ISM bands and frequencies: Comparisons and case studies

May 2, 2024

The industrial, scientific, and medical (ISM) radio frequency bands find common use in electronics systems, by virtue of their comparatively lightly regulated nature versus (for example) spectrum swaths used by cellular, satellite, and terrestrial radio and television networks. As Wikipedia explains:

The ISM radio bands are portions of the radio spectrum reserved internationally for industrial, scientific, and medical (ISM) purposes, excluding applications in telecommunications. Examples of applications for the use of radio frequency (RF) energy in these bands include RF heating, microwave ovens, and medical diathermy machines. The powerful emissions of these devices can create electromagnetic interference and disrupt radio communication using the same frequency, so these devices are limited to certain bands of frequencies. In general, communications equipment operating in ISM bands must tolerate any interference generated by ISM applications, and users have no regulatory protection from ISM device operation in these bands.

Despite the intent of the original allocations, in recent years the fastest-growing use of these bands has been for short-range, low-power wireless communications systems, since these bands are often approved for such devices, which can be used without a government license, as would otherwise be required for transmitters; ISM frequencies are often chosen for this purpose as they already must tolerate interference issues. Cordless phones, Bluetooth devices, near-field communication (NFC) devices, garage door openers, baby monitors, and wireless computer networks (Wi-Fi) may all use the ISM frequencies, although these low-power transmitters are not considered to be ISM devices.

FCC certification of such products is still necessary, of course, to ensure that a given device doesn’t stray beyond a given ISM band’s lower and upper frequency boundaries, for example, or exceed broadcast power limits. That said, reiterating my first-paragraph point, the key appeal of ISM band usage lies in its no-license-required nature. Plenty of products, including those listed in the earlier Wikipedia description along with, for example, the snowblower-mangled “fob” for my Volvo’s remote keyless system that I finished dissecting two years ago, leverage one-to-multiple ISM bands; Wikipedia lists twelve total defined and regulated by the ITU, some usable worldwide, others only in certain regions.

Probably the most common (discussed, at least, if not also used) ISM bands nowadays are the so-called “2.4 GHz” (strictly speaking, it should be 2.45 GHz, reflective of the center frequency) that spans 2.4 GHz to 2.5 GHz, and “5 GHz” (an even less accurate moniker) that ranges from 5.725 GHz to 5.875 GHz. And echoing the earlier Wikipedia quote that “in recent years the fastest-growing use of these bands has been for short-range, low-power wireless communications systems”, among the most common applications of those two ISM bands nowadays are Bluetooth (2.4 GHz) and Wi-Fi (both 2.4 GHz and 5 GHz, more recently further expanding into the non-ISM “5.9 GHz” and “6 GHz” band options). This reality is reflected in the products and broader topics that I regularly showcase in my blog posts and teardowns.

However, although when you hear the words “Bluetooth” and “Wi-Fi” you might automatically think of things like:

Smartphones
Tablets
Computers and
Speakers

I’m increasingly encountering plenty of other wirelessly communicating widgets that also abide in one or both of these bands. Some of them also use Bluetooth and/or Wi-Fi, whether because they need to interact with Bluetooth- and Wi-Fi-based devices (a wireless HDMI transmitter that leverages a smartphone or tablet as its associated receiver-and-display, for example) or more generally because high-volume industry-standard chips and software tend to be cost-effective (not to mention stable, feature-rich and otherwise mature) when stacked up against proprietary alternatives. But others do take the proprietary route, even if just from a “handshake” protocol standpoint.

In the remainder of this post, I’ll showcase a few case study examples of the latter that I’ve personally acquired. Before I dive in, however, here are a few thoughts on why a manufacturer might go down either the 2.4 GHz or 5 GHz (or both) development path. Generally speaking…

2.4 GHz is, all other factors being equal:

Longer range (open-air)
Comparatively immune to (non-RF) environmental attenuation factors such as chicken wire in walls and the like, and
Lower power-consuming

but is also:

Lower-bandwidth and longer-latency, and
(For Wi-Fi uses) offers fewer non-spectrum-overlapping broadcast channel options

Unsurprisingly, 5 GHz is (simplistically, at least) the mirror image of its 2. 4 GHz ISM sibling:

Higher bandwidth (especially with modern quantization schemes) and lower latency, and
(For Wi-Fi) many more non-overlapping channels (a historical advantage that’s, however, increasingly diminished by modern protocols’ support for multichannel bonding)

but:

Shorter range
Greater attenuation by (non-RF) environmental factors, and
Higher power-consuming

Again, I’ll reiterate that these comparisons are with “all other factors being equal”. 5 GHz Wi-Fi, for example, is receiving the bulk of industry development attention nowadays versus its 2.4 GHz precursor, so the legacy power consumption differences between them are increasingly moot (if not reversed). And environmental attenuation effects can to at least some degree be counterbalanced by more exotic MIMO antenna (and associated transmitter and receiver) designs along with mesh LAN topologies. With those generalities and qualifiers (along with others of both flavors that I may have overlooked; chime in, readers) documented, let’s dive in.

