The EMTX: How to build an 8 component 40/30 meter QRP emergency transmitter

Many thanks to SWLing Post contributor, Kostas (SV3ORA), for sharing the following guest post which originally appeared on his radio website:


Emergency transmitter: An 8-component, high-power 40m/30m transmitter to get you quickly on the air

by Kostas (SV3ORA)

Introduction

QRP is all about doing more with less. This is more than true, with the construction of this cheap, simplistic transmitter presented here. It is designed primarily as an emergency transmitter (EMTX) that can be built or serviced in the field or at any home. However, it can be used as a HAM radio transmitter as well. Do not judge by its low components count though. This transmitter is powerful, more powerful than anything the QRPers would dream of. It is just remarkable how 8 components can lead in so much output power, that lets you communicate with a big part of the world, when propagation conditions are right. It is very difficult for a circuit to match that kind of simplicity in balance with such performance.

Following my detailed instructions, the EMTX can be reproduced easily, within hours. The result is always success, this is one of the circuits that are not critical at all and a successfully working transmitter can be reproduced every time. I have built this transmitter several times, using similar components (even toroids) and it always worked. The transmitter meets the next expectations:

1. Output power (including harmonics): A few mW up to 15W (depended on transistor, crystals and voltage/current used) at 50 ohm.
2. It can drive any antenna directly, 50 ohm or higher impedance, without external tuners.
3. Bands of operation: Currently 40m, 30m
4. Mode: CW, Feld-Hell (with external switching circuit), TAP code and any other ON/OFF keying mode. AM modulation has been easily applied too.
5. Options like reverse polarity protection diode (useful in the field when testing different unknown polarities PSUs) and current meter (for easier tuning) are available.

The challenge

The purpose of this transmitter is to be used primarily as an emergency transmitter. This poses several challenges that influence the design of the transmitter:

1. It must be able to be built or serviced easily in the field or at any home, with components that could be salvaged from near by electronics sources or a small electronics junk box. This means that components count should be kept very low and they must not be rare to find but commonly available parts. As a side effect cost would also be kept small, if one is to buy any component. Also, the active components must be interchangable with many other devices without the need for the design or the rest of the circuit components to be changed.

2. It must be able to operate from a very wide range of DC voltage sources and at relatively low current, so that common house power supplies could be used to supply power to it. Such devices include linear or switched mode power supplies from laptop computers, routers, printers, cell phone chargers, Christmas lights or any other device one might have available.

3. It must be capable of transmitting a powerful signal, so that communication is ensured. An emergency transmitter that is capable of a few mW of output power, might be heard locally (still useful, but there are handheld devices for that already) but isn’t going to be of much usage if it can’t be heard really far away.

4. It must be capable of loading any antenna without external equipment required. In an emergency situation, you just don’t have the luxury of building nice antennas or carrying coaxial cables and tuners. There may be even extreme cases where you can’t even carry a wire antenna and you depend on salvaging wire from sources in the field to put out a quick and dirty random wire antenna.

5. Adjustments of the transmitter should be kept minimum without the help of any external equipment and there must be indication of the correct operation of the transmitter or the antenna in the field.

Components selection

The transistor:
This transmitter has been designed so that it can operate with any NPN BJT in place. This includes small signal RF and audio transistors and high power RF transistors like the ones used on HF amplifiers and CB radios. Despite 2sc2078 is shown in the schematic, just try any NPN BJT in place and adjust the variable capacitor accordingly. When you are in the field, you do not have the luxury of finding special types of transistors. The transmitter must operate with any transistor in hand, or salvaged from near-by equipment. Of course the power capability of the transistor (as well as the crystal current handling) will determine the maximum VCC and current that can be applied to it and hence the maximum output power of the transmitter. Some of the most powerful transistors I have used, come out of old CB radios, such as the 2sc2078, 2sc2166, 2sc1971, 2sc3133, 2sc1969 and 2sc2312. There are many others. As an example, the 2sc2078 with a 20v laptop PSU, gave 10-12W of maximum output power into a 50 ohms load.

Schematic of the 8 components EMTX for the 40m/30m bands. Components with gray color are optional.

