Where is the RF Digital Multimeter?

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 Three Laws of Electronic Measurement

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

RMHam University – Test and Measurement

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.

Topic Comments Time
1.       Introduction Measurement Concepts, trends in test and measurement 8:00 to 8:30
2.       Digital Multimeters Voltage, current and resistance measurements 8:30 to 9:20
Break
3.       SWR Measurement SWR, reflection coefficient, SWR measurements, antenna analyzers, vector network analyzers 9:30 to 10:20
Break
4.       Oscilloscope measurements Time domain, bandwidth, scope probes 10:30 to 11:00
5.       RF Measurements Frequency domain, spectrum analyzers, SDR receiver, transceiver tests, power measurement 11:00 to 11:30
Discussion and wrap up 11:30 to noon

Location and Registration

Cherry Creek School District Educational Services Center
4700 S. Yosemite St.
Greenwood Village, CO 80111
Please register with the RMHAM web site so we know how many to plan for.  There is no charge for the class.
https://www.rmham.org/wordpress/rmham-university-2018/
Here are my slides in pdf format:
Practical Amateur Radio Measurements

If you need the original powerpoint slides, send me an email.

73 Bob K0NR

This Spewed Out of the Internet #25

0511-0701-3118-0930Here’s another update of interesting important stuff spewing forth from the internet.

I put my two presentations from HamCon Colorado out on the web:  Practical Amateur Radio Measurements and Mountaintop VHF in the Colorado High Country . Also, check out Kelly N0VD’s blog posting on the event.

Having trouble finding a repeater to use on VHF? Check out my Shack Talk article on HamRadioSchool.com

KB5WIA provides some good tips on EME operating.

Hans PD0AC addresses the question: What’s the Best Chinese Dual-band HT? For best price/performance, he selected Baofeng UV-B5/UV-B6 (and I agree).

The Noise Blankers continue to publish their Ham Hijinks. Remember: Do Not Take These Guys Seriously. Seriously. Do not do this. Seriously.

There’s lots of great ham radio events coming up this summer. This weekend is the CQ Worldwide VHF Contest, the only “true VHF contest” out there since only the 50 MHz and 144 MHz bands are used. Then there’s the Colorado 14er Event, which includes Summits On The Air (SOTA) activations, on August 4th. (Don’t forget to check out the great new Colorado 14er Shirts!)  The Colorado QSO Party is another great operating event, on August 31st.

Remember: There is no such thing as ground.

Think about it: an infinitely large electrical node with zero impedance able to sink an infinite current. Not likely.

73, Bob K0NR

The Mysterious Case of Alternator Noise

When I purchased my 2003 Ford Escape, I decided to install multiple ham radios and a bunch of antennas. Mostly I use a Yaesu FT-8900 FM transceiver for operating on the 2-Meter and 70-cm ham bands. A while back, I started getting reports that I had alternator whine on my transmit audio. I was perplexed because I thought I had done a pretty darn good job of installing the radio, including connecting heavy 12V power cables directly to the battery. (See K0BG’s web page for more information on battery connections.) I really wasn’t sure if this was a day one problem (and no one ever told me about the crummy audio) or something that had just started. My first course of action was to ignore it and see if it goes away. This strategy failed miserably as my FCC-licensed spouse continued to report that I was “whining”. Finally, I decided to put my alleged knowledge of electricity to work. I got out my trusty oscilloscope and took a look at the voltage near the transceiver. There was about 800 mV of ripple on the DC voltage, as shown below.

Escape alternator noise 12V at radio - higher revs

The frequency of the ripple was in the audio range, consistent with alternator whine. The frequency of the ripple increased when I rev’d the car engine, so it was clearly coming from the alternator. I was surprised to find that the size of the ripple did not depend much on whether I was transmitting or not. The transmit current is much higher than the receive current, so I expected the ripple to be worse on transmit.

Then I decided to measure the ripple voltage right at the battery, which is shown below. The peak-to-peak ripple is smaller (about 400 mV) than at the radio but still present. I expected the the voltage to be mostly clean right at the battery.

Escape alternator noise 12V at radio - at battery

I pondered what to do next. One approach would be to install a filter to eliminate the ripple. However, filtering out a few hundred Hz signal while maintaining a low voltage drop on the 12V power feed is not trivial. More importantly, I had the sense that the Escape’s electrical system was just not operating properly. I decided to take it to my local mechanic, who tested the alternator and determined that a diode had failed. He replaced the alternator for me and the whine is now gone.

I did measure the 12 volt supply with the new alternator installed and the radio transmitting. I was surprised to find that there is still some ripple, a bit less than 200 mV (shown below). Apparently, this is not enough to disturb the FT-8900 signal.

Escape alternator noise 12V - new alternator 2

So that’s the story about my alternator whine.

My spouse says “I still whine sometimes” but it has nothing to do with my ham transceiver.

    – 73, Bob K0NR

Six Completely Useless Nerd Apps for the iPhone

One of the main uses of an iPhone is to load completely useless apps that you can use to annoy your friends. As a service to the nerd techie community, I have reviewed all available most some of the free (or at least cheap) apps available for the iPhone platform and selected the top six useless but fun apps.

Here they are:

FreqCounter (Author: Tang Peng Lab, Cost: Free) A clever little app that displays the time domain waveform of the phone’s microphone audio, displays the sound level and calculates the frequency of the waveform. This is my favorite of the list.

Sound Check Tone Generator (Author: Joe Voelker, Cost: Free) A simple app that generates audio tones (just sine waves).

Tone Generator Pro (Author: Performance Audio, Cost: $0.99) This is a tone generator that can create sine waves, square waves, triangle waves and sawtooth waveforms. Also does white and pink noise and frequency sweeps.

