Repeater Build Part 3

October of last year I set out to make a MMDVM repeater with my Yaesu FT-7800 and FT-7900 transceivers. At the time, the setup was in simplex mode as I researched duplexers for the two frequency, one antenna system. And there the project sat in the shack just taking up space and collecting dust. With my invested time accumulating and the research folder growing thicker, I found several characteristics about my repeater build that I decided to change.

Enclosures and Heat

In the former computer case with all the components situated side by side, the homebrew repeater occupied a large footprint in the shack. Taking a second look at this, with the transceivers being the same size and short in height, stacking them greatly reduces the required footprint.

Since my components already have protective cases on them, placing everything into yet another case is unnecessary and only inhibits heat dissipation. In the stacked configuration with the receive radio on top, mounting them on a single Yaesu SMB-201 cooling fan will allow better open-air circulation as well as direct fan cooling to the bottom of the transmitting radio's heat sink.

Power

For now, I am keeping the repeater in the shack, so a second power supply is not needed. My Yaesu FP-1030A has more than enough capacity to run the repeater while also running my FT-991A, FTM-7250, and a few other accessories. If and when the time comes to move it out, I have a 23 amp power supply with an attached RIGrunner 4004U ready to go. This will not only power the two radios, but the RPi controller as well with it's two USB 5 volt ports. Handy dandy cotton candy!
Raspberry Pi Cooling

The Raspberry Pi 3B+ and ZUM Radio GPIO hat setup I originally made was satisfactory, but a passive cooling setup would be better. To fix this, I put the RPi in an aluminum heat sink case, added a programmable Argon One Artik fan hat on an extended GPIO, and placed the ZUM Radio board on top of that. I placed the Pi setup on it's side (GPIO edge down) to allow better natural airflow up the warm faces.

My three other Raspberry Pi 4B's require cooling so each have an Argon One fan, but in this 3B+ application it really isn't necessary, though it's a nice feature to have if things do get too warm. I programmed the fan to turn on at 42 degrees C at 10% speed. With this, the fan rarely activates, and then only briefly. The higher quality fan, reduced run time, and modified speed should greatly extend the life if this setup. After watching this setup for some time, I found the Pi temperature stays around 39 C with only passive cooling.

Here, the MMDVM repeater RPi is on the left of my shack's four-Pi setup. All computers are on a LAN switch to help reduce RF exposure in the shack.

Wiring Harness

One thing I don't like is having a harness that's too long or too short for whatever the project is, especially in the shack where extra wires can turn into interference-producing antennas. For this reason, I modified the harness and made two ends using two standard RJ45 jacks. The DIN cables were shortened to 16 inches and combined into a single RJ45. The repeater board connection was also joined to a single RJ45 jack. The two components now connect together with a standard computer network cable of whatever length is needed for the components' location. Perfect!

Antennas

The biggest change of plans may forgo the use of a duplexer and single antenna setup. Instead, I may go with a less expensive collinear setup using two Diamond X50 antennas mounted on separate tower standoff arms. I've been running a similar setup at 5 watts for a short time and found it works quite well. So far, I have one LMR400-fed antenna up and working on the tower arm.

Considering my location in a tall forested lakefront area widely known as a difficult corridor for RF, and having a tower only 55 feet tall, there is no point in spending a lot of money on any setup here. Besides, this is for experimentation and just having fun with RF. That's a big part of what this is all about, right?

Nextion Screen

Having all this figured out left me with a little unused creativity, so I redesigned my Nextion screen appearance and layout. Thanks to WA6HXG for the original Nextion 3.2 HMI file, I just moved a few things around, changed the fonts and background images, and called it a day. The colors in this photo are off, but you get the idea. Still on the to-do list are: (1) purchase and install the X50 antennas, and (2) receive frequency allocation from Wisconsin Association of Repeaters.

