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Author Topic: 2m Bandpass Filter  (Read 5729 times)
2E0WNT
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Posts: 2




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« on: February 19, 2009, 12:43:15 AM »

I recently acquired one of these cheap 2m handhelds from China, an FDC-150 I think. It's great for the price (£30) apart from a problem with QRM.

I use the radio with a 3 element beam from SOTA activations from hill tops. It varies from location to location, but I often get strong intermodulation effects (caused by pagers I think). I suspect the radio, being wide band 136-174MHz, has insufficient filtering to reject these strong signals.

The intermod is a real problem, as I am often unable to hear stations, or only get half of what they are saying before they are wiped out. I was wondering about building a 2m bandpass filter like the one at http://www.arrl.org/tis/info/pdf/0005054.pdf

Does this look like a good bet?
Also any ideas where I can get the semi-rigid coax (UT-141 or RG-402) in the UK?
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AA4PB
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« Reply #1 on: February 19, 2009, 05:41:02 AM »

You have the right idea. If you do a google search on "2M bandpass filters" you'll get a lot of info including some other designs and several commercial models (some at distributors in the UK). The problem with many of the home made filter designs is that you need the proper test equipment in order to tune them.
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Bob  AA4PB
Garrisonville, VA
N8EKT
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Posts: 593




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« Reply #2 on: February 19, 2009, 07:50:59 AM »

Make your own 1/4 wave stub filter.

There are many online 1/4 wave stub calculators available and all you need is a length of coax and a "T" connector.

One stub cut for 152 mhz should eliminate the paging signals.
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WB2WIK
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« Reply #3 on: February 19, 2009, 10:06:55 AM »

>RE: 2m Bandpass Filter       Reply
by N8EKT on February 19, 2009    Mail this to a friend!
Make your own 1/4 wave stub filter.

There are many online 1/4 wave stub calculators available and all you need is a length of coax and a "T" connector.

One stub cut for 152 mhz should eliminate the paging signals.<

::That idea is right, but in practice I think you'll find it's impossible to do with ordinary coax, because you can't achieve a high enough Q to get reasonable rejection with a frequency spacing of only 6 MHz at 150 MHz.

Q = fc/BW if I recall...

Q required at 146 MHz for a BW of, say, 4 MHz would be 36.5.  A 1/4-wave stub (open ended) cut for 152 MHz having such a high Q would need to be made of awfully good, extremely low resistance coax.  Maybe 7/8" hardline could do it, but I don't think one could get there with RG58/U.

PAR ELECTRONICS makes an excellent in-line 152 MHz trap filter for this application.  It has BNC connectors already on it and can handle a bit of transmitter power.  I've used theirs for years, they do an amazing job.  Not expensive at all.

WB2WIK/6

 
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N7TRZ
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« Reply #4 on: February 22, 2009, 01:55:28 PM »

Assuming the paging is at 152 and maybe 158Mhz as it is in the lower 48, the PAR units are very effective and allow out of band monitoring to continue. If you're going to stick to 2m, there are bandpass filters that will drop everything out of the 2m band making your received noise lower when hilltopping, which the PAR units will not.  If your paging is on other frequencies, PAR will tune a unit to your needs
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