One of the really useful tools you can have as a Ham is an antenna analyser, which enables you to understand better how your antennas and overall RF installation are behaving/performing at various frequencies.
At the very least, an antenna analyser will let you check the standing wave ratio which, in a very simplified view, is the amount of energy that is being reflected back into your transmitter and not being sent out. A high SWR usually is not a very good thing, though a low SWR does not always mean you have a perfect antenna either, that would be too easy – a 50 Ohm resistor connected to your amplifier will always give you a 1:1 SWR but won’t help you much if you want to make contacts!
Still, understanding how your antenna behaves depending on the frequency, see exactly where it resonates, and how its impedance evolves, is extremely useful. There are many types of antenna analysers on the market, from really cheap kits, to totally awesome tools like the Agilent Fieldfox, or even more advanced stuff that costs as much as a small house… I found a good article at rigexperts which explains a lot of different possible architectures, a good read.
With this in mind, I recently purchased a Sark 110 from Seeed Studio, who sells it for $360 as of January 2014: this antenna analyser is designed for HF to low VHF bands, with a 0 to 230MHz range. This makes it perfect for hams, though being able to include the 70cm band would have been a nice touch. The Sark 110 is fairly priced, especially for what it does: you get much more than a VSWR indicator here!
Out of the box, the Sark-110 offers the following high level features:
- Vector analyser over 0-230MHz (“antenna analyser”)
- Cable analyser (time domain reflectometer)
- Signal generator
Frequency/vector analysis can be displayed as smith charts, scalar, or as four separate quadrants spanning different frequency ranges, which is a really interesting feature for testing multiband antennas.
You can and should spend more time on the Sark 110 site to get more technical info on the Sark-110, there is no point duplicating it here, so let’s spend a bit more time discovering the device.
Shipping? Yes, before you continue, a word of warning for those of you who live in the USA. You cannot currently purchase the Sark-110 in the US, due to a potential patent dispute. You can check the Sark-110 Google group for more details. Hopefully this will be resolved at some point, but in the mean time, if you want to get your hands on this device, you will need to have someone in another country forward it to you…
As you can see in the introduction picture, the Sark-110 is a tiny device! But fear not, it is mostly screen on the front side, so it is actually very readable. The Sark 110 comes in a small box, with a small MCX to BNC adapter, and an allen key. That’s it. No manual or anything else, you will have to head over to the Sark-110 site to get this. This is not a bad thing, at the end of the day: between frequent firmware updates and documentation improvements, anything that is shipped with the unit would probably already be outdated by the time you open the box.
Build quality is really very good: the unit is made out of aluminium and feels solid in the hand. The buttons have good tactile feedback. Somehow the icons below the buttons are identical to the usual “Play/Pause”, “Stop”, “Record” and “Eject” symbols. On the Sark-110, “Play/Pause” means “Run/Hold”, “Stop” means “Select”, “Record” means “take a screenshot” – close enough to a record action I guess – and “Eject” saves the current configuration…
The Allen key was intriguing, though, and a clear invitation to open the case, which I did right away:
On top, you can see a 950mAh single element LiPo battery, with a built in charge/discharge protection system, so that it can simply be interfaced with the main board through a simple 2 wire power connector.
The main board itself is well built – Seeed Studio is the manufacturer, and they know what they are doing. The two large chips are the STM32 micro controller, and the frequency generator, a dual AD9958 DDS. DDS stands for Direct Digital Synthesiser. The third and smaller chip on top is the flash memory.
Comments on the board
Though build quality is overall excellent, I was surprised at the very small amount of solder holding the MCX connector: one reviewer on eHam commented on the fact the solder joint on his unit was bad, and looking at my board, I can very well see this happening in the future. If I ever get unreliable results later on, this will probably be the first place I investigate.
Another small issue, is that MCX connectors are not really designed for numerous insertion/removals, like BNC connectors are: though choosing this connector type contributed to making the Sark-110 much smaller, a MCX connector is typically designed for only ‘at least’ 500 matings, which is not very high. So be careful when you connect/disconnect the BNC adapter if you want your device to last for a long time!
Note that I have not had problems using the Sark110 so far, and I will update this article after a few months to report how things are going in that respect.
I also reached out to Melchor, the creator of the Sark-110, to get his feedback on that particular aspect: he does recommend on his site to consider using a MCX to SMA adapter or to leave the stock MCX to BNC adapter in place whenever possible. That said, he also mentioned that his own test unit connectors have never failed until now, despite heavy use since 2011, so as long as your are reasonably careful, you should have absolutely no worry.
The USB connector is a mini-USB, a micro USB would probably make more sense in 2014, but then, like everyone you probably have tons of mini USB cables laying around, so this is not really an issue.
The screen looks awesome – the device front is mostly a screen, with a very adequate resolution.
Before doing anything else, you should probably head over to the firmware page of the official Sark 110 site, and download the latest image. Installing it is really easy: simply connect the Sark 110 to your computer, where it will appear as a USB drive. Copy the “DFU” file, eject (nicely) the unit, power it off then back on holding the “Run/Hold” button: you will see a firmware update menu, pretty much self explanatory.
