Wednesday, 27 January 2016

The Power-Lore and QRP Philosphy.

QRP operation refers to transmitting at reduced power while attempting to maximize one's effective range. The term QRP derives from the standard Q-code used in radio communications, where "QRP" and "QRP?" are used to request, "Reduce power", and ask "Should I reduce power?" respectively. In practice it is a large and growing movement within the field of radio communication; both amateur and professional. The QRP fraternity has been growing exponentially and more and more QRP clubs are thriving world wide and in a scant two decades QRP operation has become a way of life for a plural wing of radio enthusiasts and is proliferating rapidly. It is evident now that working with QRP power levels isn’t a handicap; as it was once thought..!! It's just an arbitrary restriction of the one technical aspect of radio that has consistently worked against the interests of amateur operations. Set aside power, and you are left with skill, inventiveness, ingenuity, challenge, and enthusiasm that are very similar to the attractions of low power operation with added fun at lower cost along with the incentives of simplicity of designs and portability.

The Power Lore: Most amateurs use approximately 100 watts on HF and 50 watts on VHF/UHF. Though in some parts of the world, like the U.S. and Russia; they can use up to 1,500 watts. So many radio operators believe that higher the power the longer the distance you can work with. But it is a myth and nothing more than a philosophy of quasi lore. First; a little primer on power versus gain. Keep in mind that one S-unit on a properly calibrated receiver is equal to 6 db. To get a one S-unit gain you have to quadruple your power output. In other words, you would have to go to 20 watts to increase your signal strength by 6 db or one S-unit from your 5 watts. Conversely, to reduce your signal strength by 6 db or one S-unit, you need to go down to 1.5 watts from your 5 watts. Although it varies considerably due to many factors, one S-unit is about the minimum change in signal strength to be just noticeable. Here's a table comparing various power levels to 5 watts. An explanation follows.

Forget about logarithms and focus on the business end of the equation, the received signal. Signal strength is measured in S-points, which you can usually read directly from a meter on your radio. Your concern when transmitting is how many S-points you are generating at the receiving station. The more the better you believe. It may sound astounding but it is wrong scientifically. In the first place, if your signal is perfectly copiable at S-7, increasing the strength to S-9 achieves absolutely nothing other than hissing out atmospheric noise and grunting IMDs. If you want to increase your signal level by one S-unit, for example, look in the last column for 1.00 and you'll see you have to raise your power to 20 watts to get that change. I think it is very telling to look at the figures below 5 watts. Some folks think it is much 'greater' to get a QSO at 2.5 watts than with 5 watts. In reality there is only about 1/2 S-unit difference between the two powers, hardly noticeable at the receiving end. To drop your signal 2 full S-units requires going down to a little above 1/4 watt. Curious about the S-unit difference between say 100 watts and 1 watt? Just add the absolute values in the last column for 100 and 1 watts (2.17 + 1.17 = 3.34 S-units). I think the table helps explain a lot about why QRP can be so successful. Oh, although it is not in the table, the difference between 1,000 and 5 watts is 3.84 S-units. If a kW signal is S9, your QRP will be around S5 all other things being equal. You can have fun with the table and learn more about power and signal strength ratios. If a kW signal is S9, your QRP will be around S5 all other things being equal; and you are reasonably workable with quite visceral signals..!!

Don't believe….? Ok, let's look at the power ratio in action. Say you are transmitting with 5 watts and a station gives you a report of S-5. Now double your power to 10W and what happens? Your power output has increased by 3dB and the received signal has increased by the same 3dB, which is... wait for it.... one half of one S point. Double your power again, to 20W, and the received signal is now one whole S-point stronger. Double it again, to 40W and we are at 1.5 S points. Again, to 80W and we are at 2 S points improvement on our original 5 W signal. 80W is near enough to what your typical "100W" transmitter puts out, and by now you should see what little difference an additional 20W would make. In summary by going from 5W to 80W we have increased the received signal strength by all of two S points. The reverse is true; if you are copying an 80W station at S9 and he reduces power to 5W, you will still be copying him at S7.

But let's not leave it there. Start at 100W and add 3 dB at a time by doubling power- you go to 200, 400, 800, 1.6Kw. We doubled power 4 times, picking up 12 dB or.... wait for it.... 2 S points. Talk about diminishing returns! The only caveat is that the S-meters on most radios, if they are calibrated at all, are set for the standard S9 at 50uV input- at any other input, larger or smaller, they are notoriously inaccurate.

