My G4 transmitter was about 8 months old and all of a sudden the receiver displayed the no signal icon for more than half an hour. The distance between transmitter and receiver was less than one foot with line-of-sight. I started up my second receiver and it received the signal right away while the primary receiver continued to display the no signal icon. I noticed the exact same behavior with my previous transmitter only that I did not have a second receiver at the time. Back then I replaced the transmitter right away and went on with my life. I was disappointed that I did not get a low battery warning one week ahead as promised by Dexcom.
Here comes my conspiracy theory: When the battery in the G4 transmitter ages, the internal clock drifts. Instead of transmitting the signal exactly every 5 minutes it transmits the signal a little bit too early or too late. The receiver listens to the transmitter every 5 minutes within a small window. Due to the aging battery the signal did not hit the window. Hence the no signal icon. To prove my theory I shut down the primary receiver and restarted it right away. Voilà, the primary received the signal again.
Why did I bother to write this? If this happens to you and you don't have a new transmitter ready to go then you can limp along with occasional shutdown/restart until the new transmitter arrives.
I too have been having the "replace the transmitter due to low battery" message on my receiver lately, I am bummed since it's barely 8 months that I had my G4.
With the 7+, I have used the same transmitter for 2+ years without any issue whatsoever.
Clearly the change in radio protocol is very "expensive" in terms of power used, and this becomes an expensive nuisance for us users.
Also, what I have noticed with the G4 vs the 7+ radio protocol is the fact that the Garmin heart rate monitor band and footpod that I use when I go running interfere with the G4 radio signal, and thus the G4 simply becomes unusable when I go running. And this is *BAD*.
Back in 7+ days, there were no issues whatsoever in data exchange between the transmitter, which was much thinner than the G4, and the receiver, when I was running with the Garmin GPS+heart rate monitor and footpod.
Garmin uses ANT+ on 2.4 GHz, so I presume that Dexcom G4 is also using the same frequency, but with an incompatible signal, which gets overpowered by the ANT+ signal coming from the Garmin sensors.
Overall, I am quite disappointed by the G4, and have been using it very little compared to how much I was using the 7+, which I started to use back in 2009, when it first became available.
I also wonder if an aging transmitter could be the cause of some folks not getting as much "extended" time out of their sensors? There was a thread here a while back that some were getting many multiple weeks before (I got 10 weeks out of one.) but all of a sudden getting only a couple weeks. I too have seem similar in my G4 lately getting barely 3 weeks out of a sensor, and it's transmitter is about 8 months old now. So I'm wondering if anyone who receives a new transmitter will see longer periods of sensor use. The transmitter's batter not only has to transmit the readings it also has to do the work of pulling the data from the sensor to transmit so there could be correlation there.
As far as getting a failed communication signal though, I would think the transmitter doesn't communicate until the handset requests it to do so. I'd say you probably ran into an issue with radio interference than anything else. The air waves are a busy place for any radio signal these days.
my understanding on the radio communication protocol is that it really is a unidirectional "broadcast" from the transmitter, with its own "code" that must be programmed into the receiver, so that the correct signal coming the actual transmitter can be received and decoded.
Proof of this is the fact that a friend of mine has been using a G4 receiver together with the receiver built into his Animas Vibe pump, and things were working great. Of course he needed to calibrate both devices, but the data was coming from only one single sensor and transmitter.
Hi. I have noticed there is a drift in timing, the transmitter does not send exactly every 5 minutes, but gets about 3 - 5 seconds earlier each day compared to the dexcom receiver. The dexcom receiver clock keeps very good time, less than 3s drift in a couple of months since I reset the internal time and timezone in my receiver.
So it's feasible as you say that the receiver only wakes up once every 5 minutes, since that would save a heap of battery life. On restart it would have to stay receiving until it got a transmission, then use that to set its timer for the next one(s).
As for interference. The transmitter transmits on 4 channels, 0.5s apart, in the 2.4GHz ISM band. I'm not certain, but the receiver may need to see the first transmission in order to know when to switch to the other channels to check for a signal there (if the initial one was corrupted). This is what we're doing in the wixel code for CGM in the cloud replacement for the dexcom receiver project.
If that initial transmission is missed or obscured by the Garmin, that could account for the problem. Do you wear the dexcom receiver physically close to the transmitter?
Hi Luca. Yes, it's definitely unidirectional transmission. Our dexcom receiver even has a sticker on it saying "receive only", and the FCC authorisation also confirms this. It's a bit misleading in the Dexcom receiver menu about "starting the sensor" etc, since there's no communication back to the sensor at all.
Your issue with the Garmin has given me some ideas too about how to improve our wixel code. We could allow setting of which of the 4 frequencies to mostly wait for a signal on, so if the first one is a no-go, you could set it to listen on the next one instead for the transmission to start on.
It's possible the dexcom has multiple radio receivers in it (it has the space for it), but I think unlikely - anyone taken a receiver apart?
2.4Ghz ISM? Oh boy, that would be very open to interference especially from computer devices and some modern wireless house phones. Once a signal was missed, the timing issue might play an even larger roll in keeping the messages from working well.
And thanks for the earlier clarification, that helped a lot.
My conspiracy theory turned out to be just that. The problems persisted with the new transmitter. Eventually the receiver locked up with the message: Call Customer Support. Error: HWRF. Now I have a new receiver. There is a good chance that nothing was wrong with the transmitter that I threw away.
