Weak reception when in movement!

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aurgathor
Posts: 26
Joined: Wed Apr 25, 2018 8:14 am

Re: Weak reception when in movement!

Post by aurgathor » Thu Jun 14, 2018 4:38 am

An LNA will increase signal level, but in itself it can't fix multipath distortion. Wikipedia and the patents I linked above should give you some idea on what is it, and some ways to fix it.

Brightnoise
Posts: 18
Joined: Fri Jan 19, 2018 10:37 pm
Location: Netherlands, Nuenen
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Re: Weak reception when in movement!

Post by Brightnoise » Mon Jul 02, 2018 9:48 pm

ligghts,

a car radio FM receiver has some specific signal processing functions that "hide" the loss of signal from the Rayleigh path level distribution (or variations). The dongle software has none of these functions *** (a and b; see below).

It is important to distinguish between multipath with small path delay variations (flat fades, no distortion, only signal variation) and multipath with larger path delay variations (frequency response of pass-band is not flat, audio is distorted).


1. (almost every) car radio has a soft-mute function. Without signal the audio is GRADUALLY attenuated to prevent the noise to be louder than the wanted audio; not wit a switch like a squelch function. In fact: when you use a high-end HiFi FM tuner in a car you have the noise level increase also.

The loss of audio is less annoying than the loud noise bursts when the signal is weak for a fraction of a second.

2. (good) car radio has stereo channel separation that varies GRADUALLY with the received signal level. The audio S/N difference between mono and stereo is about 20 dB (in Europe where we have 50 microseconds de-emphasis).

Figures: noise theshold for FM is in the order of 2 microvolt; audio S/N of 26 dB for FM deviation of 40 kHz is around 4 microvolt; full stereo channel separation AND audio S/N of 26 dB requires about 100 microvolt. The gradual control keeps the audio output noise at a fixed level when the input signal varies from 2 microvolt (full mono) to 80 or 100 microvolt (full stereo).

3. The best FM radios use antenna diversity with two or more antennas (switching diversity or power combining diversity) to reduce the probability that the signal level has a drop down to were the audio noise will rise. Just two antennas in a fixed combiner will NOT help: then you will just create a new single antenna with about the same Rayleigh distribution for signal level and the same probability for a signal level drop.

The diversity system will also reduce the amount of multipath: in fact the Rayleigh distribution is caused by different and parallel propagation path lengths. The audio distortion due to multipath is just a function of the delay spread (together with the relative amplitude of the different parallel signals).

4. Reduction of the post-demodulator audio bandwidth (when the signal level is low) does also help to reduce the experienced noise; at the cost of the audio clarity.


*** a and b

A squelch is a type of soft-mute that is used in stationary receivers. The soft-mute is really an improvement over a fixed-level squelch.

A stero to mono switch that is controlled by the signal level is not appreciated due to the nervous effect on the audio.
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You can write some software to insert the mentioned functions in any (SDR) radio; the car radio industry did this already for you.....

Then a better LNA: when the antenna is good, the noise matching is OK and the total system noise figure is below 5 dB or system noise temperature is below 1000 kelvin, then the addition of an LNA will not improve your results. Probably your strong-signal problems (overload, intermodulation) will rise. The dongle has already a limitation in dynamic range.

Succes,
Brightnoise

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