Post by malewithatail on Jun 5, 2021 9:26:42 GMT 10
More thoughts on radio comms WTSHTF.
I suspect most Hams, and probably preppers as well, think that the only radio comms gear needed is a short wave radio, and some form of communications gear for your group.
Whilst its true that this type of equipment will be able to keep you in touch with international events, for local happenings, a good quality, sensitive AM radio will be needed as well.
I suspect that when it happens, the powers that be will disrupt the local radio stations, both on AM and the more usual FM bands.
Don't forget that in intel gathering activities, the key is to not be discovered. I don't intend to transmit for the first few weeks at least, unless its an emergency, as your location can be determined very easily with modern direction finding gear, and compromised.
That said, you will need a decent AM radio, and having a good one will give potential access to local radio information that will be useful.
What sort of radio ?
Short of a dedicated communications receiver, the older AM only car radios, which can be picked up at the tip, are ideal. Bear in mind that some of them were cheap and didn't feature an RF amplifying stage, so will not be as sensitive as others. Short of opening it up and looking with a trained eye at the circuit, you wont be able to tell from outside weather or not it has a RF stage. Side by side tests on the bench between several radios will show those that have the required amplifying stage. Ferris brought out a series in the 1970's of car radios that were very sensitive, the 2700, 3700 and 4700 series were all very good and differed only in the power of the audio stage, which is immaterial in our case as we don't need a lot of volume.
Some of the newer digitally tuned AM/FM car radios are Ok, but they have a considerable amount of internally generated noise from the computer circuits that cause their AM performance to be quite poor, and as we are after the maximum performance, such radios cant be recommended. They were really designed for FM use, with the AM band tacked on. That's said, some of the European types can give reasonable performance, as can some of the early AM/FM types that use the digital display only, but have analogue tuning. Some of these can be determined as the frequency steps on the AM band are 1 Kc, not 9kc (Australian station spacings) or 10 kc (European) station spacings.
A modern car radio is designed to be used with an aerial of about 1 meter long, connecting it to a piece of wire 20 meters long can cause issues, such as cross modulation (where a station appears at 2 or more locations on the dial), overload (distorted audio), and susceptibility to damage by static discharges, such as during a thunderstorm, EMP and CME.
In the 1950's a range of thermionic valves were brought out that didn't need hundreds of volts high tension to operate. The 12 volts from a car electrical system was sufficient, but they needed a transistor audio stage to get enough volume to run a loudspeaker. If you can find a working one, and they are now over 50 years old, they are ideal, giving the best of both worlds of the transistor sets low power consumption, with the valve radios immunity to overload, cross modulation and EMP/CME resilience. I have a couple of them and they are superb receivers, quite able to pick up the Sydney radio Am stations, over 900 km away, during daylight hours. These types of radios are also ideal for use with short wave converters to pick up short wave radio broadcasts in your car. (The short wave converter uses 1 cheap, easy to get integrated circuit, with a simple single coil to cover the main short wave broadcast bands. I have designed several types, including a extremely stable crystal locked converter for the Ham bands. The radio isn't modified in any way at all). Be aware that any radio that offers AM stereo is a computer controlled one and should be left behind. Radios with an analogue dial and manual tuning are good candidates, and if it has a set of push buttons for station selection, that's OK as well.
If there is enough interest in the short wave converter, I can post the circuits. Its simple to build, reasonably stable, and cheap. Layout isn't critical and it can be built in the "dead bug" style if you don't want to use any chassis. It simply goes in series with the aerial of the radio and uses the standard car radio aerial for reception. Connecting it to a 20 meter piece of wire brings in the world.
The converter works by using the car radio as a tunable intermediate (IF) frequency and uses the detector /audio stage of the radio.
Even though I have a WW2 short wave radio receiver that covers the AM broadcast band perfectly well, I also have a AM/FM car radio in the shack as it can be used whilst the main receiver is being used and also covers FM for general news, weather reports etc. The car radio also draws considerably less power from the 12 v solar system in the radio room than the WW2 AR receiver does with its genemotor that provides the 280 volts dc for the valves.