Wireless multi-camera flash setups

One of last month’s teardowns was of Godox’s V1 flash unit, which supports the company’s “X” wireless communication protocol, optionally acting as either a master (for other receivers and/or flashes configured as slaves) or slave (to another transmitter or master-configured flash):

In that writeup, I also mentioned Neewer’s conceptually and cosmetically similar, albeit protocol-incompatible Z1 flash unit and its “Q” wireless scheme:

And a year back I covered now-defunct Cactus and its own unique wireless sync approach:

All three schemes are 2.4 GHz-based but proprietary in implementation. Candidly, I’m somewhat surprised, given the limited data payload seemingly required in this application, that even longer-range 900 MHz wasn’t used instead. Then again, the limitations of camera optics and artificial illumination intensity-vs-distance may “cap” the upper-end range requirement, and comparative latency might also factor into the 2.4 GHz-vs-900 MHz selection.

Wireless HDMI transmitter and receiver

Vention’s compact system, which I purchased from Amazon at the beginning of the year, has found a permanent place in my travel satchel. The Amazon product page mentions both 2.4 and 5 GHz compatibility, but I think that’s a typo: Vention’s literature documents (and promotes, versus the company-positioned inferior 2.4 GHz alternative) only 5 GHz support, and the FCC certification records (FCC ID: 2A7Z4-ADC) also only document 5 GHz capabilities. The perhaps-obvious touted 5 GHz advantages are resolution (1080p max), lip-sync latency and frame rate (60 fps), along with decent range; up to 131 feet (40 m), but only “in interference-free environments”, with a further qualifier that “range is reduced to 32FT/10M when transmitting through walls or floors.” Regardless, since this is a “closed loop” (potentially multiple) transmitter to receiver setup, Wi-Fi compatibility isn’t necessary.

Wireless video-capture monitoring systems

Accsoon and Zhiyun’s approaches to wirelessly connecting a camera’s external video output to a remote monitor, which I previously covered back in July of last year, are conceptually similar but notably vary in implementation. The two Accsoon “mainstream” units I own are designed to solely stream to a remote smartphone or tablet and are therefore 2.4 GHz Wi-Fi-based, generating a Wi-Fi Direct-like beacon to which the mobile device connects. That said, Accsoon also sells a series of CineEye “Pro” models that come as transmitter-plus-dedicated receiver sets and support both 2.4 GHz and 5 GHz transmission capabilities.

Zhiyun’s TransMount gear is intended to be used with the company’s line of gimbals, and like Accsoon’s hardware you can also “tune into” a transmitter directly from a smartphone or tablet using a company-developed Android or iOS app. That said, Zhiyun also sells a dedicated receiver to which you can connect a standalone HDMI field monitor. And for peak potential image quality (at a range tradeoff), everything runs only on 5 GHz Wi-Fi.

Wireless lavalier microphone sets

I got the Aikela set from Amazon last spring, and the Hollyland system (the Lark 150, to be exact) off eBay a month earlier. Both, as you have probably already discerned from the photos, are two-transmitter (max)/single-receiver setups. The Hollyland is the more professional-featured of the two, among other things supporting both built-in and external-tethered mics for the transmitters; that said, the Aikela receiver has integrated analog and both digital Lightning and USB-C output options…which is why I own both setups. They’re both 2.4 GHz-based and leverage proprietary communication schemes. Newer wireless lav models, such as DJI’s Mic 2, can also direct-transmit audio to a smartphone, tablet or other receiver over Bluetooth.

Joyo wireless XLR transmitter/receiver combo

I picked up two sets of these from Amazon last summer. As the image hopefully communicates effectively, they aren’t full-blown microphone setups per se; instead, they take the place of an XLR cable, with the transmitter mated to the XLR output of a microphone (or other audio-generating device) and the receiver connected to the mixing board, etc. The big surprise here, at least to me, is that unlike the previous 2.4 GHz mic sets, these are 5 GHz-based.

Clearly, as the earlier microphone-set examples exemplify, audio doesn’t represent a particularly large data payload, and any lip sync loss due to latency will be minimal at worst (and can be further time sync-corrected in post-production; that is, if you’re not live-streaming).

Perhaps the developer was assuming that multiple sets of these would be in simultaneous use by a band, for vocals and/or instruments, and wanted plenty of spectrum to play with (each transmitter/receiver combo is uniquely configurable to one of four possible channels). And/or perhaps the goal was to avoid interference with other 2.4 GHz broadcasters (such as a microwave oven backstage). All at a potential broadcast range tradeoff versus 2.4 GHz, of course.

Wireless guitar systems

I got the Amazon Basics setup last summer, and the Leapture RT10 (also from Amazon) last fall. Why both, especially considering the voluminous dust currently collecting on my guitars? The on-sale prices, only ~$30 in both cases, were hard to resist. I figured I could just do a teardown on at least one of them. And hope springs eternal that I’ll eventually blow the dust off my guitars. Both are 2.4 GHz-based; the Leapture setup also offers Bluetooth streaming support.

CPAP (continuous positive airway pressure) machine

Last, but not least, and breaking the to-this-point consistent cadence of multimedia-tailored case studies, there’s my Philips Respironics DreamStation Auto CPAP (living at altitude can have some unique accompanying health challenges). Every morning, I download the previous night’s captured sleep data to my iPad over Bluetooth. Bluetooth Low Energy (LE), to be exact, for reasons that aren’t even remotely clear to me. The machine is AC-powered, after all, not battery-operated. And that the DreamStation doesn’t use conventional Bluetooth connectivity only acts as a potential further complication to initial pairing and ongoing communication. Then again, I suppose Bluetooth connectivity is among the least of Philips’ challenges right now…

Connect with me, wired or wirelessly

As always, I welcome your thoughts on anything I’ve written here, and/or any additional case studies you’d like to share, in the comments!

—Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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