 |
 |
My Home Page             QRM from BT HomePlug and BT Vision PLT adapters Site Map MDS975 Home
|
RADIO PROJECTS & KITS One of the really fun aspects of amateur radio is making things for yourself. Probably the best and most important DIY project for any amateur radio station is building an antenna of some kind. This is often a wire antenna for use on the HF bands such as an Inverted V, Inverted L, Dipole or Doublet Quad Loop or Windom etc. For the shorter wavelength VHF and UHF bands it more practical to construct more complex antennas such as Slim Jim or Yagi for example. Other projects will be an electronic unit of some kind. For the Intermediate Level licence it is necessary to make several practical electrical and electronic circuits and also build a complete and useful device related to the subject of amateur radio. I chose to make a Morse Code Practice Oscillator from a kit of parts bought from Waters & Stanton. You can see this project a little bit further down this page. More recently I built a Field Strength Meter - "FSM". FIELD STRENGTH METER It is often said that one of the most useful pieces of test equipment in and around the shack is a Field Strength Meter. A Field Strength Meter can be used to quickly check the presence of RF energy, for example to check that a transmitter is transmitting, for use with antenna experiments such as judging the radiation pattern and efficiency of antenna and for checking RF oscillators etc. To buy a simple ready made FSM would cost around £30.00 and £50.00. Since such a device is simply a form of 'crystal set' without a tuned circuit I set about looking through the junk box to see what electronic components I had that I could use to make a suitable circuit. I found a nice aluminium case, a good telescopic aerial, a couple of germanium diodes, a potentiometer, some suitable ceramic capacitors, a nice 250µA signal meter (minus the scale which I had somehow lost) and some other useful bits and bobs. All I needed to assemble a simple yet perfectly effective Field Strength Meter that I am sure is as good as anything that could be purchased ready made - and all made from junk box componets!  Photograph showing the simple construction of the Field Strength Meter  Circuit Diagram of the Field Strength Meter All the Field Strength Meter has to do is convert the radio frequency signal into a DC current that can drive a meter movement or digital multimeter (DMM). As can be seen from the above circuit diagram the field strength meter bears a great resemlence to a simple crystal set. The differences being that since the field strength meter needs to be sensitive to a wide range of frequencies the tuned circuit (inductor and variable capacitor usually found in a crystal set) is omitted, and rather than headphones or an earphone the output is fed as DC to a signal meter or to a digital multi-meter so that comparative (rather than absolute) measurements can be made. The telescopic aerial picks up the radio frequency signal and the germanium diode converts the signal to DC. It is important that germanium diodes are used as they exhibit a very small forward bias which is needed to make the meter sufficiently sensitive. |Silicon diodes have a substantially higher forward bias which would substantially reduce the sensitivity, so for this reason it is important to use germnium diodes. On the same theme it is important to use a sufficiently sensitive meter, so a microammeter will be required. I was lucky to have an old Maplin signal meter with a sensitivity of about 250µA for full scale deflecton (FSD) in the junk box, although I would imagine that it would still be worth experimenting with any meter between 50µA to 1000µA. Alternatively a digital multi meter can be used to measure the output. The field strength meter that I built has both options selectable with the miniature DPDT switch. The meter is connected to the digital multimeter with a short fly-lead terminated with a red and black banana plug to identify the positive and negative wires. The 47K potentiometer allows for the adjustment of the overall sensitivity of the meter. The advantage of using a DMM is that it has a very high input impedance a therefore will not load the circuit to any great extent, it also enables the meter to be much more sensitive to weaker RF fields if required and also it will be easier to make more precise measurements from the digital readout, particularly small differences.I find that th e DMM is usually set to the 200mV range, or perhaps to 2000mV range if the RF field is especially strong. The value of the various components is not particularly critical, but as mentioned, the diodes must be germanium rather than silicon and any diodes such as OA90,OA91, OA80, OA81, OA47 could be used.