The crystal:
This is the most uncommon part of the transmitter. You have to find the crystal for the frequency that you want to operate on. Crystals within the 40m or 30m CW segments are not that common. Further more if you operate the transmitter at high powers and currents, you will notice crystal heating and chirp on the frequency of the transmitter. The current handling capability of your crystal die inside the crystal case, will determine the chirp and the amount of crystal heating. You can still work stations with a chirpy transmitter provided that the chirp is not that high, so that it can pass through the CW filters of the receivers. However, if a small chirp annoys you or if this chirp is too much, then you have to use these vintage bigger size crystals (e.g. FT-243), that can handle more current through them. But these are even more uncommon today.

The approach I have used in my prototype, was to connect more than one HC-49U crystals of the same frequency in parallel, so that the current is shared among them. This reduced the chirp at almost unnoticeable levels, even at high output power, just if I was using a single FT-243 crystal, or even better in some cases. Again, this is optional, but if you want to minimize chirp (and crystal heating) without searching for rare vintage crystals, this is the way to go.

A bit of warning. If you notice a very high chirp when plugging in a crystal to the EMTX, you should consider this crystal as inappropriate for this transmitter, as it cannot handle the current required. If you continue to use this inappropriate crystal, you could easily crack it inside and set it useless. Don’t use these tiny HC-49S crystals, they won’t work.

The current meter:
A 1Amp (or even larger) current meter can be used to monitor the current drawn by the transmitter during key down. The recommended current operating point is anywhere between 450mA to 1A, depended on the output power (and harmonics) level you want to achieve. The current point is set by the variable capacitor. I would avoid setting the current to more than 1Amp, although it can be done. The use of the current meter is optional, but along with the incandescent bulb, will give you a nice indication of the correct tuning of the transmitter, so that you do not need to have an external RF power meter connected to the transmitter output. If you do have, then you can remove the current meter. If you don’t have a 1Amp analogue meter available, but a smaller one, you can parallel a low value power resistor across the meter. In my case, I only had a 100uA meter and I paralleled a 0.15 ohms 5W resistor across it to scale down 1Amp to 100uA, The resistor value depends on the internal meter resistance so you have to calculate this for your specific meter. When the 2sc2078 is used at 20V, 500mA in the current meter indicates around 5W of output power, 600mA indicates around 6W, 700mA 7W, 800mA 8W, 900mA 9W and 1A around 10W. So the current meter can be used as sort of power meter without the need to do any scaling on it.

The incandescent bulb:
A current meter alone, without the use of the incandescent bulb, will not give you the right indication of the operation of the transmitter. In some cases, the transmitter might be drawing current without actually generating much, or even any RF. When you are in the field you do not want to carry extra monitoring equipment with you. The incandescent bulb will light on when the transmitter oscillates. It monitors the actual RF signal, so it’s brightness changes according to the amount of RF power the transmitter produces. Along with the current meter reading, this is just what you need to know in order to set the variable capacitor properly. Note that the bulb will not lit at very low signal levels. The one used in the prototype starts to glow up from a bit less than 1W. Miniature incandescent bulbs may not be that easy to find nowadays. However, there is a good source of these, that almost anyone has in their houses. This source is the old Christmas lights. You do save old Christmas lights, don’t you? The incandescent bulb indicator as well as it’s single turn winding on the transformer, are optional components. If you have an RF power meter connected to the transmitter, you can remove these.

The diode:
The protection diode is an optional component to the circuit. If you are in the field, correct polarity of a power supply may not be obvious. Without a multimeter it might me difficult to determine the correct polarity of the PSU. A power diode (I used a 6A one) will protect the transistor from blowing up in the event that reverse polarity is connected to the circuit.

The Cx and Cy:
The Cx and especially the Cy capacitors need to be of good quality. The Cy will get hot on high output power if it isn’t. In the tests, I have used homemade gimmick capacitor and even double-sided PCB as a capacitor for Cy and they all got hot at high power. Silver mica capacitors run much cooler and they do make a small difference in the output power, so I suggest to this type. Cy must be able to handle quite a lot of voltage, so silver mica type is ideal.

The variable capacitor:
The variable capacitor can be air variable or ceramic, although I prefer air variables in tis application. In any case it must be able to handle a high voltage just as the Cy.

