RF DIODE DETECTORS.

Simple diode detectors are not linear. There is a small forward voltage drop across the diode, and this voltage varies with diode current. Even if we allow for a fixed voltage drop across the diode, measurements will not be accurate at all power levels. At 1mA of diode current, the drop across a Schottky signal diode will be about 0.3V. At very low diode currents of around 1µA the drop will be much lower, typically about 0.1V.

This is not too much of a problem at higher power levels because an error of a fraction of a volt is quite a small percentage of a total peak voltage of several tens of volts. We can choose to ignore it or partially compensate by adding a fixed offset to the measured value. About O.6V for a silicon diode or O.3V for a Schottky signal diode such as 1N5711 or BAT 43 is close enough to be reasonably accurate. However, at very low power levels, the voltage drop is a significant fraction of the peak voltage and will lead to increasing errors as the peak voltage is reduced. For example, if the peak voltage is 500mV (+4dBm, or 2.5mW), a voltage drop of 0.2V across the detector diode would result in a measured peak voltage of just 300mV (0.46dBm, or 0.9mW). This problem gets even worse when the peak voltage input approaches 0.1-0.2V and the detector output voltage is close to zero.

 

There are a number of ways to improve the accuracy of a diode peak voltage detector. We could calibrate the meter by hand to eliminate errors at the lower end of the scale. This approach works well in practice but it is time consuming and requires unique calibration curves for individual diodes.

We could apply a small amount of forward bias to the diode which would reduce the voltage drop for RF signals or we could use a second identical diode as a reference to show us the required offset for a given level of current and diode temperature. It would be possible to apply all three methods to obtain the best possible accuracy but to keep things simple; I will adopt only the last method. Figure 3 shows how an opamp and a second diode can be arranged to compensate for the voltage drop of the detector diode. This circuit is due to KI6WX [2]. When used with a closely matched pair of 1N5711 diodes, this circuit will accurately track the input voltage down to a level of well below 0.1 V (-10dBm, or 100µW). If the circuit is to be used with a single-ended power supply, the opamp input and output voltage range must go all the way down to the negative supply rail. I used one half of an LM358. A CMOS input type like the CA3140 would be capable of even better performance.

Homemade HF Antenna Balun

 

A balun is a device that is used at the feed point of a balanced antenna when an unbalanced feed line is desired to feed the antenna. Balun is a contraction for BALanced to UNbalanced. A common example of where a balun would be desired is at the feed point of a dipole antenna when a coaxial transmission line is used. If a balun is not used it is possible for common mode currents to be present on the feed line. The effect of this could be undesirable if the directional properties of the balanced antenna are to be maintained.

Since the feed line usually leads into the shack RF could be present in the shack to create RFI as well as the possibility of receiving excessive amounts of RFI from indoor noise sources. It is often found that a balun is not necessary and everything works just fine feeding the balanced antenna directly with coax cable. When this is possible it may be found that the feed line is an odd multiple of 1/4 wavelength. In this case the transmitter end of the feed line is usually grounded and up from this point on the coax 1/4 wavelength or a multiple thereof will appear as a high impedance. When this high impedance point occurs at the feed point chances of common mode currents are low. Rather then take any chances it is often recommended to use a balun.

There are several different kinds of baluns. Some provide a 1:1 impedance ratio while others can provide 1:1.5, 1:4, and many other impedance ratios. A 1:4 ratio balun would come in handy if you were feeding a folded dipole (200 Ohms) with 50 Ohms coax. For a 1:1 ratio a balun can be constructed using the feed line itself by simply winding about five turns of the feed line around a 2" diameter piece of PVC. I preferred a 1:1 ratio balun that I could easily move from one antenna to another by simply unscrewing the coax.

My balun uses AWG 12 enameled wire trifilar wound on a 6" X 1/2" piece of ferrite rod. 7 turns are tightly wound around the electrical tape covered rod. The free ends of the windings are connected as shown below in the schematic. The whole balun is installed in a 10" piece of 1-1/2" schedule 40 PVC pipe. A SO-239 coaxial connector is installed in the bottom end cap with #4 stainless steel hardware. An eyebolt is installed in the top end cap. The antenna post consist of #10 stainless steel hardware mounted on opposite sides near the top of the PVC pipe.

