Thursday, May 26, 2016

Four Days in May 2016

Well because of my schedule it was only Two Days in May, but it was still fun.  I really enjoyed the seminars on Thursday, and was pleased to hear Paul M0XPD mention the SNA Jr. during his presentation.  The WBB was great! You would not believe how much work must have gone into getting everything ready for that.  It was more work than I thought it would be just getting 8 kits of the SWR Power Meter ready for the last club meeting.
Enjoyed the vendor night, although with the limited space it was quite crowded.  I did manage to pick up a couple of things that looked interesting. Also had a chance to talk to several people I had exchanged e-mails with over the last year or so.
After a long day Friday of trying to see as much of the Hamvention as possible, I finally made it back to the hotel in time for club night and the homebrew contest.  The only problem with having an entry in the homebrew contest, was that I did not have a chance to walk around and look at the other entries and club presentations.  But it was still a fun experience.
I spent a lot of time talking about and demoing the SNA Jr.
After suggestions from several people, I am in the process of writing an article on it for QRP Quarterly.  Hope to get that written and submitted within the next two weeks or so.
That's  all for now.

Wednesday, May 11, 2016

SNA Jr. version II update

Well, after a lot of time working on the top half of the box for the SNA Jr. v.II, I have it finished.  It took quite a bit of time with some small files, but it looks very nice.  The extruded aluminum box is nice and solid, the push button on the rotary encoder has a nice solid feel.  Earlier versions using small tin containers flexed when the button was pushed, and it felt kind of mushy. 

Did some work on the software, and now have most of the basic functionality working with the AD8307 instead of the diode detectors.   With the diode detectors I had range of about 50db, with the AD8307 I have a range of about 10 to -75 dBm.

 Finished making changes to get all functions to use the AD8307 and added a simple Frequency Correction  factor to correct for the ~11dB drop in signal output I saw when sweeping form 1 to 40 MHz.  This large drop is probably because I did not include a buffer amplifier, and just take the output of the DDS module through a 100nF capacitor.  I have not had any problems using the instrument for what I want without an amplifier, so decided not to complicate the design.

I added a Watt Meter function for use with a 40 dB. Tap.  It gives the signal in dBm and auto scales to either  a watt or miliwatt reading .  I also keep a peak power reading for each time RF power is detected.  With no power applied the screen flashes NO POWER and resets peak reading after 2 seconds.
Watt Scale
milli Watt scale

I want to add some calibration routines if I have time.  Also trying to get some preliminary assembly instructions written for the pilot run of the SWR/power meter before the club meeting this Saturday

Saturday, May 7, 2016

SNA Jr. Version II Part 1

I need to visit my daughter and help her with a couple remodeling projects around her house. I set the schedule so I can stop in Dayton for a couple of days at FDIM and decided to build an entry for the home-brew contest.  
With the success I had with the stand alone AD8307 power meter, I thought it was time to upgrade the SNA Jr. with a real power detector.  The SNA Jr. had started out with my version of the K6BEZ antenna analyzer and a need to check some filters I was working on for another project.  I replaced the SWR bridge circuit with a couple simple diode detectors and used the Arduino to compute  gain/loss in dB.  For such a simple project, I was really pleased with the results I was getting.  It was a vary small hand held instrument that I could use as a signal generator, SNA for testing filters.  With  the addition of a Return Loss Bridge I could check antenna SWR.  Then I added a simple pickup coil and a little software I also had some of the functionality of a Dip Meter.

It was very easy to edit the board layout for the original SNA Jr. to remove the diode detector and amplifier, and merge in the AD8307 circuitry from the power meter board.  The board house that I was using for the pilot run of club SWR meter project had a special going on.  Since I know of several people that would be interested in one of these boards I ordered 10 SNA Jr II boards.  It took about 2 1/2 weeks to get the boards, and am really pleased with the way they turned out.  The only issue I had was the spacing of the RF chokes I use to feed DC and signal into and out of the AD8307 circuit.  The ones I had are a little larger than the ones I had used in the stand alone power meter, and I had to bend the leads back on themselves to make them fit.  Other than that everything else works well.
In the original SNA Jr. I used a couple of 4.2 volt Li-ion batteries for power.  Only problem was that I had to remove the batteries to charge them.  In a rebuild of the SNA Jr. I used a 12V battery pack designed for use on security cameras.  This worked well, but I was not getting as long of operation on each charge as I had with the individual Li-ion batteries.

Looking around on eBay for something else I found some 2S Li-ion Lithium Battery 18650 Charger Protection Board Modules.
They are very small and take care of the charging and protection of 2 batteries in series,  there are also other versions available for different size battery packs.

I mounted a couple of battery holders , the charge controller board,  connectors and On/Off switch in the bottom half of the extruded aluminum box I bought for the project. The charge controller board is very small and after wiring it up I covered it with a piece of heat shrink and stuck it down beside one of the batteries. 

 I checked charging with a couple batteries and was very pleased with the way it operates.  I monitored charging current and voltage, it charged at about 1.6 amps with a wall wart power supply that put out 10.3 V. with no load.  While charging it put out around 8.6 V.,
and when charge was complete the current dropped to nearly 0 and voltage jumped up to the open circuit voltage. 

I built up one of the boards, and gave it a quick check to make sure everything was working.  Since I did not have the software modified to read the 8307, I checked the output of that circuit with a voltmeter.  

I got around the .25 V. I expected with no input and it jumped up to around .5V. when I touched the input with a screw-driver.  These are just about the same as I was getting with the circuit in the stand alone power meter.  Now that I know it is working I can build the shield box to cover the AD8307 circuitry

After adding the SMA connectors for RF output and input to the detector, I finished building a shield around the AD8307 circuitry using some .008" double sided circuit board.  That has become my favorite for building shields, easy to cut and solder, and stiffer than copper shim stock.  Here is a pictures of everything ready to be assembled, and one showing how tight everything fits in the box

Next to finish a little final fitting on the top half of the project box and mount everything, then on to the changes to the software.  Also thinking about adding a couple additional functions, such as directly computing VSWR instead of just Return Loss.  And add a wattmeter function, using a 40-dB tap to drop the power level down to that required by the AD8307.  Watch for updates, FDIM is less than two weeks away.