The key:
The key directly shorts the transistor emitter to the ground, therefore it is a part of the active circuit. For this reason, I suggest the key leads to be kept as short as possible. The key must be able to handle the voltage (20v) and current (up to 1A) on its contacts, which is usually not a big deal.

Transformer construction

The construction of the transformer is shown below step by step. Note that if you decide that you don’t need to drive higher impedance loads but just 50 ohm ones (eg. antenna tuners or 50 ohm matched antennas), you just need to wind 2t in the secondary and not 14t. You also don’t need any taps of course.

Step 1:

Take a piece of 32mm external diameter PVC pipe from a plumber’s shop. Alternatively, a suitable diameter pills box can be used, or any other suitable diameter plastic tube.

Step 2:

Cut a 4cm piece out of this tube. 4cm is the minimum length required.

Below a 4cm PVC tube has been cut in size.

Step 3:

Wind 16 turns of 1mm diameter enameled wire onto the PVC pipe and secure the winding in place as shown in the picture below. Notice the winding direction of the wire. This is the primary of the transformer, the one that is connected to the two capacitors. Notice that this winding is wound a bit offset to the right of the pipe.

Step 4:

Wrap the winding with 3 turns of PTFE tape. It can be bought at any plumber’s shop, just like the PVC pipe. The PTFE tape will help in keeping the second layer turns in place and it will provide extra insulation.

Step 5:

Wind 2 turns of 1mm diameter enameled wire on top of the primary winding and secure the winding in place as shown in the picture below. Notice the winding direction of the wire, as well as it’s position relative to the primary winding. This is the feedback of the transformer, the one that is connected to the collector of the transistor.

Step 6:

Wind 14 turns of 1mm diameter enameled wire on top of the primary winding, starting from just next to the 2 turns one and secure this winding in place as shown in the picture below. Notice the winding direction of the wire, as well as its position relative to the primary and the 2 turns windings. This is the secondary (output) of the transformer, the one that is connected to the antenna. At this point do not worry about the taps yet.

Notice in the picture below, the way the windings are secured in place onto the pipe. The wire ends are passed through the pipe using small holes and then bent towards the ends of the pipe and once more to the surface of the pipe, where the connections will be made.

Step 7:

Wind 1 turn of 1mm diameter enameled wire onto the pipe and secure the winding in place as shown in the picture below. Notice the winding position relative to the other windings. This 1 turn winding is placed about 1cm away from the other windings. This is the RF pick up winding, the one that is connected to the incandescent bulb.

Step 8:

Use a sharp cutter (knife) and carefully scrap the enamel of all the windings ends. Do not worry if you cannot scrap the enamel at the bottom side of the wire ends (that touches to the pipe). We just want enough copper exposed to make the connection.

Step 9:

Tin the scrapped wire ends, taking care not to overheat them much.

Step 10:

Now it’s time to make the taps on the secondary winding. Use a sharp cutter (knife) and very carefully scrap the enamel of the wire at the tap points (number of turns). Take much care not to scrap the enamel of the previous and the next turn from each tap point. Do not worry if you just scrap the enamel at the top of the wire (external area). We just want enough copper exposed to make the connection.

Make each tap, a bit offset from the near by taps, like shown in the pictures. This will avoid any short circuits (especially at the 4, 5 and 6 taps) and it will allow for easier connections, especially if alligator clips are used to connect to the taps.

Step 11:

Tin all the tap points, taking care not to overheat them.

Step 12:

This step is optional and it depends on how you decide to do the connections to the taps. You may solder wires directly to the tap points, but in my case I wanted to use alligator clips, so I did the next: I took a piece of a component lead and soldered it’s one end to each tap point. Then I bent the component lead to U-shape and cut it accordingly. This created nice and rigid tap points for the alligator clip.

Step 13:

This step is optional and it depends on how you decide to mount the transformer to your enclosure. In my case, I wanted to create three small legs for the mounting. I cut three pieces of aluminum straps and made holes at both their ends. I made three small holes onto the transformer pipe end and mounted the aluminum straps using screws. After mounting them, I shaped the straps to L-shape. Then I used three more screws to mount the transformer to the enclosure.