First I drilled all of the necessary holes, including a drain hole in the bottom end cap, and then painted all of the PVC pieces with olive drab paint the protect from the elements. Next the balun was connected to the SO-239 connector and then the pipe was slid over the balun and cemented in place with PVC cement. At this point the balun was connected to the antenna binding posts. Then the top end cap was installed with PVC cement. I tested the balun by attaching a 50 Ohms termination to the antenna posts and my MFJ-259B via coax to the coax connector on the bottom. The 50 Ohms resistive impedance was reflected back through the balun with little reactance throughout the HF spectrum. Since the design was based upon a tried and true design I am confident that it performs as expected as far as choking off currents.

I found this balun really easy to build and should easily handle a large amount of RF power as long as the SWR of the antenna remains low. A purchased balun may only cost a little more then my homemade version but I had the parts on hand and it was fun to build.

 

HF POWER AMPLIFIER

 

 

In my prototype I used IRF840 in the final. Most of the power FET are designed for high voltage operation. At lower operating voltages they saturates quickly limiting the output power. I had given 120 V for IRF840 it takes 1 Amp at peak. Gate voltage is fixed at 1V. Heavy head sink is essential for IRF. My heat sink measures 30 cm * 6.5 cm. Use mica insulator and heat sink compound for fixing IRF.

You can directly replace IRF840 with many of the power FET like IRF830, IRF530, IRF540 etc... When using a different IRF, supply voltage should be changed to less than half the maximum drain voltage (Vds). A zener diode rated slightly higher than the twice the supply voltage connected across drain and source can prevent drain source breakdown. Peak to peak gate voltage of magnitude more than 20 Volts can destroy the FET instantaneously. Two numbers of 15 Volt zener diodes are used to keep gate voltage swing below 20 Volts. Specifications for some of IRF series are given below.

Loop Antennas - Delta Loops and Square (Quad) Loops and more

 

Delta Loops for HF  -  "You'll love lower noise and relative gain over a dipole"
http://w5sdc.net/delta_loop_for_hf.htm
One Stealthy Delta - This HF antenna keeps a low visual profile while attracting plenty of attention on the air.
An excellent and amusing article by Steve Ford, WB8IMY
http://www.sgcworld.com/Publications/Articles/237qst0502.pdf
Random length multi-band delta loop antenna – A good antenna for when a dipole isn't enough by KC8AON
http://www.i1wqrlinkradio.com/antype/ch10/chiave1827.htm
An Easy to Install Vertical Loop for 80-6 Meters by John Reisenauer, Jr. KL7JR
http://www.hamuniverse.com/kl7jreasyvertloop.html
40m-10m DELTA LOOP ANTENNA - GU3WHN
http://www.rsars.org.uk/ELIBRARY/ANTENNAS%20DOCS/40m-10m%20%20DELTA%20LOOP%20ANTENNA%20-%20GU3WHN%20iss%201.3.pdf
M0PLK Multiband Delta Antenna - By Arthur M0PLK (SQ2PLK)
http://pdxa.one.pl/articles.php?article_id=17  available at http://ham-radio.urbasket.eu  and  http://www.vpa-systems.pl/
H5ANX Mk4 Delta Loop Design by Sajid Rahim
http://www.eham.net/articles/10738
Multiband H.F. Delta Loop by IW5EDI:
http://www.iw5edi.com/ham-radio/?dl2hcb-multiband-delta-loop,28
SGC Stealthy H.F. Delta Loop:
http://www.sgcworld.com/Publications/Articles/237qst0502.pdf
KL7JR Easy H.F. Delta Loop:
http://www.hamuniverse.com/kl7jreasyvertloop.html
H.F. Loop Antenna from Radioworks:
http://www.radioworks.com/nloop.html

W6ZDO Portable H.F. Delta Loop Project:
http://www.fros.com/KI0GU/w6zodelta.htm
Loop Antenna Notes by "Yukon John" KL7JR
http://www.hamuniverse.com/kl7jrloopnotes.html
Build a Multi-Band Mono Delta Loop for 40, 30, 20 and 15 Meters by Jose I. Calderon (DU1ANV)
http://www.para.org.ph/membersarticles/DU1ANV/Multi-Band%20Mono%20Delta%20Loop%20ant.pdf
DL2HCB Multiband Delta Loop
http://www.iw5edi.com/ham-radio/?dl2hcb-multiband-delta-loop,28
The Delta Loop (Skywire) Antenna - Legends, Theory and Reality
http://dk5ec.de/deltaloop-eng.htm
Loop Antenna notes and ideas from Radioworks
http://www.radioworks.com/nloop.html
Delta Loops by GW7AAV
http://www.cqhq.co.uk/2009_05_01_archive.html
More Delta Loop links:
http://www.i1wqrlinkradio.com/antype/delta_loop.html
Magnetic Loops:
Small Transmitting Loop Antennas (Magnetic Loop Antennas) by Steve Yates - AA5TB
http://aa5tb.com/loop.html

