Fullscreen HTML5Med Res (??MB)HTML4

VINTAGE RADIO By RODNEY CHAMPNESS, VK3UG Improvements to AM broadcast band reception; Pt.2 This month, we look at some practical antennas that you can make to dramatically improve AM broadcast reception. Both long wire and loop antennas are described. In the first article, the theory behind improving AM radio reception was discussed. In particular, it is important to have a good antenna and earth system, located in an area where signals are good and interference minimal. The location is particularly important, to minimise interference from man-made sources. In this article, some practical methods of improving AM radio reception will be described. In some cases, a relatively simple method will suffice. However, more elaborate systems are required in very noisy locations or where long distance reception is required. Often, it will be necessary to experiment to find which method or methods give the best results. “Long” wire antennas The “long” wire antenna is easy to erect and can give quite good results. “Long” wire is a relative term and is generally used to mean long in relationship to the wavelength of the radio wave received. However, it is obviously a short wire antenna in relation to AM broadcast band wavelengths but the term has stuck. Older receivers of the pre-transistor era almost universal­ ly have aerial/ antenna and earth terminals. Most of the later valve receivers have a Fig.1: the basic scheme for a “long” wire antenna. Note that the extra wire which goes up to the antenna proper (the wire that goes from the earth but does not connect to the antenna) can be omitted if noise isn’t a problem – see text. 62  Silicon Chip loopstick aerial as well and in most suburban areas perform quite well, as the signals are strong. However, as time has progressed, houses have been built with metallised insulation paper in the walls and sometimes in the ceiling and under the floor, or have other metal structures that act as radio signal shields. Additionally, many new domestic devices, including personal computers, often create interference. An outside antenna consisting of 5-15 metres of insulated wire taken out through the wall of the home and run along the eaves is usually sufficient to give quite a reasonable improve­ment to the reception. The antenna is away from the interference producing sources and the wanted radio signal outside is strong­er. There is nothing magical about insulated wire except that it is easier to handle and prevents shorts. If reception is still not good enough, a longer and higher outside antenna is needed. In the early days of radio, outside antennas were commonly 30 metres long and around 13 metres high. However, antennas of such dimensions are clearly not practical in the average domestic environment. For best performance, the antenna should be high and long but anything higher than 5 metres and longer than 15 metres will be reasonably effective. The antenna can be installed as shown in Fig.1. Note that the extra wire which goes up to the antenna proper (the wire that goes from the earth but does not connect to the antenna) can be omitted in this instance. It is used in another antenna system to be described shortly. The antenna lead to the outside of the home should be as short as reasonably practical and should be kept well away from any electrical appliances and wiring to reduce the likelihood of interference pickup. The mast that the television antenna is attached to, or a chimney, are convenient spots to attach one end of the antenna support cable. As shown in Fig.1, the end of the antenna is kept well away from the house to reduce interference. It is suggested that the wire between the set and the horizontal sec­tion of the antenna be insulated. It should also be resistant to the Sun’s ultraviolet rays. By using ordinary domestic electrical twin flex, it is possible to modify the antenna from an ordinary outside antenna to a noise reducing type. This is done by connecting or discon­ necting the “unused” lead from the set’s earth terminal. The horizontal section of the antenna can be insulated or bare wire. A cheap wire is tie wire which is used in the garden. At around 18 gauge, it is quite adequate for the job, being strong, light and inexpensive. Copper wire is not needed. The “egg” insulators at the ends of the antenna can be ob­tained from some of the retailers who advertise in this magazine or from suppliers who sell electric fence components. All wire joins in the various parts of the antenna must be soldered if they are out in the weather, otherwise the reception will be spoilt by crackling when wind moves the antenna (particularly if corrosion sets in). Fig.2 shows the wiring to the egg insulators. It is import­ant to ensure that minimal stress is placed on the wire where it joins the antenna proper. An external earth may not be needed to give the improvement in reception that is desired. However, if you are going to all this trouble, it is desirable to install a radio earth as well, even though the radio may already be earthed via the mains. The radio earth can be a 1.5 to 2-metre length of 19mm galvanised water pipe driven into moist soil near the side of the home. The earth wire is clamped to the pipe with an electrician’s earthing clamp or a screw-type hose clamp. The joint needs to be cleaned and, when everything is tightened