One of the most useful electronic test tools available to us is the digital multimeter (DMM). There are many different models to choose from but for less than $50, you can get a decent quality meter that measures DC and low-frequency AC voltage, current, and resistance. Using such a meter, with auto-range capability, is pretty much a matter of selecting the desired measurement function and connecting up the test leads. It is a really simple, but effective measurement device.
So where is the radio frequency (RF) equivalent of the DMM? There are some excellent multifunction RF/microwave instruments such as the Keysight FieldFox analyzer. But these professional instruments are much more expensive than a DMM and much more complicated. I am thinking of something that has the same Select-and-Connect usability of a DMM.
Low-Cost RF Instruments
In recent years, low-cost RF instruments have emerged that can make some impressive measurements. The first one that comes to mind is the nanoVNA, a compact vector network analyzer. There are several different models available but a typical configuration covers two-port measurements from 10 kHz to 1.5 GHz, at a price of around $70. The nanoVNA is quite capable, able to measure two-port s-parameters (reflection and transmission), return loss, standing wave ratio (SWR), etc.
More recently introduced, the tinySA spectrum analyzer offers basic spectrum analyzer measurements from 0.1MHz to 960 MHz. The cost is ~$60 and the SA includes a basic signal generator feature.
I own both of these devices and I think they are excellent instruments for the price. But they aren’t what I am looking for in an RF DMM. They do provide a proof point that simple and affordable RF instruments are possible.
Another device that has caught my attention is the Surecom SW-33 SWR / Power meter. This tiny meter is great for tossing into my SOTA backpack or radio Go Kit, to have some basic RF measuring capability in the field. It covers 125-525 MHz, up to 100 watts (with an appropriate dummy load or antenna), for less than $50.
But it only measures SWR and power. Not bad but not quite everything I’d like in my RF DMM.
Antenna analyzers are another category of affordable RF test equipment. As the name implies, they are focused on making measurements on antenna systems. Again, there are many different models to choose from, ranging in price from about $100 to $500. Shown below is the RigExpert Stick 230-K with a bit of a simple DMM look to it. The primary antenna measurement is SWR, but the antenna analyzers often include complex impedance, return loss, reflection coefficient, etc.
The RF DMM
OK, Bob, what is it that you do want in an RF DMM? Good question. Thank you for asking.
The device I have in mind should cover the common RF measurements that a typical radio amateur needs to perform. In terms of the use model, consider what is needed to check out a new radio installation, from the transceiver to the antenna. I want to be able to check the transmit power, the impedance looking into the coaxial cable, the SWR (and return loss) of the antenna system across the typical ham bands (160m through 70 cm).
So here is the wish list:
- Frequency Range: 1 to 450 MHz
- RF power meter (directional, inline measurement)
- RF power meter (with internal dummy load)
- Antenna measurements (SWR, Return Loss, complex impedance, other derived values)
- Frequency counter
- Basic signal generator (produce a sine wave at a particular frequency)
- Probably an N connector for ruggedness and good match at UHF frequencies. However, an SMA connector would have the advantage of small size and might be more appropriate.
- Price: <$50
I initially left out the signal generator but the antenna measurements will generate a test signal, so having a simple signal generator is not a big stretch and can be very handy. A couple of bonus features could be the measurement of FM deviation and decoding of CTCSS frequencies. I think these can be added at minimal cost but they are a nice to have feature, not mandatory.
I included an internal dummy load for simple RF power measurements. It is really handy to be able to check power level independent of the antenna system. This raises the issue of what power level it will support and for how long. It would be great to be able to measure 100-watt transmitters for a short period of time but that may be inconsistent with a low-cost, handheld device. A 5-watt dummy load should be easy and maybe a bit more…perhaps 25 watts? Of course, external attenuators can be used for measuring higher power.
It will be tempting to include frequency sweeps of the various parameters but simplicity should be the top priority, so the RF DMM probably only measures one frequency at a time. Leave out the fancy display, analogous to how a typical DMM does not provide an oscilloscope display.
It is also tempting to include standard DMM features in this device, so you’d have one meter that covers all of the basic ham measurements. Given the availability of inexpensive DMM integrated circuits, this would not be a big stretch. This would require separate DMM inputs (banana jacks). Perhaps skip the current measurement capability and just have DC/AC voltage and resistance? But everyone already has a normal DMM, so I see these features as optional.
The price point may be aggressive but the idea is to make it cheap enough that most radio amateurs own one, or several. Keep one in your Go Kit, one at home, and one in the car (similar to a DMM).
So that is my idea for an RF DMM. What do you think?
73 Bob K0NR
Good timing Bob. I could use one of those right now as I work my way through a QRP kit build, which says a few things about my current application. I am at home on the bench and in this instance I’m working on a low power HF kit build. Reading through the list of possible measurements, the initial question for me is which of those would typically be taken in the field vs on the bench? Electing to make the measurements in the field is certainly a nice option to have, key word being option. Limiting the deployment of certain measurements should lower the initial selling cost of the product. Being able to activate them later via a firmware update for a nominal fee could be a consideration. Also, the form factor and user interface should be as simple and straight-forward as possible. The size of a typical DVOM, in my opinion, would be terrific. Given the extensive list of potential measurements, how many of those could be selected using a rotary type of dial found on a typical DVOM given what would be limited size and space for identifying particular measurements at each dial stop, or would a display of some type make more sense which could combine touch screen measurement selection as well as measured results readout? I understand moving in the direction of a touch type display creates a rabbit trail with numerous things to consider, but we’ve all seen what can be done today with small displays. Circling back around, the ability to physically see one’s way around the user interface, as well as creating an environment for customized viewing of the results (e.g. change fonts or colors or the ability to select which measurements are displayed) should be a critical part of the design of the test set.
But you didn’t ask for all of that. How about the addition of an I/O interface, USB-C or whatever the latest and greatest is.
Bottom line, I think it’s a great idea Bob. I believe the market would scoop up low cost units such as you have described. I would certainly pay 50 clams for a test set with the majority of the features you have listed, but the ability to pay only for the features I primarily use with the option to upgrade later would be even more appealing.
73
Steve – K5SJC
Hi Steve,
Great comments.
When considering this concept, I was definitely thinking field/portable use, similar to the handheld DMM. Of course, handheld DMMs get used on the bench, too, and somehow the basic feature set is adequate for the vast majority of uses. At the same time, a DMM does not replace more advanced instruments such as an oscilloscope, if you really need to view time domain waveforms. I think it is critical keep this thing simple and easy to use.
Turning on features via firmware options is a common approach on electronic test instruments. Maybe less so on low end, economy products like this concept.
A touch display is certainly one way to go and does not have to be difficult to use. But then all those bunny trails open up and the usability often suffers. I think the nanoVNA is a good example. If you understand how a VNA works and how VNA calibration works, the nanoVNA is a great little device. If all you wanted to measure was SWR at one frequency, the nanoVNA user interface gets in the way.
Another approach is to keep the basic RF DMM very simple but open up additional features when the meter is USB-connected to a computer, running additional software. You get the advantage of a big display, network connectivity, disk drive and printer connectivity.
If someone creates a product like this, I’d expect to see variations appear with some of these ideas included.
73 Bob
Great idea. I’d bet you got some wheels turning and someone, somewhere will get it built.
Maybe Bao Feng could make a cheap knock off of the venerable Bird wattmeter.