End Fed Half Wave Antenna

 

The End Fed Half Wave Antenna (EFHWA) is fed at a voltage node via a parallel resonant circuit against a ‘short counterpoise’, it is a favourite of backpackers and outdoor types.  It can be considered as a half wave dipole that’s end-fed at a voltage node rather than the current node, as is more usual. This is a very handy arrangement for portable QRP work.
EFHWA Link: http://www.aa5tb.com/efha.html

End Fed Half Wave Antenna by AA5TB
http://www.aa5tb.com/efha.html

I suspect that nothing new or radical has happened in the field of radio aerials in a VERY long time, like at least many tens of decades.  Most of the new wonder aerials are really a con.  Choke off the feed-line and then see how good they really are.  Prime among the baddies is the CFA.  It doesn’t really work, at least if you place a choke in the feed-line.  With any real aerial, there should be minimal radiation from the feed system… so a choke should really make no difference at all, but for the CFA it does!  The CFA is not alone, there are others.  The popular G5RV is another design with a radiating feed, deliberately so, but of course G5RV planned it that way.  He wasn’t cheating… merely being a bit devious, to make it multi-band

Lots of stuff to pass on to my fellow radio club members, most of whom are of the  ‘if it’s not expensive, it can’t be any good’ school of thought when it comes to aerials. Nothing of course could be further from the truth!  Aerials are one area where it makes a lot of sense to build our own." Website of GM1SXX - www.observations.biz

Thanks for your email Allan. It's a good idea to point out that an antenna could be pressed into use on odd multiples of its resonant frequency, hence a 3.6MHz antenna for 80m could be useful near the 30 metre, 10.1MHz, band - near to the third harmonic of 3.5 MHz although, as you observe, the radiation pattern may be quite distorted from the traditionally expected dipole pattern and be more petal shaped. The same goes for a 7.1 MHz antenna for 40m being usable on its third harmonic of 21.3 MHz for the 15m band - a 40m dipole being three half waves an the 21 MHz band.

I have not experimented with a full size 80m dipole, but I would guess that it might be useful at 5 times 3.6Mhz in the 18 MHz / 17m band?
The point made about feeding a familiar dipole at the current node rather than the voltage node is obviously very important and, I imagine, sometimes overlooked.
PLANS: Download the pdf plans produced by G0KYA here > More from G0KYA here: http://g0kya.blogspot.com

W3DZZ antenna by the Maidstone Amateur Radio Society

 

W3DZZ antenna by the Maidstone Amateur Radio Society that adds a dedicated 10 meter (28MHz) resonant element as a 'fan'.

W3DZZ Dipole Aerial design by the Maidstone Amateur Radio Society

http://www.btinternet.com/~shaun.scannell/club/w3dzz.htm

Moonraker supply a whole range of wire trap dipoles covering from 2 to 5 HF bands (MTD1; MTD2; MTD3; MTD4; MTD5; MTD6). Diamond also produce trapped wire antennas, the W-721, W-728 and W735. Comet and Diamond each produce similar interesting 5 band wire dipoles that utilize both traps and a fan arrangement - the Diamond W8010 and the Comet CWA-1000. If space really is limited then look out for KZJ Communications (dongo1950 on ebay) - he produces 'Limited Space Inductive Dipoles'. These are inductively loaded and shortened dipoles so they will have reduced efficiency, of course, but are very nicely made, so might be very useful in a tight spot.

To obtain good efficiency and achieve a low angle of radiation, desirable for longer distance DX, a horizontal dipole needs to be installed at a good height - over 20 feet would be desirable and it is quite common to install horizontal dipoles at around 30 to 40 feet above ground level. This might be a problem at some QTH's, it certainly is at mine!

Allan Copland, GM1SXX comments: "The dipole will operate well on the band it has been sized for , if placed at a suitable height, but will also operate as a’ three-half-wave’ aerial at three times the frequency and so on, so it’s not strictly a single band aerial.  An 80M dipole (132 feet typical) will work nicely on 30 metres  (three half waves) but not on 40m (two half waves)… because on 40M the feed-point  is at a voltage node and not at a current node, for easy feeding.  Most aerials are current fed.