SHORT LOADED TOP BAND ANTENNA FOR 160 Metres / 1.810 to 2.0 MHz My experimental project during 2009 was trying to accommodate a small top band antenna in the restricted space at my QTH. A full size aerial for Top Band is going to be far too big for most back gardens, but the basic requirement really is to get as much aerial wire in the air as possible - the longer the better - and then load the antenna to bring it to resonance on the band. I used a small inductor wound on a 50mm diameter plastic tube. A top band aerial like this also needs the very best earth possible - i.e. as many ground wires as can be accommodated. I gradually refined my ideas and have now put the results on the antennas page here MORSE CODE PRACTICE OSCILLATOR  Internal view showing PCB and other components  The completed CW Practice Oscillator with Morse Key WIRING A CABLE FOR A DIFFERENT MICROPHONE I decided to use my existing Leson (Altai) TW-232 Desk Microphone as an alternative to the Icom HM-103 hand mic that is supplied with the Icom IC-706MK2G transceiver. The TW-232 desk mic is fitted with a standard type 6 pin mic plug wired for my Midland 48 Excel CB radio. The Icom 706 has a completely different RJ45 type mic socket. I needed to make a 'cross-over cable' to fit between the mic plug on the TW-232 and the Icom 706 transciever. Looking at the circuit diagram for the Icom IC706, the basic wiring only needs four wires: PTT (Push To Talk transmit switch), PTT Ground, Microphone Audio and Microphone Audio Ground. This is slightly different to CB wiring which does not have separate grounds for PTT and Mic, inside the plug on the TW-232 microphone these two ground wires were connected together. I therefore I separated the MIC Ground and PTT Ground within that plug. This would require two Cross-over cables; one for the CB that re-combined the two grounds together to match the wiring scheme required for CB and the second cross-over cable for the connection to the IC706Mk2G. Here is the wiring scheme for the TW-232 mic and the Icom transceiver: The Leson (Altai) TW-232 desk microphone wiring is as follows: White = PTT Black = PTT / Receive Ground Blue = Receive Red = Mic audio Shield = Shield (mic audio shield) Icom IC706Mk2G microphone plug wiring for RJ45 plug: 1 = +8 volts d.c. *** Do not connect & be careful NOT to short out otherwise the radio will be damaged *** 2 = Frequency up/down buttons 3 = Audio output 4 = PTT >>>>>>>>>>>>>>>>>>>> Connects to the White wire of the TW-232 Mic 5 = GND - Microphone Ground >>>>>> Connects to the Shield wire of the TW-232 Mic 6 = Microphone audio input >>>>>>>> Connects to the Red wire of the TW-232 Mic 7 = GND - PTT Ground >>>>>>>>>> Connects to the Black wire of the TW-232 Mic 8 = Squelch control
Leson / Altai TW-232 wiring
diagram
shows the microphone socket as seen from the front of the radio (Icom Corporation) The
Up / Down frequency buttons are not wired in my cross-over cable, but
could be used if required if additional switches were fitted into the
desk mic. The basic wiring only requires four wires to pins
4,5,6
& 7 in the RJ45 plug - as seen below:
 The RJ45 plug fitted to a short piece of mic cable  Fitting the RJ45 plug to the mic
cable
 Fitting the mic socket on the other end of the cable  The completed cross-over cable Thanks to Alex and Dave at the Charlie Delta ARC for the necessary plugs that enabled me to make this cross-over lead. Cheers guys!! OTHER PROJECTS There are many other useful devices that can be made, such as an ATU for portable QRP use, various types of receivers, pocket sized QRP CW transmitters, complex transceivers - the list is endless. Some projects have to be built from scratch which involves making the necessary PCB, other designs provide a pre-etched PCB while many are available in complete kit form. Another project that I wish to make in the future is a Noise Bridge. I even fancy having a go at a Crystal Calibrator - and more experimental antennas - of course! Looks like my soldering iron may
be busy!
|
|||||||||
Amateur Radio Page | My Station | Antennas
/ Aerials | Accessories
| Projects
& Kits | Data
Modes | Information |
QSL
| RSGB
|
||||||||||
Mike Smith - MDS975.co.uk © 2003 - 2009
M0MTJ
Subjects covered on this page:
Amateur Radio; Ham Radio; Radio; Transceivers; HF; VHF; UHF; Data Modes; Morse Code; RTTY; PSK31; SSTV; FSTV; Amtor; Sitor
Antennas; Aerials; Cable; Coaxial Cable; Twin Lead; Masts; Poles; Propagation; Computer; PC; USB Computer Interface; Microphone
Loudspeaker; Filters; Noise Reuction; DSP; Digital Signal Processing; Morse Key; SWR ; Inverted L; Inverted V; Dipole; Doublet;