5 Mistakes I Made Deploying Sierra Wireless Modules (And the 3-Step Checklist That Fixed It)

I'm a senior network integrator at a public safety solutions provider. For the past 5 years, I've been handling mission-critical broadband orders for fire departments, EMS agencies, and utility companies. I've personally made (and documented) six significant mistakes totaling roughly $30,000 in wasted budget. Now I maintain our team's deployment checklist to prevent others from repeating my errors.

This checklist is for anyone who's about to spec out a Sierra Wireless Airlink MG90 for a remote site, or integrate an EM9191 5G module into a custom IoT device. It won't make you a cellular expert overnight, but it'll catch the three mistakes that cost me the most money.

Before You Start: When This Checklist Applies

Use this checklist if you're:

• Specifying a Sierra Wireless Airlink MG90 (or MG90S) for a fixed remote asset
• Integrating an EM9191 (or EM series) module into a new device design
• Ordering replacement cellular modules for an existing deployment
• Planning a first responder broadband installation (FirstNet or similar)

Don't use it if you're just replacing a failed unit with the exact same model—that's plug-and-play. This is for new designs or migrations.

Step 1: Check the Power Budget & Voltage Drop

This step catches the single most expensive mistake I ever made.

The Sierra Wireless Airlink MG90 draws significant peak current during LTE transmission—especially on bands where its internal power amplifier runs at higher gain. The datasheet says 12V DC nominal, 10-32V range. What it doesn't scream is that if your power source dips below ~10V under load, the router will brown out and reboot.

What I did wrong:

In my first year (2018), I deployed 50 MG90s on a utility microgrid project. I used a 12V battery system with a simple linear regulator. The cable run was 25 feet of 18 AWG. Looked fine on paper. In the field, every time the MG90 transmitted at full power, the voltage at the unit dropped to 9.2V. The result: intermittent reboots, corrupted data logs, and a $12,000 truck roll to replace all the power supplies.

The fix:

1. Calculate the voltage drop for your specific cable run. Use an online voltage drop calculator—or better yet, do the math: V_drop=2 × I × L × R / 1000, where R is resistance per 1000 feet. For 18 AWG at 2A (peak MG90 draw), a 50-foot round trip gives you 0.5Ω × 2A = 1V drop. That's 11V at the unit—safe. But if you're using 24 AWG? That's 2.5Ω per 1000 feet, giving 2.5V drop. At 12V input, you're at 9.5V. Brownout city.
2. Add a DC-DC converter at the device end. A cheap automotive-grade converter can fix a 3V drop. I now spec Mean Well SD-15 series or similar—costs $15, saves $12,000.
3. Test under load. Don't trust the bench test with a lab supply. Use the actual cable and power source you'll deploy. Measure voltage at the MG90's power connector while it's transmitting on its highest-power band (check the FCC filing for exact max current per band).

Step 2: Verify Antenna & Environmental Specs

This is the one most people skip, and it's why I wasted $3,200 on a single order.

The Sierra Wireless EM9191 is a powerful 5G module, but its antenna ports are fragile. In one 2022 project, I ordered 50 custom enclosures with SMA connectors pre-wired. The EM9191 board uses MHF4 (U.FL) connectors internally. The cable from the SMA bulkhead to the module had to be a specific length and bend radius. I didn't check the insertion loss of that cable.

What happened:

We built 50 units. 47 failed radiated sensitivity testing. The cable we chose had 3dB of loss at 3.5 GHz (n78 band). Add that to the 1dB from the SMA connector, and we were below the module's minimum receive sensitivity. Every unit had to be re-cabled. $3,200 in labor plus a 3-week schedule slip.

The checklist item:

• Know your cable loss budget. For the EM9191 on 5G n78 (3.5 GHz), total loss from antenna to module should be under 1.5dB if you want decent throughput. At 2dB loss, you lose about 20% range. At 3dB, you lose 50%. I don't imagine that's acceptable for most deployments.
• Specify the cable type and length. For internal RF cables in the 3-6 GHz range, use 1.13mm coax or RG-178 for short runs under 6 inches. Don't use generic 2.0mm cable—it's not rated for 5G frequencies. I've only worked with domestic vendors, so I can't speak to how this applies to offshored designs, but the physics is the same.
• Test the antenna's passive intermodulation (PIM). This gets into RF engineering territory, which isn't my core expertise. I'd recommend consulting a specialist if you're deploying in high-interference environments.

Step 3: Confirm Firmware Version & Carrier Certification

This step is pure process, but it's the one that cost me credibility with a major customer.

The Sierra Wireless Airlink MG90 ships with a default firmware version that may not be carrier-certified for all regions. In 2023, I deployed 12 units for a county sheriff's office on FirstNet. The units worked on the test bench. In the field, they connected to LTE but couldn't register on the FirstNet core network.

The root cause:

The units had firmware v5.0.4. FirstNet required v5.1.2 or later for proper IMS registration. I'd bought them from a distributor who had older stock. My procurement team didn't check the firmware version before shipping. The fix was a field firmware update over VPN—which worked, but it took 2 days to coordinate. The sheriff's office wasn't happy. I wasn't happy either.

The checklist item:

1. Check the carrier's certified firmware list. Most carriers publish this internally. For FirstNet, you can find it via the FirstNet Authority's device certification page. For AT&T or Verizon, check their respective IOT certification portals. If you can't find a list, call the carrier's IoT support line. (This was painful to learn, but it's free.)
2. Verify firmware version at receipt. Before you inventory, plug in one unit and check: admin@MG90:~$ fw_printenv or look at the web UI under System → Firmware. Document the version on the packing slip.
3. Plan for firmware updates. Sierra Wireless releases firmware updates roughly quarterly for the Airlink series. Their support site (sierrawireless.com) has a firmware matrix. Schedule an update within 90 days of deployment. I maintain a table in our deployment spreadsheet with the current recommended version for each carrier.

Common Mistakes & Notes

I'm not an RF engineer, so I can't speak to antenna pattern optimization. What I can tell you from a deployment perspective is that verifying power delivery, cable loss, and firmware probably covers 80% of the failures I've seen.

My experience is based on about 200 orders with MG90 and EM9191 devices. If you're working with the smaller MC series modules (like the MC7455) or the older RV50, your experience might differ—those have different power profiles and antenna requirements.

Like most beginners, I assumed 'standard' meant the same thing to every vendor. Learned that lesson when we shipped those 50 units with bad cables. Now our pre-ship checklist includes a sample test of every new enclosure design. We've caught 47 potential errors using this list in the past 18 months. Not bad for a process that takes 30 minutes per deployment.

One final note: Sierra Wireless (now part of Semtech, as of 2024) publishes technical notes for its modules. Their document 'TN-0009: Power Supply Design for Cellular Modules' is worth reading. It saved me on my last project—we were about to spec a cheap regulator, and the note convinced us to spend the extra $2 per unit. Worth every penny.