up, painted to retard any corrosion (see Fig.1 in last month’s column). Note that the earth wire should be Fig.2: here’s how to connect the various leads for a “long” wire antenna to the insulator. Fig.3: a “long” wire antenna can be inductively coupled to a portable radio by winding a few turns around the set, as shown here. Fig.4: another way of coupling a “long” wire antenna to a portable set is to wind a few turns of insulated wire around the loopstick antenna. Fig.5: (left): an untuned loop antenna gives less signal strength but is quite effective at reducing interference. reasonably heavy gauge insulated wire. If the set chassis is earthed, it is desirable to place a .001µF to .01µF mica or polyester capacitor in the newly installed radio earth lead before it attaches to the set. This is to prevent this earth from taking the place of the mains earth. The next improvement is to make the long-wire antenna a “noise reducing” type. This is achieved as shown in Fig.1, by running a twin wire lead up to the antenna proper. This lead can be domestic electrical twin flex or 300-ohm twin black ribbon television cable. The latter will last much longer as it is treated to resist ultraviolet radiation. Don’t use the clear cable; it has no UV protection and will deteriorate within about 12 months if it is out in the weather. Which ever cable is used, it should be supported using both leads. Note that the second unterminated wire is left with its insulation intact so that the wire touches nothing and so that it can be tied to the antenna. It must be attached so that it doesn’t chafe. The advantage of this scheme is December 1998  63 nal-carrying wire. This means that a slightly larger long-wire antenna may be needed to overcome these losses. Note that 300Ω TV cable has less capacitance between its wires than electrical twin flex and will have less loss of sign­al. However, it may pick up a small amount of interference. Long-wire antennas & transistor sets Fig.6: a tuned loop antenna can dramatically improve AM broadcast reception. It is less responsive to interference sources than a “long” wire antenna and it is directional. This means that unwanted interfering stations can often be nulled out by rotating the loop. that the twin wire from the antenna proper to the set picks up very little signal, as one wire is earthed and acts as a shield for the other. This means that, if it goes through a noisy area on it way to the set, no extra signals are picked up and so the radio receives a signal that is largely noise-free. 64  Silicon Chip There are a couple of disadvantages, however. Because the “shielded” section of cable doesn’t pick up any signal, the effective length of the antenna is reduced compared to using an unshielded down lead. In addition, some signal is lost due to the proximity of the earthed wire to the sig- How can the long wire antenna be used with a transistor set that needs a boost in performance? This is quite a problem as most transistor sets have no external antenna and earth termi­ nals. One solution is to remove the back from the set and wind a few turns of insulated wire around the loopstick antenna. This is then connected to the antenna and earth wires coming into the house. However, before doing this I would suggest a different approach. This involves winding 2-5 turns of insulated wire around the set as shown in Fig.3. You then connect one end of the winding to the antenna and the other to the earth. Now turn the set on and tune across the broadcast band. If the set is a good one, it will be found that previously noisy stations are much clearer and additional stations will become quite audible. However, if the set is the typical mass-produced suburban “cheapie”, the results may be disappointing. In addition to the wanted stations, many stations may appear in odd spots on the dial, along with shortwave and Morse code stations. To add insult to injury, the stations that were originally heard well may now have other stations interfering with them. So putting up this lovely new antenna/earth system has, in this instance, been a complete disaster. What has caused this, and how can good clean signals be obtained for transistor sets, so that the expected improved reception can be obtained? The cause of the problem was mentioned in the first article: poor selectivity in the receiver’s antenna circuit. In addition, the “link” winding to the base of the autodyne converter transistor couples nicely with the link winding that has just be placed around the receiver (see Fig.4). This means that shortwave signals will easily be transferred from the anten­ na link winding to the transistor base winding. This base winding will have a tendency to be broadly reso­nant in the shortwave bands. This would not be a problem in itself were in not for the fact that the local oscillator gener­ates many harmonics in addition to the wanted oscillator frequen­cy. As a result, the shortwave stations beat with the oscillator harmonics and produce the multitudinous unwanted signals. Some of the better sets don’t suffer from this problem but most do. The way around the problem is to increase the selectivi­ty of the receiver and the procedure will be described later. Loop antennas In the past, many people simply connected 5-10 metres of insulated wire to the antenna terminal of a valve radio. This was often laid around the skirting