Notch Filters and Effects

[WA1ATA]

Are we done now?

Hopefully :roll:

The plots speak for themselves. The discussion of the deeper meaning of the plots and and the significance of various levels of attenuation is a discussion that can go on forever, because each situation is slightly different.

 

[encinorescue911]

Your plots don't seem to show anything ot me. But I checked and I can activate the attenuator on my channels and still here them fine.

 

[zz0468]

Your plots don't seem to show anything ot me.

Would you like me to explain them?

But I checked and I can activate the attenuator on my channels and still here them fine.

What that means is, the signals you are attempting to receive are strong enough that your receiver is still in saturation, even when the attenuator is applied. Try the same test on a weak signal and see what happens.

 

[encinorescue911]

Would you like me to explain them?

No, and I don't think that talking down to me is helpful. I am new to scanners and all I know is that the interference went away but I can still hear what I want.

What that means is, the signals you are attempting to receive are strong enough that your receiver is still in saturation, even when the attenuator is applied. Try the same test on a weak signal and see what happens.

With all due respect I think that is what the other guy kept trying to say. I mean with everything repeated I can hear most of LAPD just fine with the attenuator on. Before I couldn't but was afraid to use this feature. When the guy that sold me the scanner suggested an outside antenna would let me hear better I went for it and it was terrible. I kept getting inteference that is gone now. Bleeps and bleeps all day. I don't want any part of your peeing contest but it seems to me that you both made valid points. At the end of the day my radio is quieter and I am hearing what I want.

 

[WA1ATA]

Your plots don't seem to show anything ot me. But I checked and I can activate the attenuator on my channels and still here them fine.

You could also use a much smaller antenna and you would hear the signals fine.

Most of what I monitor have many dB of excess signal beyond what is needed. For those signals using a rubber ducky antenna right at the scanner is sufficient. I can also add a lot of attenuation (reduction of signal level) and still hear the signals cleanly.

The problem signals are the weak ones. For those I use a much better antenna that increases their signal level enough that they come in without much noise. Unfortunately, the better antenna also increases the signal level of a pager transmitter up to the point where it causes overloading of the first RF amplifier, which causes intermodulation products that interfere with the weak signal I want to hear.

Going back to either a small antenna, or the better antenna with an attenuator will reduce/eliminate the intermodulation interference, but it also reduces the weak signal back to making it noisy.

The only way to hear that particular signal is to use a good antenna that increases the signal level of both the pager and the desired signal. Then use a circuit that reduces the level of the pager transmitter much more than it reduces the signal level of the desired signal. If the frequency of the pager and the desired weak signal are close together, then it is difficult to reduce the level of the pager transmitter without reducing the desired weak signal too much. A very sharp (narrow) frequency response is needed.

If the desired signal is strong, then the frequency response can be much broader, or even close to flat -- as is the attenuator in your radio.

 

[encinorescue911]

Doesn't that kind of make the point of the other guy that says that signals are so strong I won't really be impacted by using my attenuator or that stub thingy? Since I put up and outside antenna I hear some much junk. I got a Uniden 796D from eBay and I'm told that because its older I cannot turn on NAC for LAPD. I hunted down the bllep bleep bleep at 462 sometyhing and thought that this stub thingy would be a solution. That's why I tried the attenuator and found that with it on I still hear LAPD fine.

 

[zz0468]

Doesn't that kind of make the point of the other guy that says that signals are so strong I won't really be impacted by using my attenuator or that stub thingy?

The point of that whole exercise is, one needs to know what the effect of inserting a filter is. In your case, if all the signals you want to hear are strong enough the overcome a 30 db notch, then it'll work for you. But for someone else, that may make the entire band go quiet. Not everyone lives in a strong signal area for the stuff they want to listen to.

Dig through the threads here and see how many questions get asked by people on how to improve their weak signal reception. Punching a big hole in the spectrum with a broad notch filter isn't the way to go about accomplishing that. It can be helpful to actually know how these things work.

That's why I tried the attenuator and found that with it on I still hear LAPD fine.

So, did you end up using the attenuator or the stub to solve your problem?

 

[WA1ATA]

Since I put up and outside antenna I hear some much junk. .... I hunted down the bllep bleep bleep at 462 sometyhing and thought that this stub thingy would be a solution. That's why I tried the attenuator and found that with it on I still hear LAPD fine.

