Branching Out

Amateur Radio Ideas

 

by Peter Parker VK3YE

You can do many different things with amateur radio and amateur equipment. Below are just a few ideas and hints for you to try. Detailed technical descriptions are not generally given, though diagrams are provided for some.

Contents

Ten Metres Pedestrian Mobile

When ten metres opens, long distance contacts with compact antennas and low power become a reality - if you can hear the other station, they can usually hear you. And the good news is that there is no need to wait until the next sunspot peak (forecast for 1999/2000) to start making contacts on 28 MHz.

Six and ten metres are affected by a propagation mode known as 'sporadic E'. This propagation, which is most prevalent in the summer months (with a lesser period in mid-winter) allows communication over distances of approximately 300 to 3000 kilometres with very low transmit powers. While such propagation can occur at any time, it seems to be most prevalent in the late morning and afternoon.

By listening to the 10m beacons between 28.2 and 28.3 MHz, it is possible to get an idea of places to which the band is open at a given time. Sporadic-E can be erratic, and there are times when the band is open but no beacons can be heard. In such times, the 27 MHz CB band, with its greater number of operators can serve as a good guide to conditions on ten.

When a sporadic-E opening is intense, and you are still a strong signal even with the power wound back and not much more than a multimeter lead for an antenna, that is the time when you wished you had a pedestrian mobile station on ten metres. Don't laugh - when conditions are good, many solid 10 metre contacts can be had from such a station.

In a few weeks last summer, contacts with all Australian states (bar one) and ZL were had from a pedestrian mobile station. And most of these contacts were made without higher power being initially used to advise the distant station to listen for a weaker portable station. Conditions are rapidly improving for the ten metre operator. On 28 March 1998, two USA stations were worked by a pedestrian mobile station in VK running 12 watts SSB to a mobile whip. Two weeks later a similar contact was had with a station in Ethiopia. With the increasing sunspot count, such contacts are likely to become a frequent ocurrence in the next few years.

As for equipment, power supplies and antennas, you can't go past a modified 27 MHz SSB CB, sealed lead acid battery (up to 7 amp hours) and a 6ft ex-CB mobile whip. Add a 2.5m long wire to act as a ground radial for best results. Placing the whole lot in a bag to give you a station that is ready for action immediately you hear the first sign of an opening. Provided that your antenna system is operating efficiently, you'll be surprised at the results. And if conditions are really good, you might try switching to AM (see below).

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Ten Metres AM

Why use AM on ten metres? Good question. Maybe it's the superior audio quality compared to SSB. Or a feeling that we should be using more of ten metres, and that AM's wider bandwidth is our little contribution to calls to 'use it or lose it'. Maybe it could be the nostalgic sentiments of older amateurs who grew up with 'Ancient Modulation'. Or, it could simply be the curiosity on the part of amateurs who have never before moved the mode switch on their brand new rigs to AM.

There is little doubt that SSB is by far the superior mode when conditions are poor. The mode's narrower bandwidth is a huge advantage in crowded bands. But, with 1.7 MHz of band space on ten metres, there's certainly room for a few AM signals. The strong signals experienced during a good sporadic E opening is an excellent opportunity to try AM on ten.

There was a time when many SSB rigs did not have AM facilities. However, most recent transceivers feature AM transmit and receive operation as standard. While you may not get much of a response if you called CQ on AM, there's nothing to stop you from first establishing contact on SSB and suggesting to the other station that you switch to AM if signals are good enough.

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80 metres during the day

Many people say that eighty metres is a night time band. However this is only partly true. Just because you hear nothing does not mean that the band is not open. In the middle of the day (particularly during winter), 80 metres can allow distances up to about 400km to be spanned, even with low transmit powers. Rather than randomly call CQ, your chances are increased if you pre-arrange tests with other operators. As signals are unlikely to be strong, CW is the recommended mode initially, switching to SSB if signals are sufficiently strong.

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80 metres from a block of flats - the world of magnetic loops

Everyone knows that you need plenty of space before you can put out a big signal on eighty metres. However, if you're only interested in local work, a compact antenna such as a magnetic loop is good enough to get you started. The space needed for one of these loops is surprisingly small - a practical square loop for 80 metres can be as small as 1.5 metres a side, though bigger loops are more efficient. A single loop is good for several bands; one for 80 metres should cover 40 and 30 metres, for example.

The most important design consideration is low resistance in the antenna - build it from 12 mm copper pipe or flat aluminium strip at least 3x20mm for best results. Your connections must also be as loss free as possible. The hardest aspect about building these antennas is obtaining the tuning capacitor - because of the high voltages present, an ordinary transistor radio tuning capacitor will not do, and something better is needed. Either use a wide spaced variable capacitor (fitted with a vernier reduction drive), make your own variable capacitor or experiment with sections of 300 ohm ribbon or coaxial cable to provide the required capacitance.