Nerd Noise (Author: Brian Shumate, Cost: $1.99) This app produces some really important, but of course, useless sounds: computer modem tones, Morse code, RTTY, MFSK, telephone busy signal, dial tone, etc. This is a MUST HAVE for the true techie.

Piano Free (Author: Better Day Wireless, Cost: Free) One of those piano keyboard apps.

Cowbell (Author: CTS MobileSoft, Cost: Free) Makes a cowbell sound. Because you always need “more cowbell”!

Just to be clear, some of these apps might actually be useful. Or maybe not. Your mileage may vary.

– 73, Bob K0NR

Added 6 May 2011:

At no extra charge, I decided to add a 7th app:

SpectraSound (Author: Francois Guilleme, Cost: $1.99) This app shows the time domain waveform from the microphone plus the FFT spectrum of the signal. This is a fun way to look at the frequency content of an audio signal.

SWR and Return Loss – There’s an Ap for That

For the RF/microwave enthusiasts out there, you may recall the old HP Reflectometer Calculator that does conversions between SWR, Return Loss and calculated mismatch uncertainty.

Well, fast forward to the 21st century and this same functionality can be found in an iPhone ap. Agilent Technologies has a free ap available on iTunes that does these calculations and some other things like decibel conversions.

Check it out at here or just search for microwave calculator on iTunes.

– 73, Bob K0NR

Agilent FieldFox RF Analyzer

Photo © Agilent Technologies 2009 All Rights Reserved

I am a bit of a test equipment junkie….it has something to do with working in the electronic measurement business for most of my career. There are many great pieces of test equipment out there but every once in a while, one comes along that really captures the imagination. Over the weekend, I got my hands on the new FieldFox RF Analyzer from Agilent Technologies. This analyzer is a combination 2-port network analyzer, cable tester and spectrum analyzer in one compact package. Add in an external sensor and it measures RF power, too. [Disclosure: I am employed by Agilent.]

With a base price of $7600, this instrument is probably out of the price range of most radio amateurs. However, the RF engineers and technicians out there in the electronics industry will appreciate its measurement capability and value.

The first thing I did was connect it up to my vertical antenna used for 2 Meters and 70 centimeters. I was able to check a few things on the antenna system and monitor some signals. Funny thing, when I tuned to the 2 Meter ham band, I didn’t see any transmitters on the air <sigh>.  So I switched over to the FM broadcast band and did a scan of 86 to 110 MHz. The spectral lines you see sticking up are the FM broadcast transmitters in my area.

Then I checked the VSWR of the antenna system (as seen looking through the end of the cable).  The sweep below shows the VSWR of the antenna system versus frequency. The center of screen is 146 MHz and the marker is set at 146.52 MHz. The VSWR doesn’t quite stay under 2.0 over the entire band.

Then I switched to the Cable Tester mode and displayed Distance to Fault (DTF). The DTF display shows the return loss of the cable as a function of the distance along the cable. (The FieldFox analyzer can correct for the propagation velocity but I did not have this feature turned on. So the distance shown is in error by that amount.)

The bump in the middle of the display is about 60 feet down the line, which corresponds to where an inline surge supressor is installed. Apparently, there is a small “impedance bump” in the line at that point. At the right hand side of the display, around 110 feet is an impedance change due to the antenna. If I had a good 50 ohm load on the end of the cable, we would not see this blip. The DTF measurement is a broadband measurement so anything that is not a good 50 ohms across all frequencyes (such as a high Q antenna) shows up as an impedance blip.

These results are not bad but I expected the impedance of the antenna system to be better than this. If I can hang onto the analyzer for a few more days, I’ll be sure to investigate the antenna system more carefully. Nothing like having the right test equipment to make useful and accurate measurements.

73, Bob K0NR

Model 400 Mini Volt Voltmeter

I’ve worked in the electronic test and measurement business for over a quarter of a century, so I have a keen interest in test equipment. My friend Steve K0SRW came across an interesting “classic” voltmeter and decided that I needed it in my collection of electronic treasures. Thanks, Steve!

This measuring device is a clever design with a neon bulb used as the voltage detector. A potentiometer varies the voltage to the neon detector, with the potentiometer knob calibrated to read out in volts. You adjust the dial until the light just turns on. This meter works for both AC and DC voltages, from a range of 65 to 660 volts. Click on the pictures to take a closer look.

The original box came with the voltmeter and it says that it was manufactured by Industrial Devices, Inc. of Edgewater, NJ. The device is marked with US Patent Number 2,644,134, which I looked up one of the free online patent databases. The patent was applied for by Nathan Schnoll on July 20, 1949 and was issued on June 30, 1953. As expected, the circuit diagram uses the potentiometer as a simple voltage divider, varying the voltage to the neon bulb.

I’ve done some searching on the web but have not found anything beyond the patent document. Does anyone have any additional information on this voltmeter?

73, Bob K0NR

Ten Uses of a Digital Multimeter in Ham Radio

The handheld digital multimeter (DMM) is a basic tool for ham radio applications. Here is a list of 10 things you can do with a DMM.

1. Check the power supply voltage on the new power supply you just purchased.
2. See if your HT battery pack is fully charged.
3. Measure the current that your transceiver draws to estimate how long your emergency power system will last during a blackout.
4. Sort the bag of resistors you purchased at the swapfest.
5. Check a fuse to see if it is blown.
6. Troubleshoot your broken rig by checking the bias voltages against the service manual.
7. Figure out if the AA batteries the kids left for you are dead.
8. Verify that your coax is not shorted between the shield and center conductor.
9. Check the level of the power line voltage in the ham shack.
10. Check for good DC continuity between the ends of the TNC cable you just soldered.

Let me know your ideas to add to the list.
73, Bob K0NR