Tag Archives: Test Equipment

Building a Field Strength Meter

I am working on some projects now that I will be documenting more fully in the coming days and weeks. First up is the addition of a field strength meter to my test equipment inventory. I found a kit on Ebay for $20 that ships out of Thailand. I went for this mainly because it is very hard to get all the parts needed for this for less money. There are a few downsides to this circuit but as with everything compromises have to be made. Here is what you get in the kit:

s-l1600

Note there is no case. The meter is about 2 inches deep so none of the project cases I had would accommodate it at least without being really much larger than the application needs. I decided to use a 4x4x2″ outlet box (such as I used in the magnetic loop project) as they are readily available at Home Depot. I have made all the cutouts and will be soldering things up soon. Here is the basic layout:

Photo Nov 02, 9 27 09 AM

I put in a 1/4″-20 insert in the bottom so I can mount this on a tripod. This is an active circuit so it needs a 9V battery for power. The meter ought to be a bit more sensitive but at the cost of not being self-powered. One additional bother with this kit is that the BNC connector has no nut to mount it to the case. I have one on order to use the nut from. I don’t expect any issues with this circuit as it is simple enough. Most of the work is in the packaging.

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Some additional Test Equipment

I have already talked a bit at length about the Mini60 Antenna Analyzer. I thought I’d mention a few other pieces of test equipment that I have collected over the years that may help with the Ham Radio Hobby. FIrst up is a basic DMM. Actually have about three that I have collected over the years. Nothing especially precise but they handle most jobs around the house and shack. I bought a USB Oscilloscope / spectrum analyzer about a year ago. It is a VT DSO-2810R 100Mhz dual channel scope. The unit is about the size of a smartphone and included two decent scope probes.Resolution is 8 bit at high sampling rates and 16 bit at lower rates. Overall it suits my needs perfectly as it is easily setup to my laptop when i need it. In the past few months I added a couple of micro-controller boards that have been configured as test equipment. These can be had at Ebay for decent prices but shipping times from China can be a bit long. One is a 0-30Mhz DDS based on the AD9850. It has a BNC output and a 1V p-p output. I am not sure what the output impedance is but seems to work ok unbuffered. I just received a similar device that is a decent L/C meter. I tested the homebrew buddistick coils and found them to read much better than on the Mini60 with a lot less hassle. Also tested some known caps and coils and found the readings matched that parts quite well.

Using the PC Link Feature of the Mini60 Antenna Analyzer

This is how I setup the frequency sweep functions on the Mini60 antenna analyzer on 64-bit Windows 7. Your mileage may vary depending on your operating system. You will need a USB cable with a mini USB connector on one end and the Mini60 itself. Start by downloading and installing the PC link software which can be found here. The software is in a ZIP file. It is a DOS based program so just decompress and place in a convenient folder. Next plug in the Mini60 to the PC USB port, turn on the MIni60 and press “set” once after it initializes until PC link is displayed. Then press down and the display will indicate that it is waiting for a link. Meanwhile Windows 7 should install the necessary drivers for the USB to serial port link. The drivers install a serial COM port that is equivalent to the USB connection used on the Mini60. Use Windows control panel to look up which COM port number is used. This will typically be either COM3 or COM4. Once the drivers are installed and the Mini60 is indicating “waiting for link” you will be ready to run a sweep. To do this open a DOS command window by typing “CMD” in the Windows search box. This opens a DOS window. Change directories to the folder containing the PCC-sark100.exe file.

Now lets say you want to run a sweep for the 10m band and collect data every 10 kHz to save into a file called 14mtest.csv. Assume the COM port used is COM4. The command line for this would look like:

pcc-sark100 -cCOM4 -s14000000 -e14350000 -t10000 -o14mtest.csv

Here is an SWR plot of my Ultimax DXtreme antenna from 1 Mhz to 30Mhz as installed at my location:

dxtreme Custom

The Mini60 then runs the sweep and saves the data into a comma delimited file called 14mtest.csv. Open the excel file called sark100-zplots.xls. Load the data file and you will see the data plotted. A variety of information can be displayed using this spreadsheet. That’s all there really is to it.

There are some direct commands that can be applied to the Mini60 when in PC link mode. See appendix I of the Sark100 manual for these commands. You will need a terminal program like hyperterm to connect to the COM port of the Mini60. You can then pass commands to the Mini60 and receive the resulting data back from the device.