Intermodulation interference happens when the signals at the radio front end exceed some threshold. Adding the attenuator increases that threshold, while reducing the sensitivity by the same amount. If the desired signal is strong enough, a broadband untuned attenuator like the one built into many scanners is the simplest solution (actually, the even simpler solution is to not have such a good antenna. A small, inefficient antenna will also reduce all signals at the front end.) The attenuator reduces the level of the interfering signal to the point where it no longer causes intermodulation distortion in the receiver front end.

With the attenuator on, instead of getting full quieting with 0.3uV you will need 10 times that or 3uV. This is OK if what you are trying to receive has a signal level of, for example, 10uV.

Stubs and cavity filters are useful when you have a strong interfering signal near the frequency of a weak signal. The coaxial stub filter and a cavity filter have the same general effect, but the coaxial stub will have much more loss of the desired signal.

Because of the attenuation of the coax stub or cavity is depends upon the frequency, you can reduce the interfering signal by a larger amount than you reduce the desired signal.

=========================

In other words, if you have intermod problems, try these solutions in this sequence:

1st solution .... smaller antenna or use a 20dB attenuator. This works if the desired signal is more than 20dB above the level needed to receive it; and the interfering signal is less than 20dB more than the intermodulation overload point of the scanner.

2nd solution ...... coax stub. This works if the desired signal is 10+ db above the sensitivity of the scanner and the interfering signal is no more than about 25dB above the intermodulation point of the scanner. .

3rd solution ..... cavity filter. This works for interfering signals that are 40dB above the scanner intermod point and desired signals as weak as 3dB above the sensitivity of the receiver, or even closer to the receiver sensitivity if the pager and the desired frequency signal are more than 5MHz apart.

See the cavity response plot at VHF SYM 152HT Specifications and Plot | PAR Electronics
and compare to the ones for the coax stubs posted above and you will see how the cavity is sharper. The equivalent plot for the attenuator internal to your scanner is a flat line across the entire band at -20dB.

So you have the choice of flat response (basic attenuator), broad notch (coax stub), or narrow notch (cavity filter), depending upon your specific needs.

 

[tomasG]

Intermodulation interference happens when the signals at the radio front end exceed some threshold. Adding the attenuator increases that threshold, while reducing the sensitivity by the same amount. If the desired signal is strong enough, a broadband untuned attenuator like the one built into many scanners is the simplest solution (actually, the even simpler solution is to not have such a good antenna. A small, inefficient antenna will also reduce all signals at the front end.) The attenuator reduces the level of the interfering signal to the point where it no longer causes intermodulation distortion in the receiver front end.

With the attenuator on, instead of getting full quieting with 0.3uV you will need 10 times that or 3uV. This is OK if what you are trying to receive has a signal level of, for example, 10uV.

Stubs and cavity filters are useful when you have a strong interfering signal near the frequency of a weak signal. The coaxial stub filter and a cavity filter have the same general effect, but the coaxial stub will have much more loss of the desired signal.

Because of the attenuation of the coax stub or cavity is depends upon the frequency, you can reduce the interfering signal by a larger amount than you reduce the desired signal.

=========================

In other words, if you have intermod problems, try these solutions in this sequence:

1st solution .... smaller antenna or use a 20dB attenuator. This works if the desired signal is more than 20dB above the level needed to receive it; and the interfering signal is less than 20dB more than the intermodulation overload point of the scanner.

2nd solution ...... coax stub. This works if the desired signal is 10+ db above the sensitivity of the scanner and the interfering signal is no more than about 25dB above the intermodulation point of the scanner. .

3rd solution ..... cavity filter. This works for interfering signals that are 40dB above the scanner intermod point and desired signals as weak as 3dB above the sensitivity of the receiver, or even closer to the receiver sensitivity if the pager and the desired frequency signal are more than 5MHz apart.

See the cavity response plot at VHF SYM 152HT Specifications and Plot | PAR Electronics
and compare to the ones for the coax stubs posted above and you will see how the cavity is sharper. The equivalent plot for the attenuator internal to your scanner is a flat line across the entire band at -20dB.

So you have the choice of flat response (basic attenuator), broad notch (coax stub), or narrow notch (cavity filter), depending upon your specific needs.

It would appear that my points were in fact received and accepted. I noted that you placed a stub above a cavity. I would disagree with you however on the order. I own numerous PAR filters. Their insertion loss is low and they are effective. Their "roll off" is usually 3-6dB within 2.5% of the frequency. I refer to this in layman terms as recovery -- how fast we get back to full strength. I respect that all scanners users speak geek Regardless of my position on the usefulness of tuned stubs the PAR filters work. Dale is a nice guy and always ready to talk and help.

