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Author Topic: Can a low pass filter help on receive?  (Read 1467 times)
KJ3Q
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Posts: 4




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« on: November 05, 2012, 08:39:40 AM »

I know a low pass filter leaving the shack on the feedline helps reduce interference noise made by my HF radio to other electronics in my home and/or the neighbors. As far as i understand, it won't make a difference to the audible noise during reception of an ssb or cw signal. Is this correct? If i have a lot of ambient noise on a frequency, would a low pass filter help in any way?

I know there are other ways to achieve this, which i have done or am considering (changes to antenna config, installing cw/ssb filters in the radio, external dsp units, etc). I was just wondering if the LP filter had any benefits on "incoming" interference. Thanks.
73,
Al
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K8AXW
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« Reply #1 on: November 05, 2012, 08:49:17 AM »

Al:  No, a low-pass filter won't help you with your ambient noise problem unless it's being generated at frequencies above the filter cutoff frequency. 

A low-pass filter simply allows everything below a cutoff frequency to flow to the radio unimpeded.  By the same token, it prevents any signals, harmonic or spurious, above the cutoff frequency to reach the antenna.

In your case you need either one or more crystal filters made specifically for your radio or an external device like the Idiom Press SCAF-1 filter.  Also there are external DSP filters that work great.

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KJ3Q
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« Reply #2 on: November 06, 2012, 05:08:10 AM »

Thank you. I wasn't sure but thought that might be the case. My first step is going to be adding the optional SSB filter to the radio. I am also building a moxon to see if this will quiet the interference a bit vs the G5RV. Experimenting and figuring it out is part of the fun.
73,
Al
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K8AXW
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« Reply #3 on: November 06, 2012, 12:12:03 PM »

Personally I don't consider that aspect of setting up a ham station "fun!"  It has been my experience though that the vertical antennas pick up a great deal more noise than a horizontally polarized antenna.... like a dipole, beam and probably the Moxon.  Not sure about the Moxon though but it should be quieter.

Considering the complexity of building the Moxon, why not try a dipole first?  They're simple to make and easy to get up.  You can also try a different orientation of the dipole.  In my location most of the noise comes from the west.  I have no idea why, but it's a fact. It is quite possible you live in a noisy location.   Sad

A SSB filter would be nice and if you work CW a CW filter is almost a necessity.
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WB2WIK
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« Reply #4 on: November 07, 2012, 02:27:53 PM »

I know a low pass filter leaving the shack on the feedline helps reduce interference noise made by my HF radio to other electronics in my home and/or the neighbors.


It does?  I've never seen one that can do that. Wink

A low pass filter, if it works, should allow you to transmit with full power, including full interference, to your antenna and usually that power -- which is the desired output -- is what causes interference to other electronics.  The low pass filter only rolls off harmonics that occur above 30-32 MHz or so, and those should already be down by at least 40 dB with almost any kind of transmitter that's been commercially built in the past 40+ years.

If a low-pass filter is actually reducing interference to other electronics, I'd say there's something wrong with your transmitter. Smiley
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K8AC
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« Reply #5 on: November 07, 2012, 05:01:29 PM »

Quote
I know a low pass filter leaving the shack on the feedline helps reduce interference noise made by my HF radio to other electronics in my home and/or the neighbors.
You need to reconsider your knowledge regarding low pass filters and just what harmonics are.  Harmonics are quite different from "noise".  A harmonic is a discrete signal that is a multiple of the fundamental signal you're radiating.  The electronics you're interfering with are responding to your fundamental signal, not to harmonics.  The noise you're hearing on reception has nothing to do with harmonics and a low pass filter in line during reception would have zero effect on anything. 
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WA3SKN
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« Reply #6 on: November 09, 2012, 08:55:28 AM »

Do you have a noise problem?

A low pass filter will not help.  The best thing to do is get your receiving antenna away from the house and in the clear as much as practical.

-Mike.
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