Homemade Filter?

[xusmarine1979]

Hello all, I have a question. Hope it doesn't sound stupid. (I'll probably get a comment on that but owell...):roll: Anyways, is there a way to make my own filter for the 400 MHz band. I believe that a local tv station (17) is interfering with my scanner. I know filters are a bit pricey at tims, so is there anyway that I can make my own or do my own mod? Thanks for any responses.

 

[xusmarine1979]

Ok, my boss told me to try this little test. Without the antenna search from 153-153 and 505 and down. These are the frequencies it stopped on, 493.7375, 489.6, 470.01875, 152.9975 and 153.005. I really don't notice much trouble on VHF or UHF, just on 800. Any suggestions?

 

[mastr]

Channel 17 video is centered on 489.25mhz, IIRC. Guess you could try a 1/4 wave open stub filter. Put a length of coax in parallel with your feedline right at the back of the scannerand 1/4 wave long at the 489.25 freq.

 

[MB]

Ok, my boss told me to try this little test. Without the antenna search from 153-153 and 505 and down. These are the frequencies it stopped on, 493.7375, 489.6, 470.01875, 152.9975 and 153.005. I really don't notice much trouble on VHF or UHF, just on 800. Any suggestions?

I would get some PAR filters. One of the best investments I have made towards scanning.

http://www.parelectronics.com/scanner.htm

 

[kb2vxa]

Hi X and all,

None of those frequencies are related to ch 17 but one is close, 493.75 is the audio carrier. If you can hear the sound on that frequency (it's WFM) without an antenna you must be very close to the transmitter. Bear in mind it's not always in the city of license. As for those other frequencies you don't say what if anything you hear on them, if it's a continuous unmodulated carrier they're probably birdies generated within the scanner itself.

Frankly I don't understand how 800MHz could be affected unless you're quite near the tower and it's spitting out spurs from somewhere in the transmitter's frequency multiplier chain. In any case somewhere along the line you would hear a heavy video buzz on AM and sound on WFM if the source is a TV transmitter. If this is actually the problem a filter will do nothing but filter out everything because the unwanted signals are in band.

FYI, 978.5 and 987.5 are the second harmonics of the video and audio carriers respectively so we may eliminate that as a possibility.

One last thing, if the transmitter IS spitting out crap in a public service band undoubtedly it's interfering with other services in the immediate area. The chief engineer would like to know in order to avoid having the station slapped with a heavy fine after a visit by the FCC field engineering team. They do periodic inspections of broadcasting facilities so perhaps you should "enquire within" and discuss your problem with the CE who can undoubtedly help you and you could possibly save him a lot of grief. Even if it's not the transmitter's fault at least an engineer can give you much better advice than anyone here except for another engineer hiding in the bushes.

 

[xusmarine1979]

Hi X and all,

None of those frequencies are related to ch 17 but one is close, 493.75 is the audio carrier. If you can hear the sound on that frequency (it's WFM) without an antenna you must be very close to the transmitter. Bear in mind it's not always in the city of license. As for those other frequencies you don't say what if anything you hear on them, if it's a continuous unmodulated carrier they're probably birdies generated within the scanner itself.

Frankly I don't understand how 800MHz could be affected unless you're quite near the tower and it's spitting out spurs from somewhere in the transmitter's frequency multiplier chain. In any case somewhere along the line you would hear a heavy video buzz on AM and sound on WFM if the source is a TV transmitter. If this is actually the problem a filter will do nothing but filter out everything because the unwanted signals are in band.

FYI, 978.5 and 987.5 are the second harmonics of the video and audio carriers respectively so we may eliminate that as a possibility.

One last thing, if the transmitter IS spitting out crap in a public service band undoubtedly it's interfering with other services in the immediate area. The chief engineer would like to know in order to avoid having the station slapped with a heavy fine after a visit by the FCC field engineering team. They do periodic inspections of broadcasting facilities so perhaps you should "enquire within" and discuss your problem with the CE who can undoubtedly help you and you could possibly save him a lot of grief. Even if it's not the transmitter's fault at least an engineer can give you much better advice than anyone here except for another engineer hiding in the bushes.

In 800 this is what it stops on. (with the antenna removed) 867.21875867.20625,857.10000. Could these be affecting it more than those others I posted?

The TRS that's having trouble is on 868.4000, 866.9500, 867.3125 and 868.1125.