But to be clear on "intermod", intermod on a scanner is not true intermod. It is caused by front-end overload, often caused by adding an external antenna in a metropolitan area, that causes the mixer to become non-linear and the scanner creates its own intermod. This is the same intermod that troubles communications systems except for its cause. When a single frequency (f1) is fed through a device whose output is not a linear function of its input, harmonics of f1 are generated, i.e. 2f1, 3f1, 4f1, 5f1, etc..

True intermod in communications systems is *usually* created when the transmitter of system 1 gets into the transmitter of system 2 creating a third frequency (or more.) Commercial repeaters almost always have circulators (isolators) that keep signals from coming down the transmitter cable.

I have found that I actually want a little loss in my scanner system using any of several antennas that I have. Although I'm not in Los Angeles we still get slop out here. Imagine what it would be like putting a scanner up on top of a mountain! Besides all the high level signals at the site, the signal strengths would be so high as to cause front-end overload that would then create non-linear operation.

My ham radio equipment suffers to some extent from this as well, but I have some commercial stuff that doesn't. I use it for serious listening. It's always fun driving through the Angeles National Forest near some of the sites, Like McDill, and my 2 meter rig goes batty! Mt. Wilson actually kills my 440MHz Bendix portable.

But back to the topic of this thread and how this ties in, using a tuned circuit to connect multiple antennas works well. I learned this trick on another site. When I couldn't find a single antenna that worked well I took advice of others more learned and got cheap knock-offs by Tram tuned for 155MHz and 470MHz. I use a duplexer reversed and this alone cut down on the intermod by knocking down out-of-band signals that contributed to front-end overload. In fact, others I know that only need a single band report using a duplexer for 2 meters / 70cm. If they need just UHF they put a dummy load on the opposite port. Or if they need VHF the opposite. I don't have an expensive analyzer that proves or disproves this but my ears know that this method works quite well.

 

[zz0468]

No, and I don't think that talking down to me is helpful.

No one is talking down to you. That was an earnest offer to explain something that you indicated you don't understand. I'm sorry you took it that way.

I am new to scanners and all I know is that the interference went away but I can still hear what I want.

And I'm one of the guys who designs and builds the systems you guys listen to, and have been doing this for a long, long time. I'm actually trying to impart some hard earned knowledge. I'm glad you found a solution to your problem.

Whatever works is fair game, as long as you understand the side effects that your solution may cause.

I don't want any part of your peeing contest but it seems to me that you both made valid points.

That's fine, but perhaps I misunderstood the tone and intent of some of your comments, just as you have misunderstood the tone and intent of some of mine. Let's leave it behind, shall we?

 

[zz0468]

But to be clear on "intermod", intermod on a scanner is not true intermod. It is caused by front-end overload, often caused by adding an external antenna in a metropolitan area, that causes the mixer to become non-linear and the scanner creates its own intermod. This is the same intermod that troubles communications systems except for its cause. When a single frequency (f1) is fed through a device whose output is not a linear function of its input, harmonics of f1 are generated, i.e. 2f1, 3f1, 4f1, 5f1, etc..

True intermod in communications systems is *usually* created when the transmitter of system 1 gets into the transmitter of system 2 creating a third frequency (or more.) Commercial repeaters almost always have circulators (isolators) that keep signals from coming down the transmitter cable.

Ok... first off, this is NOT meant as an attack, ok? I just want to point something out and provide proper clarification...

Intermod is merely the undesired mixing of two or more signals that produces unwanted products. It occurs when the source signals mix in a non-linear device. It doesn't care whether the non-linear device is a transmitter, a receiver that's gone non-linear due to overload, or a rusty chain link fence. It's ALL "true" intermod.

The fix to be applied will vary, depending on the source. Some intermod can be fixed at the receiver, as is the case of applying attenuators, stubs, PAR filters, or cavities. A trick to determine if the intermod is occuring in a receiver or outside the receiver is to put a low value of attenuator, say, 6db in the receive antenna path. If the level of the intermod decreases by 6 db, then it's external. If it decreases by more than 6db, than it's occurring in the receiver, and you now know where to apply the fix.

The really insidious cases are where the mix occurs in multiple locations.

