If you’re experiencing a video problem, it’s hard to know where it might be coming from. So many factors affect range: your transmitter, antenna, receiver, receiver antenna, and even receiver software. If you need to diagnose video issues quickly, you might want to reach for the ImmersionRC Power Meter V2.
Why Would I Need to Know?
- Most pilots buy a VTx and trust that the manufacturer’s power rating is correct. If you buy from a reputable manufacturer that calibrates their equipment, it’s probably close. Clone products or budget options usually don’t get this kind of attention to detail. Measuring power will tell you if the equipment is functioning properly, so you know whether to expect the range it claims or if you’ll cause problems for nearby pilots when you fly together.
- A race director runs into video issues more often than anyone else. If any pilot’s VTx is too strong or weak, there’s a good chance the race will run badly. This will slow down the pace of the day and cause annoying race re-runs. Having a tool to check VTx strength right before a race can prevent a lot of these issues.
- A race pilot with a bad VTx might be called off the line and asked to sit out for a heat, or worse. To prevent it, the pilot would want to confirm VTx power before entering the event.
If you encounter any of these situations, a power meter would be a useful tool for you.
The ImmersionRC Power Meter V2 has only a few parts. You get the meter itself with its battery pre-installed, a short SMA dipole antenna, and an SMA M/M adapter. Buried under the foam pad in the box is the typical ‘like/share’ business card and stickers that most Immersion products ship with. There’s no product manual, so you’ll have to look online for that.
The antenna allows you to take touch-free measurements by placing the meter next to the transmitter’s antenna. You can also directly attach the meter to a transmitter’s SMA connector by using the adapter. there’s no RP-SMA adapter, so if you want to test transmitter with an RP-SMA connector, you’ll have to use the antenna or buy a suitable adapter.
Build and Interface
The meter is very small, measuring just 68×26×22mm and weighing only 28g. On it are a button for power and a 4-way stick with button center. Readouts appear on a high-resolution screen that’s bright enough to work outdoors in the sunlight. Since the meter itself is small and the antenna/adapter that go with it are even smaller, we found ourselves wishing ImmersionRC provided a case to keep things together so they didn’t get lost. The meter itself looks pretty rugged and could take a fair amount of abuse rolling around in the bottom of a bag. The screen is recessed which offers it some protection. Probably the least durable piece is the exposed gold SMA connector, and I wouldn’t recommend storing it with the antenna attached. An 8-hour battery is rechargeable through a micro-USB port, and can be easily replaced by sliding open the case.
You can measure up to around 1W, after which you risk damaging the unit, and anything above 500mW should be limited to no more than 30s at a time. You can measure higher values if you use an external attenuator. If you do this, be sure to buy a quality one. Low-cost attenuators will give unreliable results. When you have one attached, you can dial in its value in the settings so the meter adjusts itself appropriately. After this point, it’s on you to calculate what the new maximum measurable power would be. (And be sure you know your local laws; transmitting at high power isn’t always legal.)
The user interface is just a little confusing. We think Immersion could have done a better job here, as there are very few modes and options that need to be navigated. While in the settings page, you have to pay attention to whether a selector dot is solid or filled to know what the buttons will do at that time. At least the number of options is so limited that you can’t really get lost, and none of the options will break the meter. Just fumbling around for another few moments should be enough to figure out how to do what you wanted. Another minor UI issue: there’s no indicator of charging status. You can check the battery level by turning the unit on, but you won’t know if it’s taking on power or not except by waiting and looking at the battery level again. This could take quite a while as the charge status is only delivered by a four-bar indicator which is so small you need good eyes to see.
Using the Power Meter
ImmersionRC provides the user manual online only. It’s a pretty good document, but we’d have liked to see just a little more depth on theory. For example, it tells you that “average” mode is appropriate for a VTx and “peak” mode is appropriate for a radio transmitter, but doesn’t explain what these items actually do or why you would want to set them this way.
The “Span”, changes the scope display granularity, but also the total frame width for measurements. This results in a faster or slower update rate across all of the display modes. We assume the actual sample rate is a constant regardless of the “Span”, but you’ll get the computed value from that entire duration (either the peak or average depending on the “Mode” setting).
Conducted vs. Radiated Measurement
There are two ways to take measurements: conducted and radiated.
A conducted measurement is taken when you attach the meter directly to a source. The included adapter connects to an SMA antenna mount for this kind of measurement. Measuring like this will give you more accurate, repeatable results that are less susceptible to outside interference and loss from the antenna. It’s definitely the more scientific way to measure. The downside of a conductive measurement is that you need to get the items physically attached. The desire to protect the antenna has seen frame mounts moving to tighter spaces which can be hard to get at. You may need to get an SMA extension cable—but that will introduce some amount of loss.
