# How can I calibrate a SWR meter?

I would like to build my own HF (1.8 - 30 MHz) SWR meter, however I'm not sure how I can calibrate the meter without an accurate power source. I'm targeting a power capacity of 150 W so I can use it with my HF radio. However, my test equipment at hand does not include a directional wattmeter, and I wouldn't trust the power meter on my radio when the radio is turned off.

I have a number of precision RF sources, but most of those don't exceed 1W in output power. I can get a good 100W+ 50Ω load, spectrum analyzers, and oscilloscopes, but nothing that I would really consider "power RF".

Can I calibrate or validate my meter without an accurate power source or accurate directional wattmeter?

• If you just need SWR, you don't need an accurate power source, just a consistent one, because SWR is a ratio. – Phil Frost - W8II Mar 25 '14 at 13:04
• Also I don't have time for a full answer, but here's a hint: $P=I^2 R$, and $P=V^2/R$. R is probably 50Ω, and between the scope and the spectrum analyzer I bet you can find a way to measure V or I. – Phil Frost - W8II Mar 25 '14 at 13:06
• @PhilFrost So then I'll know that my reading is good when the antenna is matched. Should I be worried about the scope adding error to the measurement by mismatching the system? How would I verify that it's measuring power correctly when the SWR isn't 1:1 (e.g. what about coupling between forward and reverse power measurements)? What mistakes might I make by (perhaps naively) hooking up a scope directly across a matched load? – W5VO Mar 25 '14 at 18:02
• You can also put an unmatched load on it. If you don't have a 25 ohm dummy load, you can make one by putting two in parallel. As long as you make the connections with wires much shorter than a wavelength (easy at HF), then you won't be introducing additional mismatches of significance. – Phil Frost - W8II Mar 26 '14 at 13:56

There are many ways you could calibrate. For measuring SWR, try this:

Attach a 50Ω dummy load to your transmitter. The SWR should be 1:1. Then, attach two 50Ω dummy loads in parallel, giving you an effectively 25Ω load. The SWR should be 2.

It seems you also want to measure power, and not just SWR.

If you have some attenuators available, you can put them on your transmitter's output until you are within the acceptable range for your spectrum analyzer. Then measure the power for that, and add to the measured power the power absorbed by the attenuators.

Or, you could transmit into the dummy load, then measure the voltage with the scope (provided it has sufficient bandwidth), and calculate the power from $P=V_{RMS}^2/50\Omega$. If you use a high impedance probe ("high" meaning anything $\gg 50\Omega$) then the scope won't affect the measurement much. 50Ω in parallel with 10kΩ is still 50Ω, within your measurement error.

Mike Bryce covered a 3:1 dummy load in QST, in Feb 2013, this will NOT give you an accurate input power reading, but WILL generate a known SWR.