filter for 140mhz

[mrscanner2008]

Hi

Do you know a good filter to cut paging on 140mhz?

The paging company in my area use 140.160 MHz for paging and the only filter that I know of are pare electronics but for the 152mhz frequency.

Does waf125 or Jim NF96X-1 are good filters???

I'm waiting for your suggestion.

Mrscanner2008

 

[prcguy]

Check Ebay item # 270334052145, this is a commercial filter that should tune down to 140MHz.
prcguy

 

[iMONITOR]

PAR will make a custom notch filter to your specified frequency for no extra charge.

 

[lmrtek]

You can make your own 1/4 wave stub filter out of coax and a "T" connector

234 divided by 140.160 X the velocity factor of the coax used for the stub.

(About 13.2 inches for solid dielectric 16 inches for foam)

Make it slightly longer and trim till it notches out the signal.

 

[trooperdude]

PAR will make a custom notch filter to your specified frequency for no extra charge.

+1

I've had him make some custom filters for problem children in my neighborhood like TV channel 14 audio. He will also do custom connectors: ie: N instead of BNC.

 

[W6KRU]

You can make your own 1/4 wave stub filter out of coax and a "T" connector

234 divided by 140.160 X the velocity factor of the coax used for the stub.

(About 13.2 inches for solid dielectric 16 inches for foam)

Make it slightly longer and trim till it notches out the signal.

What a neat idea. Why didn't I think of that? :lol: I googled this and the information I found was all related to HF. I'm going to look around a little more though. What I am wondering is if this could be done to notch out the whole 88-108 band. Any additional information you have or links to information would be great.

 

[lmrtek]

An "open" 1/4 wave stub becomes a wide notch filter

A "shorted" 1/4 wave coax stub becomes a wide bandpass filter.

 

[W6KRU]

An "open" 1/4 wave stub becomes a wide notch filter

A "shorted" 1/4 wave coax stub becomes a wide bandpass filter.

So if my math is correct, a 2.6" shorted stub of rg6 would give me a 850 MHz bandpass filter. Right?

 

[prcguy]

I thought shorted or open, a 1/4 wave stub in coax does the exact same thing. it creates a notch because it reflects the signal at one frequency 180deg out of phase (1/4 wave X 2 trips = 1/2 wave or 180 deg) and cancels the signal or at least some of it. One problem with 1/4 wave stubs are it will also notch at 3X the frequency, 5X and so on so you may suck out higher freqs you want.
prcguy

An "open" 1/4 wave stub becomes a wide notch filter

A "shorted" 1/4 wave coax stub becomes a wide bandpass filter.

 

[W6KRU]

I thought shorted or open, a 1/4 wave stub in coax does the exact same thing. it creates a notch because it reflects the signal at one frequency 180deg out of phase (1/4 wave X 2 trips = 1/2 wave or 180 deg) and cancels the signal or at least some of it. One problem with 1/4 wave stubs are it will also notch at 3X the frequency, 5X and so on so you may suck out higher freqs you want.
prcguy

This is where it gets a little fuzzy for me but here goes. If you short a 1/4 wave it will be 0 voltage at the short. A 1/4 wave from the short will be max voltage so it will be an open circuit for the design freq.

If the 1/4 wave is open then voltage will be max at the open and 1/4 from the short the voltage will be minimum so that is an short circuit at the design freq.

I think that works and this sort of makes sense to me now. I learned something tonight.

 

[prcguy]

In either case the RF gets to the end of the stub and if its open or shorted, 100% gets reflected back to the source 180deg out of phase at the frequency where the coax is 1/4 wavelength including the velocity factor of the cable. In both cases it makes a notch filter not a band pass.
prcguy

This is where it gets a little fuzzy for me but here goes. If you short a 1/4 wave it will be 0 voltage at the short. A 1/4 wave from the short will be max voltage so it will be an open circuit for the design freq.

If the 1/4 wave is open then voltage will be max at the open and 1/4 from the short the voltage will be minimum so that is an short circuit at the design freq.

I think that works and this sort of makes sense to me now. I learned something tonight.

 

[W6KRU]

In either case the RF gets to the end of the stub and if its open or shorted, 100% gets reflected back to the source 180deg out of phase at the frequency where the coax is 1/4 wavelength including the velocity factor of the cable. In both cases it makes a notch filter not a band pass.
prcguy

If you short a 1/4 wave it will be 0 voltage at the short. A 1/4 wave from the short will be max voltage so it will be an open circuit for the design freq. If it is an open circuit, nothing will flow so SWR or reflected power doesn't matter? When the freq. varies off of the design freq. the stub becomes a short.

