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Weak reception when in movement!

Posted: Thu Jun 07, 2018 10:03 pm
by liggths
hello everyone

Recently, i've mounted a nexus 7 tablet in my car to meet the purpose of a stereo radio. all works fine except the FM radio. I've used a RTL-SDR usb dongle to listen FM radio and i connected it to a the aerial mounted in the roof of the car (fresh new FM antenna), i've applied trough the roof of the car a new extension SMA cable into the front of the car to connect to the dongle. my problem is that, when i are stopped, the reception is not so bad, not too good, but ok to listen, but when i start moving the signal is weaker, in some places are too bad and in others are too good, too much oscillation.

There are any workaround to this problem, can i solved it with a LNA or other thing ?

Re: Weak reception when in movement!

Posted: Fri Jun 08, 2018 2:06 am
by w5dxp
Sounds like "picket fencing", usually caused by multi-path constructive/destructive interference.

Re: Weak reception when in movement!

Posted: Fri Jun 08, 2018 3:27 am
by rtlsdrblog
Might be multipath, but I wonder if it's something simpler like a loose connection that vibrates on and off while moving, or maybe RF noise from the engine / alternator that gets louder when the car revs up. Does the noise show up if you put the car in neutral and rev the engine? Is reception improved if you shut the engine down and leave just the battery on?

Re: Weak reception when in movement!

Posted: Fri Jun 08, 2018 4:57 pm
by liggths
Sounds like "picket fencing", usually caused by multi-path constructive/destructive interference.
cam you be more specific, please.
Might be multipath, but I wonder if it's something simpler like a loose connection that vibrates on and off while moving, or maybe RF noise from the engine / alternator that gets louder when the car revs up. Does the noise show up if you put the car in neutral and rev the engine? Is reception improved if you shut the engine down and leave just the battery on?
Loose connection is not the problem, i have all the cables and connections are shown to detect such situations, its possible that can be the noise from the dc/dc step down converter that feeds all the system. I'm going to create a cable discarding the 5v from the converter and power on the dongle from a power bank or similar battery to discard that situation, keeping just the data lines from the usb HUB to the tablet.

later i'll give feedback

Re: Weak reception when in movement!

Posted: Fri Jun 08, 2018 6:51 pm
by w5dxp
The radiated signal can take any number of different paths to the receiving antenna. Reflections from large buildings are one of the culprits in cities. There's lots of information available on the web.

http://www.radio-electronics.com/info/p ... fading.php

https://en.wikipedia.org/wiki/Fading

Re: Weak reception when in movement!

Posted: Sat Jun 09, 2018 5:32 pm
by liggths
hello, after some tests

I think you are right about the multi-path issue and I think this is what is happening since outside of the city, for example on motorways this happens less often.
The question now is, how can I correct or minimize this situation ??

Now I am sure it has nothing to do with some noise produced by the step down converter, since I now have the dongle only powered by a second battery, completely isolated from the car.

If anyone can help, I thank

Re: Weak reception when in movement!

Posted: Mon Jun 11, 2018 4:34 pm
by liggths
anyone can help with this ???

Re: Weak reception when in movement!

Posted: Tue Jun 12, 2018 6:16 am
by rtlsdrblog
Multipath isn't easy to solve. Some commercial car radios have multipath reject circuits built in, but i'm not aware of any open source SDR based solution yet.

I was thinking that it might also be interference from other signals (or other FM stations) that occurs when you drive through the city. Sometimes strong ones might poke through the buildings, causing the dongle to desensitize. Have you tested with lower gain settings? Make sure that the Tuner or RTL AGC is off.

Re: Weak reception when in movement!

Posted: Tue Jun 12, 2018 1:25 pm
by aurgathor
It is most certainly multipath. BTW, my car radio does the same thing in some places.

I think there are some DSP algorithms and other solutions that can correct it, but I don't think you'll find that in free SW, or in an $20 dongle.

Here are some patents on it:
https://patents.google.com/patent/US6173166B1/en
http://www.freepatentsonline.com/5379449.html

Re: Weak reception when in movement!

Posted: Wed Jun 13, 2018 9:11 pm
by liggths
an LNA can improve the signal ??