I suspect most Hams, and probably preppers as well, think that the only radio comms gear needed is a short wave radio, and some form of communications gear for your group.
Whilst its true that this type of equipment will be able to keep you in touch with international events, for local happenings, a good quality, sensitive AM radio will be needed as well.
I suspect that when it happens, the powers that be will disrupt the local radio stations, both on AM and the more usual FM bands.
Don't forget that in intel gathering activities, the key is to not be discovered. I don't intend to transmit for the first few weeks at least, unless its an emergency, as your location can be determined very easily with modern direction finding gear, and compromised.
That said, you will need a decent AM radio, and having a good one will give potential access to local radio information that will be useful.
What sort of radio ?
Short of a dedicated communications receiver, the older AM only car radios, which can be picked up at the tip, are ideal. Bear in mind that some of them were cheap and didn't feature an RF amplifying stage, so will not be as sensitive as others. Short of opening it up and looking with a trained eye at the circuit, you wont be able to tell from outside weather or not it has a RF stage. Side by side tests on the bench between several radios will show those that have the required amplifying stage. Ferris brought out a series in the 1970's of car radios that were very sensitive, the 2700, 3700 and 4700 series were all very good and differed only in the power of the audio stage, which is immaterial in our case as we don't need a lot of volume.
Some of the newer digitally tuned AM/FM car radios are Ok, but they have a considerable amount of internally generated noise from the computer circuits that cause their AM performance to be quite poor, and as we are after the maximum performance, such radios cant be recommended. They were really designed for FM use, with the AM band tacked on. That's said, some of the European types can give reasonable performance, as can some of the early AM/FM types that use the digital display only, but have analogue tuning. Some of these can be determined as the frequency steps on the AM band are 1 Kc, not 9kc (Australian station spacings) or 10 kc (European) station spacings.
A modern car radio is designed to be used with an aerial of about 1 meter long, connecting it to a piece of wire 20 meters long can cause issues, such as cross modulation (where a station appears at 2 or more locations on the dial), overload (distorted audio), and susceptibility to damage by static discharges, such as during a thunderstorm, EMP and CME.
In the 1950's a range of thermionic valves were brought out that didn't need hundreds of volts high tension to operate. The 12 volts from a car electrical system was sufficient, but they needed a transistor audio stage to get enough volume to run a loudspeaker. If you can find a working one, and they are now over 50 years old, they are ideal, giving the best of both worlds of the transistor sets low power consumption, with the valve radios immunity to overload, cross modulation and EMP/CME resilience. I have a couple of them and they are superb receivers, quite able to pick up the Sydney radio Am stations, over 900 km away, during daylight hours. These types of radios are also ideal for use with short wave converters to pick up short wave radio broadcasts in your car. (The short wave converter uses 1 cheap, easy to get integrated circuit, with a simple single coil to cover the main short wave broadcast bands. I have designed several types, including a extremely stable crystal locked converter for the Ham bands. The radio isn't modified in any way at all). Be aware that any radio that offers AM stereo is a computer controlled one and should be left behind. Radios with an analogue dial and manual tuning are good candidates, and if it has a set of push buttons for station selection, that's OK as well.
If there is enough interest in the short wave converter, I can post the circuits. Its simple to build, reasonably stable, and cheap. Layout isn't critical and it can be built in the "dead bug" style if you don't want to use any chassis. It simply goes in series with the aerial of the radio and uses the standard car radio aerial for reception. Connecting it to a 20 meter piece of wire brings in the world.
The converter works by using the car radio as a tunable intermediate (IF) frequency and uses the detector /audio stage of the radio.
Even though I have a WW2 short wave radio receiver that covers the AM broadcast band perfectly well, I also have a AM/FM car radio in the shack as it can be used whilst the main receiver is being used and also covers FM for general news, weather reports etc. The car radio also draws considerably less power from the 12 v solar system in the radio room than the WW2 AR receiver does with its genemotor that provides the 280 volts dc for the valves.