This VSWR analyser looked interesting from K6BEZ. Certainly simple and low cost and probably just the thing to test the multiband SOTA antennas that I have been putting together lately. I happened to have one of the DDS modules spare and would change the Arduino module in the article to a Nano as they are compact and cheap (~$10).
Rather than build it on veroboard as per the article, I made a PCB using Eagle and the toner transfer process. The board is mostly SMD to keep it compact and these days SMD is easier anyway.
The board came out nicely, it is single sided and only has one link. It uses the entire bottom copper as a ground plane. I couldn’t source the AA143 germanium diodes locally, so picked up a couple of 1N34a diodes.
After loading all the parts with the exception of the MCP6002, I got a little impatient and plugged in an LM358 temporarily so that I could play a little. I remapped the DDS data lines on the Arduino to better suit a simpler PCB layout. It all went together pretty well. Another compromise made was with the gain resistors on the op-amp. I didn’t have any 5k / 648 R resistors, so the gain was kept the close by using a 6k8 and a parallel combination of 1500 and 2200 R resistors (891 R). This gives a gain ratio of 7.624, where the original resistors had a ratio of 7.716. After a power up, it looked OK, so the K6BEZ analyser software was started. It connected OK, and when the sweep command was executed, you could see comms activity, but there was no display on the graph. After poking and proding, I could see that the DDS was indeed sweeping and there was a slight analogue voltage sweep to the Arduino. As a last resort, I tried an older version of the sweeper titled “Thrifty Antenna Sweeper”. This sweeper did give a plot on the graph although it did not redraw on every sweep. A dummy load was connected and there was a more or less straight line. The offset fed HF dipole was connected and the resultant sweep did sort of show dips where there should have been. I will wait until the MCP6002 arrives before doing anything else. The MCP6002 op-amps arrived today. Once installed, they fixed what seemed to be an DC offset problem where the SWR on a 50 Ohm load was 2:1. Now it seems that there is a frequency response problem with the substitute diodes (1N34A). See plots below:
Note-that the OCF antenna used for the test was extended with a 50m+ run of RG58 coax to reach the computer, so the coax run is probably making the match look a lot better than the antenna is at the feed. Next thing is to obtain the original diodes (AA143) to see if that is the reason for the falling response. Yahoo_Group: https://groups.yahoo.com/neo/groups/k6bez_projects/info Update 2014-09-30: Beric, K6BEZ replied on the yahoo group with a fix to the aduino code to make it compatible with the newer PC sweeper software. // Send current line back to PC over serial bus
In summary, there is an unused field that has to be removed for the newer sweeper software and the FWD and REV fields are not used.
Update 2014-09-30: 1N60 diodes made a noticeable improvement over the 1N34a diodes. There is still a rolloff in the plot when the analyser is unterminated, but VSWR is now reported out to 30MHz that correlates with the VSWR measurements using a bridge.
Update 2014-10-05: The analyser was hooked up to the SOTA End Fed Half Wave to see how it looked. A 10m feed of RG58 was used. The result is pretty good. It seems to be working OK now. The antenna was originally optimised for 7.090 MHz and it is damn close in the plot.
Update 2014-10-10: Changed R6 on the DDS board from 3k9 to 2k2 which resulted in an increase in DDS output. With 3k9, the output loaded to the sweeper was 600 mV p-p. With the 2k2 the output increased to 1000 mV p-p. This change increased the sensitivity of the analyser noticeably.
Some plots from the 40/20/10 metre end fed:
Update_20114-10-08: The PC program won’t run on my Windows XP netbook, which makes it difficult to do measurements outside.
After some software programming using processing, I have a multi-platform capable application that will at least run on the old netbook. The application is pretty rudimentary, but plan to work on it and clean it up a bit. At present it will only sweep a few pre-defined ranges as the graphing is rather basic. I am using the grafica graphing library for processing to do the plotting.
Here is a plot from a 40m end fed half wave. The processing application is working pretty well now. I have divided the sweep into 3 bands, 1-10 MHz, 10-20 MHz and 20-30 MHz. This was pretty easy to scale. I might add the HF ham bands later.
It does run on the old XP netbook just fine.
Update 2014-12-06: Lars, KN5UTE did a great job of cleaning up and enhancing the user interface on my processing version of the sweeper software. The com port can be selected now where as before it just chose the first com port it came across. It still won’t compile to an .exe for some reason, so it is still running out of the IDE.
Update 2015-01-04: Received some AA143 diodes in the mail from KN5UTE, tried these out and there was a slight improvement at the 30MHz end compared to the 1N60A diodes.
I will leave these in.