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I have to ask... what does 'VHF' mean with Ferret scout cars


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Very High Frequency, also described as FM, Frequency Modulated. There are three basic ways of sending information over the air waves. CW, Carrier Wave. This is an un modulated sine wave, which is switched in or off, such as Morse. The result is a buzz. The beauty of this system is the waveform can be very simple, so can the equipment as the only information sent is the carrier is either on or off. It is also suitable for all wavelenghths.

 

 

Long wave /Medium Wave. These are also called AM Amplitude Modulation. In this the Amplitude or height of the sine wave is varied. It was the first succesful system to transmit spech. The drawback is you need large sine waves in order to modulate enough to get all the information through.

 

 

VHF Very high Frequency. The frequency , or number of time the sine wave is sent is used to transmit information. This requires the speed of production of sine wave to be high. As the speed of the prodution is the frquency, think of a musical note, you get Very High Frequency. The beauty of this system is that it can switched very fast, and allowed stereo brodcasting . Problem is it is limited in range, basically line of sight. You can send a weak VHF signal to the moon, but it wont go to the next street.

 

All this is a very basic explination, and now Phase modulation is being introduced, but please don't ask for an explanation of that! I'm trying to understnd it, but all I have so far is a headache. :-D

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I think VHF is just referring to the frequency band. You can have different modulations at any frequency.

 

If you think about a radio wave as a wiggly line going up and down, you can change how far up and down it goes (amplitude modulation) or how many times it goes up down in a second (frequency modulation).

 

Phase modulation is when you make the wiggly line jump to a different part of the up and down cycle (so if it was going up and you put it 180 degrees out of phase it would suddenly go back down like drawing a "w" or an "m").

 

You can think of the up and down wave a bit like drawing a circle. You can make the circle bigger, draw it faster, or jump the pencil across to a different part of the circle.

 

Or you can do two or more of them at the same time (Quadrature Amplitude Modulation changes the amplitude and the phase at the same time, for a real headache)

Edited by Lauren Child
typo
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The choices of frequency and modulation are independent of each other. It is only convention that ties VHF to FM and Medium Wave to AM. From an engineering viewpoint FM is optimal for short range interference free links and AM (or better SSB and morse code) for long range links subject to interference.

 

VHF or very high frequency is 30 to 300MHz in most radio books. The military combat net radio VHF range is 30 to 80MHz nowadays and it was conventional to use frequency modulation from late WW2 until the rise of digital radios in the last 10 or 15 years.

 

The MF (medium frequency) band 300KHz to 3MHz and HF (high frequency) bands 3 to 30MHz were conventionally used with various forms of morse code and amplitude modulation. SSB as seen on the Clansman RT320 and RT321 mode switches is basically a more efficient form of AM in which all the transmitted power carries speech information.

 

These are not fundamental rules however - the aeronautical VHF bands use AM to this day, and radio amateurs use single sideband (SSB) on VHF for long range communication but FM on VHF for local chat. Conversely several Larkspur radios were designed to be able to use phase modulation on

HF - although I believe the facility was a by product of their support for frequency shift keyed teleprinter modems and seldom used for speech.

 

In terms of the Ferret there are others more knowledgeable than I (alien where are you?) but there were both mixed HF/VHF and dual VHF radio fits in both the Larkspur (1960s-70s) and Clansman (1980s-2000s) eras. I think the Clansman VHF fit was usually 1 x RT353 and 1 x RT352 - I'm not sure if there were different patrol and re-broadcast fits. The larkspur fit would have been a C42 and B47 I assume?

 

Hope this helps a bit ?

 

Regards

 

Iain

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Andy

 

You asked ... Frequency hopping and spread spectrum are the modulation methods used by modern digital radios to avoid detection and interception. Traditional combat net radios operated on a fixed channel for hours on end and were fairly easy to intercept by an enemy with a scanning receiver and even locate by taking bearings on the signal (that was the purpose of the Land Rover 101 Vampire) leading to jamming or artillery and air strikes on the transmitter location.

 

This leads to two basic requirements - digital encryption to protect the speech content from being overheard and rapid frequency (channel) changes to avoid detection in the first place.

 

Modern military radios avoid interception by use of digitally encoded speech that can then be encrypted. The Clansman DMU and BID solved that problem at least for fixed and vehicle stations but the equivalent function is embedded in all modern military radios. The requirement to change frequency is met by either spreading the signal over a wide bandwidth so it is hard to detect and decode (true spread spectrum as used in the Marconi/Selex PRR and WiFi) or by frequency hopping over a wide range of channels in a preset sequence shared by all stations on a particular net. Hopping was used by the US SINGCARS from the mid 1980s and Marconi, Plessey and Racal made similar sets for export (which might have ended up in Ferrets at that time?) but the British Army had to wait until about 2005 for wholesale replacement of Clansman with Bowman. True Bowman sets are basically fast frequency hoppers (10 or more channel changes per second) with encrypted digital modulation and integrated data terminals. The SELEX PRR is a separate project providing personal radio using different technology.

 

 

Returning to the subject (sort of) did any ferrets last into the Bowman era ? I would have thought that there would be insufficient space for a secure Clansman installation with 2 radios but was there a configuration with one RT353 and DMU ?

 

Regards

 

Iain

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