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AM transmitter construction

Discussion in 'General Electronics Discussion' started by Solidus, Mar 28, 2013.

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  1. Solidus


    Jun 19, 2011
    Hello all,

    This is a big project I have, and I doubt I will get to all the factors in this first post, so bear with me; I'll raise them into consideration as I remember them all.

    I plan on getting started with amateur radio, as it would help me not only learn more about electronics but apply it in a practical sense rather than what it is now as a dink-around hobby. However, for someone who doesn't have a ton of experience with electronics, this is a major hurdle and I have literally spent months gathering information, analyzing schematics, drawing my own, researching that, and finding out why they wouldn't work to the point of actually being able to come up with something that's viable.

    So here, I'll start with the parameters that I would like this transmitter to have and we'll go from there.

    1. I would like it to be a completely valve-driven amplifier. Every step, from audio amplifier to oscillator/carrier amplification to modulator will be valve-driven.
    2. Due to temperature-induced frequency stability issues, I would prefer this to be a variable crystal oscillator as opposed to a tuned RLC tank circuit.
    3. I am aiming for this to be in the realm of 600-800W, and I will probably construct this in multiple pieces (power supply, oscillator/audio amp/modulator, RF linear amplifier) and combine them later into a rack enclosure.

    Now, here are some of my questions:

    1. RF Output Transformer

    Regarding valve-driven push-pull stages, obviously an output transformer would be used. However, even with months of research and looking into this, I'm still blind as to a manner of calculating the plate-to-plate impedance of the stages. Most RF antennae are 50 or 75 ohms, and while I know the proper formula for calculating turns ratios of transformers based on impedance, first the primary impedance must be known.

    Now, obviously impedance varies based on frequency, resistance, inductance, and capacitance of a circuit or component. While for an audio amplifier, the frequency varies, for an AM transmitter running at say, 600kHz constant, is the impedance a definite value? That is, can it be calculated readily?

    2. Frequency range of output transformer

    I know this is a vague question, and something tells me this one will deadlock more than the first, but do any of you have any considerations for how to determine frequency range? Usually in audio usage this doesn't create much of an issue, but at RF ranges the design must be specific in a manner to reduce capacitances or inductances and other frequency-limiting variables to allow for the higher upper boundaries.

    Question is, do any of you know of equations or formulae that help in the design of high-frequency transformers? I haven't been able to come to anything surefire or anything more of sites that acknowledge that there are design considerations for that.

    3. Amplification of modulated RF carrier

    This is probably by far the most simple question - that is, can a post-modulation RF signal be amplified in the same sense as audio, provided that considerations are made for the high-frequency circuit?

    That's all I can think of for now. For awhile now, I've had the issue where as soon as I sit down to ask questions and get answers to my problems, most of the 500 million scenarios I've been pondering decide to wander into the realm of forgetfulness.

    Thanks in advance,
  2. davenn

    davenn Moderator

    Sep 5, 2009
    Hi solidus

    I admire your think BIG plans ;) but dont you think its a good idea to take smaller steps for a start ?
    Do you actually have your amateur licence yet ? mite be an idea to get that before you embark on a transmitter project.
    Do you have any experience with the high voltages associated with tube equipment ?
    Do you have a local mentor/club members to help you through your licence and construction projects ?
    If I was you I would be aiming for a transistorised transmitter in the 50 to 100W range

    You have already admitted your lack of experience and you need to take a deep breath, take a step or two back and re-evaluate your BIG project. Even a 50W transmitter would be a significant undertaking and definatly not for someone without lots of existing RF experience.
    You will need access to good test equip. spectrum analyser, power meter, dummy load, oscilloscope just to name a few
    A GDO, Grid Dip Oscillator, would also be VERY handy for building and checking tuned circuits.
    In fact a really good small starting project would be to build your own GDO

    I dont want to seem like a wet blanket, But I strongly suggest you take a breath and lean to crawl before you try to run. Working with RF is very very different to audio circuits
    I suggest you get involved with a local club, borrow books from their library ( if they have one) but at least get to know a few experienced guys that can guide your through your licence etc
    You have a massive amount of learning to do long before you even pick up a soldering iron. The ARRL amateur radio handbook would be a great start :)

  3. Solidus


    Jun 19, 2011
    Dave, you raise several good points; all of which are valid.

    First off, I don't actually have my license yet, although I will be getting started on that soon. However, it is my understanding that at least the US exams expect you to have a decent command of theory of operation, and while I don't expect to satisfy that by going and building a transmitter before, the design of one will definitely help me along that route.

