HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Pieritz on July 21, 2014, 1921 UTC
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Hallo! I want to build an AM-transmitter with a power output of approximately 10 watt for the frequency range between 100 kHz and 2000 kHz. The transmitter should be a solid-state type with an operating voltage of 12 volts and it should, if possible only use parts available in electronic stores and not too many coils to be manufactured by my own.
I want to use a linear amplifier as I want to use as oscillator and modulator a "Spitfire AM transmitter" ( http://www.vcomp.co.uk/spitfire/spitfire.htm ), as this device can be easily tuned digitally to the desired frequency and has a very excellent AM modulator, but just an output of 100 mW.
Has someone switching diagrams of a radio frequency power amplifier, which would fullfill my ideas? If yes, please post them or show my links to them. If MOSFETs and other types of FETS are required, how sensitive are these types to statics? Is it still so, that the danger of destroying a FET by soldering is so big, as one can read in most books from the 1980ies, or are there protection devices reducing the risk at modern FETs?
Can I build the desired amplifier using integrated circuits for power audio frequency amplification in a modified way? If yes, with which ICs it would work?
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Welcome to the board Pieritz! I don't know of any small linear circuits for MW although I'm sure many exist. You can Google for those. You will be looking for a linear amp that has a gain of about 13-14 dB. I have built something similar for HF that uses two IRF510s in push-pull, but that design uses several transformers wound on ferrites. Just about any MOSFET will give you usable power on MW. The IRF series performs well at low frequencies. These units have been designed for automotive and industrial uses and are quite robust - no special handling required. For simplicity, a single-ended amplifier would probably be best for you. And remember, MOSFETs can be connected in parallel for more power handling. Although this increases the capacitance problems, on MW, the frequencies are low enough where that is rarely a concern. One last thing - when using a linear amplifier you really should always use a low pass filter at the output. It will help to keep your operation cleaner. And cleaner is better, and more discrete. Good luck with your project... 73!
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There really shouldn't be much to this. Most drive-in transmitters and the like used an LM1496 modulator followed by a push pull linear amplifier. As FF said, use a pair of push-pull irf510's or similar running from either a 12 or 24 volt supply. Use material 77 on the toroids so they will work in MW. Probably T140-77 for the output wound tri-filiar, with two of the windings joined to form a center tapped winding for the primary. Circuits like this should be easy to find on the web.
Something like this http://www.sm0vpo.com/tx/15w-pa.htm (http://www.sm0vpo.com/tx/15w-pa.htm)
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Would the circuit shown on http://www.qsl.net/df3lp/projects/lftx/tx50.gif at http://www.qsl.net/df3lp/projects/lftx/ would work? Ifnot, which modifications are required?
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Would the circuit shown on http://www.qsl.net/df3lp/projects/lftx/tx50.gif at http://www.qsl.net/df3lp/projects/lftx/ would work? Ifnot, which modifications are required?
No. If you need to ask, then you probably lack the expertise to get it working anyway. My advice is to stick with tried and true designs. I'm sure you'll have plenty of "fun" getting an amp working, even with no mods. Redhat gave you a great link to a workable amp on Harry Lythall's site. I suggest you give that one a shot. And please use a low pass filter at the output. There's a good calculator for one here:
http://www.calculatoredge.com/electronics/ch%20pi%20low%20pass.htm
Good luck with your project...
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Also keep in mind when considering amplifier ratings for amplitude modulation, a ten watt amplifier means ten watts peak, not carrier level. For 100% modulation, the peak power is four times the carrier level, ie 2.5 watts carrier is 10 watts peak at 100% modulation. Above ten or so watts carrier, it becomes much more advantageous to do high level modulation verses linear amplification. A 30 watt linear am transmitter produces an awful lot of heat, while its high level cousin produces almost none. This is ever more important for battery operation.
