Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Same Spanish station contaminating 3000-8000  (Read 1757 times)
BANDMAX
Member

Posts: 11




Ignore
« on: January 06, 2019, 01:32:30 PM »

Hello eHam.
I receive the same Spanish radio broadcast interfering with all frequencies from ~3000 to 8000 KHz. The other broadcasts, commercial AM or amateur LSB all have this station as a loud overlay, on some frequencies, like 3090, 5800, 7260, 7820 it seems to be to dominant signal, but it is on top of every other tunable broadcast.

I have a 130 ft end-fed (receive only) antenna grounded at a transformer (9:1) that connects to coax. that coax runs into my office, through another transformer (1:1), and to my receiver (PL-880). A direct connect wire (short, all in office) doesn't pick up anything (no spanish channel, but nothing else either in that range).

I re-ran a different antenna today and it is still there.

How can a single broadcast end up on so many frequencies? Because I am brand new to this, I suspect I have something wired wrong.

I live in Central Florida.
Logged
K0BT
Member

Posts: 27




Ignore
« Reply #1 on: January 06, 2019, 02:00:06 PM »

The usual culprit is a nearby AM broadcast station overloading your receiver.  Search for them on Google maps or listen across the broadcast band to see if there's a match.   What receiver are you using?
Logged
BANDMAX
Member

Posts: 11




Ignore
« Reply #2 on: January 06, 2019, 02:14:33 PM »

Thanks K0BT.

Bingo, AM 1030. If it doesn't get picked up +/- 10 KHz on AM (can pick up at 25 dB), how is it contaminating everything through the SW (is still audible up through 20m)? It is getting "re-broadcast" though local structures (or my antenna) at different frequencies? I guess no coincidence that it is crystal clear at 4120 (4X) ,3090 (3X ), 2060 (2X)...

I am sure I am not the first in this boat, if you know of a thread that addresses this, please redirect me.

Logged
K7MEM
Member

Posts: 705


WWW

Ignore
« Reply #3 on: January 06, 2019, 02:17:05 PM »

How can a single broadcast end up on so many frequencies? Because I am brand new to this, I suspect I have something wired wrong.

It only seems like it's coming in on multiple frequencies. Is the radio station close by? 130 feet of antenna will bring in a lot of signal. You might want to re-orient the antenna.

It sounds like you may have front end overload. But dual conversion radios don't usually have that issue. Another possibility is the signal is getting into your radio's IF. If it gets into the IF, no amount of tuning will get rid of it.

This is an indication that the shielding/filtering in the radio is not sufficient. How are you powering the radio? It's possible that the signal is getting in through the power supply (wall wart). For that, you can try wrapping a couple of turns of the power wire around a ferrite.

If it was me, I would open the case of the radio and try adding some internal shielding (tin foil).
Logged

Martin - K7MEM
http://www.k7mem.com
K4SAV
Member

Posts: 2628




Ignore
« Reply #4 on: January 06, 2019, 02:23:29 PM »

If you look that station up you will probably find it is very close to you.

They put a lot of gain in radios like that because people typically use very short antennas.  As a result they can be easily overloaded by a very strong station when using a long antenna.  You may be sucessful by inserting a parallel tuned circuit (one L and one C) tuned the the frequency of that station, in series with your antenna.  That will be a very simple fix if it works.  If it doesn't work you may need a more complex notch filter.

Jerry, K4SAV.
Logged
BANDMAX
Member

Posts: 11




Ignore
« Reply #5 on: January 06, 2019, 02:33:34 PM »

Thanks .

The station studio is about 8 miles away, but their tower may be closer (can't find a way to locate). The tuner, Tecsun PL-880, is battery operated, no power line. I tried connecting the ant input directly to the coax because that is grounded outside and no difference. I will check inside tuner if I can't resolve with new/reoriented antenna.

Thanks K4SAV:  I will research how to implement a tuned circuit as you suggested (and what L and C means...). Should I consider any side band filtering, or just the carrier frequency?

I look forward to these challenges.







Logged
BANDMAX
Member

Posts: 11




Ignore
« Reply #6 on: January 06, 2019, 02:46:44 PM »

Oh wow, that's embarrassing. One inductor, one capacitor... thanks again.
Logged
WB6BYU
Member

Posts: 18460




Ignore
« Reply #7 on: January 06, 2019, 05:23:33 PM »

A high pass filter on the front end of your receiver is another option.

