| RF Filters |
|---|
How do I elminate those $#$@#$! paging sounds on my scanner!
You need a filter!
But, fitlers cost big bucks! WRONG!
Commercial Two-Way radio filters are very expensive & are designed for use with transmitters.
Scanners do not transmit, so you can use filters that are not designed to handle RF.
So where do I get these filters? Easy, we use Cable Television (CATV) filters.
First, we need to find out what type of filter we need.
Types of Filters:
- Notch
- Low Pass
- High Pass
- Band Pass
- Band Stop
Notch Filers - Eliminate or notch out a specific frequency.
Low Pass Filters - Pass all frequencies lower than a certain cut off frequency (Fc).
High Pass Filters - Pass all frequencies higher than a certain cut off frequency (Fc).
Band Pass Filters - Are combinations of a high pass & a low pass filter which pass all frequencies
between two frequencies.
Band Stop Filters - Are a combination of high & low pass filters to stop a range of frequencies while passing all others.
Filter Selection
The best filter for scanner users will be the notch filter, as it allows specific frequencies to be removed. The wide frequency coverage of scanners makes use of low pass & band pass impractical, although if your setting up a scanner to receive a particular band as all your local monitoring occurs in that band they make a good choice. I use a scanner with a filter and low noise amp to listen to a low powered UHF tactical frequency. High pass filters may offer relief from nearby HF interference for frequencies below 30MHz.Most scanner interefence occurs in the 152/157Mhz & 454MHz paging bands, so this page concentrates on this type of interference, but the techniques can be applied to any signal causing interference.
Steps for finding the frequency causing the intermod.
Requirements:
2 Scanners, one with a search function, and your primary scanner.
- Remove any antennas on the search scanner!
- Set the squelch to its maximum setting.
- Using a scanner with a search function, set up a search from 152.000 to 153.000Mhz.
- Start the search.
- Note all frequencies with paging data.
- Using a second scanner compare which paging channels are active when the interference occurs. When the paging chanel stops transmitting the intermod should stop, you have found the frequency for the notch filter.
Repeat the above steps for the ranges of 157.000 to159.000MHz & 454.000 to 455.000Mhz to eliminate other sources of intermod.
You may need find more than one carrier causes intermod at your site. I have a notch for both 152.240 & 157.740 to eliminate the intermod causing interference on local public safety frequencies.
Filter Use
Using filters has effects on your listening, other than eliminating offending signals.- Loss to nearby frequencies - filters are not pinpoint accurate at eliminating frequencies, there will be losses on either side of the notch filter. This will effect your listening, if these are simplex channels!
- Thru loss - a loss to all frequencies passing thru the filter, normally about 1-2dB. Helical filters have thru losses of 6-7db or more!
These losses are often over come using amplifiers.
If you use an amplifier now, or plan to use one to overcome loses from filter use take note of the following:
- Use all filters before any amp.
- Use a low noise amp, like the LNG series from Hamtronics.
Sources of Filters:
Microwave Filter Company
6743 Kinne St.
East Syracuse NY 13057
800-448-1666
315-438-4766 Fax
Email: mfc@ras.com
Microwave Filter (MFC) offers all types of filters for both the 2-Way & Microwave & the
CATV sectors, with the CATV ones of interest to scanner users.
To eliminate VHF paging signals use their 5KV style filters. The 5KV filters provide
a 70dB notch of the frequency being eliminated. 5KV's are available for a range of 54 - 300MHz.
Cost is $39.00
To eliminate UHF paging signals a Model #5891, available for 450 - 896MHz. Provides a
40dB notch, cost $50.00.
Lots of other filters available, get a copy of their C/87, Fastrap/90, & RF/88 catalogs for
complete info & specifications.
Par Electronics
6869 Bayshore Dr.
Lantana FL 33462
407-586-8278
407-582-1234 Fax
Email: parf@aol.com
Offers 3 filters of interest to scanner users for VHF & UHF paging singals. Cost is $68.
These filters can also be used with portables upto 5W output. The VHFDN153HT models is unqiue in that is allows the user to select which side of the notch is the most effected. Either the 151 or 153 side may be selected by a switch. This is important if you hvae low power/simplex channels on one side you wish to monitor. This feature is so unique its patented!
Hamtronics
65 Moul Rd.
Hilton NY 14468-9535
716-392-9430
716-392-9420 Fax
Email: jv@hamtronics.com
Offers Helical Resonator Band pass filters for VHF 136-174MHz & UHF 420-470MHz.
These are good for setting up dedicated band scanners. Cost $70-$119 depending on band.
Also offers filter combined with a low noise amp, cost $90-$140 depending on band.
Ritron Inc.
505 West Carmel Drive
P.O. Box 1998
Carmel, IN 46032
1-800-USA-1-USA
317- 846-1201
317-846-4978 Fax
Email: ritron@ritron.com
Offers a UHF Band Pass Filter (RF-450), cost $50.00. Good to, again, set up a dedicated band scanner.
Eagle Wichita
PO BOX 9446
Wichita KS 67266
316-942-5100
316-942-5190 FAX
Offers a line of notch, TNF200, filters for 1.5 to 850MHz, can handle upto 10W of power if needed. Cost $99.00 to $170.00
|
|