    
Tech833
Moderator
Username: Tech833
Post Number: 858
Registered: 8-2002
| | Posted on Friday, September 24, 2004 - 4:45 pm: | 
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Shortwave antennas
Written By ~ Tech 833
As many here already know, the antenna is more important than the radio for maximum performance. But, so far, have you always thought of performance in terms of coverage? In terms of how far your transmitter will cover? Have you ever thought of your antenna performance in terms of how much signal you can grab from the sky and deliver to your radio receiver? Well, that is exactly what we are going to do here.
In the exciting world of shortwave listening, there are a few things to remember.
First of all, you are going to be listening over a very wide range of radio frequencies. The CB band is only ½ of a MHz. Wide from beginning to end. The shortwave band covers an entire 30 MHz., that is 60 times the width of the CB band! Clearly, there is no 'one size fits all' antenna solution to this. If you were only trying to cover the CB band, a finely tuned antenna will add some signal to your receiver. However, as you drift away from the center of resonance, the antenna rapidly becomes less effective. For shortwave, we need the antenna to remain effective over a very wide area.
Next thing to consider is capturing the most signal along with the least amount of noise. Many man-made things and activities create radio spectrum noise. It is just coincidence that most of it seems to fall in the LF and MF and SW spectrum. A good shortwave antenna has to be fairly quiet in order to pull weaker SW stations out of the noise. Many antenna types have been developed specifically to null out noise sources. For example, on MW a loop can be constructed in such a way to make it have extremely deep nulls broadside to it. This is used to null out, or 'remove' the signal strength in certain directions while not attenuating signals from other directions. More on this later.
Finally, we need to consider capture area. An antenna's capture area can be compared to the diameter of a butterfly net. The larger the diameter of the net, the more butterflies you can capture with one single pass through the air. We have all been taught that for a two-way radio antenna, the ¼ wave is this much gain, the ½ wave is that much gain, the 5/8 wave is that much more gain, etc. In the world of shortwave, gain is no longer a factor. WHAT??!! I said it, gain means nothing. The shortwave antenna gain is measured by capture area. The more antenna element in phase with the signal, the more capture area your antenna has.
Let's look at some types of antennas for shortwave listening.
The most popular antenna is the longwire. Just as the name implies, it is made up of a long piece of wire. The longwire is an unbalanced type of antenna, meaning you can feed it directly with coaxial cable. Most people do not use coax with longwire antennas, and in fact, coax is not recommended. You simply connect the wire to the center (or 'hot') antenna terminal and a ground to the outer or ground terminal. The trick to making a longwire effective over a broad range of frequencies is to make it LONG! The bare minimum for a good performing longwire is about 70 feet. There is a point in which a longwire can be too long to be practical. Sometimes, this is a desirable effect. One such antenna is the 'Beveridge' antenna. It is sort of a 'longwire on steroids', sometimes extending over 1 mile in length. This type of antenna is extremely directional, and not a very good all-around shortwave antenna.
Keeping a longwire quiet is easy. First of all, use only insulated wire. I know this is contrary to what many people will tell you or what is available for sale, but bare wire is simply noisy. First of all, wind static cannot happen with insulated wire (since none of the conductor is exposed to the moving air) and second of all, with insulated wire, the insulation itself electrically 'lengthens' the wire by lowering the velocity factor. This, in turn, makes your antenna perform like a longer wire than it physically is. A win-win situation! The second rule to keeping a longwire antenna quiet is to keep the wire away from metallic objects, buildings, and electrical fields. Also, if you can support at least one of the antenna ends high above ground, this helps a lot. Putting bends or angles in a longwire actually seems to improve the capture area, so do not be afraid to experiment. My best longwire once extended vertically from the bottom of my house, up to a support, making a 90 degree bend to horizontal, and then finally extending horizontal to a pole about 50 feet away for a total of 75 feet. I have yet to duplicate its performance, even with longer wires.
The next most popular type of shortwave antenna is the inverted V. If you are standing on the ground looking up, the antenna looks like this- ^. This antenna type is also popular with the ham radio HF crowd since it has high performance and low noise. The inverted V usually consists of two equal (or nearly equal) lengths of wire supported above ground in the center and also fed in the center by either a balun or balanced line. When coax fed, a balun is a must. The good news is, this also makes the antenna quieter. The far ends of the antenna are closer to the ground, and the antenna itself should form an 'L' shape if you tilt your head. Sometimes, you can use a number of different length wires connected together at the center and insulated at the ends to widen the bandwidth of the inverted V. For shortwave listening, this makes much less difference than it does to those who will be using this antenna for transmitting.
Inverted V antennas do not like having their elements bent like a longwire does, so you should try and keep them as straight as possible. If you have a small area and need to make 90 degree bends in the wires, try to do it as near the ends as possible, and don't put more than 1 bend in it. This actually cuts down on the capture area of the inverted V. If you have a real small area, you might try an end fed inverted V, which is basically just a longwire with the highest support in the middle.
Another very popular shortwave antenna is the loop. A loop antenna is made up of several turns of wire around a form. The larger the form, the more wire you can use and the more sensitive the antenna will be. It becomes important with a loop antenna to allow ample spacing between the turns. This obviously limits the amount of wire you can use to construct the antenna. One clear advantage to the loop is that it is a highly directional antenna. As mentioned earlier, it has two deep 'nulls'. A null is an area around the antenna where radio signals are invisible to it. As the radio signal passes the antenna, if equal amounts of signal are on both sides of the form, they cancel each other out, and the signal is gone. This is most helpful on the AM broadcast band when you are trying to copy a station in one direction and another station somewhere else is interfering with it. You simply rotate the loop antenna and 'null out' the offending signal. This works great! Since rotating the loop is a precise exercise, loops are usually used indoors. Some outdoor loops are in use, but very few.
Some other antenna types worth mentioning are the wide variety of commercially available HF antennas for the ham radio market. There are all kinds of vertical multiband antennas that will also perform extremely well on shortwave. Since tuning is much less critical for receive-only purposes, you may not even have to retune an antenna from the ham bands to enjoy it on shortwave.
The 'trapped' verticals are basically just a vertical antenna with a lot of coil forms along its radiator. Each of these forms is a 'trap'. It is a filter of sorts designed to keep any radio signals above its resonant frequency from passing, but allowing frequencies lower than the resonance point to pass. This is really not very efficient as antennas go, but a good compromise indeed. Each segment of the vertical is only going to be ¼ wavelength long for its intended band. The whole entire antenna is not in use except on the lowest band. The key to success with a vertical antenna is LOTS of ground wires buried around its base. With an inadequate ground system, verticals are notoriously noisy and perform poorly compared to some wire types. The ground system is literally half the antenna, so a poor ground system is like half of the antenna being broken. This is another good reason to consider a wire type antenna for your shortwave installation. However, if you already have a multiband vertical, feel free to put it to work!
Wire antenna are the most popular on the shortwave bands, and for good reason. There are many more types of antennas we could discuss, and it is true that there are all sorts of compromises to performance and cost and size to consider. However, I feel this should give you a good starting point to future shortwave enjoyment. All of these types of antennas will handily outperform the telescoping built-in antennas that the portable shortwave radios have on them. In fact, a whole new world will open up to you when you finally do install that first outdoor antenna! Enjoy!
-Tech 833 |