The radiation pattern changes when a dipole is not used on its design frequency. The pattern will break up into multiple ‘petals’. This can be either a disadvantage or an advantage depending on what you expect from it.  Since most of us use co-ax, an UN-BAL  should really be used to connect the unbalanced feeder to the balanced aerial, but how many people actually bother? Not many I suspect.  It’s possible of course to use a balanced feed-line  system instead with a dipole and just have a delta match (no centre insulator… none needed).  There are many choices and permutations, but in general, dipoles are centre fed at a point of current maximum (and minimum voltage).

A normal dipole is current fed but of course can be voltage fed instead. This is what’s done in the EFHWA or Fuchs aerial where a resonant half wave wire is fed at one end (max voltage / min current) from an L/C tank, against a very short counterpoise wire.

Inverted L - 80 metres to 10 metres

 

A typical Inverted L antenna will be trapped for 40m/80m using a 7.1 MHz trap. It is essentially one half of a W3DZZ dipole so can be accommodated very much more easily into a small plot or garden - especially as part of the antenna is running vertically up a wooden or fibreglass (non conductive) pole. This should allow it to be fitted into quite a small garden such as mine.

The Inverted L is also a very effective aerial because it has the benefit of both vertical and horizontal radiation. While Inverted L's might make good TX aerials, like ground mounted vertical aerials they can be quite noisy on RX.

The Inverted L is extremely easy to 'home brew'. Spectrum Communications can also supply the complete aerial as shown below. It should give excellent performance on 80m and 40 metres, with 20 metres also being good but allowing use on 15m and 10m and possibly one or two of the WARC bands:

Small Loop for 20 metres to 10 metres:

A loop for 20 metres or 17 meters is relatively compact and could easily be installed in small 'postage stamp' sized gardens. A loop antenna could be triangular, square (Quad) or circular, but a square loop (and indeed a circular loop) would need more supporting points than a delta (triangular) loop, so a Delta loop is likely to be the easier option.

The loop is really a single band antenna cut for one wavelength on the band of interest, however it can also work quite well as a cheap and easy to install multi-band H.F. aerial. A loop consisting of a 17 metre length of thin antenna wire, for example, will work well on 17 metres but may also give 15m, 12m and 10m with an ATU. My own loop is made from an 16 metre length of wire, tuned for the 17m band, but can work on higher bands. A 40 metre loop will be considerably larger, but it might still possible to accommodate in many fairly compact gardens. Performance will depend on height and orientation.

Feeding the loop at the top or bottom will give horizontal polarisation, while placing the feed point on the side will give vertical polarisation. The apex can be at the top or the bottom, but performance should be better with the apex at the bottom with the flat wire across the top  - however for ease it may be more convenient to support a Delta Loop on a single pole, meaning that the apex would be at the top.

Ideally a loop should be fed with balanced line back to the shack, connected to a balanced line ATU or other ATU via a 4:1 balun. Alternatively use a 4:1 balun at the antenna end and run 50 ohm coax back to the ATU / txvr - though losses will be greater doing it by this method if the coaxial cable is quite long.

If one can install a separate antenna for the lower frequency bands of say 160m, 80m and 40m, then a Loop Antenna could be a good partner to allow operation on the higher bands of 20 metres to 10 meters or even 6 metres.

A loop should be really very easy to install using a single support pole and very cheap too! All that's needed is the supporting pole, some cheap wire, a 4:1 balun which can be 'home brewed' and some thin cord and insulators which should not be an eyesore either.

Diagram from the excellent article by W5SDC
http://w5sdc.net/delta_loop_for_hf.htm

MINI ATU WITH TOROID

 

This ATU was designed in about 1990 to allow QRP rigs to be used with portable antennas. The unit had to be small and light weight. Generally I am against using ATU's at all regarding them as a cop-out for poor antenna design. However, when portable operation is considered, it is not always possible to erect the ideal antenna. This ATU was designed with idea of achieving a better match to an antenna that was nearly right.

A later version was tried which had an additional coil switched in and also a balun to accomodate twin feeder. I was never happy with this version, as it didn't seem to work as well as the simple version. When it was deconstructed in 2007 to do some loss measurements, it was discovered that a constructional error during the mod may account for this.

