Bandwidth

[aire]

I've been studying for my UK foundation exam.

Looking at the Amateur Radio Band Plan, by the VHF 2m channels it says 12kHz, which is the bandwidth I need to set when programming a radio?
But looking at the UHF 70cm plan, the bandwidth isn't specified. Does this mean it is 25kHz?

The spacing between channels on both the 2m and 70cm plans is 25kHz, so not sure why the bandwidths would be different?
As there's 25khz between channels on the 2m band, then why does the bandwidth need to be 12.5kHz? Wouldn't that leave an unused gap between channels?

[OH8GAD]

Wouldn't that leave an unused gap between channels?

Yes, it would and does. You have to realise that when you transmit, the bandwidth of your signal does not drop to nothing 12,5 kHz from the carrier centre frequency. In theory, it goes on infinitely - Though in reality, with a well made/designed radio, it drops to virtually nothing quite close to the carrier frequency. But, amateur radio isn't just about using crisp factory designed and built radios, it's for using home built stuff which isn't always as clean in the output as a five hundred quid Yaesu, Icom or whatever. If you keep your transmitted signal bandwidth to 12,5 kHz, then you won't be slurging onto the adjacent channels 25 kHz away.

[aire]

Yes, it would and does. You have to realise that when you transmit, the bandwidth of your signal does not drop to nothing 12,5 kHz from the carrier centre frequency. In theory, it goes on infinitely - Though in reality, with a well made/designed radio, it drops to virtually nothing quite close to the carrier frequency. But, amateur radio isn't just about using crisp factory designed and built radios, it's for using home built stuff which isn't always as clean in the output as a five hundred quid Yaesu, Icom or whatever. If you keep your transmitted signal bandwidth to 12,5 kHz, then you won't be slurging onto the adjacent channels 25 kHz away.

Thanks, that's a great explanation.
On the 70cm plan, there's no bandwidth specified that I can see. What should it be?

[OH8GAD]

https://rsgb.org/main/operating/band...f/432mhz-band/

430.000 – 432.000MHz All Modes
The lower 2MHz of the band is allocated to modes with a maximum transmission bandwidth of 20 kHz.

432.000 – 432.100MHz Telegraphy (CW) and Machine Generated Modes (MGM)
This 100kHz wide sub-band is allocated to modes with a maximum transmission bandwidth of 500Hz.

and so on...

[OH8GAD]

Buuuut... legally, you are only required not to cause interference to services OUTSIDE of the amateur radio frequency allocations. So, technically, if you want to spurge a 200 kHz AM transmission in the "432.000 – 432.100MHz Telegraphy (CW) and Machine Generated Modes (MGM)" section, then no laws have been broken. That is not to say, though, that you wouldn't be chased down the street by an angry mob of tank-top wearing, balding middle-aged hams who'll lynch you from the nearest 80 metre beam with cheap RG58U coax...

[aire]

Buuuut... legally, you are only required not to cause interference to services OUTSIDE of the amateur radio frequency allocations. So, technically, if you want to spurge a 200 kHz AM transmission in the "432.000 – 432.100MHz Telegraphy (CW) and Machine Generated Modes (MGM)" section, then no laws have been broken. That is not to say, though, that you wouldn't be chased down the street by an angry mob of tank-top wearing, balding middle-aged hams who'll lynch you from the nearest 80 metre beam with cheap RG58U coax...

I love that explanation! As I understand it, OFCOM allocates a frequency range to hams, along with some fundamental rules we must follow, but after that there is an element of " self governance" where the community is expected to manage itself.

[OH8GAD]

I love that explanation! As I understand it, OFCOM allocates a frequency range to hams, along with some fundamental rules we must follow, but after that there is an element of " self governance" where the community is expected to manage itself.

That's correct.