Some can be cured at the transmitter, with circulators and cavities, and sometimes it really is mixing in a rusty chain link fence. In that case, no amount of filtering will help, and you have to replace the fence. Really.

 

[WA1ATA]

I noted that you placed a stub above a cavity. I would disagree with you however on the order.

And I placed a smaller, less effective antenna or using an attenuator above the coaxial stub.

My sequence was from simplest/lowest performance up to higher cost/highest performance.

My recommendation is to start with the easiest solutions, and work upwards. PAR filters are great, but overkill in most cases.

Imagine what it would be like putting a scanner up on top of a mountain!

I don't have to use my imagination.
I'm at 1000' elevation overlooking Silicon Valley, within a couple of miles of several different antenna farms both commercial and state. A 330 watt NOAA Wx transmitter is a bit further away, but still under 5 miles, and is on continuously.

Although scanner manufacturers don't usually include an IP3 spec as do many preamp manufacturers, it is very clear that the Uniden scanners are much more resistant to overload/intermod/compression than are the GRE products.

 

[tomasG]

Ok... first off, this is NOT meant as an attack, ok? I just want to point something out and provide proper clarification...

There was nothing that needed clarification and I don't see that your reply added anything of value. You just reworded what I had said. Perhaps it would be best that we not respond to one another at all.

Let's let a neutral third party impart knowledge on the masses.

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

@WA1ATA:

My family has a cabin at 6,000 feet up in snow country. We can't quite see L.A. but reception is awesome. And so is the front-end overload causing a non-linear mixer response intermod. I like to ride my bike on the trails with my HT and I get to a lot of the sites. My ham gear does a little better on intermod, but my scanners just can't hang without at least one PAR. I've noticed that Uniden doesn't publish specs on anything but I can use my service monitor to tell you what they do.

 

[zz0468]

Let's let a neutral third party impart knowledge on the masses.

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

He's not a neutral third party. He's trying to sell something.

 

[encinorescue911]

He's not a neutral third party. He's trying to sell something.

Comparing a commercially designed notch filter such as the PAR filters, with a quarter wave stub made from RG58 is a comparison of apples and bananas.

You didn't seem to have an issue with them earlier. Besides, isn't everyone trying to sell something? Even this site sells advertising space and its content is assumed to be of quality. Just because someone is selling a product doesn't make them less credible. The fact that people buy them and know their name means just the opposite. I checked the reviews of the Par filters and not one was bad. Because of what I've learned here I have decided to buy one.

 

[zz0468]

You didn't seem to have an issue with them earlier.

I don't have an issue with them. I just wouldn't classify any vendor as "neutral".

The fact that people buy them and know their name means just the opposite. I checked the reviews of the Par filters and not one was bad. Because of what I've learned here I have decided to buy one.

The meaning behind my comment that you quoted is not to disparage PAR filters. Quite the opposite,actually. A PAR filter is considerably better than a coax stub. So much so, in fact, that a comparison between the two wasn't valid in the context of my other comments.

As good as they are, they aren't the best out there. In 35 years of mountaintop experience, and maybe 150 sites, I have NEVER seen a PAR filter deployed on an actual working system. What you see is a lot of Angle Linear devices, preamps, filters, and duplexers. Again, that is not to disparage PAR filters. I'm just pointing out that few, if any, professional installations use them.

 

[N2ICV]

I ordered a Jim 96XI-1 Adjustable Notch Filter from HRO on tuesday. Does anyone own one or had one that can tell me how good it works? No reviews e-ham or anywere on the net.

 

[zz0468]

I ordered a Jim 96XI-1 Adjustable Notch Filter from HRO on tuesday. Does anyone own one or had one that can tell me how good it works? No reviews e-ham or anywere on the net.

There was a comment made right here at RR that you might find useful:

http://forums.radioreference.com/an...1-adjustable-notch-filter-1st-impression.html

 

[N2ICV]

@zz0468 ... Thanks for the link!

 

[W4OP]

Professional Users of PAR Filters

Interesting thread. I would only comment that perhaps 95% of our filters go to the aero, commercial FM broadcast, 2 way etc. These are not the scanner filters per se- but some are similar in topology.
We have filters at Cape Canaveral, in front of GPS systems, flying in space, on avalanche sites in Asia, on probably the majority of commercial ships on the Mississippi, hundreds of muni airports and FM translator sites- on and on.

Regards,

Dale W4OP
for PAR Electronics, Inc.