Simple Crystal Tester

 

Here is a simple circuit that can be used to test a crystal before using it in a circuit. The circuit is built around two BC547 transistors (T1 and T2) and a few discrete components.
The oscillator circuit formed by transistor T1, resistors R1 and R2, and capacitors C1 and C2 oscillates if a good crystal is connected to the test points marked as CUT (crystal under test). The output from the oscillator is rectified by diode D1 and filtered by capacitor C3. The positive voltage appearing across the capacitor is fed to the base of transistor T2, causing it to conduct.

Testing of a crystal is simple: Insert the crystal at CUT points shown in the circuit diagram and press test switch S1. If LED1 glows, your crystal is good and you can use it in a circuit.The circuit is powered by a standard 9V battery. Push-to-on switch S1 is included to prolong the battery life but it’s not needed if you use a socket for the crystal under test.Assemble the circuit on a general-purpose PCB and enclose in a suitable small cabinet. Fix the two-pin connector, LED1 and test switch on top of the cabinet. Fix the 9V battery inside the cabinet.

Crystal AM Transmitter

 

 

Here is the circuit of a medium-power AM transmitter that delivers 100-150 mW of radio frequency (RF) power. At the heart of the circuit is a crystal oscillator. A 10MHz crystal is used to generate highly stable carrier frequency. Audio signal from the condenser mic is amplified by the amplifier built around transistors T1, T2 and T3. The amplified audio signal modulates the RF carrier generated by the crystal oscillator built around transistor T4. Here modulation is done via the power supply line. The amplitude-modulated (AM) signal is obtained at the collector of oscillator transistor T4.

Fig. 1: Circuit of crystal AM transmitter

Fig. 2: Oscillator coil

Fig. 3: Modulation transformer

By using matching dipole antenna and co-axial cable, the range of signal transmission can be increased. For maximum range, use a sensitive radio with external wire antenna. The circuit works off a 9V-12V battery. For oscillator coil L1, wind 14 turns of 30SWG wire round an 8mm diameter radio oscillator coil former with a ferrite bead (see Fig. 2). For modulation transformer X1, you can use the audio output transformer of your old transistor radio set. Alternatively, you can make it from E/I section transformer lamination with inner winding having 40 turns of 26SWG wire and the outer winding having 200 turns of 30SWG as shown in Fig 3.

1 Watt FM Transmitter Amplifier

 

This is a 1 Watt FM Transmitter amplifier with a good design that can be used to amplify a RF signal in the 88 – 108 MHz band. It is very sensitive if you use good RF power amplifier transistors, trimmers and coils. It has a power amplification factor of 9 to 12 dB (9 to 15 times). At an input power of 0.1W the output will be 1W. You must choose T1 transistor depending on applied voltage. If you have a 12V power supply then use transistors like: 2N4427, KT920A, KT934A, KT904, BLX65, 2SC1970, BLY87. At 18 to 24V power supply you must use transistors like: 2N3866, 2N3553, KT922A, BLY91, BLX92A. You may use 2N2219 at 12V but you will get an output power of 0.4W maximum.

Pixie2 QRPp AM Transmitter

 

 

The Pixie2 is a simple QRP CW transmitter that dozens of ham radio operators have successfully built. (QRP is ham jargon for low-power operations, and CW is the simplest method of sending Morse code merely by turning a carrier-wave on and off.) The Pixie2 is usually built for the 40 meter band but it will work on frequencies from 1000 kHz up to at least 15 MHz. It is said to output a couple hundred milliwatts of RF.

The circuit can be amplitude modulated quite easily. A small audio amplifier feeds audio current into the 8-ohm side of a transformer. The 1k ohm side of the transformer is inserted in the V+ supply going to the Pixie's output transistor.

This modified Pixie2 is called the Talking Pixie. It has 18 components (not counting circuit board, jacks, power supply and external audio amp). Building it on a prototyping board only takes a few minutes if all the parts are available.

The level of the audio fed to the transformer is adjusted until the best sound quality is achieved. The Talking Pixie will not sound as loud as commercial stations but the user must avoid the temptation to over-modulate; nobody will listen to an over-modulated signal.

parts list

C1: 100 pF
C2: 220 pF
C3: 82 pF
C4: .01
C5: .01
L1: 150 uH
L2: 22 uH
Q1: 2N2222 or 2N3904
Q2: 2N2222A (metal can type) or 2N3866
R1: 47K
R2: 1200
R3: 33K
R4: 10 or 15 ohms (experiment!)
T1: 1000 ohm to 8 ohm audio transformer

The frequency is crystal-controlled. A crystal for the frequency you're interested in will have to be ordered if you don't have one handy.

