Opera Cake Antenna Switch for HackRF One

Following the tradition of electronic gadgets named after cakes, pastries and sweets (think Raspberry & Banana Pi, Snickerdoodle, and early versions of Android), the Opera Cake antenna switch is a device that bears no relation whatsoever to its name (at least Pi referred to Python). The Opera cake that I know (and even made a few times) is a multi-layer cake that mostly tastes like coffee. And maybe that’s the relation to the device reviewed below? You’ll need a lot of coffee to stay awake during the long hours of SDR fun that it will procure you. (Or for reading this review?)
 

The Opera Cake Antenna Switch...

As mentioned above, the Opera Cake is an antenna switch for HackRF One. Actually, it is a double four-way antenna switch, i.e. a double single-pole four-throw (SP4T) switch. You can use the two switches - also known as banks - in parallel as a DP4T switch or in series (sort of) as an SP8T switch.

Let’s start by describing what Great Scott Gadgets’ take on Opera cake looks like. Basically, it is a 120 mm by 75 mm four-layer circuit board (wait a minute, four layers, layered cake, hmm…) with a lot of connectors. There are five female SMA connectors on either end of the board, and three stackable headers mounted on it, with footprints for three more.

... Is An Add-On Board

The board has surprisingly few electronic parts. This is because the Opera Cake is an add-on board. It does not require any intelligence, as the board it plugs onto controls it. Size-wise it is intended for plugging on top of an HackRF One board. When you solder the right connectors on the board, you can also plug a GreatFET One on it (upside down). Note that when you have a boxed HackRF One, you will have to unbox it (with care!) before you can plug the Opera Cake on it.

 

Configurable

The first application of the antenna switch that comes to mind is, of course, connecting a variety of antennas to a single HackRF One so that you don’t have to rewire your setup every time you want to change RF band. Switching antennas can be done manually with a little software utility, but it can also be done automatically by the HackRF One firmware based on frequency or time. Frequency-based switching enables e.g. wideband spectrum analyzer applications, whereas time-based switching lets you do cool things like pseudo-Doppler direction finding.

When using the switch in DP4T mode, you can use it to insert for instance attenuators or filters in the antenna path. This way, the Opera Cake can function as a switched filter bank.

Opera Cake is Stackable

Also know that up to eight Opera Cakes can be stacked on top of each other (a layered SDR, hmm…), enabling different switch configurations. In manual mode each board can be controlled separately, but in automatic frequency and time switching modes all boards will switch at the same time and in the same way. So, a stack of two boards would allow for a DP8T configuration, switching e.g. eight filters.

Why Would I Want One?

Opera Cake is not a new design, it dates back to 2016 and maybe even longer. What’s new about it is that you can now buy it as a fully assembled and tested module. Up to now, if you wanted one, you had to build it yourself from the design files published on GitHub.

The nicely assembled Opera Cake antenna switch is, unsurprisingly, a perfect companion for HackRF One. Instead of obliging you to rewire your hardware or adjust the length of your antenna every time you want to work in a different band, it lets you connect (and keep connected) up to eight of your favorite antennas at once.

A Few Remarks

Depending on the age of your HackRF One, you may have to upgrade its firmware to the latest version to make it work with the antenna switch. Instructions on how to do this are available on the Great Scott Gadgets website.

Please be aware that, as on HackRF One, the SMA connectors on the Opera Cake are female types. This means that it will only work with antennas equipped with a male connector (i.e. with a center pin, see photograph above). Therefore, be careful and choose wisely.
 

A HackRF One with one or more Opera Cake boards stacked on top of it no longer fits in its (tight) enclosure. This, of course, makes the system vulnerable to dust and screwdrivers and other metal objects accidentally falling in- and onto it. Also, depending on how you wire your antennas and filters, the Reset and Power pushbuttons may become somewhat inaccessible. As the stackable headers P20, P22 and P28 only carry power and a few digital signals, and no RF signals, it is possible to insert headers to increase the distance between the boards.

Finally, a (short) male-male SMA cable to connect the Opera Cake to HackRF One’s antenna input is unfortunately not included, so bring your own.