MURS VHF vs. 900 MHz modems
At first glance, the two watt transmit power limit across five VHF channels can make MURS seem appealing for long-range license-free radio systems. In fact, companies such as GoTenna initially chose MURS for their first generation product. However as a wireless expert, I first think of global scalability whether for a Fortune 100, startup or academic research. Even my academic research systems often find global deployments, so I cannot afford to settle in with geographically-limited radio legality.
The clear choice for almost all applications is to use 900 MHz modems over MURS 150 MHz modems. Contact us for a more formal analysis for your application.
MURS data bandwidth:
| Freq [MHz] | raw data rate (kbps) |
|---|---|
| 151.82 | 9600 |
| 151.88 | 9600 |
| 151.94 | 9600 |
| 154.57 | 19200 |
| 154.60 | 19200 |
At first glance, MURS seems to have a comparable data rate to the $39 one watt 868/900/920 MHz modems, and similar hardware pricing, with seemingly longer range due to 150 MHz vs. 900 MHz frequency. This is a fallacy!
MURS interference
MURS arose in part because of the decades-long abuse of these five VHF itinerant frequencies. Like the delicensing of 27 MHz CB Radio, the license-free MURS channels arose due to chronic unlicensed use. Warehouses, hotels, shopping malls, construction sites, etc. use MURS frequencies.
The critical point is that in general MURS modems are stuck on one frequency until reprogrammed. $14 walkie talkies meanwhile are clogging up the channels. Cable TV leakage is also a problem at VHF.
Many applications of wireless modems have a latency requirement–how long will an MURS channel be blocked by noise and other users? The answer comes in using 900 MHz spread-spectrum radio.
900 MHz advantages
900 MHz is also a heavily used frequency band. The key distinction is this use is spread over as much as 26 MHz (USA). The one watt transmit power allowed in the USA and numerous other countries, and tens of milliwatts in other countries allows multi-kilometer line-of-sight range, just like 150 MHz radios.
Some 900 MHz modems have a channel list where they hop upon interference, while the high power radios typically employ FHSS. Having a clear RF channel via FHSS is key to getting even low data bandwidth transmissions through reliably with low latency.