Running the DATV Express board with Windows software requires a fair amount of computer power; an i7 processor just about copes. When you get it right the output from the Express is superb;It's not a cheap board but the excellent filtering justifies the cost.
My latest project makes operation of the DATV Express much simpler and uses less processing power. The system consists of the front end and software developed for the BATC Portsdown transmitter. While the Portsdown will run as a standalone multiband transmitter it also has the capability to drive the DATV Express board. Linux based; the software in conjunction with the Raspberry Pi 3B + makes a fast and compact front end.
After taking the various components out on portable operations I decided to construct a complete package to tidy things up. Today I finalised what I will refer to as the head unit. Some tips for my build came from Noel G8GTZ who also saw the benefits of this system.
I will show some pictures and describe what I've done. The DATV Express and associated component parts (Base Unit) will be discussed and posted in a later blog.
Given the increased touch sensitivity and clarity of the 7 inch Element 14 screen I decided to use it instead of the 3.5 inch Waveshare used in my Portsdown Transmitter. Noel G8GTZ pointed out a very nice case for the screen which can be got from MODMYPI. In the picture below you can see the 12 Volt power conversion on the back. I've also drilled the case and fitted an additional audio output as the unit won't fit my shelf with a jack plug sticking out the top.
So there are brightness issues with some LCD screens when viewed from above and I understand the 7 inch Element 14 screen is no exception. Not sure how much truth there is in it but I'm lead to believe the manufacturer decided to invert the screen to solve the problem. Whatever you'll find the picture is upside down in this case; unless you do as I did and use a software solution to put it the "right way" up. If you use it as is; the rubber feet will be on top and you'll need to make a bracket to hold it upright. In my situation the "Head Unit" will be above my eye level so no problem having the case round the right way. More on the software fix later.
In the next picture you can see the contents of the box I fitted to the back cover of the case; it consists of of a Chinese made DC-DC buck converter board to allow 12Volt input. There are a couple of additional filter components and a 5 watt zener diode crowbar at 5.6Volts. The Chinese board is rated to 3 Amps but gets scary hot; there are vent holes top and bottom of the box and it has run for many hours.
My completed head unit will require just one cable to connect to the base unit; a good quality USB cable. At the base unit the USB cable connects to a 4-way powered USB hub. The USB cables I use are the same brand as recommended for use with the BATC MiniTiouner; Lindy Cromo.
Two software mods were made to my Portsdown installation; neither have thus far been undone by the routine Portsdown updates. The first is the fix to put the LCD up the right way; you don't need this if you're happy to use the MODMYPI case upside down. Find the fix here on Github rpi-touch-display-fix
My second software mod allows command line adjustment of the Raspberry Pi sound level; see my blog entry here.
As usual this item was rushed; if I missed something please ask using the home page contact menu.
While trying to add audio output to my Portsdown DATV transmitter I found the output level from the 3.5mm jack on the Raspberry Pi was very low.
Feeling sure a web search would help I used my favourite search engine and located the solution in a post on the Raspberry Pi forum. Thanks to forum user spitecho I used a script he had posted up.
Here is the process.
Open the terminal and type the following at the prompt:
sudo nano ~/.bashrc
Now copy the following lines and paste at the end of the .bashrc file:
# Increase volume by 5%
alias volu='sudo amixer set PCM -- $[$(amixer get PCM|grep -o [0-9]*%|sed 's/%//')+5]%'
# Decrease volume by 5%
alias vold='sudo amixer set PCM -- $[$(amixer get PCM|grep -o [0-9]*%|sed 's/%//')-5]%'
Adding aliases to the bashrc file allows you to adjust the volume up or down when you login to the Pi. Having saved the editted file you should log out and back in again. At the prompt type volu to increase volume or vold to decrease.
Note: If you use this script with the BATC Portsdown it should be placed just before the following line.
If you're not familiar with the nano editor used to add the two lines of code; please read up on it first.
Using the smart phone app Zello it's possible to use your phone in "Walkie Talkie" mode. This has proved to be very useful when operating portable.
In the south of england Shaun G8VPG has setup a Zello group ATV south. Since I wasn't aware of any group in the east I have created group ATV East with an uncomplicated sign in procedure. Please feel free to register if you either operate portable or wish to listen in during BATC Activity Weekends.
Hopefully the new 71MHz allocation will soon attract other amateurs interested in DATV. My own equipment was easily adapted from earlier tests on 6m; it's not tidy yet but it is functional and clean spectrum wise.
So just a short post to say I ordered a Sirio 3 element beam and I'm very impressed with the build quality. It won't be erected at the home QTH as I'm trying to keep a low profile. It will however be used /P on the hills.
Antennas are sexy but try convincing your neighbours.
Following receipt of my 4 metre NOV I dug out my 6 metre kit and hastily got it running at 71MHz. The power amplifier is based on a Mitsubishi brick designed for 50 - 88MHz and so needed no modifications.
Next up; the transmit filter. This is a bandpass type which I originally constructed for 50MHz; luckily it works at 71MHz with a little tweak. It's good to see it put to use again.
My BATC Portsdown is designed to work on 4m and gives enough drive to couple to the PA via the filter. I had wondered if that output connector would ever get used.