Thus the effectiveness of QRP communication, and the quality of QRP equipment, can be explained very easily with a little math. I hear you groaning, but it is very simple math and in fact you can witness its effectiveness, yourself in practice. A major factor in the continued growth and success of QRP is the cohesiveness of the QRP community. It is a community in all senses of the word, from local clubs to national organizations.

Less Frustation More Satisfaction: The downside of wanting to be a DXer if you only have an average station - which is all most of us can afford - is that the joy of making an unexpected DX contact is experienced less often than the frustration of when you don't work it. You only have to listen to the behavior of people in pile-ups to see what I mean. Many of them sound stressed. They don't sound as if they are having a good time. Is there pleasure to be had in shouting the last two letters of your call into a microphone for half an hour, especially if at the end of it you have nothing to show for it? I don't think so. A few of the high-power guys like to claim that it's their ears that do all the work during a QRP contact. "QRPers brag about working DX with milliwatts but it's the guy other end who does all the work to make the contact" is a typical comment. But if you believe that QRP operators brag about making a contact, you don't understand the QRP mentality at all. What the QRP op feels is a sense of wonder that such a tiny amount of power can propagate a signal to the other side of the world. The QRO op at the other end can feel that too. And from the comments I sometimes get ("your QRP is doing great" etc.) I think that the grumpy guys who think QRP just makes work for them are in the minority.

Further Thoughts: The reason I think all this is important is that QRP amateur radio is something which is achievable by all. A high power, DX-capable station is achievable by a minority. Some simply can't afford it. Others don't have the opportunity to erect big antennas, or simply don't want to displease family members or annoy the neighbors. Be honest: how many non-hams think antennas are attractive? QRP is cleaner than QRO and does not advocate RF pollution. Advancements in RF practice are on and new avenues for QRP and QRPp (Very low power communications) are opening. Several weak signal digital modes like CCW, JAT63/65, WSPR,WSJT and QRSS are becoming popular among designers and operators. The program WSJT, is created by Nobel Prize winning Princeton professor, Joe Taylor, K1JT. Its purpose is to send and receive various weak signal modes for meteor and ionospheric scatter as well as EME (moonbounce) on VHF and UHF bands. And, it’s both free and well supported. On the other hand QRSS is a derivative of the CW Q-Signal QRS for "Please lower your code speed". By using extremely slow CW, it is possible to use a computer sound card and special software to extract CW characters from below the audible noise floor. Morse code element lengths of 10 to 30 seconds (or even longer) per dot are commonly used.

Amateur VLF operators have used QRSS techniques to span the Atlantic at 136 KHz and to receive very weak VLF beacon transmissions from distant locations. By adopting these same techniques, QRP operators can push the envelope of very low power HF communications.

                    A 6 milliwatt QRSS contact in action.

Wednesday, 13 January 2016

A Dual Band Ten Watt CW/AM Transmitter.

There is no greater feeling of accomplishment than to get on the air using your first home brew rig. But emergences of complex technologies like SSB, DSP and DDS etc. combined with limited resourcefulness of an average newbie proved to be the biggest hindrance in the way of home brewing. As I already discussed that involvement of these technological advancements, though good and beneficial in many ways but they have significantly aggravated the situation and thus the home brewing part of this hobby which was once considered joyous and was the backbone of amateur radio, has started to diminish. It is not uncommon to see that an average newbie starts his dream project exuberantly and very enthusiastically, spending a good wad of money and considerable amount of time but only to end up having that last hard to get component unavailable. Consequently, if we opt for exclusion of advance technologies in regular home brewing, we can still develop simple and effective designs. Simply put, if we choose to leave out SSB for example, for its complex phasing methods or all those expensive filters, we can still evolve simple to go transmitters similar with designs of the yore, using CW and AM.

Here is a simple CW/AM transmitter capable of putting 10 watts of output on both 80 and 40 meter bands. The circuit is simple and quite straight farward and needs little explanation. I used a VXO based on digital gates to drive Q1. You can copy this from my "Nano Transceiver" project described in my last post. Such a VXO provides frequency agility with lesser cost and components. It will put you on right segment of band without any initial alignment and will offer good stability. If you wish to use your existing VFO you can include Q6 as shown. D1 is included to retain proper base bias in low battery condition. L1,2,3,4,7 and L8 are wound on pig nose balun cores.

