Building and Testing an RF Load

Updates:

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.

A03_4832

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.

s11_decibel_db4305

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.

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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.

s11_axial

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

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

s11_smt

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:

A04_3665B

 

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:

A04_6639

All for now, and wishing you good DX!

Jeff 9V1AS

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