Radiated power measurement is much easier. Just stick an antenna onto the power meter’s SMA and wave it around. this picks up whatever power level is being passed through the air. You can do this if you can’t hook up the quad directly: maybe the antenna mount is RP-SMA, too hard to get to, or directly soldered. You can also use this method if time is absolutely critical, such as an organizer testing for power output compliance at a race event. There are many sources of loss and interference this way. Getting an exact measurement is basically impossible. You’ll get in the ballpark, but that’s it. Moving the meter around will get you vastly different results, so some trial-and-error is needed. Take a look at these two measurements, taken just a few millimeters apart. One suggests the power is below 25mW, the other nearly 200mW. You can assume the actual power is somewhere near the highest number you can find.
Meter Display Mode
The first screen that appears after boot gives you the amount of power the meter currently detects. There’s a large number for the power in dBm, and a small number for the power in watts. Also on screen is the battery level indicator, average vs. peak measurement mode, frequency currently being measured, duty cycle, and a bar graph.
The units on the power display are auto-ranging, meaning that it displays in microwatts unless you have it near a power source. As the detected power increases, the scale will change over to milliwatts. The effect is that you must read the unit each time to be sure how much power is detected. The unit indicator is only a few display pixels tall. I like the bar graph at the bottom with tick marks. These appear to be a lower bound at 0mW/−∞ dBm, then at 1mw/0dBm (far left), 25mW/14dBm, 100mW/20dBm, and 400mW/26dBm, and the upper bound at 1.600W/32dBm. It’s easiest to find the hot spot with a radiated power measurement when using the bar graph since you don’t have to mentally parse the values; just wave it around until you find the highest point on the graph. Graph markers are also useful as a reminder of the power limits of the device. If you go a little above the fourth third line, the meter should not be used longer than 30s at a time. If you max out the graph, you’re at risk of damaging the device and should stop immediately.
Immersion believes the dBm measurement scale makes more sense and it’s a mistake for hobbyists to think in milliwatts. They make a point of saying so in the Power Meter’s manual. But that’s not how equipment is spec’d, discussed, and regulated. This is one of those trivial ideological arguments that nobody is going to win in the end. We think a lot of users would appreciate the option to have the larger number—the one they care about most—in mW, instead of only the very small one below.
PowerScope Display Mode
The second screen is a rudimentary oscilloscope. It’s not useful for measuring VTx power, but can give you just a little insight into other kinds of radio transmissions. Here, you get power level (only in dBm) and duty cycle again. The “Span” is visible and easily changed with up/down while viewing this mode. Here, “Span” acts as a zoom so you can more closely inspect the on/off cycling of the transmission power or get a wider overview.
With this, you can check if your radio is transmitting, and at what power. Uses for this might be checking if the radio is working, seeing how long the transmission packets are, comparing one transmitter to another, or double checking after completing an antenna mod. It’s not good for figuring out what frequency something is transmitting on, and can’t capture data for detailed analysis. This is a spot-check tool for hobbyists, not something to be used for serious development.
Sniffer Display Mode
The sniffer is really just a simplified power measurement display, using relative units as compared with a reference power. You set the reference by connecting the meter to a source and clicking upward on the stick. This saves the reference as the center of the graph, and the device remembers this reference between power cycles. The numeric display is the dB variance from the reference. (It’s labeled as dBm, but should actually be dBr for relative decibels. If Immersion wants to be pedantic about using the dB scale, it’s only fair to point this out!) If the meter reads +3dB, you’re currently measuring a power level that’s twice your reference. +6dB is four times your reference, and so on. The idea here is to be able to set the reference for your required race output, say, 25mW. You can then wave the meter near to other quads and check the graph and relative level. If you can get it to read +6dB or more, the quad is probably out of spec for the race. If it never reads more than -6dB, there’s a likely transmitter issue. This mode is really designed for quick measurements of very obvious differences. The mode doesn’t even allow quick change of the measured frequency. Any variance of +/-3dB should probably just be ignored.
If you have a known-good reference nearby, it’s super easy to place the meter and tap ‘up’ to store it. But it’s also super easy to accidentally overwrite the reference point, and there’s no undo. An option to manually choose a reference power would have been welcome, so a race director wouldn’t need to carry their known-good transmitter to every race. A quick reset to 25mW as reference would probably cover the vast majority of race directors’ use cases.
The ImmersionRC Power Meter V2 doesn’t do a lot of things, and it doesn’t have a lot of fancy features or high precision. But it does something really important, and it does it well enough, and it does it far cheaper than existing commercial solutions. That makes it pretty valuable for a lot of people. It’s not a necessity, but it could tell you if your VTx is fried or out of spec. It could save you from burning up equipment after making a mistake working on it. It could help you pinpoint what to fix after a crash started causing video issues. It could save your race group a lot of time dealing with failing equipment or incorrect settings from its pilots. If that’s worth $80 to you, this is the product to do it.
If you’re looking to buy a Power Meter V2, the best deal right now seems to be the Rotor Riot edition. This version comes with additional connectors so you can directly test a VTx with u.fl or MMCX connector. As of this writing, it’s being offered at the same price.