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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

The post Where is the RF Digital Multimeter? appeared first on The KØNR Radio Site.

On Saturday, I had the privilege of talking to a group of radio amateurs on the topic of electronic measurements. I opened the session with a short discussion of “why do we even need electronic measurements?” This was captured in three “laws” listed below:

Bob’s First Law of Electronic Measurement

With electricity, most of the time we cannot observe what is going on without measuring instruments.

Bob’s Second Law of Electronic Measurement

When we can observe electricity directly, it is often a bad thing.

Bob’s Third Law of Electronic Measurement

Lord Kelvin was right

The post The Three Laws of Electronic Measurement appeared first on The KØNR Radio Site.

The Rocky Mountain Ham Radio group offers an educational series they call RMHAM University. I am honored to be able to present on the topic of Practical Ham Radio Measurements on Saturday morning Dec 15th.

As many of you know, this topic is an intersection of my hobby of amateur radio and 40 years working in the electronic test and measurement industry. I’ve written two books on the electronic measurement and enjoy talking about it.

Here’s the agenda for the class.

Location and Registration

The post RM Ham University – Test and Measurement appeared first on The KØNR Radio Site.

The nature of a technical hobby gives way to the ‘buying of stuff’. Sometimes this is the tool required to carry out the crux of the hobby, in our case I’m referring to the transceiver. Sometimes this is the tool required to check that the main tool is working correctly, in our case this could any number of tools such as an antenna analyser.

There are also many hams that like to buy the box, or series of boxes and do the minimal amount of testing to ensure safe operation and there are those that will only operate what they have built themselves. Most of us fit on that spectrum. I certainly do, it just varies on what I’m doing.

For longer than I care to admit I have used an antenna analyser that belongs to my local club. It is free to be loaned but I’ve used it more than my fair share of times. I also like to build the odd antenna. This means I measure the length and then cut the wire and check it’s various characteristics and generally cut it again until I’m happy that I have a suitable compromise. Its a really useful tool. I assume its quite accurate because it cost a lot. But do I really need it?

Separating needs from wants is not that easy, partly because what starts out as a want can quite easily become a need. I made a decision recently, I was going to buy my own analyser. But which one? Que the usual looking through specs and performance criteria, guess what happened next. I started with a small requirement for a HF analyser and ended up looking long and hard at analyser >£300. Reflecting on this it becomes easy to bump up the needs because I never really noted down what I actually wanted.

So what did I actually need? In this case I wanted to ensure that I wasn’t going to damage a transceiver and transmit spurious rubbish (as opposed to my usual cw rubbish). So I didn’t really need a tool to measure, I needed a tool to indicate. But ah ha, I still needed to cut the antenna to the right length, so I did need something to measure but did it need to be really accurate?

It turns out it didn’t. It’ll be a good idea to get a rough idea but to 3 decimal places? nope, it just isn’t that important. So I duly purchased a cheapo SARK analyser off ebay for around £30 and it’s allowed me to measure and get the right length(s) (I built a multiband end fed antenna this time) and use a cheapo end fed tuner and the analyser to get the correct swr and impedance for safe and fairly optimised operation.

The lesson learned is that if I don’t set out the requirements first then I’m going to end up spending 10 times on a product that I probably don’t need.