    I do have some experience working with the voltages valve-based equipment runs at. I don't have thorough experience, although I'd venture to say I have a certain degree of common sense and safety. I used to say only halfway jokingly I'd perform the first power-up of my builds using a 10-foot stick to hit the power switch, and I've given myself enough shocks and electric burns from touching the wrong places at the wrong times to know to use absolute caution when dealing with anything close to high voltage.

    The lack of experience and the magnitude of this project isn't to be downplayed; I've done months of research prior to even sitting down for a serious attempt at a design and right now on my desk is a small breadboard where I'm experimenting with using a 555 astable to make a dinky little transmitter. Sure, it'll only be around the mW range, but it will teach me some of the principles of AM transmission.

    I decided to build a valve-based transmitter for a few reasons.

    First of all, I seem to find valves more interesting than transistors. Maybe it's because my entry to electronics only started when I ripped apart a Heathkit signal generator expecting to find PCBs and only found valves and sockets, but I do have much more experience constructing valve circuits than transistor circuits, and the biasing of transistors and their dynamics seems to be much more complicated than vacuum tubes to me. Also, when a tube is functioning, it glows and produces a nice orange glow. The same can't be said for a bunch of black squares! :)

    Also, a lot of homebrew transmitters are valve-based, and looking at the schematics allows me to more directly determine what each stage of the system does due to familiarity. The concept of "borrow what's right" applies here.

    In terms of equipment I currently have, I only have a few multimeters and an old valve-driven oscilloscope, although I wouldn't be starting a build without having a dummy load, frequency meter, and spectrum analyzer. I will also look into GDO circuits and possibly start constructing one. Can you give guidance on the role of that, to be exact?

    Thanks for giving lots of constructive advice, and helping me with starting points. It's a lot better than being shot down with no help at all!

    Eli Fedele
  4. Merlin3189


    Aug 4, 2011
    Can I add my weight to Davenn: this is not the way to go, if you are a beginner, even with good electronics experience. I know a professional electronics engineer who 10 years after getting his licence decided to design his own transceiver. He found it difficult. There's a lot of knowledge and deep understanding needed to get good results.

    I'm not sure what your US licence says, but ours allows us to build and operate a radio station as part of our self training in radio communication. You need to learn the basics to get your licence, but that is only the start of learning to do it well.

    You have made a reasonable choice in going for an AM transmitter, though most people start with CW for their first attempt at building a transmitter. But you could learn most of that from a low power rig, be that transistor or valve. That would keep you clear of some of the dangers and difficulties (my less than 100W valve transmitter carries 1000V on the plate.) Also, if you are not quite as successful in your design & construction, then low power reduces both the signal you want and the spurii that you don't want: That is important because it will avoid your becoming a bad nieghbour and possibly breaching your licence, which sets limits on the amount of interference you may cause.

    As for answering your questions, especially since you are learning for your licence, again I would support Davenn: the ARRL Handbook is great and contains chapters on everything you need to know. Join a club: and again, QRP (low power) groups tend to be much more interested in home constructed equipment in my experience.
    If you are keen on valves (and you could build a valve rig with only 100V on the plate), try to get hold of old editions - 50 + years ago, magazines regularly published designs for valve gear and usually gave a lot of explanation of design principles and warnings about problems. QST magazine from ARRL and Radcomm from RSGB were great sources, as were many books that they published specifically for people like you (& me!)

    I'm a bit ashamed to admit that more than 35 years after getting my licence, I've never built a transmitter myself, though I've built several receivers (& designed some), which was difficult enough for me. So I've some admiration for your aspiration, which I hope you will achieve some day. But for now I'd say, go for something simpler and safer and avoid the problems of higher powers. You can learn all the stuff in your first exams from that.

    Good luck & 73 de G4YZA
    Last edited: Apr 12, 2013
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I have a DVD of all the issues of Radio & hobbies, and later Radio, Television and Hobbies from the first issue (1939) up to the last issue (before it became Electronics Australia in 1965). Oh and they also have EA from 1965.

    If you find any references to these magazines, I may be able to assist you.

    I think you'll fine the CD for sale on the Silicon Chip web site if you're really interested.

    If you're not quite so interested, maybe I can locate the indexes they used to publish (I know they did with EA, not sure about the earlier magazines). Alternatively I'm pretty sure our reference Libraries will have these magazines indexed and they're available over the web (the index at least). Perhaps this applies to other magazines of interest too (If not from an Australian library, from one more local to you).

    Here id the index entry for Radio and Hobbies

    I work within cooee of the State Reference Library. If you're interested (and can't find it yourself) I'll go up there and enquire as to how well these are indexed and how to search for individual articles, view the index, etc.

    It's been a while since I used their system and it appears to be a little (a lot) different from last time.

    A list of projects from 1965 onwards is available here

    Alas, much of this information pre-dates electronic records, so you need to rely on manual indexing :(
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