Fighting Noise and QRM

(1/3) > >>

Doug Rathman:
Hello to all,

I spend most of my radio play time trying to make CW contacts on 40 meters. I'm using a Yaesu FT-840 with a 500Hz CW filter and it has IF Shift (RIT). Other than that, there are no other features to fight noise/QRM.  As a result, I don't make as many contacts as I would like because much of the slower CW is in the 7100-7150 area and QRM/noise is masking many of the signals.  

My question is whether a newer rig with features such as DSP noise reduction/filtering, auto-notch, etc would make a noticable RX improvement?   I'm considering a rig like the Kenwood TS-570 or a similar one in that price range. I would appreciate any advice.  

BTW, I use a 44ft dipole antenna/tuner combo that is respectable.  I tried a full size 40m dipole too, but it didn't make much difference on RX.

Doug KC0SOG  

Dee D. Flint:
Narrower filtering will make a vast improvement.  This can be DSP like the newer rigs or the regular filters like the older rigs.  Or you can even get an outboard DSP for your current rig.

When I am running under conditions of severe noise or interference, I typically use a filter of about 200Hz.  Typically I will scan the band using a 500Hz filter and then when I find a signal, switch to the narrower setting.

40m at nite is a challenge for sure. Nothing you can do about the "noise" there, it's coming in just like the signals are. You might try adding some front end attenuation just in case you are experiencing some rig generated intermod. Try adding 6 dB. Also, go look around the 80m novice band for some slower CW. Not so much noise there at nite.

Steve Jackson:
How/where is your antenna mounted?  Please describe height, relative location to building(s), general description of environment (city lot, rural ram, subusban neighborhood, etc.), feedline type, length, and manner of installation, location and type of tuner, and placement and type of any earth grounding arrangements you might have.

The noise you're hearing comes in from the antenna, along with the desired signal, so, the antenna is a good place to start eliminating the noise if it is possible to do so.  Trying to get rid of noise once it is picked up is hard...

Bob Lewis:
I doubt that a rig with DSP is going to make a huge difference in your ability to copy, especially if its DSP is outside of the AGC loop. A filter at the IF level (like your 500Hz filter) is able to eliminate QRM outside the passband before it hits the AGC and affects the overall receiver gain. Audio DSP (either inside the rig or as an add-on box) is not able to do that. The biggest improvement might be to go to a bit narrower IF CW filter, like 250Hz if you can get one.

An auto notch filter won't do you any good with CW because in most cases it will try to notch out the CW signal you are trying to copy. A good auto notch is quite effective at taking out carriers when copying a SSB station. A manual notch can be effective at removing an unwanted CW signal that is inside your passband but still sufficiently removed from the signal you are trying to copy.

A good DSP is capable of reducing the steady rushing sound background noise but not usually too good at taking out QRM, static crashes and other intermittant type noises.

On 40M if the desired signal is not too weak I find it quite effective to insert some attenuation or to turn down the RF gain control. This is especially true if you have a problem with strong broadcast stations overloading the receiver.


[0] Message Index

[#] Next page