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Longwave, Medium and Shortwave Listening

Week 1: Longwave, Mediumwave and Shortwave.

Firstly I have decided to write a series of guides to the radio spectrum, this is the first of them. Next week I will post the next one which will cover the VHF part of the spectrum 30 to 300 MHz.

So you have decided to begin messing about with radio. This week I will explain reception of the bottom end of the radio spectrum. This spans from D.C up to 30 MHz. In this band you will hear things from natural radio emissions from the earth and space, to things like air and shipping beacons, time signals, amateur radio operators, international broadcasting and if you are lucky things like numbers stations and the like (Secret government comms and whatnot).

To do this you will need a radio receiver capable of receiving this band, If you cannot afford a radio receiver capable of receiving below 100 KHz, your PC sound card will pick up signals from just above D.C to at east 25 to 30 KHz if you connect a large coil of wire to the mic input and use the correct software which I will explain in a minute. If you are going to buy a cheap shortwave receiver ensure it is capable of receiving SSB signals (Single Side Band) which commercial aircraft, shipping and most amateur operators use to transmit. If it is AM only chances are you will only receive commercial radio stations and some time stations like WWV or WWVH which are the US operated time stations. The software I use is called Spectrum Laboratory and can be downloaded from http://www.qsl.net/dl4yhf/spectra1.html

So you have gone out and finally purchased that receiver, and you can’t seem to pick anything up. Don’t give up, chances are you have set it up incorrectly or you are in a “noisy location”, Try setting your antenna up far from electrical appliances like computers and TVs. These are extremely noisy and will block reception from a fair distance, especially if you are receiving a AM modulated signal like a commercial radio station or anything below 5 MHz as most electronic appliances emit radio energy in this range.

I find trying to receive WWV or WWVH gives me an indication of what reception is like in any given area, WWVH is located in Hawaii and can be received on 2.5,5,10,15 MHz and if received will sound like a chiming clock with a voice announcement giving the time in UTC (Coordinated Universal Time or GMT) and a tone at the beginning of the minute. WWVH is announced in a female voice, WWV is in the same format and is transmitted from Colorado on 2.5,5,10,15 and 20 MHz and is announced in a male voice.

Lower frequencies are better received during the night while higher frequency signals are better received during the day. This is because of a change in a layer of the atmosphere called the ionosphere which is effected by the sun and depending on whether or not the suns radiation is striking it depends on what frequencies are absorbed and which are reflected of of it. AM radio stations (those between 500 kHz and 1600 kHz or those received on a regular radio) are best received at night as the signal travels further because of the state of the ionosphere. This is why stations from hundreds of miles away come in clearly at night but cannot be heard during the day. This also applies to anything from roughly 5 MHz and below. (I will try to use soundcloud to post the recording when I find it).

Description of some radio terms

Ionosphere

This is a layer of the earth’s atmosphere which reflects radio frequencies from around 30 MHz down to around 5 MHz. Within the ionosphere exists around four layers they are…

D layer: This absorbs frequencies from 10 MHz and below. It sits around 60 Kilometres above the earth’s surface and exists only during the day. It disappears during the night.

E layer: This reflects frequencies below 10 MHz. It sits at around 90 Kilometres above the ground and contributes to long range communications during the night. Although it exists during the day, because of the fact that the D layer is below it, it has no effect during the day.

F layer: This is the cause of ground wave signals being able to travel long distances. it is the uppermost layer at 200 Kilometres above the ground. Because the D layer is below it, it has no effect during the day because of absorption by the lower layers of the ionosphere.

During high sunspot activity the layers can be more reflective or absorbing depending on the layer. This is because they receive more radiation from the sun.

DXing

You may have heard of this term before. What it means is the reception of signals from hundreds to thousands of miles from your location. Shortwave stations are easier to DX than medium wave (AM broadcast band included). This is to do with the fact that the ionosphere has less to do with reflecting the signal and more to do with guiding it between the ground and itself. this is referred to as “ground wave”

Ground wave

This is when the space between the ground and the ionosphere acts like a duct for the signal to flow through. This allows low frequency signals to travel long distances, This phenomenon exists only at night and has no effect during daylight hours.

Sunspot

these are areas of intense magnetic activity on the sun. These cause a large amount of solar radiation to strike the ionosphere which in turn effect shortwave communications and even satellite communications.

Radio can be very rewarding and it is awesome when you finally receive your first transmission from half a world away. My first long distance reception of a station was Deutsche Welle (German international broadcaster) from their transmission facility in Portugal (I’m in NZ) on the opposite side of the planet from me, and I managed to record it.

Daktologist.

Discuss http://www.totse.info/bbs/showthread.php/18642-Longwave-Medium-and-shortwave-Listening

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