The completed transformer is shown in the pictures above and below. The 6 connection points at the bottom of the pipe, are the low voltage points, whereas the 2 points at the top of the pipe, are the high voltage points.

If you have built the transformer as described, the bottom connections are as follows (from left to right):

Wire end 1, connected to the incandescent bulb
Wire end 2, connected to the incandescent bulb
Wire end 3, connected to the current meter
Wire end 4, connected to the current meter
Wire end 5, connected to the GND (ground)
Wire end 6, connected to the transistor collector

The top connections are as follows (from left to right):

Wire end 1, connected to the 25pF variable capacitor and the Cy fixed.
Wire end 2, is the 14th secondary tap and it is left unconnected, or tapped to the appropriate impedance antenna.

Videos of the EMTX in operation

I have made two small videos of the EMTX in operation.

The first 13.5MB video (right click to download), shows the operation when the transmitter is set for a bit less than 10W of output power.

The second 3.5MB video (right click to download), shows the operation when the transmitter is set for about 5W of output power.

EMTX chirp analysis

Every self-exited power oscillator (and even many multi-stage designs) exhibits some amount of chirp. Chirp is mainly considered as the sudden change in frequency when the power oscillator is keyed down. Apart from chirp, there is also the longer term frequency stability that may be considered. The chirp in the EMTX is surprisingly low, if it is built properly. Hans Summers, G0UPL has performed a chirp analysis on my EMTX (PDF) and the EMTX built by VK3YE and presented on YouTube. Hans, performed the analysis from the video/audio recordings of both transmitters. I sent him two videos, one with the EMTX set for an output power of 10W and one where it is set for 5W. The chirp at worst case (10W) was about 30Hz and at 5W in the order of 10Hz or so. Being so small, the chirp is almost undetectable by the ear and it surely poses no problems when passing the tone through narrow CW filters. This is an amazing accomplishment from a transmitter so simple and so powerful.

EMTX harmonics measurement

Every unfiltered transmitter will excibit harmonics at it’s output. This means that the output waveform has some distortion in comparison to a pure sinewave. Many of the transmitters I have seen, present a very distorted output waveform and absolutely need a LPF if they are to be connected to an antenna. I can’t say that this is true for the EMTX, because surprizingly, it has low distordion, despite the high output power it can achieve. Although a LPF is always a good idea, it is not that much needed on the EMTX. However you have to use one to comply with the regulations.

The image above, shows the measurements on the output of the EMTX, when it is set closely to 10W at 50 ohms. The main carrier is exactly at 9.9W and all the harmonics are less than 50mW! Also, the harmonics, do not extend into the VHF region.

The image below, shows the measurements on the output of the EMTX, when it is set closely to 5W at 50 ohms. The main carrier is exactly at 5.17W and all the harmonics are less than 9.6mW! Again, the harmonics, do not extend into the VHF region.

These small harmonics levels aren’t going to be heard very far at all, compared to the powerful carrier. This means only one thing. A LPF, although a good practice, is not mandatory in this transmitter. But you should better use one so that you comply with the regulations.

Many HAMs use just a watt meter to measure the output of their homebrew transmitters. This is not the proper way of doing it, because the watt meter is a non-selective meter. It will measure both the fundamental carrier and the harmonics, without being able to distinguish them. So in an unfiltered transmitter, or in a transmitter with a simple (often non measured) LPF, this way will give a totally false reading of the output power of the transmitter at the set frequency.

The proper way of accurately measuring the output power of a transmitter and the harmonics levels, is a spectrum analyzer. The FFT available in many modern oscilloscopes, having a dynamic range of approximately 50-55dB, is adequate for this purpose as well. A 50 ohms dummy load must be connected at the transmitter output and then the high impedance probe of the scope, is connected to the output of the transmitter as well. This was the way that the above measurements have been performed.

WebSDR tests

Here are some test transmissions, to determine how far one can get with such a transmitter. I have to say that there is an antenna tuner between the EMTX and my inefficient short dipole (not cut for 40m and not even matched to the coaxial). However I could still cover a distance of more than 2500Km even on the 5W setting.

A screenshot of the transmitter signal, as received on a WebSDR 2500Km away and when the EMTX is set for an output power of 10W.

Below, is a picture and an audio recording of the transmitter signal, as received on the same WebSDR and when the EMTX is set for an output power of 5W.