The transformer must be rated to handle at least half a watt of audio; a very tiny transformer will not sound good and will have too much resistance on the 1K winding.

L3, C6 and C7 form a low-pass filter to attenuate the harmonics generated by the circuit. Specific values for various frequencies can be found on the Medium Wave Alliance's filters page.

L1 and L2 are factory-made axial molded chokes.

variations

The impedance and bandwidth of the antenna will affect the sound quality of AM transmitters like the Talking Pixie. What sounds good on the test bench with a 50-ohm dummy load attached to the output might not sound as good with real-world antennas like short end-fed wires. Some kind of antenna tuner might be helpful. (By the way, two 100-ohm resistors in parallel make an adequate dummy load for this rig.) Needless to say the size and efficiency of the antenna will have a major impact on the range.

If you build the circuit on a prototyping board (as shown above), you can experiment with many variations on the circuit design.

Here are some modifications that have been suggested...

Martin Spencer suggested using a FET (such as 2N7000) instead of an NPN transistor for Q2. This could give more linear modulation. Replace L1 with a 5K variable resistor; remove the crystal for a moment and adjust the resistance for about 2 mA drain current.

Mark Weiss wrote: "You can put another transistor in series with the PA and use it as a series voltage source. By varying this voltage control element with the audio signal, highly linear modulation is achieved. Transformers tend to present variable impedances, causing the PA to be less stable under varying load conditions. A direct-coupled modulator can offer the potential for great tolerance of loads that aren't precisely +50 j0."

Other QRP CW transmitters can also be modified for amplitude modulation. You will find schematicsfor such transmitters in ham radio books and magazines, and on websites operated by QRP clubs.

TOBACCO TIN TRANSMITTER

 

I would like to show you my novelty transmitter project which I have enclosed into a tobacco tin. It requires very few parts, they are very easy to obtain, most of them came from my junk box with the exception of the crystal (7.030 MHz) which is an international QRP calling frequency. Any crystal will do as long as the operating frequency is within the amateur band, preferably in the CW section.

CIRCUIT

This is a one transistor crystal controlled transmitter, it uses the 2N2222 transistor in a basic oscillator arrangement, and has a simple output filter section for any unwanted harmonics. If the output filter was not used  the transmitted frequency would not only be 7.030 MHz but also 14.060 and 28.120 MHz etc. These unwanted frequencies are called harmonics. The output power is only 250 milliwatts (quarter of a watt) but high harmonic output is illegal even at this low power.  

CONSTRUCTION

I have used the ugly style construction, this is where you start with an off cut of copper clad board material, and build the circuit on the copper side up. The copper surface makes a low impedance ground and a anchor point for components. The grounded or copper surfaced components make a good solid support for the rest of the circuit to be built on. It is up to you as the builder which style you use, but the ugly construction is a lot cheaper than buying expensive vero board or making a printed circuit board. As you can see from the diagram there are very few parts, and it is straight forward to build. RFC1 is 6 turns of 32 s.w.g. enameled copper wire wound on a tiny ferrite bead, any thin wire and ferrite bead should work. The toroid in the output filter section is 14 turns of 26 s.w.g enameled copper wire wound around a T50-2 core.

OPERATING

When you have finished just press your morse key, no tune up procedure is necessary. You will also need an HF receiver, or shortwave radio with a BFO to operate with, I use a Realistic DX-394 receiver with an indoor wire and the transmitter next to the receiver with any of my outdoor antennas, but you could operate both from one antenna with a changeover switch. Although the transmitter only has 250 miliwatts this circuit when connected to a good outdoor antenna such as a dipole in favorable conditions has worked DX over 7000 miles. You will notice I have left a space on the right hand side of the box, this is to put an optional PP3 9 volt battery inside if I am going portable.

OTHER BANDS

The transmitter can also be  built to work on 80, 30 and 20 meters with the crystal of your choice, and the following changes=

80 meters =T50-2 toroid 21 turns capacitors 1, 2 and 3 =750 pfd

30 meters = T50-2 toroid 13 turns capacitors 1,2 and 3 =330pfd

20 meters =T50-2 toroid 12 turns capacitors  1, 2 and 3=270pfd

Please note It is illegal to operate this transmitter without an hf license.

