AM broadcast stations on evry band to 30 MHz?!

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bubblegumpi
Posts: 49
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AM broadcast stations on evry band to 30 MHz?!

Post by bubblegumpi » Fri Oct 28, 2016 11:56 am

I have been little by little improving my sdr. So now it seems I just pick up AM stations on every signal below 30MHz; i.e. When a time signal beeps on say 3.300 or 5.000 the AM comes through loud and clear. This is after my last upgrade to the setup
What I did in order.
1. Make a 58' long wire inside, shaped in a square around the ceiling
2. Ran the long wire outside from 2nd story window to a tree ( nice straight line 15' abouve ground to a tree 10' abouve ground), ran coax from the long wire in the window to the SDR 20' away (goes through a closet down a hallway into my bedroom). This seemed to makes a huge improvement in gain and started to pick up a lot of signals while leaving the noise down.
3. Put a USB cable from the SDR to the USB hub with chokes on it, noise floor went down a bit.
4. Replaced the noisey Switch mode power supply that runs the USB hub (needed as my computer is a tablet/laptop deal with only one USB port) with a modified ATX power supply that sits in another room. The USB hub is powered from a really long wire that runs +5 volts down it. This powers the USB hub with clean power while the unit is far away from SDR/hub/ computer that runs off the battery when I listen to the SDR (laptop supply also creates noise and 60Hz buzz).

So now that I got rid of all the noise I get too much AM signal and its actually harder to hear the stuff on the SW bands, this got bad after I got rid of the noisey USB power supply. Its almost unusable now. WTF? I thought interference was bad?

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g1hbe
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Location: Cheshire, UK

Re: AM broadcast stations on evry band to 30 MHz?!

Post by g1hbe » Fri Oct 28, 2016 2:08 pm

I wonder if your SDR is overloading due to strong signals on MW(AM)? Try turning off the LNA and the AGC, then adjust the gain with the slider. Failing that, an outboard filter which rolls off sharply below 1.8 MHz should do it.

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Andy

bubblegumpi
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Joined: Fri Oct 14, 2016 11:41 pm

Re: AM broadcast stations on evry band to 30 MHz?!

Post by bubblegumpi » Fri Oct 28, 2016 5:12 pm

So click the EXW button and adjust which NB? too low and I get nothing then too high is over load, no sweet spot. Where is LNA? I'm going blind so this is tough.

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bubblegumpi
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Re: AM broadcast stations on evry band to 30 MHz?!

Post by bubblegumpi » Fri Oct 28, 2016 5:24 pm

Every signal that comes in below 30MHz has the same religious station right on top of it, totally swamping out the real signal. I can see the real signal but can't hear it. This sucks, its useless now I;m tempted to put the noisy power supply back. I would like to build a trap filter but I have no idea where to get the parts so a premade one might be better. If I can build it by soldering wire wrap through hole parts to a project board that would be best, but it might need traces to satisfy the RF Voodoo gods.

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g1hbe
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Location: Cheshire, UK

Re: AM broadcast stations on evry band to 30 MHz?!

Post by g1hbe » Fri Oct 28, 2016 7:36 pm

The LNA and AGC buttons can be found in the 'main' window (the one that has the PLAY button). The gain slider is also in there.

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Andy

NN4F
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Re: AM broadcast stations on evry band to 30 MHz?!

Post by NN4F » Sat Oct 29, 2016 1:43 am

Sounds like your gain is set WAY too HIGH.... Set around 50-60db with the AGC/LNA OFF and see

Paul

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Paul Jones - NN4F, Charleston SC
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PA2EM
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Joined: Fri Apr 29, 2016 7:24 pm

Re: AM broadcast stations on evry band to 30 MHz?!

Post by PA2EM » Sun Oct 30, 2016 9:41 am

Did you use a 9:1 UNUN ("BALUN") like in

http://www.ebay.com/itm/100K-50MHz-RTL- ... Sw9IpXyX05

That would be needed for any longwire, not sure that it will help with your problem, but nice to give it a try?
Last edited by PA2EM on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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bubblegumpi
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Joined: Fri Oct 14, 2016 11:41 pm

Re: AM broadcast stations on evry band to 30 MHz?!

Post by bubblegumpi » Mon Oct 31, 2016 4:16 am

PA2EM wrote:Did you use a 9:1 UNUN ("BALUN") like in

http://www.ebay.com/itm/100K-50MHz-RTL- ... Sw9IpXyX05

That would be needed for any longwire, not sure that it will help with your problem, but nice to give it a try?
Just ordered that although I just realized its from Hongkong, I hate ordering stuff from China, but we sometimes don't have a choice, thanks free market capitalism! Now I just need to play "find the F to BNC adapter" again, does it have to be 50 ohms? What is the approximate impedance of a long wire anyways? SWR seems tricky to figure out on these things, you also have to gauge how important is it. Is gain reduction just a backward way of having the gain setting? If so I have played with ALL the settings on the face of the program trying to get this to work right.

Today I added the 2' copper pipe ground rod and it seems to help but varies greatly on time of day. Maybe I can just call this AM radio station and ask them nicely to turn off their transmitter every time I play with my SDR? Nah I don't want the hassle of having to make a phone call.

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DanubeBCL
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Re: AM broadcast stations on evry band to 30 MHz?!

Post by DanubeBCL » Mon Oct 31, 2016 10:02 am

bubblegumpi wrote:Now I just need to play "find the F to BNC adapter" again, does it have to be 50 ohms
Do you mean BNC/SMA?
bubblegumpi wrote:What is the approximate impedance of a long wire anyways?
"Longwires" do no have a constant impedance over the frequency range. That depends on length, height above ground a.s.o. The impedance can be between appr. several kOhms on MW and several 100 Ohms on SW. The popular 1:9 Unun assumes that the average impedance of a longwire is 450 Ohms, which is an acceptable compromise. The Unun is recommended for longwires, but it will not help against oversteering of the receiver from strong signals. When reducing LNA gain does not help or goes at the cost of sensitivity, additional RF filters are needed.
bubblegumpi wrote:SWR seems tricky to figure out on these things
Normally for reception purposes an SWR of appr. 3 (or even worse) is acceptable. For transmitting it should be better, of course.
73s, Heinrich

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g1hbe
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Location: Cheshire, UK

Re: AM broadcast stations on evry band to 30 MHz?!

Post by g1hbe » Mon Oct 31, 2016 11:36 am

Heinrich
The early SRP's used an F-type connector. I bought my SRP early on and it was in a blue & white case with an F connector for the antenna. I actually prefer it as it is far less fiddly than the SMA type!

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Andy

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