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