With no 4m antenna installed at the QTH I had to rummage around the garden shed and find my old Sirio vertical. The experience reminded me that the shed needs a clear out; large spiders scampered about annoyed by my intrusion.
Raising the antenna just a few feet off the patio I discovered the VSWR wasn't ideal but probably safe enough for today. Given a bit more time I can get it lower using my MFJ analyser.
Putting a call in to Arthur G4CPE we agreed I would transmit to him; my signal looked OK on the analyser. This was going to be a one way contact because my up-converter was in storage.
Here is a screen grab taken by Arthur showing the result; it shows a slightly asymmetric constellation but the spectrum was clean. Power output was 14 Watts.
Tomorrow we'll make it a two way contact
Two way contact now completed.
My receive converter has been swapped out thanks to Arthur. The current one has an amplified 70MHz SAW filter on the input and seems to be much more sensitive.
Arthur G4CPE has modified a PA originally intended for use at a much higher frequency; new coils and capacitors have been substituted and a clean 10 Watts is output at 71MHz.
These are early days and further tests and adjustments will be made. Here is a screen shot showing Arthur's received testcard.
Arthur G4CPE and I had a test arranged for 10.30 this morning with Ian G3KKD. It was scheduled for a few weeks back but Ian went down with flu and so had to defer it. We both had new kit to test and were hopeful for some good results.
On arrival at Dunstable Downs I could see we were going to struggle with all the new hedge and tree growth; it had been much clearer on my last visit. Ian gave us a call just a little before 10.30 and told us he hadn't been able to access the spot he wanted; all considered it wasn't looking too promising.
My latest rig is running about 8 watts into a grid antenna so under the right conditions we should have been a cracking signal over 37K. Sadly it was not to be and after several attempts we had to settle for a fail.
So Arthur and I decided to send in a report regardless; we can't win all the time.
At least we got some good stills and video courtesy of our good friend Don G0WFT who risked the wrath of the National Trust to get some airborne shots.
Today I did some preparation for the BATC Activity Weekend which coincides with the IARU ATV Contest. Charging batteries and packing all the 5GHz kit into a crate for Saturday was top of the list.
During a break I remembered a Russian SSTV test was scheduled for today and tomorrow; I popped into the shack and switched on my TS2000. We don't have a satellite antenna so I just left the collinear hooked up; it often works well with low angle passes.
Well before the ISS came over the horizon I could hear she was active; it was a shame because the signal was quickly very strong, all I got was the last third of a picture. I waited for the second picture and received the one below when the ISS was at best 14 degrees above the horizon.
At the start picture three was strong and clear; alas LOS was too soon.
Back to my picking list for the weekend.
Let’s hope the current good weather is still with us then.
On test this week my mk2 transceiver with power amplifier was producing just over 1 Watt. One of my friends; Arthur G4CPE suggested checking output directly on the pa as he wasn't convinced the connectors were ideal. In my haste to get the job done I had grabbed what was in my connector stock and used this open type. I should have considered suitability at 5.6GHz.
With the test gear connected directly to the power amplifier I was amazed at the difference and saw 35.9dBm on the meter. Arthur produced an SMA to N bulkhead connector with a short lead and urged me to do a quick swap.
Wow what a silly mistake I had made and what a difference. Next I changed the receive antenna connector for the same type. When I get out portable in a week or so I hope to see a big difference.
I'm hoping to use my new power amplifier during the next BATC activity weekend in June. First though a temperature test was needed as it's not very efficient. Testing power consumption a couple of weeks back it was using 85 Watts to generate 10 Watts RF output.
Following discussion with a knowledgeable friend I added a small Papst fan found in my spares cupboard. Advice was to have it cut in at about 60 degrees C; so I bought and fitted a normally open bi-metal device.
Initially I thought about buying some of the adhesive temperature strips and couldn't believe the cost; luckily I was loaned a measuring device with a probe which tucked nicely between the temperature sensor and one of the heat sink fins.
Starting at 20c which was shack temperature at breakfast time I fired up the amplifier with loads on input and output; at the same time running a stop watch. From an earlier brief run I had suspected the sensor and fan would be a safety feature only, I figured I would be OK with short overs.
It took 20 minutes and 40 seconds for the fan to kick in and the probe showed that it was exactly 60 degrees. After 2 minutes with the fan running the probe temperature dropped to 54 degrees; and 51 degrees after 5 minutes. Confident the amplifier is protected I'll run it in June.
There is one more issue to deal with; and that's where and how to mount the heavy lump without using long cable runs. Maybe the subject of another post.
While out and about with my 5GHz ATV portable set-up my modified survey tripod has been a talking point. As standard they have a 60mm hole in the centre of the head; a gimbal fitted with a 3/8" threaded screw is used to secure the measuring instrument.
What I needed was a pole mount to secure either a dish or WiFi grid; I removed the gimbal and frankly I had no idea how to make the required change. It was later the same week I recalled another club member had some brackets specially made by a company in Nottinghamshire; I gave them a call. Photographs and measurements were emailed and within a few days I received a drawing.
In this view of the underside of the tripod head you can see the peg on which the gimbal was mounted.
Below is a picture of the completed modification; which was created without making any changes to the tripod head. Apart from refitting the gimbal there's nothing more required to restore the tripod to its original condition.
If you're looking for any special brackets for your antenna installation I recommend you have a chat with Brian; see link below.