Photos

Pictures of the finished transmitter. You don’t have to build it that nice-looking if you don’t care.

EMTX prototype built on a breadboard. Yes it worked just fine onto a piece of wood.


This is a phenomenal project, Kostas. Thank you so much for sharing it with us. I love the simplicity of this design–truly form following function. With a little patience, anyone could build this transmitter.

Check out this project and numerous others on Kostas’ excellent website.

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9 thoughts on “The EMTX: How to build an 8 component 40/30 meter QRP emergency transmitter

  1. Premchand

    Super and simple circuit easy to build here in South India we used 2 no bfw10 for 7 mhz vfo and one sl100 sk100 and one bd139 as final getting 7w on 12 volt in India it was called down qrp. so when compared This I component is so superb and I will assemble one the long distance communication made by me in India is only 700kms hi 73s de vu2rpc

    Reply
  2. Michael Black

    We really need to end this fixation on extreme simplicity.

    Everyone should build a crystal oscillator, that was the first radio thing I built that worked. But you don’t need power, it’s something to hear in your receiver.

    Yes, some like to build simple for the challenge, but they generally know how things work. Beginners need to start simple, but those don’t make the best rigs, and nowadays low power, CW and crystal controlled will present obstacles. It was intimidating the first time I operated with my license, and I was using someone’s Collins KWM-2.

    And simple is never enough, someone recently showing a circuit that “had no hard to make coil”. But that caused impure output, something a beginner might not know. Winding a coil is part of building a simple transmitter, learn the basics and advance.

    There is no way that someone would have an RF capable power transistor, but not have a bunch of low power transistors. Adding a separate oscillator before the power stage adds
    virtually nothing to complexity, but gets rid of some of the problems.

    In an emergency, a crystal will be the biggest problem, not parts to build a two transistor transmitter. You won’t scrounge a ham band crystal (unless there’s an analog tv set to offer a 3.58MHz color subcarrier crystal), and out of band means it had better be a real emergency. An odd frequency reduces who is listening, and CW eliminates most people from understanding what’s sent. Do the Boy Scouts even bother with morse nowadays?

    Ham radio has focused too much on “emergency communication” rather than technical. This seems to represent that. Simple rather than good. There seems to be a lot of hams who never go beyond simple, finding some excuse. So people are stuck in 1971, which is just a solid state version of the 1920s. And synthesizers and frequency counters and receivers with higher IFs existed that year I found the hobby electronic and ham magazines.

    Ham emergency communication is about providing communication to third parties, not being stuck somewhere and having to build a transmitter. Carl and Jerry in their fictional emergencies at least built spark gap transmitters from cars and tv sets.

    Reply
    1. Kostas

      Hi there Michael, thanks for your comment. I have to disagree in quite a few points, so I explain them below inline.

      >But you don’t need power, it’s something to hear in your receiver

      If this is your intention, you don’t need power. EMTX is meant to be a transmitter, not a crysal oscillator to test near receivers. I did not mention this is a beginners transmitter, no matter if it is simple it can be messed up even by experienced folk. See the examples I mention in the website.

      >“had no hard to make coil”. But that caused impure output, something a beginner might not know.

      Sure. However, I have done ACTUAL measurements on the EMTX which are present in the website just for this reason you mention. The output is surprisingly clean as you can see for such a simple TX. Although an output LPF is recommended to meet the regulations.

      >There is no way that someone would have an RF capable power transistor, but not have a bunch of low power transistors. Adding a separate oscillator before the power stage adds
      virtually nothing to complexity, but gets rid of some of the problems.

      Never say no way. People that collect components, find components in all shorts of things even in old CBs, and in fact the transistor used in the EMTX is such. But I do not see a debate here.
      Adding a separate oscillator WON’T be able to give you that kind of power, this might be new to you, but this is the case. There are barely designs out there with 2 stages that can reach 10W of output power. The ones that exist are only class-E designs. The reason is that the final amplifier can never have enough drive from the single transistor oscillator to reach these levels. EMTX is a self-exited oscillator with an adjustable feedback. This in simple words means that the transistor input gets all the “drive” power it needs in order to produce the maximum output power. It’s as simple as this.

