Why do I receive hams in the shortwave broadcast band? Help!

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radiomaniac
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Joined: Mon Jul 31, 2017 4:31 pm

Why do I receive hams in the shortwave broadcast band? Help!

Post by radiomaniac » Wed Aug 02, 2017 11:53 pm

Hi, so I bought a chinese sdr receiver about a year ago (It has a built in upconverter to be able to receive hf) and yesterday I noticed I had configured the frequency shift in the sdr# wrong.

The chinese guy told me I had to use a -40.000.000khz shift, but using that shift, frequencies appear on the wrong numbers. For example I received a signal comming from the wooferton hf transmitter and I thought I received them on 10.7mhz and They told me they only transmit on the broadcast bands, so it's impossible that I listened to them on 10mhz.

After a little bit of research I found that I should set the frequency shift to -28.800.000khz. According to this site (that guy and I have the same sdr)
I read somewhere that the frequency shift should match the internal oscillator frequency. I opened my sdr and I saw a 28.8mhz cristal oscillator, so that makes sense to me.

After doing that, I checked by selecting 990khz , which is a 25kw Am radio station located pretty close to where I live, or not that close but still powerful in 900khz another 25kw transmitter, and sure enough, the numbers match the frequencies on the spectrum, so I thought I had solved the problem. I just added the difference in khz to the frequencies I had stored in the frequency manager in order to not loose the frequencies I had stored over the time. After that, I can see that I really heard that wooferton test signal on the shortwave broadcast band. So everything looks to be ok until a few hours ago, I heard multiple hams in cw on 21.7/21.8 mhz, and hams do not transmit on the shortwave broadcast band, do they?

It's weird because I also receive an israeli number station on 6607khz and that's the frequency (I checked on a websdr and online frequency lists)

By the way, If I uncheck the frequency shift while I'm on direct sampling, nothing changes, Is that normal? I think I shouldn't be able to receive hf without the frequency shift. I'm confused and I need help.

Is there anyone here who can help me? I don't know what to do, is my chinese sdr faulty? do I have something wrong on my sdr#? ...

Thanks in advance
Last edited by radiomaniac on Thu Aug 03, 2017 1:38 pm, edited 1 time in total.

rtlsdrblog
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Re: Why do I receive hams in the shortwave broadcast band? H

Post by rtlsdrblog » Thu Aug 03, 2017 1:55 am

Are you sure that your using an upconverter and not direct sampling mode? Can you send a link to the exact model that you bought? I'm a bit convfused as you say that its using an upconverter, but you are using direct sampling.

radiomaniac
Posts: 19
Joined: Mon Jul 31, 2017 4:31 pm

Re: Why do I receive hams in the shortwave broadcast band? H

Post by radiomaniac » Thu Aug 03, 2017 12:18 pm

rtlsdrblog wrote:Are you sure that your using an upconverter and not direct sampling mode? Can you send a link to the exact model that you bought? I'm a bit convfused as you say that its using an upconverter, but you are using direct sampling.
Oh, I thought that I was using an upconverter but I use direct sampling to hear hf, you can see the model on the link in the main post, I post some pictures of the pcb here too, but I thought I was using an upconverter... I couldn't be more wrong I guess.
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radiomaniac
Posts: 19
Joined: Mon Jul 31, 2017 4:31 pm

Re: Why do I receive hams in the shortwave broadcast band? H

Post by radiomaniac » Fri Aug 04, 2017 1:12 am

Problem "solved".After calibrating the sdr, I checked I could receive shortwave broadcast stations and local commercial am stations on their frequencies. I still hear hams where they shouldn't be, but that's because of a thing called nyquist rule.I didn't read that on KA7OEI post. I quote it here just in case somebody is having the same problem:

"The other problem is that the local oscillator frequency is 28.8 MHz. What this means is that there is a Nyquist limit of 14.4 MHz - and that means that any signal above 14.4 MHz will reappear again as an "image" at a lower frequency, approaching Zero Hz as it approached 28.8 MHz. In other words, if you were tuned to 7.1 MHz in the 40 meter band, you would also hear a signal at 7.1 MHz below 28.8 MHz, or 21.7 MHz! This also means that if you were trying to listen on 21.1 MHz in the 15 meter band, you would hear a signal at its 7.7 MHz image! The situation is worse on 10 meters since the 28.8 MHz local oscillator is nearly in the middle of this band: An FM signal on 29.5 MHz would also appear on 28.1 MHz!"

So I have to remember that nyquist thing. This post can be closed.

rtlsdrblog
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Re: Why do I receive hams in the shortwave broadcast band? H

Post by rtlsdrblog » Fri Aug 04, 2017 5:19 am

Yep was going to say that your SDR does not have an upconverter. It uses direct sampling.

So you should not be setting anything in the SHIFT box.

And yes you'll see images around 14.4 MHz due to Nyquist. A filter for your band of interest, or a HPF/LPF can help there.

radiomaniac
Posts: 19
Joined: Mon Jul 31, 2017 4:31 pm

Re: Why do I receive hams in the shortwave broadcast band? H

Post by radiomaniac » Fri Aug 04, 2017 5:42 pm

rtlsdrblog wrote:Yep was going to say that your SDR does not have an upconverter. It uses direct sampling.

So you should not be setting anything in the SHIFT box.

And yes you'll see images around 14.4 MHz due to Nyquist. A filter for your band of interest, or a HPF/LPF can help there.
Hi, I just bought an upconverter with a 125mhz oscillator (the "ham it up" upconverter) so when arrives I shouldn't have any problem once I put it in a nice metal box to shield it. So I guess no more nyquist problems since the nyquist frequency would be 62.5mhz, which is well above the hf spectrum. Now I have some doubts about How am I going to put the dongle+ham it up+filters all in the same box (if possible) with some switches, so I'm going to open another thread explaining my doubts about this and see if anyone knows. Thanks

Username
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Re: Why do I receive hams in the shortwave broadcast band? H

Post by Username » Sat Aug 05, 2017 9:47 am

can you make the image smaller? we cant see anything. :evil:

radiomaniac
Posts: 19
Joined: Mon Jul 31, 2017 4:31 pm

Re: Why do I receive hams in the shortwave broadcast band? H

Post by radiomaniac » Sat Aug 05, 2017 1:49 pm

Username wrote:can you make the image smaller? we cant see anything. :evil:
I know, I'm sorry, I just uploaded the images to the forum and they appeared like that, I can't modify how the forum behaves xD But optionally if you want to see the images (although the problem on this post has been solved already) you can press "control" key while you scroll backwards with your mouse and you will see them, or you can right click the images and open them in a new tab.

Username
Posts: 602
Joined: Sun Oct 09, 2016 7:27 am

Re: Why do I receive hams in the shortwave broadcast band? H

Post by Username » Sat Aug 05, 2017 2:18 pm

so before you upload it resize it! :evil:

radiomaniac
Posts: 19
Joined: Mon Jul 31, 2017 4:31 pm

Re: Why do I receive hams in the shortwave broadcast band? H

Post by radiomaniac » Sat Aug 05, 2017 6:28 pm

Username wrote:so before you upload it resize it! :evil:
Ok relax man, I'm new in this forum, in other forums no matter the size of the pic you upload it always displays it in a small sized box you can click in to expand, I didn't know how this forum works.

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