+-RH
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Redhat makes an excellent point Pieritz. If your Spitfire AM output is rated as 100mW PEP and the linear has a 13-14 dB gain figure, the exciter will drive it to about 2 watts PEP output. If the exciter is rated as 100 mW carrier output, its PEP will be 400 mW, if fully modulated. This will drive a linear with a 13-14 dB gain figure to about 8-10 watts PEP. The overall point is to ensure that your linear amp will safely develop the power that its being driven to. If not, POP goes the MOSFETs. Of course, even before that point is reached, you will be scraping off more and more of your modulation envelope, the more you overdrive the linear.
Also what Redhat said about producing heat and power wastage is absolutely true. You will have to decide whether a linear is really the way you want to go. Although I have dedicated rigs for 43M and 48M, I use linears a lot to get a pirate signal up on the higher bands. I find it easier to build and tweak many band-specific exciters and low pass filters (and dipoles), and bring the power level up with a broadband linear. I can easily run a couple hours from a 90aH Marine Battery. It works for me. Only you can know if a linear is the right way for you to go.
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There is a guy on Ebay whose handle there is pbsn6040. His company is called BootsParts. (I think?) He uses clock oscillators for his transmitters and sells the audio transformers to feed them. He's also got diy linear amps for them. The best thing about his stuff is it's dirt cheap.
You could easily buy a clock oscillator that's resonant at say, 1710 khz., and amplify the thing to a couple or three watts.
The big problem with MW is the antenna. It's tough to build or buy one that's efficient down there and meets FCC Part-15 regulations. It doesn't mean it can't be done, but it's a PIA.
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I'll have to check out pbsn6040, Pigmeat. There are a few ebay gems out there amongst the ripoffs. I buy a lot of my RF transistors from "weezle66" over in the UK. He is a distributor for Eleflow, a company that manufactures legacy RF semis that exceed the original specs. I'm always looking for other reliable parts suppliers. Thanks for the tip!
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Hi Pieritz :).
Here is another option from Germany. Works very fine linear (2.5W AM = 10W PEP). It is kit you have to assemble, I use even 3 of them in 3 different designs.
http://www.qrpproject.de/UK/qrppa2008.html
Remember.. You most have 40W PEP to make 10W AM ;).
Maybe this one:
http://www.rfsource.gr/pm62fdc-am-pallet.html
One thing more.. You must change your Spitfire output to 50 Ohm..
Best 73 from Ole (Denmark) :)..
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Don't sweat it FF.
Ole, that's some nice stuff you posted. The beginner stuff is priced right and the plug and play reasonable.
BTW, I'd wondered where the suppliers for Elecraft disappeared to. Thanks for the info.
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BTW, I'd wondered where the suppliers for Elecraft disappeared to. Thanks for the info.
A correction on the Eleflow guy: "weazle66"... my bad...
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A good station setup it's not all, you must also have a good antenna. Look on this site:
http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html
You can learn more about my setup here and look in the 'PhotoGallery':
http://radiooznrh.webnode.com/
Best 73 Ole :)..
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Hi Ole! Really nice website with lots of interesting info and pix ("Polka to you all" - LOL!) Congrats on getting the MW antenna repaired... 73!
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@OZNRH: How sensitive are the MOS-FETs used in the kits described at http://www.qrpproject.de/UK/qrppa2008.html and http://www.rfsource.gr/pm62fdc-am-pallet.html?
Which impedance has the output of the spitfire AM transmitter?
Can I realize the impedance matching well with standard inductvities ( I mean that inductivities, which look in size and design normal resistors) or should better coils be used manufactured by oneself?
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@Pieritz: When you ask about the sensitivity of used MOSFET, do you talk about as modules or single MOSFET?
The Greek modul use an IRF640 and the German use two types RD06HHF and RD16HHF, here you must 'Google' the datasheets.