      >In an emergency, a crystal will be the biggest problem, not parts to build a two transistor transmitter

      I see that the title of the page bothers you a bit. This is a transmitter that can really do the job, because of it’s power. Compare that to the thousands of QRPp transmitter designs that just do not have enough juice to do the job. In an emergency situation you need some power to get out of your yard. If the title bothers you, well, call it a spy set like the paraset, a self-exited oscillator, a quick and dirty design, or whatever. The result is the same. A powerful, low cost, low components count, stable transmitter to just do the job and give you fun and satisfaction that you have built something that is not really a toy.

      >Ham radio has focused too much on “emergency communication” rather than technical. This seems to represent that.

      Maybe ham radio where you live focus on em comms, of maybe people that do these videos on youtube. But there are a huge lots around the world, who do not do videos, and spend their time try to make simple things that work reliably and this involves much much more technical challenges to do. SIMPLICITY is CHALLENGING and it is the most difficult thing to achieve!
      If you say that the EMTX focuses on em comms and not on the technical side, then just read the article up to the end, I do not have anything more to say on this.

      >Simple rather than good. There seems to be a lot of hams who never go beyond simple, finding some excuse.

      I do not think you understand the article right. If you see the measurements and the tests, this is both a simple AND a good transmitter. And this is the challenge I explained above, that drives all these people you judge.

      >So people are stuck in 1971, which is just a solid state version of the 1920s. And synthesizers and frequency counters and receivers with higher IFs existed that year I found the hobby electronic and ham magazines.

      The statement that circuits in 70s are ssd version of the 20s is totally inaccurate. Late 60s and 70s is where people were trying new techniques in things that were not possible or too difficult to experiment with tubes. Amateurs could build more complex circuits and try new things due to the easier to handle transistors and the lower cost (later on). With tube designs, limited budget amateurs would only build novice transmitters. But at the end, these higher IF receivers, and even single conversion receivers built from scratch were and are bigger projects with many parts. Not everyone has the budget or the time to build these. Home experimenters seem to like a lot even today the simple regen designs, because they just do the job, no matter their disadvantages. After all, the superhet has disadvantages that can be handled too.

      >Ham emergency communication is about providing communication to third parties, not being stuck somewhere and having to build a transmitter. Carl and Jerry in their fictional emergencies at least built spark gap transmitters from cars and tv sets.

      Em comms can be many things that can be life saving. Nevertheless I am not advertising this as a life saving transmitter. I think you are still stuck on the title. Well, I built mine out of a CB radio and wire out of a transformer. The coil former can be a pills box. I have even tried gimmick capacitors which worked great. And this is certainly not a spark gap transmitter and certainly non-fictional!

      When making statements, one should first read the articles well and understand the exact operation of the circuits. Then try them himself to verify the results. This is a painful process and many people just don’t do that, but instead judging things by taking one or two sentences from an article and comment on these, ignoring all the other info in the articles.
      What is published, has to be accurate for the people. Measured results on an article, is the best way to evaluate a circuit. Many hams do not have the equipment or taking the trouble to do measurements. So unless someone can’t read the presented measurements, I think he should be grateful when these measurements are presented.

      Reply
  3. Mike

    Is the 32mm measurement inner or outer diameter? If it’s ID, what should the OD be? That’s not a standard plumbing pipe size in the US, where we use Freedom Units™!

    Reply
    1. Kostas

      The 32mm is an outer diameter. It can be close to that within limits. Don’t be too extreme on it. The variable capacitor does the job of bringing the circuit into correct resonance.

      Reply
  4. Ron F

    I’ll admit I did have a small chuckle when I got down to the schematic, imagining a SHTF scenario of “zombies have attacked the Capitol and eaten the VP, you get on air with this, and the only responses you get are the Band Police gang berating you for chirp!”

    Then I read the next paragraph and saw Kostas had that bit covered 😉

    Reply
    1. Kostas

      Yes, that’s a chirp-free transmitter (in practical terms) if built properly, as explained in the article. The greatest care is the crystal/s to be used. Care that and you’ll be fine really. I am not sure whether regulations mention things about chirp anyway, I think only the harmonics content is the one they care. But I may be wrong though.

      Reply

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