 

[prcguy]

Ahh, found the answer to open vs shorted stubs. For a notch filter with an open ended stub it will be 1/4 wavelength in coax at the desired frequency but if its shorted it needs to be 1/2 wavelength in coax at the desired frequency. They are both notch filters but on the 1/4 wavelength the open end appears as a short at the T junction and a a short on the same stub will appear as an open back at the T connector so the length is doubled in this case to reverse the phase.
prcguy

 

[W6KRU]

Ahh, found the answer to open vs shorted stubs. For a notch filter with an open ended stub it will be 1/4 wavelength in coax at the desired frequency but if its shorted it needs to be 1/2 wavelength in coax at the desired frequency. They are both notch filters but on the 1/4 wavelength the open end appears as a short at the T junction and a a short on the same stub will appear as an open back at the T connector so the length is doubled in this case to reverse the phase.
prcguy

Nope, we can't agree yet. A 1/2 wave stub that is shorted will be 0V at the short. So 1/4 wave away the stub will be an open, but 1/2 wave away will be shorted again.

 

[prcguy]

An open 1/4 wave stub will show as a short at the source and a shorted one will show as an open at the source. Thats why it you would need to make it 1/2 wavelength in coax at the desired frequency if its shorted. Its not my speculation, just google coax stub and you'll find lots of engineering info and projects for building them.
prcguy
Do a google search on coax stubs and you'll find the answers.

Nope, we can't agree yet. A 1/2 wave stub that is shorted will be 0V at the short. So 1/4 wave away the stub will be an open, but 1/2 wave away will be shorted again.

 

[W6KRU]

An open 1/4 wave stub will show as a short at the source and a shorted one will show as an open at the source. Thats why it you would need to make it 1/2 wavelength in coax at the desired frequency if its shorted. Its not my speculation, just google coax stub and you'll find lots of engineering info and projects for building them.
prcguy
Do a google search on coax stubs and you'll find the answers.

Been there done that. All of the information I have found so far is for HF freqs. but the principle is the same. Here is an excerpt:

Here is a trick that not many people know about.

A 1/4 wave shorted 40 meter stub:
passes 40
rejects 20
passes 15
rejects 10

A 1/4 wave open 40 meter stub:
rejects 40
passes 20
rejects 15
passes 10


Yes, open and shorted stubs behave as you describe so they do opposite jobs as shown in the example above. Please send me a link that talks about 1/2 wave stubs.

 

[prcguy]

Your own info below shows the shorted vs open relationship. Using 20m as a constant, the 1/4 wave shorted on 40 rejects 20 because its a 1/2 wave shorted on 20m. Another thing to consider is low loss coax like LMR400 or hardline will provide a deeper and narrower notch than lossy coax like RG-58.
prcguy

Been there done that. All of the information I have found so far is for HF freqs. but the principle is the same. Here is an excerpt:

Here is a trick that not many people know about.

A 1/4 wave shorted 40 meter stub:
passes 40
rejects 20
passes 15
rejects 10

A 1/4 wave open 40 meter stub:
rejects 40
passes 20
rejects 15
passes 10


Yes, open and shorted stubs behave as you describe so they do opposite jobs as shown in the example above. Please send me a link that talks about 1/2 wave stubs.

 

[W6KRU]

Your own info below shows the shorted vs open relationship. Using 20m as a constant, the 1/4 wave shorted on 40 rejects 20 because its a 1/2 wave shorted on 20m. Another thing to consider is low loss coax like LMR400 or hardline will provide a deeper and narrower notch than lossy coax like RG-58.
prcguy

The chart shows that a shorted 40m 1/4 wave stub is a bandpass filter at 40m and a 40m open 1/4 wave stub is a notch filter at 40m. Remember how we started this discussion? A shorted stub does one thing and an open stub does the opposite unlike what you stated to start with.

I thought shorted or open, a 1/4 wave stub in coax does the exact same thing. it creates a notch because it reflects the signal at one frequency 180deg out of phase (1/4 wave X 2 trips = 1/2 wave or 180 deg) and cancels the signal or at least some of it. One problem with 1/4 wave stubs are it will also notch at 3X the frequency, 5X and so on so you may suck out higher freqs you want.
prcguy

 

[kb2vxa]

Let's just clear one thing up. A 1/4 wave open coaxial stub does the same thing as a 1/2 wave shorted stub. So why complicate the matter? Oh yeah I forgot, you guys just love to show off and stray far afield while doing so and then you wonder why the one who raised the question disappeared in confusion.

 

[W6KRU]

Let's just clear one thing up. A 1/4 wave open coaxial stub does the same thing as a 1/2 wave shorted stub. So why complicate the matter? Oh yeah I forgot, you guys just love to show off and stray far afield while doing so and then you wonder why the one who raised the question disappeared in confusion.

Not at all Warren. I just learned about stubs yesterday. I was trying to help prc with this but it seems to have degenerated into an argument. I agree about the 1/2 vs. 1/4 wave thing and I don't know why that tangent was taken.