The second sensible thing to do, while you have the device connected, is to backup the “detcalib.dat” and “oslcalib.dat” files, which contain the various calibration parameters after factory calibration. This way, should you do anything wrong at a later stage, you will always be able to retrieve your initial calibration values.
You can then connect a load to the analyser, and start working! Refer to the manual for an explanation of how the various buttons work. The two right-most controls (called “Navigator” A and B) include a “push” action but it is not very convenient to use, and you can instead use the “Select” button instead of pushing the navigators.
Getting a quick overview of your antenna performance is super easy: use the “scalar” mode, select the HF band you want to test, and you will get a real time graph of your VSWR and impedance modulus. This device is very ham friendly, and all HF bands are preconfigured. Screen refresh is fairly fast too.
There are two markers, M1 and M2, that you can of course move manually. More interestingly, you can also configure them to track various min/max values on both curves or more advanced operations such as value on one curve while the other is crossing an arbitrary value. Simple but powerful. Below is an example: on my MP-1 antenna, I am checking how it is doing around 20m. I have set M1 to tell me the lowest SWR, where M2 is telling me SWR when the impedance curve crosses 50 Ohm:
This antenna clearly needs a bit of tuning to perform as it should – my KX3 antenna tuner still happily compensates for this less than ideal tuning for digital modes on 20m…
Smith Chart Mode
One great strength of the Sark-110 is that you can also create Smith charts in real time: a Smith Chart will give you much more insight on how your antenna behaves over the frequency range that you are testing, in terms of capacitance and inductance. Below is the Smith chart for the same antenna with the same slightly mis-tuned settings, in Smith Chart mode:
The red circle is set at a VSWR value of 2, and Zo at 50 Ohm. As was already clear on the scalar graph, the antenna VSWR never quite enters the circle, and the antenna is resistive around Marker 1 at 15.1MHz.
One important note on how to get accurate readings: ideally, you should do an “OSL” (Open / Short / Load) calibration of the Sark 110 when you first use the MCX to BNC adapter, and every time after you change to another type of adapter, in order to remove the adapter’s effect on your measurements. The user manual recommends you also do this periodically. Doing this will require building four test loads, which is not very difficult and is described very well in the Sark-110 user manual.
This is a really nice feature of the Sark110: it will let you test of your feed line, and more importantly, detect any fault occurring along its length: cable pinch, bad connection, etc.
In order to get a correct cable length measurement, you will need to select the cable type that matches yours in the Setup menu: the Sark110 comes with a good list of standard cable types, and you can also define your own measurements. That said, even without the correct cable type, you can quickly check if your connections are in good shape, such as shown on the printout below:
M1 shows a first reflection at about 15.8m, which is about right since the coax is 50ft long. This is where I have a 1:1 current balun inserted. M2 is at 19.6m, again consistent with the 12ft extension leading up to the antenna, which is the large reflection on the graph. the first discontinuity shows a small inductance (refer to the Sark 110 manual for more details of how to interpret the graphs) which is consistent with a current balun. And the second large echo shows that the antenna presents a higher impedance than 50 Ohms to the feed line.
One small comment here: it would be really nice to get more flexible zoom capabilities: the same “span” control on the left hand side as in other modes, but which would control the displayed length rather than frequency would be perfect.
As a side note, I have not tried this yet but you should also be able to use TDR mode to detect wiring issues on computer network cabling installations. You will need to build an appropriate MCX to RJ45 adapter, of course, but it should not be too difficult. This will help you troubleshoot and locate cabling faults with great precision, a feature that is usually only available on network qualification testers that cost over a thousand bucks! Just make sure none of your network cables are connected to a powered hub, computer or switch before you start testing…
Field mode is a simplified scalar mode display meant for, you guessed it, field measurements. It is a ‘high readability’ scalar display that is very useful if you are using any type of antenna that requires adjustment depending on operating frequency. You just get the essentials and you can do your adjustments quickly.
By the way: you can choose to display all screens either with a black or a white background: depending on ambient lighting, you can select what works best.
“Single Frequency” / SWR Indicator mode
Similar to field mode, you can also display the antenna SWR on a bargraph. The device can also provide audio feedback on how the SWR changes, very useful for handsfree operation! You also get an indication of the equivalent RLC model of your load, a nice touch:
Signal Generator mode
Another nice feature is the signal generator: it outputs a simple sine wave signal to any frequency from 0 to 230MHz, at various signal levels. Below is a scope capture of a 10MHz signal: clean output, level is consistent with what the generator indicates when coupled in low impedance mode… The “Power Max” setting outputs a -3dBm signal at 50 Ohms.
I hope you enjoyed this quick tour of the Sark-110. It is a great addition to my toolbox, and has already enabled me to improve my antenna installation a great deal, and validate some of the assumptions I had been making until now, always a good thing. Its build quality is excellent, the feature set is very complete, and I have not even covered its more advanced operation modes such as computer connection, custom settings, and so on. As I always say… read the f*** manual if you really want to get the most out of it!
As a last note, during my initial tests I asked Melchor, the creator of the Sark-110, whether it would be possible to add a “screen flip” option so that the device can be operated in left or right handed mode depending on where the antenna coax is located: within 12 hours, Melchor had released a beta version of his firmware implementing this option, absolutely outstanding support! Thanks again.