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Author Topic: QRM on 40 meters  (Read 1729 times)
KB3ZBE
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Posts: 19




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« on: January 08, 2013, 10:49:34 PM »

I'm a relatively new Ham, just received my tech license 6 months ago.  I've had alot of fun on the 10 meter band with voice and with PSK.  I've just started to listen in on some of the nets on 40 meters.  I wanted to get familiar with procedures and protocol so I could join in when I get my General license soon.

What I'm noticing is that the level of QRM on 40 meters for me is in the 7-9 range.  I can hear the
stronger signals at 9+  but the marginal signals get lost.  I've read that this is a common problem on 40 meters and that neighbors with plasma tv's are among the causes.  I'm listening on LSB
around 7.192 MH where there is a nightly QSO net.  Receiver is an ICOM 756 PRO III.

Any advice on how to lower this noise level?  I've read some of the info on this net and MFJ has a device that some recommend.  However, I wanted to hear from you "experts" what you recommend.

By the way, the antenna I use for 40 meters is a G5RV junior.

Thanks,  Mike (KB3ZBE)
« Last Edit: January 08, 2013, 10:54:58 PM by KB3ZBE » Logged
G8JNJ
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« Reply #1 on: January 09, 2013, 03:28:13 AM »

Hi Mike - welcome to the hobby,

Local noise tends to be more problematic on the LF bands.

My own experience of operating in an urban environment. Is that you need to work you way through a number of different noise sources and solutions to reduce the overall noise floor.

Before buying an noise canceller, which will only help under certain circumstances. I'd suggest doing the following:-

Try and reduce the amount of noise being generated in your own property first. A quick test is to power the radio from a battery and flip the main breaker. if nothing changes that's great. But the chances are you will find one or more sources of unwanted noise are actually being generated in your own home. So the next stage is to do some direction finding with something like a portable receiver or loop antenna, and track down the sources. Cheap wall-wart switched mode supplies and phone chargers are frequent offenders.

Suitably applied and sized ferrite rings can help a lot to tame this sort of interference source.

Some of my experiences are documented here:-

http://webzoom.freewebs.com/g8jnj/EMC%20and%20Amateur%20radio.pdf

Once you have sorted out your own home. I'd take a look at your antenna. Adding a common mode choke to the G5RV coax just below the open line section can make a big difference to the level of received noise on the LF bands. However many commercial designs and those published on the internet. Do not provide sufficient choking impedance to make a difference. Especially on the LF bands. A few turns of coiled coax as recommended by many. Will make NO DIFFERENCE !!!

Take a look at the choking impedance charts on Steve G3TXQ's website and pick the most suitable design for the frequency range you are interested in. Personally I always try to get as much RESISTIVE choking impedance on the LF bands as possible. As locally generated noise seems to be much less problematic on the HF bands.

http://www.karinya.net/g3txq/chokes/

Once you have sorted out common mode induce noise on the coax. And the G5RV is working more like a truly balance antenna. You may be able to try changing its orientation slightly so that the worst noise sources lie within one of the nulls off the end of the antenna.

Once you have worked your way through these issues. You will have a much better idea about what other noise sources are likely to be local to you, or your neighbours properties. How to trace them, and if it is possible to do anything practical to fix them once you have found them.

Only when you have investigated all of the above. Would I consider it to be worthwhile looking at noise cancellers.

I hope this helps - Good luck,

Martin - G8JNJ

www.g8jnj.webs.com



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W8JX
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« Reply #2 on: January 09, 2013, 07:28:18 AM »


Any advice on how to lower this noise level?  I've read some of the info on this net and MFJ has a device that some recommend.  However, I wanted to hear from you "experts" what you recommend.



Well start by turning pre-amps off, leaving them on just amplifies QRN and QRM too. Myself I very rarely use a preamp on 40 or 80.  Noise is kinda normal on 40 though it can vary with location, antenna type and time of year you have to learn to "work" with it at times. 
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AA4PB
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« Reply #3 on: January 09, 2013, 07:36:19 AM »

Be sure its actually locally generated noise. 40M at night receives a lot of European broadcast stations the can cause a lot of squeals and interference.
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G8JNJ
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« Reply #4 on: January 09, 2013, 12:09:49 PM »

Good points raised by you guys.

I'd assumed it was actual noise.

But as suggested, reception on 40m is frequently plagued with problems associated with strong broadcast signals on adjacent frequencies.

I'd agree - sure pre-amps are switched off, and consider switching the attenuator in.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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W4VR
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« Reply #5 on: January 09, 2013, 01:23:56 PM »

The other thing you should do is turn off your noise blanker.  It tends to cause strong signals nearby to appear on your receive frequency.
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KE3WD
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« Reply #6 on: January 09, 2013, 02:14:19 PM »

And what you are describing is really QRN, not QRM. 

QRM is interference from another station that is on or near your tuned frequency.

QRN is noise. 


73
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KB3ZBE
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« Reply #7 on: January 09, 2013, 02:48:22 PM »

Guys, thank you very much for all of the good suggestions.

I did a few simple things like shutting off the preamp and trying the
attenuator on a few different settings and this helped quite a bit!  Now I
can start trying some of the other suggestions.

What a hobby!  I've never run into a group that is so willing to share
their knowledge and help.

Mike
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WB2WIK
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« Reply #8 on: January 09, 2013, 03:00:29 PM »

Surely no reason for preamps on 40m.

If it's man-made noise (very likely) coming from within your home (fairly likely), simply getting the antenna farther away from the house often helps.  Even if it's very close to the house, simply raising it up higher can help, as any more separation between the antenna and sources of noise almost always reduces the noise received.

[This doesn't apply for atmospheric noise, though!  Atmospheric (lightning, weather-created static, etc) noise often gets even worse as you raise the antenna higher; but then, most signals will be stronger also, so this kind of balances out.]
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KF7DS
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Posts: 181




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« Reply #9 on: January 10, 2013, 09:18:01 PM »

I live in the City and having a Pixel Sat Radio magnetic loop Rx antenna has made operating on 40m and 80m possible, and a pleasure.

Don KF7DS
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