VNA

One of the most desirable instruments in the RF engineer’s arsenal is a vector network analyzer. It gives you the ability to directly measure all of the key parameters of two-port networks, such as gain and return loss. The one I’m building was conceived by Paul Kiciak, N2PK. Paul’s website has all the design information. He’s a very talented RF engineer and has given the amateur community an amazing new capability. There is a Yahoo group for the VNA (membership required). There, you can see info from many happy builders.

It’s based on a pair of DDS generators (RF and LO) and a direct-conversion, or zero-IF, detector with a 24-bit ADC. It’s a unique approach that is simple and yet offers absolute accuracy and dynamic range equal to or better than commercial analyzers over its 60 MHz range. A transverter may be added to expand the frequency range. For now, I’ll stick with the basic unit.

The instrument itself contains no intelligence; rather, a PC talks directly to the hardware via SPI and digital I/O bits via a parallel port connection. Mine has a USB interface, designed by G8KBB, and available commercially from Elrasoft. I run the software under a virtual machine (Parallels) on my trusty MacBook.

Vector Network Analyzer

About My Build

The PC board I’m using contains two detectors, which will support a full S parameter test set. Even without the test set, this enables simultaneous measurement of, for instance, gain and input return loss. The board is available from Ivan Makarov, VE3IVM. Ivan’s site contains also a great deal of practical information on the VNA.

Features of my particular VNA include:

• Dual detectors

• Ovenized main VNA board

• USB interface

• RF output switching and step attenuator to support full S-parameter test set

• External 12 VDC power

• Front-panel accessory power jack to support external amplifiers, etc.

• Machined and anodized aluminum enclosure

Enclosure

Since I have a basic machine shop, I enjoy fabricating mechanical parts for my projects, so I decided to make a heavy aluminum enclosure for the VNA. Walls are .375” 6061-T6, the bottom plate is .125”, and the cover is .063”. Parts were anodized black through the help of a local machine shop that we do business with. The owner was wiling to add my small batch of parts to a bigger job gong to the platers, so the work only cost me a bottle of Merlot. When anodizing, you can ask them to mask off parts where you want to maintain electrical contact, including the inside of tapped holes.

Reflection bridges, also built on boards supplied by VE3IVM, are fitted to small machined housings. This adds mechanical reinforcement to the end-launch SMA connectors and provides a bit of shielding. I tried out a silkscreening kit (StencilPro) for the labels on this, with mixed results. Using their finest mesh yields clear edges on text down to about 9 point, but there are some clogging problems during developing that I was never able to overcome. Also, the kinds of stenciling inks available to the amateur are not as resistant to wear as the commercial stuff, which is incredibly expensive. So I’ll be “sticking” with labels for now.

The bottom plate is Alodined. I got my Alodine materials online from Aircraft Spruce and Specialty Co. but there are certainly other sellers. Preparation starts with sanding with a belt sander, and also some hand wet-sanding with P120 grit silicon carbide paper, followed by a detergent and water wash. Next, the part is cleaned with Metal Prep #79, a phosphoric acid etchant. That has to be thoroughly rinsed in water. While it’s still wet, the part goes directly into the Alodine #1201 Solution. I use a shallow pan with just a few ounces of solution in it for each of these steps, and a wide paintbrush to apply the solution while the part stands on edge. Keep turning the part so the solution runs over it from all directions, otherwise you will get streaking. The Alodine step takes about 10 minutes, and leaves a golden brown color that is conductive with excellent corrosion resistance, even outdoors. It is also an outstanding preparation for painting. I will never make another aluminum part that goes without this treatment, except if it will be anodized. By the way, for anodizing, I sand the parts very carefully to 120 grit and VERY thoroughly wash them. What you see is what you will get; anodizing makes odd scratches and nicks stand out!

Panel labels are white-on-clear, made on a Brother labeler. I have one of their models with USB that allows you to paste in graphical items, like the USB and ground symbols I put on the back.