Building and Testing an RF Load


28 Jul 2013: Replaced VSWR graphs with larger versions.

3 Sep 2013: Added description of Mike 9V1LX’s homebrew load


RF loads help you test your RF amps without radiating (well, maybe just a little bit). They are essential for you, if you are an RF amp builder; helps you measure your amp’s output into a known load. Also helps you stay on the correct side of the law.

I have several commercial loads which I use regularly.

One of them is this one from Decibel Engineering, which I rescued from an old pager band transmitter. It is rated at 150W.


Here is the S11 plotted in VSWR units. The VSWR is < 1.02 up to 200MHz. I am sure it will stay at a respectable value up to at least 1GHz. All the following measurements have been made on an Anritsu S331E Sitemater.


About 2 years ago, I decided to try building some after reading construction plans in QST etc. The common advice was to parallel common garden variety resistors in order to lower the inductance value. I was curious to exercise all the advice I read. Nothing better then to try out and then measure.

Here are the results of my effort. Both units are built on LPKF rapid prototyped PCBs.

Here are 2 views of the unit built of axial resistors.



Here is the VSWR plot. As you can see, the VSWR stays below 1.5 for all frequencies below 152MHz; good enough if you are experimenting with HF  and low VHF amps.


Here is the 2nd unit, this time built of SMT resistors.



The VSWR results are surprising. The VSWR crosses the 1.5 mark at 141MHz, lower than for the load built of axial resistors.


If you have stayed on this blog till here, you might be wondering just how commercial RF loads are built. Well, there are special RF power resistors. I have built an absorption wattmeter using some, but that’s the topic of another blog. Here is a picture of how they look like:



On 20 Aug 2013, Mike 9V1LX paid me a visit with his Yaesu FT897D and his homebrew dummy load. Just for fun, I measured the S11 using the Anriysu Sitemaster S331E and the results were better than my homebrew loads shown above. Unfortunately, I lost the Sitemaster files, but I recall that the VSWR was below 1.5 up to 500MHz. Here is a photo of Mike’s load:


All for now, and wishing you good DX!

Jeff 9V1AS

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