Way Down Low. 27MHz: An Unexpectedly Useful Radio-control Band Allocation
When considering wireless systems, there is an increasing tendency in recent years to think in terms of the highly visible, frequently encountered high UHF to low microwave bands (800MHz to 2.5 or 5.8GHz) where mobile phones and domestic and commercial WiFi and Bluetooth networks are found. Even moving into the industrial low power radio niches we are more likely to think of defined network implementations like BLE or Zigbee (or one of their other imitators) operating in the same bands with multi-megabit per second data speeds (or hundreds of kilobytes at least) and enough range to cover a typical house … or at least a large room.
Even the simplest “wireless” devices (things you hardly consider to be radios: things like your car’s keyfob or your garage door opener) use allocations that start around 300MHz and run up to beyond 900MHz (and sometimes end up in that busy 2.4GHz band again).
The thing is, a lot of simple “remote control” functions (the garage doors mentioned above, but also a whole host of other simple but very useful things, like light control, access gates and doors, irrigation pumps, and water level/flood detection to randomly guess a few) only need a tens of bytes to be sent at most and aren’t especially time-critical into the bargain. These tasks don’t need highly complex networks, and they certainly don’t need megabits per second data rates … but they often (especially in outdoor or agricultural applications) need a fair bit of range. At least line-of-sight, if not more.
Go online and look for products offering this sort of “push the button, switch the lamp” functionality and you will find no shortage of them … but they are still usually stuck in the UHF bands. What I am suggesting for these applications is something much lower frequency: 27MHz
Yes. 27MHz: An HF allocation, usually thought of as the sole province of the trucker’s CB radios, or maybe your children’s toy airplane command link.
At first inspection, 27MHz seems like an odd, archaic band. With a wavelength of around eleven meters (!), the antennas would seem to be prohibitively large, while the allocated spectrum is, by the standards of modern “digital” communication, a tiny handful of 10KHz wide channels. But now think again. We all know already that simple control tasks require very little data, so a 1-2kbit/sec maximum data rate isn’t any handicap, while (borrowing again from CB usage) a very good inductively loaded whip can be reduced to only a couple of feet long, while the helical “rubber duck” aerials used on handheld “walkie talkie” type sets (while less efficient) are only five or six inches in length.
The circuitry for 27MHz radios is proportionally simpler, and (from an engineer’s perspective) easily yields higher performance. It also helps that there are no mediocre single-chip products to be had for the HF bands, which makes for better modules designs in the market. If you buy a 27MHz receiver (like the Lemos LMR0-27) then you KNOW it will have decent selectivity and spurious rejection characteristics.
The laws of physics have something to say about 27MHz too.
Path loss (and consequently the amount of transmitted power you’ll need to achieve a given range) is proportional to frequency, to the tune of 10 x log (1/F ^2) in relative dB terms so a lower frequency radio link gets “more miles per watt” than a GHz band one will.
If you chew through the maths (and I have so you don’t need to) you find that a low end 10mW 27MHz link (assuming good and equal performing aerials) offers impressive performance
|
Frequency |
10 x log (1/f^2) |
Relative dB |
Equivalent power for same path loss |
|
27MHz |
-28.6 |
0 |
10mW (10dBm) |
|
151MHz |
-43.6 |
-15 |
300mW (25dBm) |
|
446MHz |
-53.0 |
-24.4 |
2.7 watts (34.4dBm) |
|
928MHz |
-59.4 |
30.8 |
12 watts (40.8dBm) |
|
2.4GHz |
-67.6 |
-39 |
79 watts (49dBm) |
Now, in reality, these figures aren’t quite so rosy. 27MHz aerials are (in the small-sized versions) quite inefficient, and penetration of buildings at the lower frequencies can be inferior, but even a 10mW 27MHz system can provide surprisingly good range performance at low power levels, while the 4 watt US types (see below) have ranges you’d measure in miles
Finally, we come to the regulations associated with 27MHz. It is often said that the 2.45GHz band is the only “all nations” allocation … but if you look for low power 10mW 27MHz bands you will also find them in practically every region on earth. Actual regulations vary (the FCC part 90/95 documents and the European 300-220 ones don’t look especially similar) but a properly designed radio should meet both with very little trouble
In the USA however, things are even better.
Assuming that your application can be kept within the definitions of a “radio control” system (a one-way point to point link where an input at one end causes an effect at the other: in other words a simple “push the button, ring the bell” type job) then the fascinating FCC part 95.701 (and on) special case is open to you, and this has a couple of very valuable allowances.
§95.731 Permissible RCRS Use
RCRS transmitters may only be used to transmit one-way communications and only for the purposes set forth in this section. (One-way communications are transmissions which are not intended to establish communications with another station.) (3) RCRS channels in the 26-28 MHz frequency band may be used to control or operate any kind of device. (b) Telecommand. Any RCRS channel may be used by the operator to turn on and/or off a device at a remote location. (c) Telemetry. Any RCRS channel in the 26-28 MHz frequency band may be used to transmit a signal from a sensor at a remote location that turns on and/or off an indicating device for the operator.
- The allowable transmitted power (on any of he six stipulated spot frequencies: 26.995, 27.045, 27.095, 27.145, 27.195 and 27.255 MHz) is MUCH greater .than 10mW. These regulations allow a solid 4 watts, and on 27.255MHz that can be raised to 25 watts (!!!)
- The testing and certification requirements are greatly relaxed, which makes putting a 27MHz product into the US market (legally) a great deal less expensive than (for example) a MURS band VHF design. In fact, a competent RF engineer (with suitable test equipment to confirm performance) would be able to design, build and sell such a product with no further certification.
§95.735 RCRS Equipment Certification Exception
Notwithstanding the general requirement of §95.335, a non-certified RCRS transmitter that transmits only in the 26-28 MHz band and complies with the applicable technical requirements in this subpart may be operated in the RCRS for the purpose of controlling a remote device.
Actually implementing a 27MHz control link is relatively easy. The CB marketplace provides a wide variety of antennas at low (commercial/domestic) prices while either radio modules (to be connected to encoder/decoder equipment) or complete control products are sold by a number of suppliers.
It is well worth considering this not especially well understood or widely used band when looking at remote control applications, as it offers significant cost and range/performance advantages over more industry-mainstream solutions
Useful Links
The relevant part of the 47 CFR part 95 regs for 27MHz RCS operation is here:
Note that there has been a recent update, and the old 47 CF 95.201 is now at 95.701. The rules remain materially identical, but the text is a lot clearer and simpler