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Author Topic: New apartment, interference inside but not outside  (Read 733 times)
KC9HXG
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Posts: 16




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« on: August 04, 2005, 08:13:37 AM »

OK, I've done some research on this issue, and I'm dumbfounded. We've just moved into a new apartment. We're on the 3rd floor. However, it's a loft apartment, meaning it's also a 4th floor. The window in our bedroom leads directly onto the roof. Now, window leading onto a 4th floor roof excites me as far as antenna possibilities. Especially since my boss is actually the landlord. Meaning I not only get discounted rent, but with a phone call, I can pretty much get permission to do whatever I want.

I haven't purchased a home rig yet, I'm still planning and studying my code so I can make use of the rig. However, something troubles me. When I plug in my scanner, on several frequencies, I get severe interference. I can't tell if it's radio/television/etc. Sometimes it's 2 second bursts of music, other times it's 'in and out' interference for several minutes strong. However, it's never strong enough for me to pinpoint it's source. It's mainly occurring on 146.700 and 147.105. However, when someone transmits, the voice negates all interference and is clear as a bell. It might exist elsewhere in other bands, but my scanner is rather inexpensive and limited in reception. Local authority channels (154.xxx-155.xxx) are clean as can be.

One thing I've found in my research is that perhaps it's coming through the electrical lines. I've attempted to plug the scanner into various outlets throughout the apartment, and it exists on them all. So it's either very very bad RFI, the entire apartment is on 1 circuit, or it's not coming through the electrical.

When I hop in my car and turn on my mobile, there's no such interference.

Does anyone have any ideas? I don't want to spend the money on a rig/antenna setup if it will be unuseable. Perhaps some things I can test in order to narrow this down a bit? The scanner functioned perfectly before the move 5 days ago.
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M0AFJ
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Posts: 37




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« Reply #1 on: August 04, 2005, 08:48:12 AM »

HI, just a thought, it sounds like you are getting cross modulation from a local transmitter. If you are in what in the UK would be a `high rise' it is possible that you are getting a much stronger signal from the local FM broadcast transmitter, this is overloading the front end of the receiver. Try removing the scanner aerial and see if the problem goes away.
You should not see this problem with a more normal amateur rig because they have a much better designed front end. There are some VHF mobiles though which are not so good. Look at the reviews on EHAM before you buy.
Good Luck, CU on the Bands
Tim M0AFJ, Milton Keynes (UK)
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KB4QAA
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Posts: 2407




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« Reply #2 on: August 04, 2005, 09:04:19 AM »

Welcome to the world of radio.

Don't dispair.  Especially in the city, interference on various frequencies and at varying times is normal.  This is part of the challenge of being a radio operator.  

You mention that your scanner is inexpensive, so it makes sense that other strong signals may overwhelm it at times.   A quality ham radio will probably do much better.  There are many things to experiment with, such as different antennas, different locations, filters of all sorts, even different operating modes, FM, SSB, Digital.  

Keep studying for your license.  Join a local club so you can meet others and get advice.  You won't learn everything about hamming and radio in twenty years.  That is part of the fun and challenge.  BTW, I have horrible Rf interference in my apartment, so I mostly have to operate mobile for HF, VHF is a bit better.  It has made me learn about mobiling.    

73,  Bill.
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WA6BFH
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« Reply #3 on: August 04, 2005, 09:23:37 AM »

You might consider what the 3rd order intercept point is on your scanner receiver, as compared to what I would describe as a “real radio” with both good “Front-end” as well as a good slope factor for the filtering of the I.F. stage.

Ask yourself these questions:

Was my scanner designed to have very tight frequency bandwidth response?

Will a good radio, such as some multi-mode Ham radio have similar problems?

If I had a 2 Meter multi-mode radio, what kind of Front-end design would it have, and what would be the slope factor at the I.F.? In fact, might it have an I.F that was chosen to help avoid intermodulation mixes?
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K7PEH
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« Reply #4 on: August 04, 2005, 09:50:37 AM »

As someone has suggested, disconnect the antenna just to make sure this is not something coming over the AC lines.  If you disconnect the antenna and the problem persists, it is very likely to be an AC line RFI problem unless you are next door to a high-power radio station.

