We have touched all important stages of Ham Band Transmitters and Receivers. One thing we left untouched was the electronic keyer. Serious CW operators know that a paddle is the way to make CW easy and fun. Building one for oneself also is not a difficult task except it requires careful soldering and IC mounting. The electronic keyer is configured in such a way the moment one touches the paddle the entire system changes over to transmission. The speed of a keyer also can be set. Those contact problems with Morse keys are also do not affect the performance of a electronic keyer. The pad contacts at the electronic keyer requires no pressure at all. In C-32/1 see the circuit of an electronic keyer.
Monday 28 September 2020
Chapter 32 - Electronic Keyer
Chapter 31 - Multiband German Quad Antenna.
As we all have understood by now, the most important stage with regard to a transmitter is certainly an antenna. Unfortunately, I doubt if at least most of the home brewers give due importance to it. If the antenna is not resonant to the frequency, if the SWR is not matched and if the transmission feeder and antenna arms are not quality pieces, there would not be any power to go out into the space. Dipoles stand out among simple and effective of antennas. Another type of antenna which can be used at situations of space limitations is ground plain about which we already discussed. The length of a round plane radiator element is calculated with the formula = 234/f (MHz) in feet.
Chapter 29 - Transmitter Antenans
Even with a similar transmitter and antenna, the report received and the contact experience need not exactly be the same especially when contacting Dx stations. It was already told that not only the peculiarities of the atmosphere but also the height of the antenna are determining. Directional qualities of low level antennas will be low and the radiation angle will be high. Only low radiation angle signals go to more distance. But at low angle radiation the dead zone effect will be higher. Away from line of sight of antennas the next point of the same signal will be where the nearest reflected wave reaches the ground. Th area in between these two points where no signals from that particular antenna is available is called the dead zone. See fig: C-29/1
There is also the tradition of fitting loading coils in the centre of both the arms to bring the antenna into an impedance matching situation. All these short cuts but increases the 'Q' of the antenna and reduces the bandwidth. Make 60 turns at one and half inch dia. with 16 SWG wire leaving one foot at one end and eight feet at the other end. Connect the feeder cable at the one foot side keeping terminals at half inch distance (just like we do in horizontal dipole. Fix the other ends of the arms (eight feet long part) as shown in C-29/3. This is the details of a 40 M antenna. With any ATU illustrated already, this will also give good results, with slight decrease in gain.
Study of antennas is quite interesting. Basically it was mere enthusiasm that led Louis Varney (G5RV) to the invention of the very popular multi band G5RV antenna in 1946. This antenna is very popular in the United States and is found best for 20 meters. It should be noted that this antenna can be erected as horizontal dipole, as sloper, or an inverted-V and with a trans-match, it can be operated on all HF amateur radio bands (3.5–30 MHz).
In effect this is a dipole only with each arm measuring 51 feet each. If that much open space is not available, each arm can be bent in 90 degree at 31 feet to hang the remaining 20 feet vertically down. Height is vvery critical for G5RV Antennas. Even if this works in 25 feet too, 3o feet is the ideal recommended height. it is 300 ohms TV ribbon cable that is used for feeder cable. The length of the feeder cable is fixed at 29 feet 6 inches. If the height is low the lower part of the feeder cable can be bent slightly. Because the length of this cable is critical in impedance matching at different bands, this part of G5RV antenna is called matching stub. This antenna can be erected in inverted 'V' pattern with a 30 feet high pole in the centre. If it is open wire that is used as matching stub the suggested length is 34 feet. This matching stub is connected to a trans-matching arrangement. After trans-matching antenna is connected to the transmitter through an SWR meter. That portions that is not matching stub can be 75 ohms coax cable, ribbon wire or open wire. Compared to a dipole a 3 db more gain is marked for G5RV Antenna.
Chapter 30 - Antennas
I think the Indian Hams are particular on having copper wires for antenna arms. Because the power loss increases with higher resistivity and electrical contact is not easy in many conductors as that in copper, most people recommend copper for antennas. This does not however mean that metals like aluminium are not usable here. During Kuwait war an Amateur from Kuwait used an Aluminium antenna and successfully contacted Kerala.
This is usable at any HF bands. The disciplines require for multi band antennas had been discussed before. Since a phase shift of 180 degrees between magnetic electric waves, electrical noise is almost fully eliminated in this design. If its' features, especially that of its' 'Q', are used appropriately this design could be very effective for Receivers.
Chapter 28 - Simple Dipole Antennas
That's why from 14 MHz on height also turns critical. Before choosing a particular type of antenna, know details like the expected points and areas where we mean to reach the signal, including the financial and technical capacity of the user. There is no limit to the possibilities open.
For a half wave dipole the length for a wave travelling in free space is calculated and this is multiplied by a factor "A". Typically it is between 0.96 and 0.98 and is mainly dependent upon the ratio of the length of the antenna to the thickness of the wire or tube used as the element. As said, the 'K' factor is approximately 0.97. This also means that for absolute matching the full length of antenna arms found out through the formula needs some cutting, for maximum impedance matching. 'K' is the factor that shows the necessary difference, in the formula.
In C-28/1 the picture of a 7 MHz half wave dipole antenna is given. in the centre, distance between dipole arms is half inch. It is good to water proof the centre separator and the joints. Only if the dipole height is in perfect divisions of the operating signal wave length or near to it, the impedance also come closer to the proposed value. Simply because at 14MHz and above it is Dx communication that is intended antenna height is decided to match low radiation angle.