I used an IRF510 for its low gate capacitance but almost any MOSFET used in power invertors or computer UPS will work on these frequencies. In case of non availability of 2N4427, 2N3866 or BD 139 can be substituted or you can even use two BC547 in parallel. VC1 provides fine tuning of the output matching network.

The final adjustment is simple. Just connect a multimeter in series with the drain of final MOSFET and adjust gate bias of it with VR1,  just to keep the mosfet on the edge of conduction, and that's all. You are ready to go.....! Feed in your VFO/VXO , adjust VC1 for maximum output power and fire your transmissions...

Thursday, 7 January 2016

Nano: A Minimalist's CW Transceiver

Lately I felt the necessity of an 80 meter portable rig for use during traveling and camping that proved to be the main impetus behind the development of this little transceiver for portable use. As the design was incepted I zeroed my choice for my favorite direct conversion technique for its simplicity added with the incentives of no-nonsense high performance (that of course SPRAT has always been for...!). After all real flavor of QRP advocates doing more with the least possible and I abide it. The design presented here is the final result of the efforts done.


I kept the transmitter section as simple as possible and to do that I used the CD 4069 gates for the oscillator and the driver sections of the transmitter chains. The oscillator uses a common 3.58 Mhz. ceramic resonator VXO that has a wider tuning range than a similar X-tal unit and covers a larger portion of the CW segment of the band. The VXO exhibits excellent stability. A VN10KM provides the final RF amplification and gives some good QRP power for real milli-watting..!! However you can power the final RF stage from a separate 12 volt power source, if you expect more output. This way you can get about 2.5 Watts out. If you opt to do so mount an LM7806 regulator to the enclosure side to provide a regulated 6 volt to the rest of circuit. So there is considerable degree of flexibility for the varying QRP needs. I have included L5 a miniature molded RF choke to the circuitry that along with a polyvaricon capacitor forms a simple on-board antenna matching unit for long wire aerials. The idea actually came from an old issue of Lo-Key magazine of CW operators club, Australia. You just need a simple field strength meter to tune the transmitter to get on to air. It is simple enough. I have not included any LPF at the output but it can be incorporated if desired. The two spare gates of CD4069 are used to provide a simple type of side tone.


The receiver is a simple direct conversion type. L1 and L2 form simple pre-selectors. However this simple arrangement proved sufficient to eliminate broadcast breakthrough of the close by local commercials. The stage following it is a famous W7ZOI RF feedback amplifier that ensures a linear input-output impedance and good dynamic performance. I used a simple single, common source JFET as mixer stage. Despite its simplicity it performed extremely well. The output of mixer is amplified to a reasonable level by the proceeding AF stage. A simple CW filter constituted of C1-L6 provides some degree of CW filtering. The idea came from G3RJV. Both these parts sit directly on the volume control terminals on the front panel. And of course my ever favorite TDA7052 provides the final AF amplification. It is chosen because it is quieter, louder and has lesser number of bulky components around it. Consequently, with a smaller foot print it is in-dispensable for QRP designs. The receiver, despite being simple and small; is very hot and sensitive. Even with a small piece of wire as an antenna it is capable of pulling in some good DX stuff.


I have assembled of the project using VK3XU patchy board construction technique. It is easy to give it a go that way. However a suggestive PCB template is given for those who wish a real commercial look. My version of assembled rig is housed in a cabinet made of double sided PCB laminate pieces. I used polyvaricons from old transistor radio sets for various tuning controls as shown but they can easily replaced with varicap diodes for portability or in case of non-availability of the former.

Schematic & PCB Template


Wednesday, 6 January 2016

Funsceiver: A Super Simple Dual Band SSB/CW Receiver.

Here is a super simple SSB/CW receiver for 80Mtr. and 40Mtr. The receiver uses a handful of normally available components.

The complete circuit of the receiver is given in figure below. The input signal from the antenna is coupled through a varicap tuned tank circuit wired around L1, an air core coil. I employed varicap tuning as normal variable capacitors are difficult to find these days. I used varicaps but if you couldn't find, you can use normal diodes like 1N5804 etc. 

The desired band of input signals are selected by this tuned circuit and is fed to the Polyacov’s mixer wired around high speed switching diodes D1,D2. Both of these are connected back to back and thus they function as a second harmonic mixer. For coverage of 80Mtr. band you can remove either one of mixer diodes or for reception of both bands a suitable switching arrangement can easily be devised.

