Every Tuesday evening, there is a local net here in Cork on the VHF bands where a few of us get on air for a chat for about an hour. Over the course of the month, we alternate between 2m FM, 70cms FM, 4m FM and 70cms DMR.
If there is a fifth Tuesday in a month, we sometimes use it for experiments. As Tuesday the 31st of December was the fifth Tuesday in the month, a few of us tried the Rattlegram app.
Rattlegram is an app for a smartphone and it allows users to send short text messages over the radio. The beauty of the app is its simplicity. There is no need for wires or interfaces, the user just needs to hold their smartphone next to the microphone and loudspeaker of their radio.
The burst of data from the Rattlegram app only lasts for about two seconds. It's really simple to use. You just compile a short message on the app, press PTT on the radio and tell the other person you're sending a message, hold the microphone of the radio next to the smartphone and press 'TRANSMIT' on the app. The person on the receiving end gets the error free message on their app a second or two later.
Test Results... There were four of us on air for the net... Denis EI4KH, Robbie EI3GGB, Don EI8DJ and myself EI7GL. Over the course of about two hours, we successfully exchanged short text messages with the app over the air.
The tests on FM on the 145 MHz and 50 MHz bands were nearly all 100% successful. The main thing here was to make sure that the volume on the radio was turned up to a reasonable level and the phone was held near the radio.
It was interesting to see that the tests on 50 MHz SSB were also successful. The digital burst of Rattlegram is made up certain audio frequencies and on FM, these frequencies are copied exactly.
On SSB, you will never tune exactly to the other persons frequency. You can tune your radio to make the other person sound ok but if you were to relay something like music then you'd know something wasn't quite right.
With Rattlegram over SSB, the tones must be off slightly but it didn't seem to matter.
We also tried using Rattlegram with DMR via a local repeater and simplex. Both tests were a failure and it looks as if the Rattlegram audio signal is distorted to such an extent that decoding wasn't possible.
As for why DMR failed? One theory put forward was the time division multiplex nature of DMR as the signal is chopped up.
(Addendum: Brian, EI8EJB reports using Rattlegram without any issues on FM simplex but there could be issues when trying to go through a FM repeater. See comments to blog post)
The image above shows what the Rattlegram app looked like on my phone for the tests. The oldest traffic is at the bottom and the newest messages appear at the top of the screen.
The three dots at the top right allow the user to access the menu items. From there, you can select CALL SIGN to put in your call as otherwise, you will appear to others as ANONYMOUS.
To clear the screen of all the text messages, select DANGER ZONE and then Delete Messages.
The longest message that you can send seems to be around 3-4 short lines of text.
Rattlegram - What's its purpose??? ... The question that obviously arises is why would you need to Rattlegram anyway? The example often offered is that in the case of a natural disaster like a flood, storm, hurricane, wildfire, etc, mobile phone cell towers can be destroyed.
In that scenario, your mobile phone without cell coverage reverts to being a fancy camera with a calculator. Rattlegram allows the user to send short text messages over the radio until full communication is restored.
I can also see Rattlegram being used a simple tool in an scenario where I just want to pass on some basic information like say a phone number to someone.
In conclusion... The purpose of our experiment was to introduce participants of the net to the Rattlegram app and to gain experience of seeing it in use. As with the conclusion of many experiments, it raised for me more questions.
1) How well will it perform under weak signal conditions on FM or SSB? How far down into the noise will the signal go and still work?
2) Will it work on the HF bands with multipath? Will it work on a circuit with multiple hops?
I get the feeling that Rattlegram is a bit like a solution looking for a problem. It's a tool that I'm sure some resourceful operators will find a use for.
Further information... There are plenty of videos on YouTube about Rattlegram or under its older name Ribbit.
Andreas HB9BLA has a nice video below which gives a good overview of the Rattlegram app.
4 comments:
Just not my bag, just jump on the local repeater, and chat, so everyone else can join in... Same with all this DMR, C4FM, all tucked away in their own little world...
Repeater in Cork is dead as is, and without activity, would be great to hear some activity from time to time. These local nets are fine but are LOCAL.
The local VHF net in Cork has been going for almost eight years so we must be doing something right.
Hi John, we had been using Rattlegram locally up here in Co. Louth and found that while trying to use it via a repeater, it very much depends on the audio filtering used in the repeater and whether or not CTCSS was also being used on the input and output, limited success via the repeaters, but as you mention, absolutely fine on FM simplex, regardless of band. Tha is John and continued success
Thanks for the info Brian. Interesting that going through repeaters can be an issue.
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