HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Radiotech on June 29, 2022, 1909 UTC
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I have spent some time messing around with the Hellschreiber beacon, and got some new ideas for construction and also managed to get a bit more power from it. The circuit is a simple crystal oscillator running at 7140kHz into a C-Class amplifier IRF510. There is a low pass filter after both the oscillator and the power amplifier and the output is nice looking.
At 13,8v i get 12W at 65% efficiency. The enclosure is a 3" PVC pipe with 3d printed end caps, i hope to seal it from moisture with epoxy glue. In the pipe i will try to fit a small battery pack, and connect a small solar panel. The PVC pipe is better than PE and PP since those wont glue at all.
I put this one in 7140kHz to not disturb people in the CW part of the band.
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You might want to consider conformal coating the board for long term stability and resistance from the elements. Conformal coating has been applied to PCB for decades and is in wide use in military applications. It also looks like MG Chemicals has this available in aerosol format which will avoid dipping the PCB or trying to brush it on which is mainly used in repair and rework applications.
https://www.mgchemicals.com/category/conformal-coatings/ (https://www.mgchemicals.com/category/conformal-coatings/)
For the end caps, I would use a silicone based RTV. The same stuff you use around your bathtub will work like a charm. Don't forget to put a small weeping hole in the bottom of the pipe to allow the moisture to escape.
Good luck with the project. It looks good.
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If you do conformal coat, keep in mind that it might affect the operation slightly due to the capacitance of the coating against ground. Yes, the frequency is crystal controlled but that's not what I am talking about.
Also, conformal coating is not a particularly great heat transmission medium; you may find the circuitry also behaves differently due to the increased temperature under the coating. Definitely do not coat the heat sink and the final transistor.
The conformal coating that I have used professionally made rework a pain in the backside.
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I continued working on the enclosure today, i am trying to get the board as high as posible in the tube to be able to store batteries under it.
On standby the circuit draws 17mA and the microprocessor 3mA. Probably the voltage regulators use most of it, one 5v for the mcu and one 8v for the oscillator. I will half to think about getting that a bit lower.
Edit: I got the power consumption down to 9,1mA by adjusting the IRF510 bias voltage. The current users are probably as below, it would be nice if i could get the oscilator to run on 5v, since i could take away one of the regulators and get down to 6mA.
On TX at 14,4v it draws 1540mA and puts out just below 15W of RF power.
MCU: 3mA
5v regulator: 3mA
8v regulator: 3mA
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Why not run class E, way more efficient and will consume far less current for same O/P.
Gave you loads of advice as have others but you seem stuck in the past with a design from the 1950's
Ah well, your batteries will just go flat sooner than they would with a better design.
Str.
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I got a 14,4v 2500mAh batterypack(from two 6-cell NIMH batterypacks), the tube needs to be a bit longer to fit them how ever. 14,4v will give me 12-ish watts of RF output. Hopefully i can get a small solar panel to charge the batteries. I dont want a complicated charging circuit, but will look into a the possibility of using a relay to cut charging, or short the solar panel.
Without a solar panel the 6mA standby current would exhaust the battery after two weeks or so. Non-stopp TX with 50% duty cycle should last about three hours.
Thank you for the sudgestions on using a better amplifier, I know that there are better designs out there, i might continue experiment with other amplifiers as another project.
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Another suggestion (I really don't know why I bother!)
Why are you using NiMH?
LiPO, everytime.
Or maybe you should use NiCads ?? ;)
Str.
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I thought a bit about what battery to use, and decided to use the 7,2v 2500mah NIMH battery because it doesnt really need a charge controller. The price was about the same for both Lipo and NIMH, but the NIMH has a couple up sides.
Pros:
- Can handle 1/10c(250mA) continous charging
- Does not break down due to voltage getting too low.
- Isnt as much of a fire hazard that LiPo is if you handle them wrong
Cons:
- Lower capacity by weight/size
- High self discharge
- Lower current capacity
One possibility with the NiMh is to get a solar panel that provides maximum 250mA, then i dont need a charge controller, the panel could charge the battery indefinetly without any risk of over charging the battery.
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What's wrong with a charge controller?
Plenty from CH using multiple cells and cost virtually nothing.
3Series 11.1V 12V 12.6V Li-ion Lithium 18650 Battery Charging Protection Board PCB BMS Module(10-13A) https://amzn.eu/d/7XpfAJs
You seem bent on using old inefficient technology at every step.
I'll leave you to it bud.
I'll await V3!
Str.
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I have not got much experience working with MOSFETs, but this perhaps could work as a charge controller. When the batteryvoltage gets above say, 15volts, the attiny45 puts a high on PB5 and shorts out the solar panel to stop over charging the battery.
In this way, if the battery goes dead, there is no voltage needed to start charging the battery. R1 is a simple 10k pull-down. IRF520 should be able to deliver 3 amps with 5v on the gate. Do you think this would work?
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Just buy the real thing, it works and has the terminals to solder the batteries in. Just supply wi the correct voltage and leave alone. Charge will terminate at correct voltage and go O/C when near flat. A total no brainer. Comes in LiFePo and Lipo flavours.
Anyone building a beacon would be well advised to use such.
So LiPo, Attiny and class E, + a PCB.
Maybe get round to it one day..
Str.
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You are probably right streachyman, my designs will never become very usefull or efficient, designing things my self(without propper expertise) and soldering together old parts is probably a waste of time. :) It is summer time so i will throw out this old project and spend some time in the sun instead 8)
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Ha ha! Just that being an Engineer or 'Racio Tech' it's good to try to make the best design is it not? There's load loads of old published designs around that weren't much good when they were current and now with new devices, parts and techniques there's a fairly decent and efficient design to make I think.
Please look at the advice given as I think if heeded you'd have a really good design.
Please design a PCB, they're very simple and so cheap making it a piece of cake to put the circuit together.
Sounds like you're having fun.
Keep it up!
All the best from Blighty.
Str.
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You seem bent on using old inefficient technology at every step.
I'll leave you to it bud.
Another suggestion (I really don't know why I bother!)
Why not run class E, way more efficient and will consume far less current for same O/P.
Gave you loads of advice as have others but you seem stuck in the past with a design from the 1950's
Not satisfied with rubbing people the wrong way on another forum with a similar line of criticisms, you have to continue to do it here too. (Shaking my head.) I actually happen to agree with you in your general thrust on that other board about using obsolete transistors and boat-anchor-style construction techniques but I would not have conducted myself in the same manner.
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Stretchy is from the UK. The Sun, the thing that powers solar panels, is just a rumor over there in Cloud Land. Being an island nation they prefer windmills. They're building a huge array of them on Guernsey. I hope the cows and sheep aren't lifted off to France by the vortex.