boards and reception in most cases was satisfactory. However, it was soon shown that if the wire was run along the picture rail and then doubled back along the skirt­ing board, the reception was just the same. The next step was to connect the end that had been doubled back to the set earth. “The set earth!”, you might say. “That will short the signal out!” Not so – the antenna wire in fact becomes a large untuned loop antenna and its effectiveness in picking up signals is governed by the area within the loop. What will be noticed is that while the signals are a little weaker, the interference completely disappears in many cases. In this case, the antenna system has been changed from a “long” wire (electric field pick-up) antenna to a loop (magnetic field pick-up) antenna, just by earthing the end of the antenna. This is a simple way of assessing the effectiveness of the two types of antennas. So let’s now take a look at the loop antenna types that can be used. Loop antennas have been used since the very early days of wireless (radio) in a variety of forms. Some of the early sets had a loop antenna sitting on top of them. They were rather bulky and so were the sets. Gradually, the loop gave way to the “long” wire antenna, which meant less bulk in the lounge room. As radio progressed, the valves and components became smaller and portable battery radios were developed. The early sets used a spider web weave loop antenna (coil) in their back which nominally measured Fig.7: an alternative scheme for a practical loop antenna. It uses a loop made from 13mm polythene pipe and 10-conductor rainbow cable. The bottom ends of the loop are secured to a standard plastic case using saddle clamps – see text. around 25 x 18cm. These were reasonably efficient although not as good as the ones used in the sets of the 1920s which measured up to 60cm square. In the early 1950s, the flat wire loop was gradually re­placed with the new ferrite loopstick antenna. These units were more compact than the large loops in the back of portables. However, they weren’t particularly small in Australian-made high-performance sets (transistor sets in particular), commonly measuring 200mm long x 13mm in diameter (and a few were even larger than that). Cost considerations meant that the size was reduced in later years and some ferrite rods are now just 40 x 8 x 4mm. These are found in sets intended for use with signals from strong local stations. Practical loop antennas Let’s now take a look at two loop antennas that you can build to dramatically improve reception and reduce the deleteri­ o us effects of interference. The first antenna has a loop dia­ meter of nominally 1 metre. It consists of a frame made of wood or plastic, as shown in Fig.6. The tuned wind- ing consists of 7 turns of wire spaced around the extremities of the loop frame. The beginning and end of this winding terminate to the stator and rotor terminals re­ spectively of a single-gang variable capacitor (or you can use one gang of a dual-gang variable capacitor). The seven turns will tune across the broadcast band with a tuning capacitor of around 400pF. If the gang has only about 300pF maximum capacity (eg, if two sections of a miniature tuning gang for a transistor radio are paralleled), an additional turn or two may be required to cover the broadcast band completely (you may have to experiment to get the best results). An additional (separate) pickup turn is also wound around the frame and this is terminated on the insulating plate and then connected to the antenna and earth terminals of the set via a 300Ω ribbon cable (TV twin lead). Provided it is suitably weatherproofed, this antenna can be located outside, away from noise sources. The disadvantage is that it can only be tuned to nominally one station, which means that you have to go outside to December 1998  65 The rainbow cable leads for the antenna shown in Fig.7 are brought into the plastic case and wired in series by terminating them on tagstrip. The end of the brown lead joins to the start of red lead, the end of the red lead to the start of the orange lead and so on, as shown in Fig.7(c). Note that the yellow wire isn’t connected to anything, to reduce the distributed capacity across the winding. retune the loop. However, if you only wish to listen to one station, that is no problem. Another approach is to use varicap diodes instead of a mechanical tuning capacitor. This will enable the antenna to be remotely tuned via a variable DC voltage which can be fed down the antenna twin lead or coaxial cable from the receiving loca­tion. This loop antenna has two advantages over a long-wire antenna: (1) it is less responsive to interference sources; and (2) it is directional, so that (in some situations), unwanted stations can be nulled out by rotating the loop. The second loop antenna Another variant of this loop antenna – which in some ways is easier to construct – uses 13mm-diameter polythene pipe as the former for the wires. The wires are slid inside the pipe but you don’t have to slide them in one-by-one. Instead, the trick is to use 10-strand rainbow cable. You will need 3.15 metres of 13mm polythene pipe plus four saddle clamps. In addition, you need a plastic case measuring at least 130 x 68 x 41mm, 3.25 metres of 10-conductor rainbow cable, an 11-lug terminal strip, a tuning capacitor, a knob, a SPDT toggle