How can you measure what frequency your radio is on?

Foundations of Amateur Radio

The frequency you listen and transmit on in a modern radio is derived from a crystal master oscillator, in my case 22.625 MHz. That master frequency is multiplied and divided to determine the final frequency. To get to 2m you need to multiply by 6. To get to 70cm, multiply by 20. Similarly, to get to 40m, divide by 3.

Any slight variation of crystal frequency has an impact. 100 Hz variation in the master oscillator causes the radio to be off by 600 Hz in 2m, or 2000 Hz in 70cm. The higher you go the bigger the error.

This leaves us with two problems. If the crystal changes frequency over time, your radio wanders with that change which is especially noticeable on the higher frequencies. I've previously discussed how you can deal with the variation by correcting for temperature.

The other problem is the actual absolute frequency. If the radio is set-up for a crystal with one frequency and you replace the crystal with a different one, how do you know what frequency you're actually on? Your dial says one thing, but is that the actual frequency? How do you measure any difference?

Is a new radio the same as an old radio, does the frequency change over time?

Measurement is the act of comparing two things. Think of a ruler, wooden stick with markings on it. If the lines on the stick are not drawn in the right place, anything you measure with that stick will not match other sticks. That won't matter if you only ever use your stick to build everything, but typically you use parts supplied by someone else with their own measuring stick.

In your radio the same is true. What the actual frequency is doesn't matter until you need to compare it to the frequency of someone else. Like say, another radio station.

The first thing we need is something to compare with, a reference frequency. As it happens there are several of those around. As an example, you'll find reference broadcasts on 5 MHz, on 10 MHz, 15 MHz and 20 MHz. There are countless other frequencies where you'll find radio time signal stations. These stations broadcast on a steady frequency with a defined signal that you can use to do measurements against, even your local broadcast stations have a carrier that you can get started with.

A typical radio time signal will be an AM station with all manner of information encoded on the transmission. You can tune your radio to the station and hear a talking clock, second marks etc. Unless your radio is seriously out of whack you're unlikely to be able to notice any frequency errors.

If you tune to the same station with side-band you'll hear some artefacts, but essentially you'll hear nothing. However, if you tune slightly off frequency, you'll hear a tone. This tone is the central carrier frequency and it's very accurate.

At this point you can do many things. I'll cover one of them.

I'll explain this with 10 MHz.

If you set your radio to Upper Side Band and tune to 9.999 MHz on your radio, you should hear a 1 kHz tone. Similarly if you set your radio to Lower Side Band and tune to 10.001 MHz you'll also hear a 1 kHz tone.

In essence you're listening to the carrier as a 1 kHz audio tone.

You can swap between the two frequencies, by setting one on VFO-A and the other on VFO-B and switching between them with the A/B switch on your radio. If the tone changes, your radio is off frequency. How much off frequency is determined by the difference between the two tones. By lowering both frequencies by the same amount, or raising both by the same amount, one of the tones will go up while the other one goes down and vice versa. Once you've got both the tones the same, write down both frequencies. Split the difference and you'll know what frequency your radio thinks 10 MHz is on.

You'll need a radio with both Upper and Lower Side-Band and the ability to switch between two frequencies and before you get started, you need to make sure that your radio doesn't have any frequency changing stuff turned on, RIT, Clarifier, Offset, whatever it's called on your radio. All of them need to be off.

There are countless other ways of doing this, a procedure called zero-beating with a signal strength meter, using a tone and listening for a wobble in the sound, using an external second receiver and zero beating against that, using a computer to generate tones, using the FMT software included with the WSPR software and likely many more.

The point of all of these processes is to detect a difference where there shouldn't be one.

One final comment.

The most accurate process at this time without specialist measuring equipment is by using your WSPR enabled computer and the FMT software that's included. I'll look at that next time if I can understand what Joe K1JT wrote on the subject.

Happy measuring!

I'm Onno VK6FLAB