I have a notch filter question

[kmacka]

I am looking into getting rid of some interference and adding a notch filter to my setup may be a solution. But I have some questions about their use. Do notch filters come in just vhf and just uhf versions? Or do they just come in certain frequencies. I say that because I am getting some interference at different frequencies in the vhf and uhf bands. I figured if I could just order a vhf and uhf notch filter that may solve the problem, or not. If they don't come in just a broad version (vhf/uhf) I guess I will have to write down all the frequencies and then go from there, right? Let me know what you think I should do.

 

[Audiodave1]

Hello,
Notch filters are generally fairly narrow band at low (VHF 150 ish) frequencies but get wider in the UHF band due to the limitations of passive electronics (The type we can afford anyway)

You will always be attenuating a range of frequencies with passive filters. There are specific filters for FM broadcast, VHF pager bands, NOAA WX band and the like. You can also get custom tuned filters like I had to at my last QTH.

Have a look at Parelectronics.com Fair pricing, great performance.

This link should answer most of your questions.
http://www.parelectronics.com/faq_scanner_filters.htm

Humans answer their phones and will discuss your needs with a smile.

Dave

 

[zz0468]

What should you do? You should find exactly what mechanism is causing your interference, and then take the appropriate measures. And you haven't given anywhere near enough information to be of much help beyond that. Notch filters can be a great help in certain circumstance, but by no means are they a panacea in removing interference.

 

[kmacka]

What should you do? You should find exactly what mechanism is causing your interference, and then take the appropriate measures. And you haven't given anywhere near enough information to be of much help beyond that. Notch filters can be a great help in certain circumstance, but by no means are they a panacea in removing interference.

Well I'm not expert and when I say interference, I think I'm getting paging interference. But I have nothing to compare what I hear to what paging interference sounds like. I also have one frequency that gets something of a mix of weather station and my local ems dispatch. I guess I will just start from scratch and copy down my problem frequencies and then record what I am hearing and then post what I find. What other information should I provide? Should I try to locate paging transmitters in my location?

 

[zz0468]

Ok... there's a couple of things to go by. Yes, look up some local paging frequencies and plug them in to listen what they sound like. Paging is a frequent component of interference complaints. Hearing a mix of weather and your EMS dispatch sounds like it could be intermod. Are you running any kind of amplifier, like a preamp or an amplified splitter? Are you physically located near (like a mile or less) a busy radio site?

 

[prcguy]

Another thing to consider is simple notch filters will also notch at odd multiples of the design frequency, so a VHF notch filter can take a chunk out of your UHF reception.
prcguy

 

[kb2vxa]

First of all a notch filter takes a notch out of the band as a chisel takes a notch out of a piece of wood. The most common is a pager filter that notches out that portion of the band.

A good example of a band stop filter is an FM filter that removes the entire band. The opposite is a bandpass filter that allows only one band to pass and blocks the rest. That is particularly useful to the 2M ham operator plagued by pager interference and intermod caused by strong out of band signals.

Now armed with this information you can figure out what sort of filter you need IF yo need a filter. It sounds to me however you have multiple sources of intermod likely caused by a preamp or just a bum receiver.

Oh prcguy, where did you get the idea that a notch filter also notches odd harmonics of it's resonant frequency? Please explain the source of your information, practical testing experience, RF design mathematics or hearsay? Frankly this is the first time I've heard of such a thing in 30+ years experience in radio communications so I'd just like to know.

 

[zz0468]

Oh prcguy, where did you get the idea that a notch filter also notches odd harmonics of it's resonant frequency? Please explain the source of your information, practical testing experience, RF design mathematics or hearsay? Frankly this is the first time I've heard of such a thing in 30+ years experience in radio communications so I'd just like to know.

Full size highband cavities have a 3rd harmonic response. It doesn't matter if they're configured as a pass or notch filter.

 

[kmacka]

Ok... there's a couple of things to go by. Yes, look up some local paging frequencies and plug them in to listen what they sound like. Paging is a frequent component of interference complaints. Hearing a mix of weather and your EMS dispatch sounds like it could be intermod. Are you running any kind of amplifier, like a preamp or an amplified splitter? Are you physically located near (like a mile or less) a busy radio site?

After hearing a couple posts I think I might have an intermod problem. I am currently running a scantenna into a electroline 4 port multicoupler. The multicoupler is a version of which amplifies the signal...I have to purchase the version which does not amplify the signal and has no loss.

 

[zz0468]

Try running a single receiver direct to the antenna and see if your interference goes away. If so, it's the amplified multicoupler. Those things are not designed for that application, and in spite of what some folks here will say, they can do a piss poor job when connected to an antenna.

 

[prcguy]

It's not my idea, its physics at work. Make a 1/4 wave open or shorted stub (T connector with open or shorted 1/4 wavelength in coax), which is a notch filter and sweep it with a network analyzer. At the first frequency where the open or shorted stub is a 1/4 wavelength in coax (velocity factor taken into account) it will give you a 10dB or so notch due to the reflected signal arriving 180deg out of phase at the starting point (round trip of two 1/4 wavelengths is 1/2 wavelength = 180deg phase shift) and the notch will repeat when stub is resonant at 3/4, 5/4, etc of the design frequency. zz0468 mentions that VHF cavities are also resonant at the 3rd harmonic and I have several Sinclair 3/4 wave UHF cavities that are identical to their VHF cavities except the pickup loop is smaller to provide better matching at UHF frequencies. So, if you use a simple notch filter at say 152MHz to snuff out paging it will also snuff out desired signals at 456MHz and so on. A more elaborate filter can be designed to have minimal impact on frequencies outside the stop band.
prcguy

Oh prcguy, where did you get the idea that a notch filter also notches odd harmonics of it's resonant frequency? Please explain the source of your information, practical testing experience, RF design mathematics or hearsay? Frankly this is the first time I've heard of such a thing in 30+ years experience in radio communications so I'd just like to know.