Repeater Build Part 1 - Repeater Build Part 2 - Repeater Build Part 3

FT-7900R ZUM Repeater Build - Part 1

Repeater Build Part 1 - Repeater Build Part 2 - Repeater Build Part 3

 

Here is a project involving a ZUM Radio duplex MMDVM and a Raspberry Pi 3 B+ combined with my trusty old Yaesu FT-7900 and FT-7800 radios, and a couple of mini-DIN cables. Together, a trial version of small digital all-mode repeater capable of full duplex D*Star, DMR, NXDN, P25, POCSAG, and Yaesu System Fusion should be the result. If this works out as expected, I will likely be seeking out an old GE Master II, or any other robust analog repeater, to handle whatever mode is selected.

Pins and Colors

So it's been a little while, and I finally figured it all out. Part of the problem I found was the Yaesu CT-39A mini-DIN wire color was incorrect for the pin location. So lets forget about colors and talk straight up pin number to pin number. Conduct a simple ohms test on your mini-DIN cable pins to determine what color goes to what pin (my Electronics-101 oversight...)

As stated on page 10 of the Yaesu FT-7900R user manual, the pins are identified by both number and function. Using this, combined with the pin information from the ZUM Radio, I found the following connections to work properly using one FT-7900R as a simplex digital mode repeater.

Note: Two wires on the CT-39A mini-DIN are not used: Yellow (RX1200), and second Black (shield ground).

The pins - looking at the back of the FT-7900R - are identified in the following locations. The locator tab will be at the bottom center.

 

Pin 1: top left - PKD (DATA IN)
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)


Keep in mind, this will transmit when anyone keys up on the WIRES-X, YSF, or FSC rooms you are connected to. Be sure to keep the power as low as possible for your needs, and share the air.

Now to get a repeater frequency allocation from the Wisconsin Association of Repeaters and get this working as a duplex repeater!

Pi Cooler

The Raspberry Pi ZUM repeater board gets a new case and a custom fan install to keep things nice and cool. This is a HiFiBerry box, which is tall enough to have a GPIO hat and a fan inside. I cut a 1.125" in the top and mounted a Pi fan inside, blowing directly down on the ZUM Radio hat and Raspberry Pi. Temps stay under 40 degrees C.

Repeater Case

There it was, down in the basement. A lonely old Windows computer that has seen it's better days. Perfect! I gutted all the hardware out, saved everything that was still good, and prepared the case to be converted into my new ZUM repeater box.

I was able to rewire the existing power supply to reduce the number of wires clumped together for the previous occupant of this case. Now I have 17 amps of 12 volts, 18 amps of 5 volts, and 11 amps of 3.3 volts available, all regulated and fan-cooled. This works out perfect for all my needs.

Getting everything positioned in the case is pretty easy, so long as the fans have coordinated air movement over the radios and through the case.

I'll need to fabricate a new front face plate to incorporate the two radio faces, a Nextion touch screen, the voltage and temp monitor, and the power switches. The Raspberry can be controlled via SSH, but a re-program of the Nextion screen can provide the shutdown capabilities I am looking for. It should work well and look pretty nice when it's all done.

Propagation Estimation

I found a tool online to help estimate propagation from the repeater's site at Radio Mobile Online. The parameters I used for the estimate are a 6 db vertical antenna at 55 feet on 146 MHz. This, of course, only gives a general idea of coverage. The actual specs will be used as soon as I get the frequency allocation from the Wisconsin Association of Repeaters.

This map is at 5 watts output. Since the goal is to serve the surrounding communities, I would like to keep the repeater at 5 watts to prolong the life of the FT-7900R driving the repeater, but a real-world test today around the area had the 5 watts barely making 5 miles; at 6 miles there was almost no squelch break on the Yaesu FTM-400XDR in the car. I was always told to expect one mile per watt on the ground, so I'm not really surprised. My Diamond X300 antenna, at 55 feet, is basically on the ground, here in the woods.



For comparison, I can hit the WE9COM repeater in Plymouth from my QTH using my FT-70D and FT-2D on 5 watts. The RFinder app lists this repeater as 14.1 miles from my QTH. This is 2.82 miles per watt. Here is an example of antenna height making all the difference.

Below is a map I made with a 5 mile, 10 mile, and 15 mile radius. If one watt per mile on the ground holds true, this map should be better aligned with real-world expectations and performance.

Repeater Build Part 1 - Repeater Build Part 2 - Repeater Build Part 3