In C-28/2 there is the details of a half wave dipole antenna using the ribbon wire used for TV antennas. If this is used for receiver only, the capacitor tuning is not necessary. The attraction of this dipole is that perfect matching at SWR 1:1 is possible. The cable from antenna to the equipment should be vertically hanging. Length adjustments in coiling is not recommended. The value of the tuning capacitor used in between may be close to that of a metal gang capacitor. It may be used here. The length between capacitor and final transistor is output is = impedance of the transistor divided by the frequency = - feet. For example, in 40 meters it is 75/7(MHz) feet. Both the ends of this horizontal dipole antenna are fixed to insulators at either sides in the atmosphere. This antenna belongs to folded dipole class.
Chapter 27 - SWR Protection
The RF power from the transmitter is let through a 4" long coaxial cable having the same impedance as that of the out put and input. Since the shield of the cable works like electrostatic shield, only one side of it shall be grounded. The toroid used here can be any toroid that works in RF ranges. The coil should have 14 turns of 24 SWG wire. If the deflection seen in the meter is very high change coil connections.
Fig. C-27/2 gives the details of a simple Antenna Tuner (ATU) that can be used with low power transmitters. Another ATU circuit that had been successfully tried wit high power transmitters are given in fig. C-27/3
As shown in the picture, either the arrangement should be such that switching is possible to each turn of the coil or a soldering and testing at each turn pattern can be followed. In word sense what we require here is a variable roller inductor, which may be available from junk/flee markets. Another ATU circuit close to C-27/3 is shown in C-27/4.
The construction details of a simple SWR meter that shows standing wave strength at forward and reverse conditions is shown with details in fig.C-27/5. We have given here only a short account on VSWR and ATU.
Sunday 27 September 2020
English Articles
Introduction
These are English translations of an article series that was published in a Malayalam science magazine, 'Electronics For Everybody', from 1994 - 1998. These articles are mostly on home brewing strategies, explained in a simple way, while the book (Gateway to Ham Radio) intends to give all the basics of electronics, along with giving a powerful support to all, for their advancement.
These articles were written at a time when home brewing was at its zenith and there were components shops in every town in India. Etching PC boards of own design was a craze those days. Soon there had been a paradigm shift, mostly because all types of gadgets appeared in the market comparatively at much lower costs and better performances. One another reason was the changing HF band condition that made low power AM QRPs difficult to strive through. Also, the general enthusiasm towards research and innovation also deteriorated heavily, as the trends changed.
These articles, however, serve as virtual support to those theories that are established in the book. It may make things more digestible. It will heavily help a new comer, especially if he hails from some arts categories.
About me, I come from an English literature background. I could make it, only because there were a lot of friends and seniors ready to offer. What that pushed me forward was the truth that many of the licensees were not science students at all! Yes, in 1991, after 7 years of my entry, I could design and make a 12 Volt HF transceiver of 5 watts output. This 5 watts was enough to contact almost all stations in South India. For a few years I was on that and have contacted far and wide on AM or CW. I thought that writing everything down might help some one else. Shri B Soman Nair, then Chief Editor of the magazine invited the series and it all happened one by one every month. OM Mukund VU2 DRL was the driving force behind. I'm happy, even though I lost seven to ten days every month.
The real satisfaction was when I came to know that these issues were still intact with many new Hams. A lot of people had been asking for old copies of the magazine. I was helpless! But OM Jayachandran VU3BWB and OM Madhu A65DE helped me to recover all those issues, easily. Thank you both. The soft copies in both languages were prepared to be shared free. But everything ended up in creation of a totally new book - Gateway to Ham Radio.
There is a humble request to all the readers - begin doing something only after reading the full texts. Even though you do not propose to assemble anything, the basic information imparted through these articles will certainly widen your knowledge base, considerably.
Thank you all. Kindly do not forget to contact us in case of clarifications/comments/suggestions.
Chapter 1 - Radio Signals and Ionosphere
Chapter 2 - BFOs and LSB/USB/CW Signals
Chapter 3 - QSL Cards
Chapter 4 - Receivers and transistors
Chapter 5 - DC Receivers and VFO
Chapter 6 - Oscillators, Transistors
Chapter 7 - Low Power AM transmitters
Chapter 8 - 3.5 MHz doubler VFO
Chapter 9 - Communication Receivers
Chapter 10 - PLL, AGC controlled IF stages
Chapter 11 - Receiver parts
Chapter 12 - Multi band BC Receiver
Chapter 13 - Band pass filters
Chapter 14 - Make custom coils
Chapter 15 - Toroids
Chapter 16 - Modulation techniques
Chapter 17 - SSB Transmission
Chapter 18 - SSB Generators
Chapter 19 - Crystals and Single sideband
Chapter 20 - Multipurpose Test meters
Chapter 21 - SSB Mixers
Chapter 22 - SSB linear stages
Chapter 23 - RF Power amplifiers
Chapter 24 - Linear Power amplifiers
Chapter 25 - Linear/Output amplifiers
Chapter 26 - QRP Module layout
Chapter 27 - SWR Protection
Chapter 28 - Simple Dipole antennas
Chapter 29 - Transmitter Antennas
Chapter 30 - More on Antennas
Chapter 31 - Multi-band German Quad antenna
Chapter 32 - Electronic Keyers
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