Also, given that you may be thinking of putting an antenna up on the roof of this apartment building, be aware that the RF field right under an antenna can be strong enough to cause RFI.  With a regular single family home this is not usually much of a problem to neighbors but if your neighbors are also living under the antenna then you might want to be mindful of causing RFI.  In my experience, the first place this RFI will show up is in telephone circuits and stereo speakers.  This is because the lead for the speakers and the telephone wire in the walls of the apartment are pretty decent antennas at the higher frequencies (above 40 meters).
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WA9UAA
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Posts: 319




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« Reply #5 on: August 04, 2005, 10:21:26 AM »

Good Day,
It might be coming from leaks in the local cable service intermixing with other signals in the scanner. GL HTH
73,
Rob WA9UAA
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KC9HXG
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Posts: 16




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« Reply #6 on: August 04, 2005, 10:34:43 AM »

Great advice. I called my wife and had her remove the antenna while the interference was occuring. It stopped.

There really isn't any transmitters that I know of near my location. A high school 2 towns over has an FM radio station, but they only broadcast until 6pm. Plus, the old apartment was about 3 miles closer then we are now, and there was no interference there. I was thinking it might be somebody using an FM transmitter to transmit an IPOD to their home stereo or perhaps a wireless mic system for a guitar/amplifier, but the interference is constant at all times.

We are within 2 miles of the main business district, so I'm wondering if one of the business may have a strange FM intercom setup running 24/7.

Since my mobile rig has none of this interference, it supports the poor front-end theory that some of you have suggested. Perhaps it's time to pick up an HT and use it to scan the ham bands while at home and gift the scanner to my younger brother. And that would also have the added benefit of being able to volunteer to work the county fairs and other events. Or, perhaps I should ask a ham to come over for dinner and bring his HT to see if it picks up the noise.
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WA6BFH
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« Reply #7 on: August 04, 2005, 10:35:18 AM »

One good guess would be leakage from Cable TV. Maybe one of your neighbors has an un-terminated “splitter”.
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W0FM
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Posts: 2056




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« Reply #8 on: August 04, 2005, 10:41:50 AM »

The "2 second" to "several minute bursts" of music and other info suggests an intermodulation issue to me.  The off and on nature of the interference suggests a potential paging or two-way FM transmitter in which the transmissions are varied.  

A frequency from one such transmitter can "mix" with other transmitted frequencies in the area creating an unwanted signal on a third frequency (perhaps the 2 meter freqs you've described).  Intermodulation is much more common in urban areas with lots of commercial, two-way and paging transmitters bunched up on the best (highest) sites in town.

The better the "front end" of your radio, the more capable it is of dealing with intermodulation.  Filters may be necessary to handle your RFI on an inexpensive scanner.  I have filters on my more expensive scanners to reduce the intermodulation interference, but found I can listen to the same frequencies on my ham rig without the help of filters.  

Good luck.

73,

Terry, WØFM
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W0FM
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Posts: 2056




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« Reply #9 on: August 04, 2005, 10:45:18 AM »

I also think you'll find this review area helpful.  These are the filters I use on my scanners.

http://www.eham.net/reviews/detail/3086


Terry, WØFM
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W3JJH
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« Reply #10 on: August 04, 2005, 11:42:17 AM »

Since the interference is in short bursts, it is likely that it is a two-way radio transmission intermodulating with commercial FM stations.  A quick check of the FCC data base shows that KSH720 is licensed to operate a 100-W base station on 42.96 MHz in Portage, IN.  Receiver overload caused by being too close to their site could cause IM with WJMK (Chicago) and WXRD (Crown Point) that would fall in the 2-m band.
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W0FM
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Posts: 2056




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« Reply #11 on: August 04, 2005, 02:15:57 PM »

And you don't have the problem in your vehicle because the mobile antenna is lower than your fixed antenna.


Terry, WØFM
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N6AJR
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Posts: 9910




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« Reply #12 on: August 04, 2005, 03:10:45 PM »

looks like intermod to me, usually from either a station real close over driving the front end of the reciever, or 2 or more signals hetrodyning and making a 3rd freq that you rig is picking up on eoither the main freq or a harmonic... it happens a lot in the city, with taxi, police and fire, etc all around 150-174 mhz..

try setting the scanner over here for a while and see if it is better or worse then put it way over there and listen again, that may help you spot it.

When I have my 2m on 147.24 and transmit on certain freq's on 20 meters with 300 watts or more, it causes my 2 m rig to actually make signals from other  repeaters come out the rig.  

on 15 m and more that 500 watts I get into my neighbors dvd player,  on low power no problem, so if they are watching a dvd and I am on hi power they call and I go bare foot, problem solved..

any time you mix 2 signals, you end up with 4, the two origanals , a, b,  and one which is  a+b and one which is a-b, and any one of those can fall in your reciever or a harmonic of them.. so it can be a bit of a mess.

sometimes a couple of ferrite beads will help or a dirty balun ( about 10 loops of coax in a 6 inch circle..) or even a counterpoise, ( a couple of quarter wave wires acting as a "tuned" ground)

good luck
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