The loop is directive off its ends - you will notice a null when the antenna is broadside to the direction of the incoming signal. The antenna's narrow bandwidth means retuning every time you change frequency. This is an annoyance but is a must for best performance.

These antennas work surprisingly well indoors, though you should keep transmitting power down and not position it where other people can touch it while you are transmitting. An article on building a magnetic loop is available on Peter Parker's Projects Page. Novice Notes Online contains several links to other webpages featuring magnetic loops.

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Bus and Train mobile

The signs inside most buses and trains say that radios with loudspeakers are not allowed. However, they do not prohibit conversations between two or more passengers. Talking into microphones does not seem to be banned either. So, it looks as that provided one conducts oneself with decorum and respects the rights of other passengers, amateur radio and public transport commuting are not necessarily mutually exclusive.

I have never heard of anyone hook their laptop to their HT and play packet radio whilst bus mobile. But, I'm sure it can be done provided your route does not stray away from a bulletin board's service area, and there's not much competition from other signals on the frequency.

A standard hand held transceiver will perform well inside a bus or train carriage. However, provided that your bus or train is not too crowded, you can replace the small helical whip with a full-sized quarter wavelength antenna for two metres for better signals into the repeater.

Depending on the length of your commute, the standard battery pack supplied with the handheld may not be large enough to provide power for all the trip. If this is the case, consider using an external sealed lead acid battery to power the rig (see the Novice Notes column on portable operating for more details on this).

If you have a choice, make sure you get a window seat. Ideally it should be on the side of the carriage that is in the direction of the repeater - this seems to reduce attenuation of signals due to shielding. And, always wear an earphone so that other passengers are not disrupted by your communication activities. And, if the bus/train is very crowded, it may be best to refrain from transmitting at all because of widespread concern over the effects of RF exposure. Of course, if you did this 10 years ago, many people would stare at you as you muttered words into a handheld box. Now with widespread mobile phone use, attitudes have changed, and no one would bat an eyelid at your communication activities.

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HF on your handheld

Do you own an HF transceiver, VHF scanner, 2 metre handheld and a portable HF shortwave receiver capable of receiving SSB signals? If so, you have all the equipment needed to operate pedestrian mobile HF without having to carry bulky batteries, transceivers or antennas. The information presented here will also be useful if you are called upon to relay a WIA or club broadcast or need to set up a station in a confined space for amateur radio public relations purposes.

This is how to do it:

Note: This type of operation is quite popular in the USA. However, regulations about retransmitting signals onto various bands, unattended operation and station identification vary from country to country. As a safeguard, you may wish to experiment with means of access control (eg CTCSS subtone) to prevent others from unintentionally accessing the link.

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Experiment with SSB repeaters

All currently licensed repeaters in Australia are for either FM, packet radio or amateur television. There are no repeaters for SSB or CW modes known to be operational in Australia. However, repeaters using these modes are likely to have a longer transmitting range than repeaters that use FM.

Repeaters that handle SSB signals are on the air in NZ, the USA and other countries. However, their most common use is on amateur satellites. These types of repeaters on satellites (called linear transponders) can handle a number of different signals at once. They do this by repeating a whole band of frequencies (typically 40 - 100 kHz wide) so that several SSB and CW signals can be retransmitted at once. These devices generally receive in one band (70cm or 2m) and transmit in another (2m or 10m).

As discussed in a previous Novice Notes column, a linear transponder is a challenging project for the experimenter. However, it should be possible to simplify it a great deal by narrowing the transmitted bandwidth to 3 KHz so only one SSB signal can be repeated at a time.

The simplest way of achieving this is to simply connect the audio output of an SSB receiver to the microphone input of an SSB transceiver. This system will work, but has a number of limitations. One of these is the need to ensure that the transmitter is fed with a signal of the right level to assure full power output on both weak and strong incoming signals. This can be done with some sort of audio compression between the receiver and the transmitter. However, this wastes power as significant power would be radiated even if no signal was being received.

A way to overcome this is to use some form of squelch circuit. However, an ordinary squelch as used in FM receivers would render the unit insensitive and lead to weaker signals dropping in and out. More advanced squelch systems (eg 73 Magazine, August 1982) can be built to overcome this problem. Power consumption would be much reduced as the transmitter is only on when there is an incoming voice signal.