switch, a 1.2-metre length of timber or plastic conduit to support the top of the loop and some screws to mount the pieces of hardware. The first step is to thread the wire through the pipe. To do this, attach a small nut to some cotton and feed this through first. This done, attach some string to the cotton and pull this through, then use the string to pull through the rainbow cable. Trim the rainbow cable to length, leaving about 80mm exposed at either end. The assembly of the loop antenna can now commence – see Fig.7. Drill holes for the cable to go into the side of the box plus holes to accommodate the screws that go through the saddle clamps. The tuning capacitor is mounted inside the box. Very small variable plastic capacitors are easier to mount – if you can get suitable screws. In some cases, epoxy adhesive can be used instead but be careful how you apply it. The 11-lug terminal strip is also mounted in the box to terminate the leads of the rainbow cable. The loop is now fastened to the back of the box using the four saddle clamps. The 1.2-metre length of timber is attached to the side of the case using two screws and is used to support the loop at the top. This ensures that the loop remains vertical A 1.2-metre length of timber is attached to the side of the case and supports the top of the antenna loop, so that it remains vertical. In operation, the radio is placed inside the loop (on top of the plastic case) and the loop tuned and rotated for best reception. 66  Silicon Chip capacitance between turns, making it necessary to tune the broadcast band in two stages. If the turns are spaced away from each other, the distributed capac­itance would be low and the whole band could be covered in one sweep. This type of loop is more difficult to make though. Nulling unwanted stations Fig.8: a large untuned loop antenna. It is more elaborate than the one shown in Fig.5 and is also a better performer. This type of antenna is more suited for use with a radio that is equipped with both antenna and earth terminals. One very convenient feature of these two loop antennas is that by rotating them horizontally, it is possible to null out unwanted stations. This can make a big difference where a wanted station is being interfered with by an unwanted station. As long as the apparent directions of the wanted and unwanted stations are greater than 45 degrees apart, the results can be very satis­fying. Untuned loop antennas and stops it from flexing – see photo. The ends of the rainbow cable are brought in through the hole in the back of the box and connected to the terminal strip. Note that each wire is wired in series with the last one – see Fig.7(c). Begin by soldering the brown wire at the “start” end of the cable to the end terminal lug. Its “end” is then connected to the second lug, along with the red wire of the “start” end. The “end” of the red wire then goes to lug three, along with the “start” of the orange wire, and so on. Note that the “end” of the orange wire attaches to the “start” of the green wire. The yellow wire (which comes after the orange wire) is not connected to anything. This is done to reduce the distributed capacity across the whole winding so that the loop will tune properly. The sequence of the wiring then continues with the normal colour code progression, finishing with the “end” of the black wire going to the 10th lug. Unfortunately, the distributed capacity is still too great for the loop to tune the whole of the broadcast band in one sweep. To overcome this problem, the brown wire from the loop connects to the rotor of the tuning capacitor. The SPDT switch is then used to connect the tuning gang stator (s) to either the junction of the green and blue wires or to the single black wire. The circuit diagram shows the connections. Remember to make sure any trimmers mounted on the tuning gang are adjusted for minimum capacity. The loop is now ready to test. Tune a transistor set to a weak station, then place it in the loop and rotate the loop’s tuning capacitor for best reception. All being well, a very noticeable improvement in reception will be observed. Now tune the transistor radio to both ends of the dial to determine whether or not the loop covers the whole band. It may be necessary to vary the number of turns in use to cover the whole band, depending on the tuning capacitor used. Rainbow cable has high distributed Because of their size, tuned loops are usually not well accepted in a domestic environment. However, untuned loops can do all that the tuned loops can do and more, with the exception that they cannot be rotated to null an unwanted station out. They are also rather large but because they are mounted outside, they don’t cause any inconvenience inside the home. Fig.8 shows the large untuned loop antenna. It is more elaborate than the one shown in Fig.5 and is also considerably better. It is installed away from interference sources, usually in the back yard. It must be orientated so that the horizontal sections nominally point towards or away from the stations of interest. As with the tuned loop antennas, there will be a signal null at right angles to loop. This type of antenna is more suited for use with a radio that is equipped with both antenna and earth terminals. That’s all we have space for this month. Next month, we’ll describe how to make an antenna booster. SC December 1998  67