If you talk about as modules, the Greek has about 27dB. at 28V. Look here:
http://www.rfsource.gr/files/PM62FDC.pdf
The German has 30 dB Gain at 13.5V, 10mW in, 10W out (P/P):
http://www.qrpproject.de/UK/qrppa2008.html
My setup: 'Home made Exiter --> QRP-PA2008 (inside the exiter) --> PM62FDC' --> SB1000 --> 1 kW P/P (250W AM).
The Spitfire: One of my friends has a Spitfire and I will ask him about he has a diagram, but i think it has a kind of LP filter there adjust to a '3 meter wire', so the output impedance is 'unknown', but if he has a diagram, I will see what there can be done so it works as 50 Ohm.
Best 73 Ole :)..
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Hi,
I just found your 2014 forum's topic about your search of an RF amplifier
to make more power out of your Spitfire transmitter.
I'm looking for the exact same type of amplifier for my SSTRAN transmitter.
So far I managed to get more power from it by using around 25 feet of wire
'stapled' to my basement ceiling. Doing so I went from 10% reception to around 60-70%
on my Harman Kardon AM receiver ! The wire placement is very important and picky though..
So what RF amplifier did you find since then ??
Thanks.
Jean-Pierre Desrochers
Canada
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Hi,
I just found your 2014 forum's topic about your search of an RF amplifier
to make more power out of your Spitfire transmitter.
I'm looking for the exact same type of amplifier for my SSTRAN transmitter.
So far I managed to get more power from it by using around 25 feet of wire
'stapled' to my basement ceiling. Doing so I went from 10% reception to around 60-70%
on my Harman Kardon AM receiver ! The wire placement is very important and picky though..
So what RF amplifier did you find since then ??
Thanks.
Jean-Pierre Desrochers
Canada
https://www.ebay.com/itm/AM-MW-RADIO-BAND-DIGITAL-LCD-DDS-TRANSMITTER-40-WATT-PEP/323960968980 (https://www.ebay.com/itm/AM-MW-RADIO-BAND-DIGITAL-LCD-DDS-TRANSMITTER-40-WATT-PEP/323960968980)
This guy normally has EXACTLY what your looking for, but it looks like he's out of stock right now. It takes about 0.1 watt and makes about 6-8W.
This is a fully build transmitter that he makes. Maybe you would find it interesting. Keep an eye on his shop, the amps will probably be back. I've seen them disappear briefly before.
The real name is Lesvos Electronics of Greece.
Check this (RF INPUT:0.1-1W)
But it is a bit overkill: https://www.ebay.com/itm/AM-LINEAR-AMPLIFIER-150W-600W-PEP-MF-HF-0-4-3MHz/233237832811 (https://www.ebay.com/itm/AM-LINEAR-AMPLIFIER-150W-600W-PEP-MF-HF-0-4-3MHz/233237832811)
It probably won't drive to full power, but it should still produce a lot of power.
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... you will need a tuner/SWR analyzer for that kind of power though. An MFJ SWR analyzer should do.
And of course an antenna in your basement is not optimal for this.
Try putting a long wire outside and then ground to a water pipe in the basement.
Here is another option: https://www.ebay.com/itm/20W-HF-PA-20-watts-24v-HF-amplifier-for-HAM-radio-CW-SSB-FM-digital/332643829840 (https://www.ebay.com/itm/20W-HF-PA-20-watts-24v-HF-amplifier-for-HAM-radio-CW-SSB-FM-digital/332643829840)
You need an attenuator -20db for it: https://www.ebay.com/itm/Attenuator-SMA-MF-6GHz-Male-to-Female-2W-1-60DB-RF-Coaxial-Attenuator/323827300226 (https://www.ebay.com/itm/Attenuator-SMA-MF-6GHz-Male-to-Female-2W-1-60DB-RF-Coaxial-Attenuator/323827300226)
Start a new thread next time. It's not good to resurrect old threads from 2014 even if they are somewhat relevant.
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100mW with a wire taped to the ceiling?
?