Sunday, July 20, 2014

JT65A Sandbox Environment

After working with JT65A on HF now for a couple of weeks I wanted to better understand it, and explore some of it limits, particularly how weak a signal can be and the effects of poor time synchronization. I set out to connect up a simple sandbox environment. I wanted the environment to be independent of the Internet so that meant I needed a local time source using NTP. The sandbox environment has two (2) PC stations running WinXP and JT65 HF v1.0.9.3 HERE and a Meinberg NTP client HERE. Theses are connected to each other via a passive audio mixer. In addition a third PC is running Linux with NTP and Audacity. All the PC are on a common LAN. The following drawing shows the basic layout:

The first step is to configure the NTP server on the Linux PC to act as a time source. The main point here is to provide a common time source for the two WinXP PCs to sync their clocks with to within +/- 1 second per the JT65A requirements. They don't have to be sync'd to the "real" time just a time. The NTP package used is the standard Linux NTP. The /etc/ntp.conf file just needs a simple addition. The following two lines need to be added the the /etc/ntp.conf file:

fudge stratum 10


Once this change is made the NTP server will need to be restarted or you can just restart the Linux PC. Now with the time server on the network the two WinXP PCs will need to know about it so the ntp.conf file on each PC will need the time server name or IP address added to the the current list of time servers. The Meinberg NTP application has a shortcut to edit the file just by going to Programs>Meinberg>Network Time Protocol>Edit NTP Configuration. Just add the following line:

server  mytimeserver iburst


After this edit you will need to restart NTP which is available via Programs>Meinberg>Network Time Protocol>Restart NTP. Now you can use the Meinberg NTP status command via Programs>Meinberg>Network Time Protocol>Quick NTP status. You should see your NTP server listed with a delay, offset and jitter values well under 1000. These are in milliseconds, so you should see values under 200-600 milliseconds for the offset. If all values are zero, something is wrong and your NTP client is not seeing your NTP server or your NTP server is not running or configured correctly.

Now you are almost ready to use the sandbox environment. Next you will need to simulate the band noise. I am using Audacity on the Linux PC as noise source. I just start up Audacity then I generate 30 seconds of noise which is a built-in function of the tool. Then I loop that noise in the player which will keep playing over and over until you stop it. Look at the Audacity web site for details and tutorials HERE. Next we get JT65 HF running on both WinXP PCs and adjust the noise level so that both are adjusted to 0 db in the JT65 HF client using a combination of its level controls and the passive mixer. The passive mixer has is just a resistor network. Each mixer channel is a 50k ohm potentiometer followed by a 50k ohm resistor. The input comes from the audio out of one PC and the output of the mixer goes to the mic input of the other PC. Below is one channel of the mixer. 

All the the outputs of the passive mixer are tied together to go to all the mic inputs. So for this environment three channels are needed (one for each PC plus the noise source), see the basic layout drawing above.

With this setup, I have been able to simulate signal levels from -3 db to -22 db. I may use this environment to demonstrate JT65A and other digital modes like PSK31, RTTY, FELD HELL and others.

Sunday, July 13, 2014

Portable Antenna Tripod

Since I will be needing two general purpose WiFi antenna portable tripods for HSMM range tests I set out to build one based on this DESIGN. I had most of the wood and just needed the hinges and some bolts. It turned out better than I expected and is amazingly strong. I have not measured the weight of it yet but I would say it is at least 10 pounds (4.5 kg). Here is a picture of the first one that I finished last night just before dark with one of the Cisco antennas attached.

It is not as tall as I wanted but it needs to fit in the back seat of a small car. The original Cisco dish mount allows for some articulation. I will be able to get +/- 45 degrees elevation and about +/- 15 degrees in azimuth. If I need more range than that I will have to pick up the tripod and move it. Hopefully I will get some more time this coming week the complete the second tripod. My first range test will be a 1-2 mile path. 

Monday, July 7, 2014

JT65 HF mode

I decided to try JT65 HF. W6CQZ has packaged up Joe Taylor's K1JT JT65A mode in a easy to use program.

I briefly read the ARRL guide HERE, and I was able to make a contact (WE7P - Bruce in Tacoma Washington) in under 5 minutes on 15 meters.

This is not a rag chew mode but uses a very simple set of exchanges, however it makes the best of low power and simple antennas (which I like). I have only worked a few state side contacts so far but I am hearing DX stations too. I also worked another station (N3GTY -  William in Spring Lake North Carolina) on 20 meters.

The other thing that is neat about the JT65 HF client is that everyone running it can be a reverse beacon monitor. I noticed that one of my CQ's was heard in Japan by a stations client setup to report the spots into PSK Reporter.

Crystal Set in storage for the Summer

This last winter I built a simple crystal set radio receiver using oatmeal containers for the inductor forms and a cats whisker assembly along with a couple of variable capacitors. It has been setting on my bench taking up too much room so I thought I would document the build and then store it away. I should  create a chassis or base for it the next time I assemble it. This is the basic schematic.

The antenna was a simple slinky stretched in the attic of the garage, it of course would work better with a larger antenna but my outside dipole is in use for other radio activities. The antenna connection goes to a dual ganged 365 pf variable capacitor (C1) then through a 35 turn inductor wound on the first oatmeal container and then to a cold water pipe ground. The ground is very important for crystal sets. Next I have about 60 turns of wire on the second inductor with taps every 5 to 6 turns. The second inductor is another oat meal container. The two inductors are sitting on top of each other as you can see in this photo. No electrical connection between the two inductors.

Ideally if I set this up again, I will position the inductors horizontally on a wooden rod or something to be able to adjust  the air coupling between the two. The first inductor circuit is the antenna tuner and the second inductor forms the resonate tuning to the desired frequency with capacitor C2. The antenna tuner section could be with a series capacitance like I am using or it could be in parallel depending on the size of the antenna. Some crystal sets let you change all the connection which makes them very versatile. I was using clip leads so I could easily adjust the configuration. My diode detector is a modified MidnightScience unit. It came with a galena crystal, however I found that iron pyrite worked the best for me. I used a 1/2 inch copper pipe cap with adjustment screws to hold the pyrite crystal in place. The chunk I had was much larger than the galena sized holder so I needed to build my own holder.  The pyrite detector is typically twice as loud as the galena with the antenna I use. I have also tested germanium as well and in this set, the pyrite is the best.

With my simple indoor antenna and a good ground, I was able to receive several stations. The strongest were KFI (640 khz) and KNX (1070 khz). Fortunately  these stations had some separation. Two strong stations near each other on a crystal set can be a problem. KNX is actually near a weaker Spanish station that required delicate tuning but with the separate antenna tuning and frequency tuning it did pretty well. The other key to crystal sets are the headphones. I have two pairs of antique high impedance (2000 ohm) magnetic headphones. The ones I used on this set don't look that great but work well. I have also used the small piezo ear pieces but they fail if left connected to the radio for any length of time and in general are not very rugged. I have built many crystal sets over the years but they still bring great enjoyment and fun.