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A downlink diversity system also solves one other problem, fading and nulling, the most perfect diversity
system would have 3 antennas to handle signals from any angle perfect, but the gain from doing this is
often minimal and cost and complexity is big, a 2 antenna diversity is the most common compromise.
All cellular systems use diversity on the receiver side and brute transmitter power on the Tx side to perfect
the link, a cell side Tx is over 26dB more power full over the handset transmitter to obtain an equal quality
link, also handset side have a cheaper receiver with less sensitivity, we do not have space for a diversity
antenna system on a cell phone.
Wavelength and frequency
- a double frequency will have halve the wavelength. Long wavelength cannot
pass thru small holes, like take a 27MHz walkie-talkie and try to use it inside two cars, (cars are made of
metal, end the window holes are much under the wavelength) the useable range is then really bad, now go
out of the cars and see you get 10 times the range, at higher frequencies you don’t have the car window
problem, but air attenuation is higher with higher frequencies, so a longer range is easier to get using lower
frequencies.
Gain
- Adding more antenna gain will only make the beam more narrow, point the antenna right and you
get more range, point the antenna wrong and you lose signal, Any gain over 8-10dB will be hard to point to
a moving target like a plane, you need a tracker system or a cheap friend that will work for free to point the
antenna.
An 18dB gain yagi antenna on the receiver side and 500mW 2.4GHz video transmitter with 0dB antenna,
have a proven LOS of 51km when both polarizations are right.
Range and dB
- Improving a systems link budget with 10dB will increase the range by a 3 times factor, 10dB
power is the same as 10 times the power.
20dB more power gives 6 times the range, 100 times more power, combining more power with more gain
is often the way to get longer range, and also improving receiver side sensitivity is a good way to go.
Bandwidth vs. range
- Video transmitters with audio, stereo and mono exist, the video signal is 5MHz wide,
and the audio is 5KHz wide, A factor 1000 in bandwidth, so in theory the same range will be achieved on
the audio as the video link with only 1/1000 of the power, that is why those systems have a much lower
power in the audio, often see -20dBc to -30dBc, dBc means dB under the main Carrier -30dBc is the same as
1/1000.