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Author Topic: Cordless phones again!  (Read 1935 times)
N3MVF
Member

Posts: 21




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« on: March 06, 2003, 10:00:49 PM »

Never had a problem on my cordless phones until I installed a new beam.  I assume my neighbors now have the problem.  I installed a MA5B beam above the roof of my 2 story house.  Coax is LMR 240 ultra flex.  Never had cordless problems using my dipole and a few hundred watts.  Since the beam went up, I have strong cordless RF at 80 watts.  Here's the deal.  The RF is extremely strong when I use 20 meters, slight on 17 meters and none on 15-10 meters.  Also, my phone is 900 Mhz.  My neighbor has a 2Ghz cordless and hasn't said anything yet.  Why 20 meters?  Is the 2Ghz phone going to make a difference in receiving RFI?  I have the tower grounded to an earth ground too.  Would a low pass filter eliminate some of the harmonics for a cordless phone?  

Any suggestions?

73
Greg
N3MVF
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WB2WIK
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Posts: 20595




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« Reply #1 on: March 07, 2003, 12:50:29 PM »

It would be very unusual for the RFI to be occurring at the 900 MHz operating frequency of the cordless phone, and much more likely that it's occurring in the "land line" connected base unit.

Try 4-5 wraps of the telephone cord (between the base unit and the wall, very close to the base unit) through a ferrite toroid form of Mix #43, and see if that takes care of the problem.  If it does, and your neighbor complains, offer to do the same thing for him.

If the ferrite helps a bit but doesn't quite eliminate the interference, try another ferrite toroid, in addition to the first one!  Also, sometimes the interference is conducted into the phone via the power supply which powers the base unit.  Most of those are "wall cube" type supplies which plug into the AC mains and then have a long DC cable providing power to the base unit.  Wrapping that DC cable through a ferrite toroid close to the telephone base unit often helps eliminate RFI altogether.

In my case (five cordless phones, throughout the house), I had to add ferrite to both the telephone cords and the DC power cords on all the phones to completely eliminate RFI from all bands and power levels.

WB2WIK/6
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