Air Band Filters

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iceman977th

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Looking to get a filter to attempt to set up a radio feed at our club's tower site. It stares directly at the local airport, but sits below a 50kw FM transmitter. I doubt it will do any good, but does anyone have some recommendations on an air band filter to try? I've seen the $20 ones on eBay from China but I don't know if they are any good or not.
 

prcguy

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If you have an FM transmitter at the same site or nearby you will probably need a good FM notch along with a band pass filter for air band. The best notch I've found is the MiniCircuits ZBSF-95-N here Mini-Circuits

I've found some really good commercial US made VHF air band specific filters on Ebay for good prices. The Coaxial Dynamics filter in this picture is ok and much better than any Chinese filter you will find and the DB Products is really great, I think the skirts are a good 60dB down in the FM broadcast band and at 2m amateur.

If the 50kW FM station is really close you could have a huge increase in noise floor across the VHF air band even with all the filtering. Some testing with a spectrum analyzer or good SDR receiver with spectral display can tell you a lot. Just test across town in a known quiet area and look at the noise floor across the VHF air band and do the same at the tower site using the same antenna and feedline.

air band filters.JPG
 

vagrant

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@prcguy Hey, could a person use two of those ZBSF-95N+ filters with a particular length of coax, depending on the velocity factor, to almost double the attenuation, or would it be too much or not really make a difference?

K4EET, can you convey exactly what "should answer" the OP's question for $20 + eBay + China? I am not that good with cryptic answers. Thank you.
$20 + eBay + China should answer your own question...
 
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iMONITOR

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prcguy

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I don't know what you would get but it should be close to double the specs. If you have one filter with 40dB attenuation of some frequency and cascade that with another identical filter, I would expect close to 80dB attenuation. The MiniCircuits filters have a male and female connector and you could just screw them together.

I have two of these filters and could connect them together and measure the result but will have to climb my tower to fetch one.

@prcguy Hey, could a person use two of those ZBSF-95N+ filters with a particular length of coax, depending on the velocity factor, to almost double the attenuation, or would it be too much or not really make a difference?

K4EET, can you convey exactly what "should answer" the OP's question for $20 + eBay + China? I am not that good with cryptic answers. Thank you.
 

kruser

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I don't know what you would get but it should be close to double the specs. If you have one filter with 40dB attenuation of some frequency and cascade that with another identical filter, I would expect close to 80dB attenuation. The MiniCircuits filters have a male and female connector and you could just screw them together.

I'm not so sure it works exactly like that as far as doubling the attenuation.
I've never looked at the results before from doubled identical notch filters so I can't vouch for the below but I believe it to be true coming from Dale.
From PAR's article about filters, Scanner Filter FAQ | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

"Can identical filters be cascaded to achieve a deeper notch?
Yes, with a qualifier. If the filters are directly cascaded, the resulting increase in notch depth will not be additive.
That is, two –35dB filters directly cascaded may only increase notch depth by 6dB or so. The proper method of accomplishing this would be to connect the filters together with a coaxial jumper whose length is ¼ of an electrical wavelength.
This takes into account the velocity factor of the coaxial cable."
 

Ubbe

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The proper method of accomplishing this would be to connect the filters together with a coaxial jumper whose length is ¼ of an electrical wavelength.
Then Dale probably assumes, or have measured, that his notch filters gives a phase shift equal to 1/4 wavelenght in the notch so that total phase shift when reaching the next filter will equal to a 1/2 wavelenght of the notch frequency.

/Ubbe
 

prcguy

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I believe this is true but it may also depend on the type of filter. Some filters are easily influenced by their surroundings like a 1/4 wave cavity band pass or notch filter or a coax stub, they need to be spaced apart with a critical length of coax to play together. Other types of filters are sort of their own little "island" and will not react the same.

I'm not so sure it works exactly like that as far as doubling the attenuation.
I've never looked at the results before from doubled identical notch filters so I can't vouch for the below but I believe it to be true coming from Dale.
From PAR's article about filters, Scanner Filter FAQ | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

"Can identical filters be cascaded to achieve a deeper notch?
Yes, with a qualifier. If the filters are directly cascaded, the resulting increase in notch depth will not be additive.
That is, two –35dB filters directly cascaded may only increase notch depth by 6dB or so. The proper method of accomplishing this would be to connect the filters together with a coaxial jumper whose length is ¼ of an electrical wavelength.
This takes into account the velocity factor of the coaxial cable."
 

Ubbe

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One guy hade 3 notch filters for different frequencies from ParElectronics directly cascaded and they measured very differently depending of in which order they where placed. So they interacted with each other in different ways.

/Ubbe
 
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