John_M
Member
How do these effect us scannists. While doing some researching I stumbled upon some filters to wipe out this band.
FM Broadcast Interference.
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Seeing as most scanners do NOT cover the FM broadcast band.... it really doesn't affect us much. Sometimes if your near the transmitter site.. You may get a lil bleed in... of if the things not set just right, this can also occur,.. but its not that often,.. or that bad.
musicman
Member
FM Traps
I find that FM traps often help in the aircraft band, but not much for anything else.
I find that FM traps often help in the aircraft band, but not much for anything else.
John_M
Member
I just bought a FM notch filter for $34.95 from www.Radiofilters.com. I mainly boug...far
is the one from [url]www.radiofilters.com. You can also find this filter on ebay
for $29.95.
for $29.95.
N
N_Jay
Guest
JMedley_1 said:
John_M
Member
N_Jay, I just purchased it online today 4/21/05. When I get it I will do some testing and I will post my opinion on this thread.
John.
John.
John_M
Member
Voyager said:
From what I gather I suspect the effect on the Aircraft Band 108-136Mhz is because the FM Broadcast band is so close to it 88-107Mhz.
My Uniden BC895XLT gets considerably better reception across *all* of VHF-High (108 - 174) with a cheap Radio Shack FM trap on the antenna. I have a 50,000-watt FM station nearby at 99.9 MHz. The trap has little to no effect on VHF-Low, UHF, or 800/900 MHz.
I assume that my other scanners are built to not desensitize as much because of stations in the FM band. I have an old Regency/AOR MX-7000 that can actually tune wideband-FM broadcast stations, and it only gets "muddied" by the local flamethrower from 98 to about 102.
The rule of thumb I live by is experimentation. If you have an FM trap, give it a shot. If it doesn't work, save it for a TV that's getting interference on channels 9 - 13 or something (that's the first harmonic of most FM stations), or channel 6.
I assume that my other scanners are built to not desensitize as much because of stations in the FM band. I have an old Regency/AOR MX-7000 that can actually tune wideband-FM broadcast stations, and it only gets "muddied" by the local flamethrower from 98 to about 102.
The rule of thumb I live by is experimentation. If you have an FM trap, give it a shot. If it doesn't work, save it for a TV that's getting interference on channels 9 - 13 or something (that's the first harmonic of most FM stations), or channel 6.
I have exactly the same experience as Phil with my Pro-96 using a discone on a roof mounted 30 foot mast and RG-6u cable. VHF reception was terrible compared to other scanners that I would hook up to the same antenna. I bought the $6 RS FM trap and an F to BNC adapter after reading about this on another board. The difference was night and day. Lake County S.O. on 155.970 is about 75 air miles from my location. Before I installed the FM trap, it would barely break squelch. It now comes in with almost full quieting. I saw similar improvement in all my VHF-Hi and air band reception. No degradation on any other band either. If you're using a "hot" receiver like the Pro-96, I'd sure spend the $11 it cost me for the FM trap and adapter first before buying a pre-amp since it's pretty obvious that a strong FM signal was causing front end overload and that's why VHF reception was so bad.
John_M
Member
SAR2401 said:
Very, very, interesting. I emailed www.Radiofilters.com and asked
him if the Optoelectronics FM Trap ($149.00) would let signals pass with no attenuation all the way to 2.8Ghz. That is the only reason I suspect maybe why the Optoelectronics FM Trap is so expensive. He told me that he had bought an Optoelectronics FM Trap and said to himself, "I can build one for alot cheaper than $149.00". He also told me that over the weekend that he was going to test the Optoelectronics trap with his HP
Spectrum Analyzer. Unfortunately, his Spectrum Analyzer only goes up to
1.5Ghz. So maybe we will never know why the Optoelectronics FM trap
is so expensive. He did tell me that he would send me an email with his test results. I will update this thread when I get the email. Now, on the flip side of this would be to see if the traps attenuate signals below 88Mhz. From the graph on www.radiofilter.com there actually looks to be a little gain from 20mhz to 60mhz on it's filter.
John.
John_M
Member
The input and the output impedance of the FM trap From RadioShack is 75 ohm, the gerneral input impedance of a TV attenna input. The attenna impedance of our scanners is 50ohm. This is a mismatch and if you are using the RS FM Trap with your scanner you are paying for it somewhere. (Most likely signal loss)
Guys, don't use this RS FM trap!
Guys, don't use this RS FM trap!
astrodanco
Member
JMedley_1 said:
How many times do we have to re-hash this?
The system impedance will vary by way more than 25 ohms over the low to high frequency range of the scanner. 75 vs. 50 ohm coax impedance is practically insignificant to a scanner.
Hi group,
Just wanted to chime in with my FM issues, I have 2 Optocoms that are DEAF below 400Mhz without a FM notch filter. I also have 2 50-80Kw stations in my area (within 2 miles).
I got the PAR electronics filter and it made a worlds difference...
Dave
Just wanted to chime in with my FM issues, I have 2 Optocoms that are DEAF below 400Mhz without a FM notch filter. I also have 2 50-80Kw stations in my area (within 2 miles).
I got the PAR electronics filter and it made a worlds difference...
Dave
John_M
Member
Practically? What does that mean? I don't know. All the FM Filters designed for us scannistsastrodanco said:
design their filters with 50ohm input and output impedance. The FM Trap from RS was not designed with us scannists in mind.
bsavery
Member
JMedley_1 said:
Practically... meaning if you have enough signal strength at the antenna, you can overcome the loss from the impedence mis-match, the connector adaptors, and the other non-radio comms designed components.
Or... as one fellow put it.... if your trying to pull in weak signals, you might as well put your money into a decent handheld antenna and hang it off the back of your scanner before wasting your money on using CATV hardware.
BUT... there are a lot of people using it, and quite happy with it, so I guess the old saying "Your Milage May Vary" holds true for scanners too!
Bob
John_M
Member
I doubt very much that the RS FM Trap designed with TV signals in mind was tested for attenuation of signals 25Mhz to 88Mhz and 107MHz to 1.3Ghz. You are in the dark if you use the RS FM Trap. In other words you do not know if it does attenuate the signals we want. Obviously, it lets some of our signals pass but if you want to be sure and have piece of mind you should use a FM Trap designed for scannists.
John_M
Member
bsavery said:
Good point Bob.
gr8amp
Member
JMedley_1 said:
I guess I am in the dark then. :roll:
This is the frequency response of the average "inexpensive" TV FM notch filter. The radio shack version is nearly identical (only difference is the label). Does this look good enough for you?
This topic has been discussed here many times. There is nothing wrong with using 75 ohm devices for scanning. Find me an antenna system that maintains a perfect 50 ohm feedpoint impedance over the entire range of frequencies that you monitor, and you might have an argument there.
A scanning setup is a compromise, and is far from an ideal system, and where FM interference is a major problem the RS filter provides great results.
Another solution is to make a small LC circuit with a tunable cap, and place it in the radio. Usually, there are only one or two major stations causing interference (around here anyways)so the cap can be tuned to notch out the desired station. This works just as well in certain situations. Pictured below is one that I recently installed in a BC780.
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