Sunday, February 14, 2010

Hot Amplifier

With my recent experimentation with the WSJT protocols on 2m, I've run into a new problem. The FSK441 protocol is effectively 100% duty cycle while it's transmitting, and with the sequencing it's 30 seconds of transmit, 30 second of receive. This has the side effect of creating a very warm amplifier, to say the least. I'm currently using an old RF Concepts amp, 45 watts in/170 watts out.

To reduce the chances of melting your amplifier, it's recommended that you drive the amp at reduced power. That past week I've dropped my input power to about 20watts PEP. Even at that rate, the heat sink on the amp was very warm to the touch. I figured the easiest solution to solve this problem would be to put some fans on top of the heat sink to try and draw away the heat, create some constant airflow.

I went down to the work shop and rummaged through some old junk boxes and pulled out 3 working 12v fans that were pulled from an old PC (it occasionally pays to be a pack rat). So, now I just needed a mounting method. Over the course of the week I had time to think about it, and after a trip to the hardware store I came up with an overly complicated mounting bracket that would hold the fans down to the amp. I'm not sure where the quote came from, but i've always subscribed to the thought "Anything worth doing, is worth OVER-doing!".

I found I had some small aluminium strips that I had purchased at the local home center, it seemed to be the right material for the job, but I didn't quite have enough of it, so off to the store I went. Luckily for me they recently built a big box home center 2.0 miles from my house. Said company now extracts money out of me on a weekly basis.

So, with crude measurements, a few whacks of a hammer, some drilling and I have a basic bracket. I simply tied the lead wires together and crimped them into an Anderson Power-Pole connector, which is the standard low-voltage connector in my shop.

The three fans together draw about 350ma at13.8v. Ideally I'd like to find a small circuit that would detect the amp keying and would turn the fans on for a set period of time. Normal voice operations doesn't really require it, but extra thermal protection never hurts. For WSJT modes having the timer be longer than the typical sequence would effectively keep the fans on constantly, which is a good thing.

The whole assembly is currently sitting snugly on top of the amp, but it's not permanently attached. I think I can get away with not having to drill any holes into the heat sink since it's fairly tight.

Now, the dangerous thoughts are running through my mind. Since I've greatly increased the thermal dissipation, how much harder can I run my amp? Could I run at full power?

I don't currently have the ability to measure the surface of the heat sink, but I'm guessing I could probably run closer to 30w input, maybe 130w output. I think it might be time to look for an inexpensive IR thermal sensor, like I saw at show-n-tell at the last NEWS group meeting. I'm guessing Harbor Freight probably has something that will do the trick.

Monday, February 8, 2010

JT65b Digital mode on 2M SSB

Over the course of the past year I've been working on getting my home VHF/UHF station permanently on the air. I've had various antennas up in temporary configurations, but never with any sense of permanence. Recently, while reconfiguring the workshop, I also reconfigured all my feedlines and radios. I now have a permanent 2m ssb radio, 2m 160w amp and 12 element yagi up around 25'.

Part of the reconfiguration also got me back on HF again, something that I've been missing since I moved to Colorado back in '04. With the big feedline re-shuffle I re-routed my 75m inverted vee up to the same radio that is the permanent 2m ssb station (Yaesu FT-847).

One of the main HF modes that I enjoyed was PSK-31. I really enjoyed watching the DSP work wonders with nearly completely in-audible signals. Part of the new station configuration included purchasing a Tigertronics Signalink USB soundcard/radio interface. This is a nice little unit that greatly simplifies the whole radio interfacing, in my case, eliminating the need for overly complicated wiring schemes.

While working PSK-31 one day it occured to me that now that I had this digital interface I could also try some of the new WSJT modes on 6m and 2m. Since I didn't have a 6m up in the air, 2m it was! This all happened during the weekend of the ARRL January VHF Sweepstakes, so it was a good time to test it all out.

Well, Saturday night of the contest weekend I listened on the meteor scatter calling frequency of 144.140, but I wasn't hearing any pings at all. I decided to leave the program running over night to see if I heard anything. In the morning I was very happy to discover that I had decoded a couple of pings! Wow, actual reflections off of meteors! Over the course of the next couple of weeks I didn't have much time to play, but this past Saturday afternoon I found myself on Ping Jockey talking to other folks. I actually heard some pings, but unfortunately had the WSJT program in the wrong mode. D'oh! Russ, K2TXB, asked if I'd like to try some JT65b over tropo, I was excited to try!

Off we go, I see Russ's signal and I start clicking buttons, quickly realizing that I don't know what I'm doing :) All the reading I had done was on FSK441, not JT65, so the sequencing was a bit different. Then, much to my dismay....WSJT starting crashing! Needless to say Russ and I didn't complete, but Russ was a great sport and gave me some private pointers that really helped me out. I was very appreciative that Russ took the time to elmer me, it really does make a difference, thanks Russ!

I'll skip over the saga of me getting WSJT to work, but it involved downloading the source code and compiling on mutliple machines. The bleeding edge is often a painful place to live. In the end I had a workable system. The "end" was about 11pm that evening.

Sunday morning 5:45 I wake up with hopes of actually completing with someone, I jump on Ping Jockey and to my suprise there are a handful of guys awake and ready to run. My first victim was VE2DSB, Dan up in Quebec, FN35hs. Dan wanted to try QRP first, just to see if we could do it. I figured with me being a complete amateur at the digital modes I would have failed completely, but to my surprise I was able to see his signal on the waterfall! The QSO went very well, no issues at all (other than one operator error, clicked the wrong button). A few minutes later we had a QSO in the logbook! Dan then told me that he was only running 15w into a fairly modest antenna!

Here is a shot of the spectrum with the messages added on top in red.

15 Watts into a 12 Element yagi, and we were able to work each other over a 285 mile path! It's absolutely amazing what can be done with these digital modes.

My next step is to sucessfully work someone on FSK441. I've had some decodes, but never enough to actually make a contact. Saturday, during one attempt, I had several folks out in the midwest report hearing *my* pings, but I never heard any in response. Part of the station improvement that will happen shortly is the addition of a SSB Sp-2000 pre-amp that will be mounted up near the antenna. I just need the weather to cooperate and give me a nice 50-60 degree Saturday afternoon. Wishful thinking?

Friday, February 5, 2010

Work has begun on my new (to me) Elecraft K2

A number of years back I purchased a partially-built Elecraft K2 kit from a co-worker who lost interest in building it about 1/3 of the way into it. I had always planned on using it as an IF rig to drive VHF/UHF/Microwave transverters, since it's very well suited for the purpose.

Well, the kit sat on the shelf for years and gathered lots of dust. Oddly enough, a few years later the co-worker was re-invigorated in his interest, bought and built a new K2 kit! A few weeks back I completely tore apart my workbench and got it finally configured the way I wanted it, with all the test gear powered up and in the right position. I had previously only been able to power up two devices at a time which made it difficult to build anything.

Feeling like I could fix anything, I found a little bit of motivation and dug up the K2 kit. Several layers of dust later and I was into it! When my co-worker had first started building the kit, it was the first major electronics project he had taken on which resulted in a few problems that needed to be worked out. Simple things, like missing soldering a ping on an 8 pin DIP, etc. Simple, but will stop you dead in your tracks if you can figure out why it's not working! At this point the rig is working on 40m CW, and I'm starting the next build phase.

I get the feeling that I'll need to re-visit 40m in the future, I think it's suffering from a lack of sensitivity, but I feel the need to march forward. I figure at the rate that I'm moving (slow) it will take me a couple of weeks to complete. I still also need to build and install the SSB module and I also plan on picking up a KIO2 module.