Happy building and good DX M0DAD.

http://users.whsmithnet.co.uk/m0dad/construcion/tobacco_tin_transmitter.htm

THE PIPPIN QRP TRANSMITTER

 

 

---

Basically, it is a conventional Colpitt type crystal oscillator but with the output taken from a low value collector load resistor and direct coupling is made into the base of the PNP device used as an amplifier. The result is a circuit even more simple than the OXO and with considerable advantages.

The small amount of forward bias developed for the PA stage makes it very much easier to drive but is less than the voltage required to actually bias the stage "ON". Keying is in the emitter circuit of the oscillator stage and when the key is up and no current is being drawn there is no forward bias at all on the PA stages.

The isolation of the PA from the oscillator by taking the drive from the low value oscillator collector load, is most impressive and there is virtually no pulling of the oscillator even if the PA load is briefly shorted to ground.

Input to the PA stage runs at 120 to 150mA. at 12 to 14 volts and output on 7Mhz runs at better than 1 watt measured into a 50 ohm load. The PA transistor has a "Stove Pipe" heat sink attached and has been left running continously for more than 1 hour without any complaint from the PA stage.The collector choke is the usual type and uses 6 turns of 29 swg on two ferrite beads in tandem.

http://www.qsl.net/g3pto/pippin.html

The NOGAnaut QRP Transmitter

 

The crystal oscillator is the simplest form of transmitter. Normally, oscillators are used to drive buffer amplifiers and power amplifiers, which provide increased output, as well as prevent the output circuit from adversely loading the oscillator.

Most transistors exhibit a characteristic impedance different from the 50-ohm impedance of a well-tuned antenna system. An improper match between the impedance of the transistor and the load (e.g. antenna system) can cause severe power degradation, and worse, can seriously affect the signal, including shifting the oscillator frequency in unpredictable ways.

In the NOGAnaut transmitter, the 2N2222A transistor, which exhibits a characteristic impedance of approximately 200 ohms, is matched to a 50-ohm load via the pi-network filter composed of C1, C2 and L2. The values of these components were chosen to provide a close match between the 200-ohm transistor and a 50-ohm antenna (it is therefore critical that a good 50-ohm antenna system be used with this transmitter). It so happens that these values also form the familiar half-wave harmonic filter, thus satisfying FCC spurious emissions requirements.

Figure 1. NOGAnaut 80M Transmitter Schematic.

Capacitor C5 provides the necessary feedback to begin oscillation. You may find that you can operate your NOGAnaut without this capacitor--stray capacitance in the circuit provides a certain amout of feedback without C5. However, it was found during development of this circuit that the oscillator can have troubles starting at times, therefore it is recommended that you leave C5 in the circuit.

The 0.01 uF capactor, C3, serves as a DC-blocking capactor. At 3.6864 MHz, this capacitor is essentially a dead-short to RF, but blocks the DC current from flowing into the load.

This is a familiar Colpitts oscillator, operated in "common-base mode." The usual base-bypass capacitor is replaced by the capacitance of the crystal. With a 15V supply, this transmitter has been measured to deliver as much as 134 milliwatts into a perfectly matched 50-ohm load ("your mileage may vary"). With a 9V supply, about 20-50 milliwatts should be expected.

The transmitter is keyed by interrupting the positive supply voltage. You can modify this to be grounded keying, if necessary (just interrupt the negative supply voltage instead of the positive voltage). This may be necessary if you use a keyer that expects grounded or negative keying.

For a very good description of crystal oscillators, check out Solid State Design for the Radio Amateur by Wes Hayward, W7ZOI, and Doug DeMaw, W1FB. This is one of the most popular amateur radio books ever written and is packed full of practical information about how solid state circuits behave. It is published by the ARRL, and can be purchased directly from them, as well as from many electronics retailers.

Further information about pi-network filters can be found in The ARRL Electronics Data Book, by Doug DeMaw, W1FB, also published by the ARRL. This book contains most of the nuts and bolts of basic circuit design, and is a must for any ham shack.

http://www.nogaqrp.org/projects/noganaut/circuitdescription.html

25 part transmitter for the QRP Minimal Art Session

 

My Minimal-Art-Session rig shown here was made for class B of theQRP-MAS competition which is in May every year. In class B the transmitter should have a maximum of 50 components, and the fewer the number of components, the more points one will get per contact. With such simple rigs, only CW is viable.

