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Local frequency bleeding into feed

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trnbuf

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#1
I provide a railroad scanner feed that I have hooked up to an outside antenna. While listening to the feed my local fire frequency and weather frequency bleeds together into my railroad frequencies. It gets quite annoying at times. Is there something I can do to keep this from happening?
 
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#2
You are likely experiencing an intermodulation mix, probably 3rd or 5th order like 2A-B = C or 3A-2B = C. You can find some IM caculators on line. A good calculator will consider the effects of modulation and bandwidth , some calculators simply do simple math, so if you get no direct hit , remember your receiver frequency is C, so if a product is C plus or minus 100 KHZ, it is probably indicitative of a "hit".

Ok how to fix? If the fire transmitter is on same site as the weather, The IM could be occurring in one or both those transmitters. However more than likely, it is occuring inside your receiver itself due to overload by the stronger of the weather or fire transmitter. Assuming you have no preamplifiers external to your receiver which can aggravate the IM, you will have to attenuate one of the two signals with a notch filter. The effectiveness of this filter is inversely proportional to the seperation in KHz between the undesired TX and the desired RX frequency.

If you can post the exact frequencies involved, I can help . Otherwise remove any preamplifiers and try using an attenuator if that option is acceptable.
 

trnbuf

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#5
I do have 2 amplifiers hooked up between the antenna and scanner. If I disconnect even one of the amplifiers, my reception for my railroad frequencies are not that good. The frequencies that I know off that are bleeding in are digital frequencies. The frequency is 151.1525 that bleeds into several of my railroad frequencies.
 
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#6
What type of scanner are you using? Possibly switching to a triple conversion scanner would solve the problem if currently using a double conversion scanner.

Shawn
 
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#8
I do have 2 amplifiers hooked up between the antenna and scanner. If I disconnect even one of the amplifiers, my reception for my railroad frequencies are not that good. The frequencies that I know off that are bleeding in are digital frequencies. The frequency is 151.1525 that bleeds into several of my railroad frequencies.
Cascaded amplifiers;

..... There's your problem!

All of that excess gain will degrade the intermodulation performance of your receiver. Which is probably not so good as it is. Is even one of these amps mounted at the antenna?
 
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#10
A variable attenuator should always be part of scannerists arsenal. Often when amplifiers are used you will need to attenuate to a certain degree to overcome overload. Sometimes the reception increase when attenuating a few dB. Look for someting like this ATTENUATOR IEC or F type in line aerial signal reducer 3 6 9 12 15 20dB variable | eBay

/Ubbe
I did a lot of master antenna work in my past career. Preamplifiers at the antenna mounted on top of a structure (Chicago Sears Tower) and multicouplers to feed several repeater receivers. At Gannet Tower in North Miami FL I came up with a scheme to mount a relay controlled directional coupler and dummy load at the tower mounted preamp (~ 700 feet AGL) with a sampling cable so that a generator could be used to feed a reference signal right at the antenna and therefore system SINAD and noise floor measured at the receivers.

I had a switchable attenuator that I would install before the multicoupler which had its own set of amplifiers. I would dial in attenuation until the 12 dB SINAD faltered and then back off about 2 dB of attenuation. Then I would install fixed attenuators at the same value. This worked pretty well and the systems were very hot and interference free.

I got to see the work by some of the other companies on shared sites. They would cascade multicouplers incorrectly, instead of branching and balancing them, they were cascading an unused port of one 16 channel multicoupler into another.
 
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#11
There is a company called PAR who makes notch filters for commercial and scanner radios. They have a notch filter that is specifically for notching out the weather transmitter frequencies. However the railroad channels are so close that about half would fall above the lower skirt of the 3 dB bandwidth of the filter. I have provided the links and the filter bandwidth for you to study.

Before making any decision, you need to determine

1) Do you really need all that pre-amplification? Can you live without it?

2) What are the exact channels involved in the interference?

3) What other channels must you be able to monitor that cannot be filtered?

4) The relative strength of the weather transmitter and the fire frequencies (151.1525?) . This may be difficult without a spectrum analyzer or some calibrated attenuators and some math.

5) Which of the interferers is the strongest? If it turns out the fire transmitters at 151 MHz are the strongest, filtering them will be easier as they are further in MHz from the railroad frequencies.

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

PAR can probably build a custom filter for you

PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

VHF SYM 162HT Specifications and Plot | PAR Electronics

Rail Road Band
"AAR stands for: Association of American Railroads. This is an organization comprised of American and Canadian railroads. The band plan they use for radio communications consists of 96 VHF frequencies or 'channels'. Each channel is spaced by 15 kilohertz. The band begins at 159.810 MHz and ends at 161.565 MHz. Most railroads have radios that will cover all of these channels."
 

Ubbe

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Stockholm, Sweden
#13
The easiest and cheapest thing to test is a 1/4 coax stub filter that attenuate 151MHz.
If it attenuats other frequencies too much you could still use it with a variable attenuater in series with it
and balance it for optimal reception. The total lenght of the stubfilter must then include the attenuater.

https://www.youtube.com/watch?v=aXU0JdUhtyk
Coaxial Stub Notch Filter Designer

/Ubbe
 

mikewazowski

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Central Ontario
#14
Cascaded amplifiers;

..... There's your problem!

All of that excess gain will degrade the intermodulation performance of your receiver. Which is probably not so good as it is. Is even one of these amps mounted at the antenna?
Best answer.

Forget about all the other suggestions until you remove that second amplifier.
 
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