So a basic single channel linear translator (probably better referred to as an SSB repeater) should consist of a receiver, transmitter, audio compressor and SSB squelch circuit. A beacon may also be provided to demonstrate to the user that the system is operating.

On what bands should a unit such as this operate? This depends on what you want from the device. If you want a reliable full-duplex link between two sites of 100-500km distant (but separated by mountain ranges), two metres and seventy centimetres would be a good combination. An old IC202 and IC702 pair (perhaps assisted by a linear amplifier) should serve well here. Other applications may include operating on HF via a VHF or UHF link. This application could be useful for a person in (say) a retirement home who may be able to receive HF signals, but cannot put up an antenna sufficient for good results to be had on transmitting.

The VHF and UHF amateur bands are lightly used in Australia. This means that there would be little objection in experimenting with a linear translator device using double sideband. The techniques would be very similar to that used by those who build DSB transceivers with direct conversion receivers. The incoming signal would be converted to audio, fed through a low pass filter and coupled to a DSB transmitter on a different band. The receive signal to noise ratio of this system would not be as good as a true SSB device, and half the output power would be wasted in the other sideband. However, the technique may be promising to the experimenter with little money.

Linear translators have many advantages. They allow a range of modes to be transmitted at the one time. The use of SSB and CW permits far longer transmitting ranges to be obtained. Full duplex, telephone-style contacts are the norm - quite different from conventional FM repeater operation. The author has done little work in this area, but the field is a promising one for the amateur experimenter.

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Handheld antenna

Can't afford the high cost of commercially-made antennas for your hand-held? Why not build one yourself? A quarter wave whip for two metres offers considerable gain over the supplied helical, and gives a stronger signal into the repeater or on simplex. Take a standard PL259 plug (preferably the type made for thick cable such as RG8 or RG213) and a 50cm length of stiff copper wire or rod. This should be thin enough to slide inside the PL259's centre pin. Coat both the inside of the PL259 inner connector and one end of the wire or rod with solder. Then apply heat to the centre pin of the PL259, insert the rod through the rear of the plug and allow to cool. Use either insulating tape or a rubber grommet to ensure that the rod cannot touch the rear of the plug. With the use of an appropriate adapter, the antenna can now be used on your handheld transceiver. A finishing touch may be to glue a toothpaste tube cap or toggle switch nipple to the end of the antenna for safety. As the antenna is three-quarters wavelength long on UHF, it should also be effective on 70 centimetres.

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Labelling leads

In many radio shacks, there is often a plethora of leads behind the equipment. It is very easy to make mistakes when changing connections. In extreme cases, this can lead to equipment damage, for example when full power from a transceiver is applied to the antenna socket of a receiver. Cables should be labelled to minimise this risk. A good way is to write (with a ballpoint pen) labels onto strips of paper 5mm wide and as long as the label requires. Clear adhesive tape is placed over the front of the label and around the cable. The tape is then continued so that it sticks to the back of the paper and around to the front of the label, where it is cut with scissors. The result is a descriptive 'flag' at the end of the cable near the connector. A refinement could be to write on both sides of the paper strip instead of one.

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Cardboard Circuits

Many amateur experimenters have their own favourite means of putting together prototype circuits. Here's a method that is simple, cheap and is even good enough to use for permanent circuits; handy when the prototype works so well that you're afraid to pull it apart.

The technique involves the construction of your own printed circuit boards. What's different is that no chemicals, circuit board material and dalo pens required. As well, the method is so cheap that you can simply throw away boards after you've finished with them.

The method is based on the ready availability of self-adhesive copper tape. This material is used by stained glass craftspeople. Available on rolls of 30 metres, the 5mm-wide copper tape can be cut with an ordinary pair of scissors. What makes the tape useful for our purpose, however, is the strong adhesive backing on one side of the copper. This backing can stand high temperatures, such as that applied by a soldering iron.

The cardboard is simply used as an insulated surface on which rectangles of copper foil are placed; as components are soldered directly to the copper, there need be no holes drilled through the insulating material, as is the case with conventional printed circuit boards. While cardboard from the side of an ordinary cardboard box is satisfactory, any non-conducting material, such as glass, wood or fibreglass could be used.

After looking at the circuit's schematic and drawing a diagram of where components are to be mounted, determine the size of the cardboard required. After cutting the cardboard to the right size, start sticking on rectangles of copper tape. This is easy to do; simply cut the tape to the length required and peel off the paper backing, which protects the adhesive. Pressing the copper with your index finger lightly against cardboard should result in a strong bond, which will withstand the heat of a soldering iron.

To form a right-angled bend, place two pieces of copper strip over one another, with their ends overlapping. To ensure a good connection between the two pieces, solder the overlapping edges of the two pieces together.

