1- Having a listen for free

1- Having a listen, what can you do for free, or under £50?

This is to help you understand the basics of radio transmission modes to give an overview of what is out there.
Frequencies, bands and modes

we’ll go into each of these in some details as it’s important to understand these principles.

FREQUENCIES also known as BANDS which refer approximately to WAVELENGTH

Frequency(Hz) = C (3x 10^8M/s) / Wavelength (Metres) where C is the speed of light 299792458 m⋅s⁻¹. Call it 300 million M/sec.

Let’s simplify the digits for 100 Mhz

Frequency 100 MHz = 300 / 3 Metres (we divided both sides by 1 Million)

Or Wavelength of 3 Metres = 300 / 100Mhz. Enough with the maths for now!

A waveband is an approximation to communicate a band, not an exact measurement. 

A ¼ wave antenna at 145 Mhz would be 2M/4 each element

That’s 0.51724137931 * 0.95 velocity ratio, = 0.491379310344 so just start longer than  0.5m

Velocity ratio allows for radio waves travelling slower in a conductor than a vacuum.

https://codingace.net/physics/frequency_cal_antenna_equations.html

All band allocations vary across the world so you may hear things that you do not expect in places you don’t expect them.

0-30 Mhz is High Frequency. There are sub-bands within especially at the low end, HF is mainly for long distance work, broadcast and amateur radio, CB. Long distances 50-5,000 miles!

HF bands for Amateur Radio also known as Short Wave

Main Bands, often referred to by wavelength 40m, 20m, 15m or the lowest frequency, 7Mhz, 14Mhz, 21mhz.

160 m 1.6Mhz to 2.0 Mhz

80 m 3.5 to 3.8 Mhz

40 m 7.0 to 7.20 Mhz

20 m 14.0 to 14.35 Mhz

15 m 21.0 to 21.450 Mhz

11 m 27.60125 to 27.99125 Mhz * UK CB Radio (license free)

10 m 28.0 to 30.0 Mhz

WARC Bands (not used for contesting)

30 m 10.1 to 10.150 Mhz

12m 24.89 to 24.999 Mhz

17m 18.068 to 18.168 Mhz

https://rsgb.org/main/operating/band-plans/hf/

https://www.red-radio.co.uk/CB-Frequencies

VHF

30 Mhz to 300 Mhz, portable and hand-held radios, aviation, television, emergency services and maritime, medium distances often to High antenna base stations or repeaters 5 to 200 miles.

6 m 50.0 to 52.0 Mhz
4m 70.0 to 70.5 Mhz
2m 144.0 to 146 Mhz

UHF

300 - 1000 Mhz , portable and hand-held radios amateur radio, PMR446, emergency services, mobile to base line-of sight, medium distances 5-100 miles
70cm 430.0 to 440 Mhz

https://rsgb.org/main/operating/band-plans/vhf-uhf/

PMR446 – (The Modern CB) is license free!
PMR446 stands for Private Mobile Radio, which is a licence-exempt personal radio service operating on frequencies between 446.0 MHz and 446.2 MHz, primarily used for short-range communication in Europe and some Asian countries. It allows users to communicate without needing a license, making it popular for both recreational and business purposes.

You cannot legally use a radio that is not specifically designed for PMR446 only.
https://www.ofcom.org.uk/siteassets/resources/documents/manage-your-licence/business-radio/business-radio-guidance/ir_2009_analogue_and_digita1.pdf?v=335306

PMR – Private Mobile Radio The acronym PMR is not the same as PMR446. That refers to Private Mobile Radio and needs an Ofcom business radio license.
https://www.dcrs.co.uk/what-type-of-radio-licence-from-ofcom-do-i-need/

UHF/Microwave – uses dishes rather than antenna and is strictly line of sight

1000 Mhz (1 Ghz) to 3 Ghz UHF, but generally referred to as Microwave, drum dishes connecting other services, very focussed line of sight < 200 miles (I personally received aircraft 1090 Mhz ADS-B signal at 230 miles)

https://globe.adsb.fi/
https://radio-hobbyist.com/how-to-track-aircraft-with-ads-b-and-an-sdr/

3 Ghz to 30 Ghz - Radio astronomy, microwave devices/communications, wireless LAN.

To be honest these require specialist equipment that is often expensive, or bought send hand from clearance sales or for you to build yourself which need RF antenna design experience.

https://rsgb.org/main/operating/band-plans/

Modulation

It is not generally enough to transmit a radio signal, we need to put some information on it and we do that by modulating it. There are of course many ways to do it, but we only cover the main ones here to keep it simple, just understand that there are many more modes appearing all of the time.

AM & FM - They are modulations not bands!

CW = Continuous Wave, turn the signal on or off with Morse Code a dot or dash, a dash is 3 times the length of a dot.

AM = Amplitude Modulation, make the signal stronger or weaker with the audio or input signal.

FM = Frequency Modulation, make the signal frequency higher or lower with the audio or input signal.

PSK or QPSK and many similar - Change the phase modulation of the signal

SSB Single Sideband, Upper Sideband or Lower Sideband - remove the carrier and one of the sidebands, add it back in the receiver to save power.

And many digital modes too. DMR, YSF.

If you choose the wrong demodulation, you will probably not hear the transmitted audio at all.