filter for 140mhz

[prcguy]

Neither open or shorted stubs are a band pass filter, they only notch. I've built lots of open stubs and many years ago a shorted stub but in all cases they were tested on a scaler network analyzer and were very clearly notch filters. If you find any info that states a shorted stub is a "band pass" filter I'll send you some flowers for Valentine's day. Otherwise my credit card will be chillin in my wallet.
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.

 

[W6KRU]

Neither open or shorted stubs are a band pass filter, they only notch. I've built lots of open stubs and many years ago a shorted stub but in all cases they were tested on a scaler network analyzer and were very clearly notch filters. If you find any info that states a shorted stub is a "band pass" filter I'll send you some flowers for Valentine's day. Otherwise my credit card will be chillin in my wallet.
prcguy

From the chart I posted earlier:

A 1/4 wave shorted 40 meter stub:
passes 40


If you made a shorted stub that was a notch then it wasn't 1/4 wave. I prefer chocolate. :lol:

 

[prcguy]

If there is an argument its your comment that a shorted stub is a 'band pass" filter, which its not. I agree that a shorted stub needs to be 1/2 wavelength at desired frequency and an open stub is 1/4 wavelength but neither are a band pass filter.
prcguy

From the chart I posted earlier:

A 1/4 wave shorted 40 meter stub:
passes 40


If you made a shorted stub that was a notch then it wasn't 1/4 wave. I prefer chocolate. :lol:

 

[W6KRU]

If there is an argument its your comment that a shorted stub is a 'band pass" filter, which its not. I agree that a shorted stub needs to be 1/2 wavelength at desired frequency and an open stub is 1/4 wavelength but neither are a band pass filter.
prcguy

OK. Here is another quote from another web page:

5) My current setup...I have 4 1/4 wave stubs on a coax switch to
use with an all-band dipole. The stubs are closed on the end, so I
switch in the one that is 1/4 wave on the band I'm operating on.



Why can't you see that the opposite conditions are going to produce the opposite results. Isn't it logical that if an open stub is a notch that a shorted stub would be a bandpass.

 

[prcguy]

I can't see it because its not true. You can drive about 60mi up the coast to my garage and I'll show you on my network analyzer that both are only a notch and there is no band pass function to a stub. They will both have harmonically related notches in the spectrum as you go higher in frequency.
prcguy

OK. Here is another quote from another web page:

5) My current setup...I have 4 1/4 wave stubs on a coax switch to
use with an all-band dipole. The stubs are closed on the end, so I
switch in the one that is 1/4 wave on the band I'm operating on.



Why can't you see that the opposite conditions are going to produce the opposite results. Isn't it logical that if an open stub is a notch that a shorted stub would be a bandpass.

 

[W6KRU]

I can't see it because its not true. You can drive about 60mi up the coast to my garage and I'll show you on my network analyzer that both are only a notch and there is no band pass function to a stub. They will both have harmonically related notches in the spectrum as you go higher in frequency.
prcguy

And yet another quote from a chart on another page:

Length End Functions as
0.25 open short at input - null at F

0.25 short open at input - pass at F

 

[prcguy]

Your reading the statement out of context, its not a band pass filter. It just means the shorted stub will notch at twice the frequency. I'll make one of each, sweep them both and post the results here this weekend. Since I have a high quality type N open and short it will be easy to measure the same cable in both modes.
prcguy

And yet another quote from a chart on another page:

Length End Functions as
0.25 open short at input - null at F

0.25 short open at input - pass at F

 

[DaveH]

Talk about a thread hijack....the OP asked for the time, not how to
build a watch!

Now, back to the OP: what else are you trying to listen to (bands,
frequencies)? That will have an impact on the filter that's suitable.
For example, RENIR frequencies fall in and around 140.160MHz so
a notch filter may be problematic.

Dave

 

[breadtrk]

With almost 300 views for 30 posts......Watch building seems a very popular subject.

 

[W6KRU]

With almost 300 views for 30 posts......Watch building seems a very popular subject.

For those of you are still hanging around to see how a watch is built, I apologize to prcguy. We had more discussion via PM and I finally got it. A stub can't be used as a bandpass filter. It will not meet the definition of a bandpass filter because it passes other freqs in addition to the design freq. Sorry all. BarBQ crow at my house.

 

[kb2vxa]

"We had more discussion via PM and I finally got it."

Now you see how I do it, take him aside and explain in simple, easy to understand terms so he won't be subject to all the confusion caused by the arguing in the thread and all the "I know what I'm talking about and you're an idiot" attitude.

 

[nanZor]

The great thing about this thread discussion is that ALL of you guys spurred me on to learn more about them!

Because of this, I now have a nice open stub hanging off my handheld for 860 mhz. But I wanted to go further...

What I've read here are two things:

1) An open quarter-wave stub will notch at the design frequency and odd harmonics.

2) A shorted half-wave stub will notch at the design frequency and all even harmonics.

I built and tested both!

(246*VF/Fmhz) * 12 = inches - where VF = velocity factor of the line. Measured from the center of the tee! (I wasn't doing this before, and I think that's what was throwing me off for a long time)

Application: Pro-136 desktop dedicated mainly to civil air on 118-136, and suffering from desense from either VHF or UHF transmissions or both.

I'll admit that I did this as a theoretical exercise, since I'm not having problems. However I tried to attenuate VHF or UHF generally without any specific notch freq in mind.

First test:
I cut an open quarter-wave stub for 150 mhz, hoping that it would also notch/attenuate at 450 mhz. I cut it at 150mhz hoping that the attenuation slope wouldn't go too far into the civil air band up near 137 mhz.

Worked great! Both vhf-hi and uhf were attenuated. I didn't really hunt around for the exact notch freqs.

Second Test:
I cut a half-wave shorted stub for 150mhz. Attenuated vhf-hi but not too much on uhf! Since it is a half-wave, the attenuation on the even harmonic would end up around 300 mhz or so, and the Pro-136 doesn't cover mil-air. So if I wanted to attenuate vhf-hi, but not uhf, this did it. Again, I wasn't looking for any specific notch freq, but just a general overall attenuation near the design freq or harmonic.

I'm using crappy RG-58 on purpose for a less-deep notch and wider attenuation slope - although not very deep. I just wish I had some real test gear to see the slopes and notch depths.

In either case, 118-136mhz seemed just fine, HOWEVER, with either stub inline, there appeared to be some minor overall attenuation - I can't pin it down, but if I had to guess maybe 3db loss just by having it attached. Yet if I was having desense problems, 3db insertion loss would be easily sacrificed to help protect the front-end. So depending on s-meter calibration, that would be about half of an s-unit! I can handle that.

Either way, you guys taught me a lot by forcing me to look into this and see why I wasn't getting the results I thought I should. Thanks!

Although my stock of jumpers has gone down considerably with all the testing.