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Noob here, strange things to question.

Posted: Wed Jul 17, 2019 10:20 pm
by Hystorm
I have a ton of questions but will keep it short each time.
I have an RTL-SDR dongle I purchased while I'm waiting for the Airspy Discovery.

I'm using the latest SDR#.
First question, why the freak are there so many FM stereo radio station ALL OVER the place?
The band marked as Military Air by SDR# is simply full of my local FM stations as seen here:

https://screenshot.net/gmkr0f3

I wanted to check out the CB band but no, its also full of FM music broadcast stations:

https://screenshot.net/g3kjmi5

Is this because the dongle is cheap?
I sure hope so as this rots...
If not, I've head talk of an FM blocker, what is that and is it what I need?

I'm using an expensive Diamond Discone antenna on the roof with LMR400 cable direct to the dongle.
(it's setup for the Airspy D)

I have more questions but good for now.

Re: Noob here, strange things to question.

Posted: Thu Jul 18, 2019 11:14 am
by Username
Is this because the dongle is cheap?
depend there are good cheap and crap cheap one.

1) What (kind of) Antenna do you use?
2) Which LNA?
3) And what Cable?
If not, I've head talk of an FM blocker, what is that and is it what I need?
depend where you life and how strong the FM Station transmitt. You can use an Bandpass Filter to.
For example: AOR ABF128

Re: Noob here, strange things to question.

Posted: Thu Jul 18, 2019 8:34 pm
by radiolistener
Hystorm wrote:
Wed Jul 17, 2019 10:20 pm
First question, why the freak are there so many FM stereo radio station ALL OVER the place?
This is because these stations have very strong signal, and your receiver has insufficient dynamic range. Very strong signal leads to overload of your receiver. It leads to distortions and phantom stations at wrong frequency.

You can fix it by using the following steps:
1) Disable AGC and reduce gain to the minimum value
2) Use FM rejection filter, or bandbass filter. Bandbass filter is better, but it needs to have different bandbass filters for different frequencies

Hystorm wrote:
Wed Jul 17, 2019 10:20 pm
Is this because the dongle is cheap?
yes, the dongle has weak dynamic range and when you listening it near a strong transmitter (such as any FM station or GSM cell) it leads to such issue. But many other cheap receivers also has small dynamic range. If you want better, it will cost you a lot of money. In such case you're needs to look for a professional SDR equipment, such as FlexRadio.
Hystorm wrote:
Wed Jul 17, 2019 10:20 pm
I've head talk of an FM blocker, what is that and is it what I need?
This is FM reject filter, it can help in your case.
https://www.rtl-sdr.com/rtl-sdr-com-bro ... -for-sale/

Re: Noob here, strange things to question.

Posted: Thu Jul 18, 2019 10:31 pm
by Username
I'm using an expensive Diamond Discone antenna on the roof
a 0db Discone without a LNA is funny...
Every Cable have an loss to.