Review of Mini60 Antenna Analyzer

I mentioned in a previous post that I had purchased a Mini60 antenna analyzer. I am going to post a quick review on this device now that I have had a chance to use it. The Mini60 is a Chinese made copy of the Sark100 antenna analyzer developed by Melchor Varela, EA4FRB. This was originally an antenna analyzer provided in kit form. He has since discontinued production of the SARK100 kit and is now building a much better featured analyzer called the Sark110 which sells for about $389. It should be noted that although the Mini60 is a derivative of the Sark100, Melchor provides no support of any kind to these devices nor is he associated with their manufacturers.

mini60

The Mini60 is sold on Ebay from a variety of sellers with the average street price of about $130 delivered. This review focuses solely on the ones marked Mini60. I mention this because there are similar devices marked as SARK100 that also sell on Ebay for about $100. The Mini60 arrived at my QTH in a box and was wrapped in bubble pack. Nothing else arrives with it. No manual, no cables…just the device itself. It turns out that the instructions for operating the SARK100 apply completely to the Mini60. The following are links to the most useful files available for the Mini60:

The SARK100 Manual

The SARK100 Basic PC Control Software

Zplots for the Sark100

The complete archive of the Sark100 can still be found on Melchor’s website here. Regarding the manual, you can skip the sections devoted to assembly and test of the Sark100. The interesting bits are related to how to operate the Mini60 as well as items in the appendix on using a serial link for PC control. The Basic PC control software is DOS based software which allows you to run a sweep across a range of frequencies and saves the collected data into a comma delimited (.csv) file. This file can be opened within the Zplots Excel spreadsheet for graphic display of a variety of antenna measurements. Note that the Mini60 can be used stand-alone but the addition of the PC based sweeps provides considerably more data across a wider range of frequencies.

The Mini60 is powered either by its internal rechargeable battery or by an external 12V power supply (not included). I typically plug the Mini60 into a USB port for charging of its internal batteries. One thing to be careful of is that if the device is left on for more than a minute or so it shuts down to save power. It is important to note that this only shuts down the display and continues to draw current if the power switch remains in the “on” position. The timeout can be adjusted but I have on occasion drained the batteries by forgetting to switch the power back to “off” after a timeout.

The Mini60 is built solidly with a sturdy metal case. It is about the size of a pack of cigarettes. There is a single PL-259 connector for attachment of the antenna or feedline, a single power switch a mini-USB port and an external power port for 12V DC. Upon Power up it displays frequency, SWR and impedance magnitude. The device powers up in impedance mode by default. Other modes include complex impedance, capacitance, inductance and off mode. The off mode can be used to measure any RF feilds in the vicinity. Frequency coverage is from 1Mhz to 60Mhz in 13 bands. Pressing scan will scan across the selected frequency band and will beep when an SWR match below 2:1 if found. After a successful scan, the device will display the bandwidth and be set on the frequency of the minimum SWR found.

The Set button allows the device to adjust step size, suspend timeout, calibration and the PC link functions. I’ll describe the operation of the PC link functions is a separate post.

Overall, I have found the Mini60 easy to use. I verified the output frequency with an oscilloscope and found good matching. The default suspend timeout is a bit short but is easily extended. I like the USB charging feature and PC link function. The PC link feature is a key feature for me and now that I have used it I would not want to use an analyzer that did not have some sort of data download feature. The analyzer made short work of setting up a homemade Buddistick antenna on both 20m and 10m and is very useful in comparing actual with modeled antenna results.

The only negatives I have noticed have been fairly minor. Adjusting the frequency is a bit tedious particularly at the band crossings. This can be compensated by running a PC link sweep but that is not always convenient as a PC is required. The Inductance measurement feature on its own is somewhat strange. I get different readings on different bands. I ultimately used a series RLC circuit with the L being unknown and running a frequency sweep with the PC link feature. It is fairly easy to find the inductance value by reading the frequency of minimum SWR on the sweep and calculating the Inductance value. I’d say that the inductance values are only good within 15 to 20%.

For $130 this has turned out to be a handy and useful tool. Used analyzers can often be found for similar pricing but lack the PC link feature that really differentiates this unit at this price point. I plan to keep using it and will update if there are any quality issues noted down the road.

First impressions of the “MINI60” Antenna Analyzer

I mentioned in a previous post that I had ordered a “MINI60” antenna analyzer on Ebay. Well it finally arrived today and not a moment to soon as I started building a homebrew Buddistick. There are quite a few negatives floating around the internet but I took a chance anyway. These units are Chinese made clones of the Sark100 originally designed and built by EA4FB. The unit arrived wrapped in bubble wrap in a small box with nothing else. These are notorious for not having a manual or documentation. It looks pretty sturdy as it uses a metal case. It is about the size of a pack of cigarettes and has a SO-239 connector on the end. I have plugged the unit into a USB charger and it indicates it is charging. I’ll let it charge overnight and see if the light turns green.

I did connect it to a laptop and it found the drivers with no issues on my Windows 7 machine. It appears as COM4 on my machine. There is a command line program originally for the Sark100 that seems to work well in scanning across a range of frequencies and dumping the data to a comma delimited file. The Z-Plots program can then be used to plotĀ  a variety of variables such as SWR, Zo, Rs, Xs etc. I did a quick test with an oscilloscope to very the output frequency as this was one of the issues discussed on the internet. The waveform checked out ok at 14.1 Mhz, I measured the end fed antenna and got back reasonable looking results. I’ll be posting more on this device in the next few days.