Once the board has been completed, the components can be mounted; these are soldered straight to the copper tape. That completes the construction of your project. A blob of Blu-tac can be used to attach the cardboard circuit board to the interior of the case housing the project. You will find that because it is easy to add or remove components, experimentation is far easier than if you used an ordinary printed circuit board. As mentioned before, the cost of assembling circuits is very low, and the technique is ideal for beginners' projects.

A 30 metre roll of the tape costs around $7 from suppliers of stained glass craft materials. Assuming you use a metre of tape for every circuit, the cost works out at less than 25 cents per project. Just one roll will last most people months, if not years. Of course, there are other applications that lend themselves to the use of this tape. These include repairs to printed circuit boards and even using the foil for VHF/UHF antenna elements.

The diagram below shows what your completed board should look like.

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Build your own multiband transceiver?

Many modern HF general coverage receivers have a first IF in the low VHF range. The advantage of this for the manufacturers is that it is possible to obtain high image rejection without banks of expensive band pass filters at the front end. A stable PLL-type frequency synthesiser is (also operating in the low VHF range) is typically used as the local oscillator.

Let us assume that the receiver has a first IF of 70 MHz. The local oscillator tunes 40 - 70 MHz to allow 0 - 30 MHz coverage - for example, to receive WWV on 10 MHz, the local oscillator would be set to 60 MHz.

Suppose one was to take the output of the local oscillator to a socket on the rear panel of the receiver. Furthermore, what if one mixed it with a signal from a crystal oscillator operating on 70 MHz? The result would be an output signal on the frequency to which the receiver is tuned. If the signal were to be fed through a 1.8 - 30 MHz broadband amplifier module (supply rail keyed via a switching transistor) and a pi-network for each required band, one would have a very economical multiband CW transceiver for all HF bands. Adding a balanced modulator between the external 70 MHz oscillator and the mixer should allow DSB transmission.

SSB should also be possible. Either the phasing method (requiring no additional frequency conversions) or the conventional filter method could be tried. For the phasing method you would produce an SSB signal at 70 MHz and mix it with the output of the local oscillator. For the filter method, it might be possible to mix a 10 MHz SSB signal (perhaps using a ladder filter made from 10 MHz crystals) with a 60 MHz crystal oscillator to produce the desired 70 MHz SSB signal.

The author has not yet tried any of the above but it promises to be an interesting approach to constructing your own SSB station that would rival any commercially-produced transceiver.

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Use an AM car radio for VHF FM reception

If you're looking for a means of receiving amateur VHF FM signals, an AM car radio may be just what you are looking for. Though not capable of providing outstanding audio quality, this approach is simple, cheap and particularly suited to reception of local signals.

The incoming VHF signal is converted to a signal in the 0.53 - 1.6 MHz range. It is then 'slope-detected' on an AM car radio. The radio is used as a tunable IF.

A crystal-controlled converter is preferred for best frequency stability. The converter would include a crystal oscillator, frequency multiplier and mixer. Thus a converter suitable for reception of two metres need only use three or four transistors. Of course a RF preamplifier could be added for improved sensitivity. For two metres, the oscillator injection frequency could be 145.9 MHz. This would allow reception of 146.4 - 147.5 MHz. This range covers the main simplex frequencies and all repeater outputs in Australia.

Note that image rejection will be poor because of the low IF. In this case the image responses will be within the two metre band, around 145 MHz. However, in many parts of Australia there is little activity in this part of the band, so the poor image response will seldom be a problem in practice.

A practical application of this approach is as a simple FM transceiver for repeater use on either six or two metres. Supposing you would like to built a station to operate through a six metre repeater which receives on 52.8 MHz and transmits on 53.8 MHz. You build a conventional crystal-controlled FM transmitter for 52.8 MHz.

However, unlike with a conventional transmitter, you leave the oscillator and frequency multiplier stages powered on during receive. The purpose of this is to provide a 52.8 MHz local oscillator signal for the receiver. This is mixed with the incoming signal in a one transistor mixer stage (possibly using a dual-gate FET) to produce a difference of 1 MHz. This difference signal is slope-detected on an AM car radio tuned to 1 MHz. The advantage of this is that no receive crystals are needed for this arrangement to work. Also you can tune into repeaters on adjacent frequencies just by tuning the car radio. The principle is similar on two metres except the receiver is tuned to 600 kHz to suit the repeater frequency offset.

(A converter using this technique has been published in Amateur Radio magazine)

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This page was produced by Peter Parker VK3YE parkerp@NOSPAMalphalink.com.au. Material may be copied for personal or non-profit use only.