This rig was made by combining ideas from several designs that I had seen and it was finished the same day as the competition. I ended up with 22 components in the transmitter except for the output filter. The rules say that no matter what, the output filter counts as three components. I guess this is in order that no one is tempted to simplify the filter too much and start emitting harmonics. 

It was made directly on two experimental boards. The right-hand one is for the VXO and the PA and the left-hand board contains the output filter. The transmitter is housed in an enclosure which once was a network converter. The advantage was that there already was a BNC-connector and a power supply plug there. All the original surface mounted components were blown off with a hot air gun so that the original board could be used as a base for my boards.

QRP-MAS transmitter for 80 m with 25 parts, drawing by LA4YW, Liv

My transmitter is made to give as close as possible to the maximum power of 5 Watts, but I ended with 3.5 Watts even though the IRF510 is capable of more. But there was not enough drive signal for that. I also had some problems with high frequency oscillations so therefore there is a series resistor to the gate and some extra decoupling which raise the component count. The transmitter frequency can be pulled about 1 kHz for each crystal. I have used it with an antenna tuner and a 75 m horizontal loop and with my Elecraft K2 as receiver. Antenna switching was manual.  

I didn't really count on many contacts because I only had crystals for 3579 kHz, not for the QRP-frequency 3560 kHz. But surprisingly I had 6 contacts with D (Germany) and one each with ON (Belgium) and OK (Czech republic). The "best" rig component-wise that I contacted was DK0VLP with only 12 parts.

Next time I plan to make a tube transmitter - inspired by the AA8V/W8EXI One-Tube Transmitter - hopefully cutting the component count in two myself also.

Homebrewed Off-Center Fed Dipole

 

Building A Homebrewed Off-Center Fed Dipole Scanner Antenna.

Aluminum/copper tubing construction:

You will need to check the fit of the tubing with the T connector and the caps while you are at the store. One combination that fits nicely is 3/4" copper pipe with 3/4" CPVC fittings (not to be confused with 3/4" PVC fittings which will be too large). The tubing/connector is held in place with 2 stainless steel sheet metal screws for connecting the balun to each element.

Find a "U" bolt to fit your mast. Drill two holes in the support pipe to fit the U bolt.The support pipe is 18" from the "T" to the mast.

Remember, bandwidth increases as diameter of the elements increases. I think, if I remember correctly, at the hardware store, that a few CPVC fittings will fit copper tubing perfectly!

Some say that the 18" element on top mounted works best,Some like the 48" element on top.It does'nt matter,it works the same.

If you use the copper tubing,be sure to paint it with some good,non-conductive paint.I used to paint mine light grey. -Have fun! (Teraycoda)

For an alternate/temporary mounting option, drill a hole in one of the end caps and put in an eye bolt with a nut on the underside of the cap to secure. Be sure to secure this end cap to the copper tubing somehow, perhaps with an additional small stainless sheet metal screw. Be sure that the eye bolt itself doesn't make electrical contact with the tubing. Also, drill a small weep hole in the bottom end cap to allow any moisture to escape that may accumulate inside. Use the eye bolt and some rope to pulley the antenna up high in a tree, or use a hook to hang it somewhere. Give careful consideration to safety and grounding depending on your particular usage scenario. (Qdude)

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Variation for Off-Center Fed Dipole Using Simple Wire and 75 to 300 ohm TV Balun Transformer

Electrically, this version is the same as the one using copper tubing (above) but can be assembled quickly and is quite portable. While not as broadbanded as an OCFD using copper tubing or other metal with a larger diameter, the OCFD made from simple wire turns in great receive performance in all the commonly scanned bands, as reported here on RR in multiple message threads.

The legs/ends of the dipole are simple bell wire and shown here coiled up. Uncoil them and hang them vertically; doesn't matter if the long or short leg is at the top... works the same either way. The wire terminal lugs shown at the end of the legs of the dipole antenna should NOT be connected electrically to the wires - just crimp them on over the wire insulation. They are used as convenient hangers for the antenna, and not meant for electrical connection. Obviously, the lugs at the TV transformer/balun ends of the wire should be stripped before crimping on the terminal lugs to ensure contact with the antenna wires when you attach the TV transformer. Ensure the 75 ohm coax feedline that you connect from the balun/transformer runs away from the antenna at as near a 90 degree angle as possible.