TR2 is wired as a collpit’s oscillator to provide local oscillator signal for mixing. A 3.58MHz ceramic resonator is used to generate RF signal to avoid the need of any alignment of the receiver. These resonators are common in local markets or can be salvaged from any old digital desk phone, since almost every phone uses them in dialing circuit. The oscillator provides good coverage of both 80 and 40 meter band. The output signal from the oscillator is coupled through bifilar transformer T2 and C14 to the diode mixer. T2 is wound on a surplus pignose balun core. As I mentioned earlier the pair of back to back diodes acts as a harmonic mixer providing reception of 7 MHz band. For receiving 3.5 MHz band either one of the diode can be removed. For a dual band reception a switching arrangement can be implemented. 

As a signal of equal frequency to the input signal is mixed in the diode mixer, the detection of the signal occurs and the output audio signals are filtered through low pass filter built around L2/C2. L2 can be wound on a 47K half watt resistor. About 100 turns of 36 SWG or similar thin wire are sufficient.This filter filters out any RF component from the audio signal and the filtered audio signal is then amplified through a two stage direct coupled AF amplifier designed around transistor TR1 and TR3. This low noise AF amplifier provides about 95 dB of gain. The output signal trough VR3 is then routed to the output phono socket. I used BC547 transistors but they can be replaced by 2N3904 or similar general purpose  NPN transistors.

The output of this receiver can directly drive a pair of high impedance headphones or can be coupled to the input of any audio player or to a set of computer speakers with internal amplifier circuit. The receiver provides reception of CW and SSB signals as well as it has the ability to copy many digital modes if connected to a PC with appropriate software. In spite of its ultimate simplicity, with a resonant antenna like a dipole it provides a pleasing audition of an entire gamut of weakest radio signals, just like a window in the air.

Those desiring a standalone receiver can use the AF amplifier circuit ahead of VR3 to avoid need of its output coupling to an external source. A suitable amplifier is given in figure below.

Saturday, 2 January 2016

A Minimalist VHF Receiver.

What is the cheapest receiver you can make for VHF? Here is a candidate where all you need to do to modify a small FM headphone receiver is to desolder one end of two capacitors, and connect a short cable with an antenna connector. 

One 22 pF capacitor lifted on the right-hand side near the headphone connector for connection to the external antenna, and another one lifted on the left-hand side, above the volume control for increasing the tuning range (The receiver IC is on the foil side of the board).

Find a simple pocket-size headphone FM receiver with tuning wheel (not push-button search). I found mine at a road side shop and it is designated "HS-822, UK design," and runs from two 1.5 V AAA batteries. See picture of complete radio above.

Open the receiver case and check the FM receiver chip. Mine has KA22429 which is equivalent to a TDA7021. It is a 16-pin surface mount device with an FM receiver with a 76 kHz intermediate frequency. Although the TDA7021 is specified for 1.5-110 MHz, don't let that scare you. The oscillator tuned circuit from pin 5 to Vcc (pin 4) consists of 56 nH in parallel with a fixed 22 pF capacitor + a tuning capacitor. Unsolder and lift the hot end of the 22 pF capacitor (the end connected to pin 5).

This receiver uses the headphone cable as the antenna. There is a coupling capacitor from the RF input on pin 12 which is connected to the headphone socket. Unsolder and lift the headphone side of this capacitor, and connect the RF input via the capacitor to a BNC antenna connector. Connect the BNC ground to ground (pin 3) or Vcc (pin 4), whatever is more convenient.

The tuning range was 88-108 MHz. Now it is approximately 112-163 MHz. Mine receives airport communications (AM), amateur repeaters in the 2m band (144-146 MHz), and some public service transmissions in the 150-160 MHz range. If I connect my TV cable, channel S9 (sound 161.25 MHz) will be received at a setting of 108 MHz. It receives wideband FM, as well as AM and narrowband FM with somewhat reduced output level.

Don't except miracles in terms of signal handling however the receiver exhibits excellent sensitivity . If there are two active repeaters in the 2m band, only the strongest will be received, this is due to well known capturing effect of FM.

Compared to wideband FM, narrowband FM/AM requires more accurate tuning, and the receiver is somewhat sensitive to the placement of your hands.

I have not tried this with other chips, such as the SC1088 = TDA7088, or the TDA7000. Both the Philips chips are specified for 1.5-110 MHz, but who knows how high in frequency they will cover?

This is your super simple and inexpensive VHF receiver well under 100 bucks. I would be interested to hear from others who try to convert other single-chip FM receivers. Happy home brewing friends.