 

[N_Jay]

Oh prcguy, where did you get the idea that a notch filter also notches odd harmonics of it's resonant frequency? Please explain the source of your information, practical testing experience, RF design mathematics or hearsay? Frankly this is the first time I've heard of such a thing in 30+ years experience in radio communications so I'd just like to know.

This is where an understanding of the theory trumps your "years of experience"!
Almost all resonant circuits show some resonates at odd harmonics.

Of course, if I had made the original statement, Warren would have come back with some "all knowing" snide comment.

 

[zz0468]

After hearing a couple posts I think I might have an intermod problem. I am currently running a scantenna into a electroline 4 port multicoupler. The multicoupler is a version of which amplifies the signal...I have to purchase the version which does not amplify the signal and has no loss.

If it has "no loss", then there is an amplifier in it. It might not work any better.

 

[kmacka]

If it has "no loss", then there is an amplifier in it. It might not work any better.

http://www.electroline.com/en/products/drop_amplifiers/eda_ug/index.html

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=170178589197&ssPageName=STRK:MEWA:IT&ih=007

Here are two links to the product that I am looking at. Here is a bit a text that is on the Ebay auction site.

"The EDA-UG2802 is used in applications where the cable signal has already been amplified, and additional ports are required to distribute the amplified cable signal. In cases where more than one amplifier is present, it is possible to overdrive the cable signal and cause additional problems. The EDA-UG2802 takes the guesswork out of this situation, as it provides the exact same signal level at the output that is provided at the input."

 

[zz0468]

There's a couple of potential problems with those.

First, is noise figure. If the noise figure of the amplifier in the multicoupler is higher than the noise figure of the scanner (very possible), then your ability to receive weak signals WILL be diminished - NO MATTER HOW MUCH GAIN THAT AMPLIFIER HAS.

The second potential problem is the amplifier's 3rd order intercept point (IP3). In very simple terms, that's a measure of how a receiver or amplifier behaves in the presence of strong signals. These amplifiers are designed for a relatively narrow range of signal levels, not the wildly variable signals received by your antenna. If the 3rd order intercept point of that amplifier is exceeded by a received signal, it doesn't matter whether or not the multicoupler has gain to each port or not - it will produce its OWN intermod and send it on to every receiver attached.

Even the zero loss multicouplers have an amplifier. It's just padded down so that the net gain/loss of the overall device is zero to avoid overloading other devices downstream. All of the above still applies.

 

[kmacka]

There's a couple of potential problems with those.

First, is noise figure. If the noise figure of the amplifier in the multicoupler is higher than the noise figure of the scanner (very possible), then your ability to receive weak signals WILL be diminished - NO MATTER HOW MUCH GAIN THAT AMPLIFIER HAS.

The second potential problem is the amplifier's 3rd order intercept point (IP3). In very simple terms, that's a measure of how a receiver or amplifier behaves in the presence of strong signals. These amplifiers are designed for a relatively narrow range of signal levels, not the wildly variable signals received by your antenna. If the 3rd order intercept point of that amplifier is exceeded by a received signal, it doesn't matter whether or not the multicoupler has gain to each port or not - it will produce its OWN intermod and send it on to every receiver attached.

Even the zero loss multicouplers have an amplifier. It's just padded down so that the net gain/loss of the overall device is zero to avoid overloading other devices downstream. All of the above still applies.

So do you think that I am better off just getting a higher end multicoupler, like the Stridsberg?

 

[trooperdude]

So do you think that I am better off just getting a higher end multicoupler, like the Stridsberg?

Who said Stridsberg was high end ?

Look at the internal electronics parts list.

A lower noise pre-amp MAY help.

I've rolled my own multicouplers in the past from a good LNA and a minicircuits
splitter. But it's not cheap.

 

[kmacka]

Who said Stridsberg was high end ?

Look at the internal electronics parts list.

A lower noise pre-amp MAY help.

I've rolled my own multicouplers in the past from a good LNA and a minicircuits
splitter. But it's not cheap.

Lol I guess I just assumed. I mean an electroline drop amp is $80.00 compared to a 8 port Stridsberg at $330.00

 

[kmacka]

"A lower noise pre-amp MAY help."

I guess I should just ask. What would some of you guys do in my situation. I want to get rid of the intermod. I need something to split my antenna signal into about 8 ways. What would you recommend for something with low noise and will help get rid of the intermod.

 

[zz0468]

So do you think that I am better off just getting a higher end multicoupler, like the Stridsberg?

No! I didn't say anything about Stridsberg. I have nothing against them, I've just never used their stuff. A quick look at their web page tells me it's probably an order of magnitude better than cable tv stuff but...

A few questions for you:

1. Did you try running without the multicoupler? If so, what are the results.

2. Are you located near any radio sites?

3. How many receivers are you trying to run?