Fm trap recommendations
- Thread starter Bonkk083
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The Flamingo FM appears to have lower loss outside the band. The RTL is slightly cheaper and should do the same function. It's capacitors and resistors, so no real science here except component tolerance and whether the designer understood electronic theory.
Ubbe
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You can look in thread where most FM trap filters have been tested and then depending of if you need to receive the lower part of civil air band or must have a lot of attenuation of a nearby broadcast transmitter you pick the one that seems to fit your needs the best.
forums.radioreference.com
/Ubbe
FM bandstop filter comparison
I rolled some numbers on the RTL-SDR FM bandstop filter and the HPN-30118 AM/FM filter using a VNA. The results are interesting compared to the ZBSF-95+, especially when you hit VHF Air voice at 118 - 137 MHz. The ZBSF-95+ numbers come from their spec PDF as I do not own that yet. There are...
/Ubbe
Dale Par al the way!
The Radioshack fm trap @ 150mhz = 0.42 and the ZBSF-95 is 0.27 What does that mean exactlyYou can look in thread where most FM trap filters have been tested and then depending of if you need to receive the lower part of civil air band or must have a lot of attenuation of a nearby broadcast transmitter you pick the one that seems to fit your needs the best.
FM bandstop filter comparison
I rolled some numbers on the RTL-SDR FM bandstop filter and the HPN-30118 AM/FM filter using a VNA. The results are interesting compared to the ZBSF-95+, especially when you hit VHF Air voice at 118 - 137 MHz. The ZBSF-95+ numbers come from their spec PDF as I do not own that yet. There are...
/Ubbe
Ubbe
Member
The values are the amount of dB the filter will attenuate at that frequency. 0.42dB are nothing and also 1dB but 2dB might be noticed if you have an extreamly noise signal that barely open squelch.
The amount of attenuation in the FM broadcast band might be important if you have a a radio transmit tower nearby, then you will probably want a filter that attenuates 60dB and not 40dB, that are a 100 times difference.
/Ubbe
After reading these forums, I bought a MiniCircuits FM filter.
Will the Radioshack fm trap add more attenuation at that frequency than the ZBSF-95
Ubbe
Member
If you look at the values for the $10 RadioShack filter and the $100 ZBSF you'll see that they are pretty equal at all frequencies but if you have a local FM transmitter at 88Mhz then the Radio-Shack filter are useless.
If you want to monitor air frequencies between 118-120MHz then the RTL-SDR filter are useless as it attenuate 10dB. If you only need to attenuate 30-40dB of the broadcast band then the HPN30118 will work fine.
If you have a SDS scanner then you can set it to 100KHz step size in the 88-108MHz range and manually tune over the whole band and check how big signal strength you get. If it goes up to -30dBm then you'll want a filter that attenuates more than 40dB at that broadcast frequency.
/Ubbe
If you want to monitor air frequencies between 118-120MHz then the RTL-SDR filter are useless as it attenuate 10dB. If you only need to attenuate 30-40dB of the broadcast band then the HPN30118 will work fine.
If you have a SDS scanner then you can set it to 100KHz step size in the 88-108MHz range and manually tune over the whole band and check how big signal strength you get. If it goes up to -30dBm then you'll want a filter that attenuates more than 40dB at that broadcast frequency.
/Ubbe
Ubbe
Member
You'll have to evaluate the FM band either using a scanner that somewhat accuratly can show the signal strength of the broadcast channels or a $25 SDR dongle connected to your PC and using something like the SDR# program to look at the spectrum. Some scanners has a spectrum feature that scans over a frequency range and show the signal levels.
If there's only low signals in the broadcast band then focus to get a filter with the lowest attenuation in the airband. If you have a very strong broadcast signal then focus on getting the filter with the biggest attenuation at the biggest signal in the broadcast band and at the same time a low attenuation in the airband and other bands you monitor.
/Ubbe
If there's only low signals in the broadcast band then focus to get a filter with the lowest attenuation in the airband. If you have a very strong broadcast signal then focus on getting the filter with the biggest attenuation at the biggest signal in the broadcast band and at the same time a low attenuation in the airband and other bands you monitor.
/Ubbe
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