So I've been thinking about amplifying my AM transmitter. I looked online and it seems all the sources for IRFP amplifiers that work in the AM broadcast band are no longer available. The only AM amplifiers I can find are on ebay and they use very expensive high power mosfets.
I also think it would be fun to learn how to build an amplifier because everything I have up to this point, I just bought (or it was a super simple crystal oscillator into a 2n222 or whatever).
Right now my AM transmitter produces 5W RMS. I'd like to make an amplifier with around 100W RMS using these IRFP devices. I saw there was a 30W amplifier posted somewhere on here, but I already have a 15W RMS amp, so I don't think there'd be much of a difference.
After a bit of research I found that most AM transmitters are built with either a high power audio amplifier or PWM because driving the amplifier linearly like you do with FM would cause audio distortion. Is this true? Would it actually be audible.
After a bit of research I found that most AM transmitters are built with either a high power audio amplifier or PWM because driving the amplifier linearly like you do with FM would cause audio distortion. Is this true? Would it actually be audible.
There is absolutely no audio issue with using a linear RF amplifier as long as the amplifier is not overdriven. The problem is when people do not take precautions and drive the amplifier too hard not leaving enough headroom for the 100% modulated RF which then causes RF clipping and you would guess it lots of splatter. For example a 10 watt AM transmission would require at least 40 watts of headroom in the linear amplifier if being low-level modulated. The obvious issue with this is low efficiency but at low transmission power it isn't really an issue.
Modulating before the power RF amplifier is called low-level modulation, whereas modulating the power amplifier itself is called high-level modulation. Both forms of modulating an AM transmitter will give you an equal quality signal and audio. It just happens to be easier to low-level modulate smaller transmitters. There are even instances when some transmitters use a mix of both low-level and high-level modulation, like with a lot of the older citizen band two-way radios.
It is true that modulating the output amplifier requires a more powerful audio amplifier like you stated. PWM is a good way to get good power efficiency but lots of transmitters out there are still "plate modulated" using a modulation transformer and a powerful audio amp. This is why with small transmitters sometimes it is easier to low-level modulate it and then use a linear RF amplifier at the output because it does not require a powerful audio amplifier, but it's not much of a trade off, just depends on which setup is more convenient for the application.
Personally my setup consists of a 2.5 watt modulated transmitter running into a 40 watt linear amplifier. Carrier is set to 10 watts, and the signal peaks at 40 watts when 100% modulated (carrier is always 1/4 of 100% modulated power). I have also used transmitters with high powered audio amplifiers driving the output RF stage itself and both methods work equally well.
If you want a 100 watt signal you must ask yourself if that is the peak power you want or carrier power. If you want 100 watts carrier you will need at least a 400 watt linear RF amplifier. If only peak power is what you want then with a 100 watt linear you are only going to get 25 watts carrier. Keep in mind that even 25 watts on AM is a lot of power and would be close to the equivalent of 100 watts on the FM broadcast band. Remember all that power is in the sidebands of AM. With a 5 watt transmitter like you describe having you will need a linear amplifier that can take that much power at its input. Some linear amps do have "padding" options but you would just be wasting power.
If you don't understand the different classes of amplification then now would be a good time to learn. Class C is often used in FM work where linearity is not necessary or in plate modulated AM. With AM that is low-level modulated (like you are considering using) you will *need* to use linear amplification, aka: Class A or Class AB. Class B is not really suited for transmission equipment. Class D and E are other options often using MOSFETs and PWM modulation and purely digital signal inputs but beyond the scope of this.
Thanks for all the info! Don't usually get such in depth responses on a forum.
OK, so it seems like low level modulation should be fine as long as the amp is a linear design. I am aware that I will need an amp capable of 4 times the carrier power. So what I am looking for is a 400W PEP amplifier. I know that sounds like a lot, but I've tested the range of my current 5W transmitter into a tuned long wire and it goes about a mile (maybe 3 with a lot of static). My state has the least ground conductivity so that could be a factor. I was once able to get around 12 miles I think, but that was using a balloon and a full quarter wave vertical. I plan on putting up an inverted L using a crossbow or maybe my brother's drone.
For the transmitter's safety, is it possible to tap a dummy load with an antenna and shorten until the most power is going into the antenna or would this cause a mismatch and actually be pointless in protecting the TX?
stretchy: I may be interested in your amp. Is it linear? Does it have to be used with PWM? Would it work with at least 5w carrier input? When you say 100W do you mean carrier or PEP? What form does it come in (kit, schematic, pre-built)? And what frequency range?
