#### gustav2

Joined Mar 22, 2018
26
Hello everyone,
Recently I got interested in wireless communication and radio signals. I am still an absolute beginner when it comes to RF signals so in this post I wanted to ask some basic questions to increase my understanding about radio communication a bit.
I already started experimenting a bit and managed to make a AM transmitter with a 555 timer using the harmonics of the created frequency (Schematic below, demonstration:
). Firstly here is what I think to understand about RF transmission:

- The antenna used to transmit a signal must be a multiple of the signals wavelenght (like 1/2, 1/4, 1/8 wavelenght).
- The lower the frequency the longer the range of the signal but worse quality.
- The higher the output power the longer the range.
- The concept of AM and FM.

So for example in my 555 timer AM transmitter I used a LC resonator on the output of the 555 to amplify the created signal a bit (measured 35Vpp from 12V input and a frequency of 116kHz) and tuned my AM radio to ~580kHz and sure enough I was able to hear the transmitted signal. However the signal only had a range of ~1 meter. Some day i'd like to be able to transmit a radio signal over long range. But still some questions remain:

- How does an electrical signal trough wires actually get sent into the the air?
- How do radio transmitters pull more power to increase the transmission lenght?
- Why are all radio signals these days in the MHz or GHz range while they offer shorter range?
- The longer the wavelenght the longer the antenna, is there a minimal lenght for an antenna, like can i use a 1/128 wavelenght antenna for really low frequency? or would it be completely useless?
- What is the difference about all the antenna's out there (dipole monopole loop antenna's etc.) performance wise?
- Do I need to know any other subjects to fully understand radio transmission?

Thanks for helping me out,
Gustav

Last edited:

#### Janis59

Joined Aug 21, 2017
1,232
- How does an electrical signal trough wires actually get sent into the the air?
--> Any air have sth about "vacuum electric impedance" =377 Ohms, thus the vawe is speading in

- How do radio transmitters pull more power to increase the transmission lenght?
--> increase the voltage= increase the wattage of decrease the antenna input impedance (no-one is doing like this except very rare). Just if typical antenna impedance is 50 Ohms, then 1 Volt makes N=U^2/R=1/50=20 mW but 12V makes 144/50=3W.

- Why are all radio signals these days in the MHz or GHz range while they offer shorter range?
Because between 100 kHz and 400 kHz (LW range) may be squeezed in only (400-100)/20=15 operators (20 kHz=BW), instead in the FM range between 80 and 110 MHz may work simultaneously (110-80)/0.100=300 operator stations (and BWis 100 kHz). More over, LW the Italy will interfere with Suomi and Equador will make a trouble to Germany, whilst FM any town will not disturb even a neighborous town, what is the ultimate benefit.

- The longer the wavelenght the longer the antenna, is there a minimal lenght for an antenna, like can i use a 1/128 wavelenght antenna for really low frequency? or would it be completely useless?
--> theoretically yes, but the effectiveness are catastrophycally falling when physical dimensions comes below about 1/10 of lambda. Thus the coil-type of antenna I use on my VW vehicle roof for 27 MHz have absolute good SWR, 50 Ohms etc blablabla, but if the 25W distance on full size 2,5 meter antenna would be about 40 km, mine have about 1 km. That is pay for I have dimension of 25 cm.

- What is the difference about all the antenna's out there (dipole monopole loop antenna's etc.) performance wise?
Most one must look is the VERTICAL angle of lobe diagram. Some antennas irradiate the Milky Way senselssly, whilst some have 10-15 deg angle what means many times better performance for long-range communication at equal power etc parameters. Some example of such antennas are J-pole (Slim-Jim), Yagi-Uda, threesome quadrat, skewed plane-frame etc.

- Do I need to know any other subjects to fully understand radio transmission?
Yes. Must read a books. Lot of.

#### MrChips

Joined Oct 2, 2009
22,084
Oh, so many questions. Have you tried Google or Wikipedia for some answers?

In antenna design, the transmitter must match the impedance of the antenna.
https://en.wikipedia.org/wiki/Standing_wave_ratio

The range is affected by antenna directivity, profile and polarization.
https://en.wikipedia.org/wiki/Directivity

The ionosphere influences the frequency and range of radio communications.