IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

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glovisol
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IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by glovisol » Mon Mar 18, 2019 3:10 pm

Hallo all,

What follows will seem obvious to many, nevertheless it costs nothing to post this warning. When scanning above 30 MHz ALWAYS USE THE MW/FM NOTCH FILTER. In areas with strong or very strong FM stations (e.g., all over!) the SDR receiver front end will be surely knocked off to overload & noise and the scan function (in the airband especially) WILL NOT WORK AT ALL. As soon as you switch the notch filter in, noise will drop by 30 dB or more and scanning will work like magic.

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Patrick73
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Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by Patrick73 » Tue Mar 19, 2019 10:06 am

The downside is that engaging the FM notch seriously deteriorates reception on the lower part of the airband (108 - 122 MHz).
In my case, increasing the RF Gain to compensate does not help for frequencies < 120 (the corresponding stations, usually weak, remain unaudible).

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glovisol
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Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by glovisol » Tue Mar 19, 2019 2:53 pm

I wonder about what kind of antenna you are using if the very low insertion loss of the built-in notch impedes reception of signals in the Airband. Please consider the following, which is based on theory and on PRACTICAL EXPERIENCE as well.

1) Signals in the airband come from high flying aircraft, so there are no obstacles and the only attenuation follows the distance attenuation law.

2) A small Discone antenna mounted on the roof or on a small balcony will have a noise floor, as measured on your SDR, of between - 130 to - 125 dBm, depending on local noise, which starts to be quite low in this frequency range.

3) Airband signals strength is always from -115 to - 80 dBm, depending on how far the aircraft is.

4) Setting the Scanner threshold at -105 dBm will allow you to operate the Scanner and to copy a multitude of signals.

In my installation I use an $40 Discone antenna mounted on the roof, with 20 m. of (very lossy!) RG56 coaxial cable going to the SDR, the scanner works faultlessly and I copy a multitude of signals thanks to it and my RSPduo.

With the antenna disconnected and the RSP tuned to 115 MHz, you should read a noise floor on the SDR power meter of less than -130 dBm, -133 dbM typical. Now you connect the antenna: noise floor will raise to between -97 and -100 dBm: THIS NOISE IS GENERATED BY OVERLOAD CAUSED BY THE STRONG FM SIGNALS IN THE 88-108 MHz BROADCAST BAND. Now you click on the MW/FM button and noise will go away. Turn the scanner on and enjoy the Airband!

FINALLY: if the insertion loss of the MW/FM notch is high, you should open a ticket with SDRplay.

Cheers,

glovisol

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Patrick73
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Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by Patrick73 » Tue Mar 19, 2019 3:34 pm

I'm using a ground plane designed for the airband and quality coax (15VATC sat cable and Aircell 7)

The problems only occurs in the lower part of the airband (< 120 MHz) on weak signals. You're probably aware that not only "high flying aircraft" can be audible : VOR, TACAN ILS, towers, all ground stations.

My remark was not about operating the scanner feature, but about receiving weak signal on the lower part of the airband when enabling the FM notch.

For example, nearby VORs, on 115.4 (CBY) and 115.55 (LTP) totally disappear when engaging the FM notch.
What I mean is that the FM notch does not "cut" at 108.001 but attenuates up to 120 MHz, thus weak signals in the range 108-120 are too attenuated to remain audible.

Tune to a weak VOR in the range 110-118 MHz, then engage the notch ; do you still hear it ?

Attached are 2 screenshot showing reception of CBY VOR without and with the notch : weak but readable first then not audible (not even by increasing RF Gain to max)
Attachments
NO NOTCH.JPG
NO FM notch
NO NOTCH.JPG (133.03 KiB) Viewed 2004 times
NOTCH.JPG
FM notch engaged
NOTCH.JPG (129.74 KiB) Viewed 2004 times

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Paul
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Location: SW UK

Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by Paul » Tue Mar 19, 2019 3:38 pm

Having read this, I wouldn't mind betting that someone will try to scan the Marine Vhf Band with the notch activated ;)

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glovisol
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Location: Piedmont, Italy

Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by glovisol » Tue Mar 19, 2019 5:37 pm

1) If the point is that we are talking abot Scanner and scanning, I do not see the scope in scanning to receive VOR and/or other fixed signals. You know they are there, you can tune and copy them, end of the story. If they are weak, they are weak, but if you receive them at - 118 dBm (I guess because you do not show the power meter readings in your pics) it means your SDR is not interfered by FM broadcast stations and you do not need the notch filter in any case.

2) So, in my opinion, Scanner and scanning in the Airband is useful to LOCATE and COPY signals coming from aircraft and from ground stations, which are relatively powerful, but which need the scanner because every contact lasts only one to three seconds.

3) Losses of the FM notch filter can be appreciated in Figures 1 & 2. In Figure 1 we are receiving a wideband noise spectrum (noise generator) at -91 dBm, with Notch off. In Figure 2 Notch is on and signal drops to -110 dBm @ 118.5 MHz, a loss of 19 dB. At 123 MHz notch loss is 5 db less, or 14 dB. This is better than the SDRplay published specification. So you are right in stating that the Notch has appreciable loss, however this is of little consequence in scanning the airband, because air traffic signals are strong enough.

4) Finally marine band frequencies are above 156 MHz, where the published Notch specifications show a loss of 3 db or less, so the use of the FM notch is surely recommended here as well.
Attachments
-90 dBm no notch.jpg
RSPduo response 118 to 123 MHZ, no FM notch
-90 dBm no notch.jpg (188.32 KiB) Viewed 1982 times
-90 dBm Notch.jpg
RSPduo response 118-123 MHz FM notch in
-90 dBm Notch.jpg (188.54 KiB) Viewed 1982 times

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Patrick73
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Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by Patrick73 » Tue Mar 19, 2019 6:30 pm

I was not talking about scanning here but only about the incidence of the notch during 'normal' listening.
Thus, we agree on the fact that the loss can make a weak signal to disappear in this particular instance. We also agree on the fact that scanning is totally useless for tracking VOR, TACAN or ILS ! But this was not the purpose of my comment.

Case closed ;-)

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glovisol
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Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by glovisol » Tue Mar 19, 2019 6:48 pm

Yes, case closed...it all came about just because I was talking about scanning VHF here and therefore I misunderstood the scope of your post. In any case I hope our discussion was useful to bystanders!

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Paul
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Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by Paul » Tue Mar 19, 2019 6:52 pm

Re: Marine VHF Band - live and learn.....! :oops:

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glovisol
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Location: Piedmont, Italy

Re: IMPORTANT WHEN SCANNING VHF WITH SDRuno VER.1.3!

Post by glovisol » Wed Mar 20, 2019 4:55 pm

In areas with strong & very strong FM Broadcast stations the MW/FM notch filter is a very effective tool in eliminating wideband noise killing receiver sensitivity at frequencies above 108 MHz. This noise is caused by SDR front end overload by very strong incoming local FM stations. The picture below shows a 10 db noise reduction through the use of the Notch Filter in the 144 -148 MHz ham band. Thanks to Paul for causing me to check this out. A sharper external notch filter could be a boon in this respect!
Attachments
Notch Filter advantage in VHF.jpg
Notch Filter cuts spurious noise by 10 dB
Notch Filter advantage in VHF.jpg (209.74 KiB) Viewed 1868 times

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