OK the amp, well it's more of a transmitter, just feed it with twice the frequency you want (uses a div by 2 on the PCB to drive Push pull FET's). It's class E it'll prob do 200W (PEP) I limit it to 120W peak @ 24V with 40W carrier @ 12V. VERY efficient (85%)and the design can be used form 0Hz to about 10MHz. Class E has been around for a while now and there's nothing to touch it for efficiency. The PWM is class D and being a 'Series' modulator you have to (generally) run a higher voltage but because most power loss relates to current, volts are a good thing! Forget Mod trannies and ALL that, waste of time, inefficient, non linear, cant do more than 100% mod, ALL of these things are NOT so with PWM. Your going to have to send me a mail to; stretcyhyman2000ATyahooDOTcoDOTuk.
I also sell a LuLu style TX, have some on FleaBay right now.
'Class E Transmitter Shortwave 15W'
Yep it uses a mod tranny as I got moaned at so much for not producing a design that did!
Main issue with folk is they think they cant get any power at 6V (this would be the carrier voltage at 12V) but they've been using Class C. I can easily achieve 10W @ 6V and 40W @ 12V with a push pull class E amp.
I want to be able to easily experiment with different antennas. Can I do this:
For the transmitter's safety, is it possible to tap a dummy load with an antenna and shorten until the most power is going into the antenna or would this cause a mismatch and actually be pointless in protecting the TX?
Thanks for all the info! Most of the circuits you posted are kinda over my head. I don't really know much about electronics, so I'd just be mindlessly constructing it. I just want to stick with the amp for now. The amp you describe doesn't seem to be what Im looking for. You have to feed it with double the frequency? Why? That won't work with my existing transmitter.
I can find general info on how an amp should be constructed, but the part that I really don't get is the matching networks (I think that's what it's called). I understand the basic theory, but I can't do it because I don't know what capacitance/inductances are required and I can't find any info on how this is done. It differs per transistor as well, right? So what do you look for?
I do really appreciate all the help though. If I knew more about electronics, I'm sure it would be very valuable.
For the transmitter's safety, is it possible to tap a dummy load with an antenna and shorten until the most power is going into the antenna or would this cause a mismatch and actually be pointless in protecting the TX?
The problem with that is once you put a 50ohm dummy on the output attaching any antenna to that same line will not show you any practical measurements. The transmitter will see the proper load, but the matching of the antenna will not I believe. There is a neat trick though to test low power transmitters using this method so that if you only want to run a test broadcast across the house you can just connect a random length of wire up to the dummy load. Power goes into the dummy and a bit is leaked from the wire
You would be better off just tuning up the antenna to the transmitter itself since it is likely able to take a mismatch without going up in smoke, and then connect the power amplifier once adjusted. With 100 watts carrier you are going to need a heavy duty RF ammeter to tune the antenna for maximum current. It is hard to find SWR meters that work well at mediumwave frequencies unless you have a BIRD meter.
For the kind of power you want you are probably best off converting some 160 meter ham radio linear amplifier to mediumwave. This will take some skill even dealing with those kind of power levels. 100 watts is not childs play, the voltage/amperage required to run that will be dangerous, and RF burns will bite you bad if you make a mistake.
Not to sound discouraging but I still think you would be better off with lower power and spend most of your time on the aerial and radial system until there is no further improvements to be had given the setup and location. Then worry about higher RF power levels if you feel it is worth it. The antenna is going to be the least efficient part of the whole project because of size and practicality, so it is best to get that as good as you can get it first instead of throwing wasted power into it that will just be burnt up as heat.
The amp is OK, but it's just an amp and it AB so not very efficient. For A.M. use Class E and PWM the O/P.
SO Simple! why bother with anything else?
Maybe you guys are buying some MW TX kit and then trying to amplify it?
I'm not sure?
Please look at my stuff and also the FAT5 and all the work on the Class E Website. The FAT5 and my variant are just cut down versions of the WA1QIX design.
They are the most simple and efficient designs out there, period.
I tried to contact that guy and there was no response. I suspect he's closed because of the Greek instability. Have you ordered from him recently? Can you comment on his products?
The amp is OK, but it's just an amp and it AB so not very efficient. For A.M. use Class E and PWM the O/P.
SO Simple! why bother with anything else?
Maybe you guys are buying some MW TX kit and then trying to amplify it?