Basically the AM BC signal is overloading the amplifier stage in your radio, probably to the point where
the amplifier shuts off entirely on the peaks of the signal.  That allows the signal to modulate all other
stations that you are receiving.  So every other signal will include the audio of the AM BC station.

The fix (other than improving the receiver) is to reduce the signal coming into the receiver.  A shunt
coil from the antenna to ground, and a series capacitor between the antenna and the receiver (or
more than one of each) will attenuate the lower frequencies more than higher ones.  Even passing
through a 1 : 1 transformer with a critically small number of turns might help.
Logged
KE6EE
Member

Posts: 2804




Ignore
« Reply #8 on: January 07, 2019, 08:30:10 AM »

Simply use the built in telescoping antenna on the radio. That's all you need and, quite evidently, all the radio can handle.

I have one of those radios.
Logged
KE6EE
Member

Posts: 2804




Ignore
« Reply #9 on: January 07, 2019, 08:55:35 AM »

An alternative to building your own preselecting LC filter is to purchase a commercial one from various makers, like MFJ. Such filters can be tuned to the frequency you want to listen to. They run in price from about $70 to about what you paid for the Tecsun radio.
Logged
VU2NAN
Member

Posts: 313


WWW

Ignore
« Reply #10 on: January 07, 2019, 10:52:17 AM »

Hi Bandmax,

The problem may be caused by an open 1N4148 diode (D17 or D18 connected back to back from the external antenna terminal to ground to prevent front end ESD damage).

With one diode open, the other would act as a multiplier for the strong 1030 kHz AM signal from the transmitter nearby.

The result would be multiples of the 1030 kHz signal causing interference across the spectrum.

Should that be the cause, replacement of the damaged diode should solve your problem.

Regards,

Nandu.

Logged

KL7CW
Member

Posts: 577




Ignore
« Reply #11 on: January 07, 2019, 11:15:51 AM »

BYU's advice is right on.  Your antenna length is a significant percentage of a wavelength of BC band stations, so naturally it is probably very good at picking up these signals.  Most inexpensive (and even expensive) radios like yours are designed to work with short antennas like 6 or 8 foot whips or possibly slightly longer wires.  One easy experiment to try is to just use a small capacitor in series with the center coax connection right at the antenna input of your radio.  In the "old days" this was often all that was required to "tame" a radio with too much RF gain at the antenna input.  Perhaps something as small as 5 or 10 pF or as large as 100 pF might be a starting point.  Often we just made a "gimmick" capacitor consisting of a few inches of twisted insulated wire to perhaps a foot at the most.  You may be able to select a value of C large enough to let through SW stations above the BC band, yet greatly attenuate BC band signals.  I have even used this method to just cut down all BC station levels to a low enough level so the receiver would not overload.  My guess is that with your long antenna, even if the 1030 kHz station was not on the air other strong stations further away could also overload your receiver.  Your idea to get your antenna away from the house and it's many noise sources is valid.  If the simple C idea does not work, then better filtering like BYU suggested will work even better.  It sounds like re orienting the antenna, or even slight shortening would probably not do the job.  A simple series LC trap (tuneable) shunted across the antenna jack at the radio could greatly reduce the 1030 kHz station, however as I stated probably the signal from other sources is still too hot for your radio with its too long antenna.  You could replace your 130 foot wire with something like a 20 or 30 foot wire located as far away from your house and other noise sources.  This shorter wire may or may not receive enough signal for your frequencies of interest, and very possibly will not reduce the 1030 kHz enough since it is so strong.  Personally, I would shorten the antenna to perhaps 25 to 50 feet, and also use a series C (or filter if necessary) at the radio.  This should greatly reduce 1030 signal and probably other signals which also overload (degrade) your radio performance.  I would try the gimmick capacitor first, then other ideas could be a small compression trimmer (perhaps with a maximum C of 50 pf ? or even a section of a scrounged variable capacitor from a junk radio.  I am sure one of these ideas will work just fine.  You could build something like a 10 or 20 db resistive attenuator, however this would not be the best solution since in might attenuate SW signals more than necessary, but something to consider anyway, or in addition to the other measures.
      Rick  KL7CW
Logged
VU2NAN
Member

Posts: 313


WWW

Ignore
« Reply #12 on: January 07, 2019, 10:22:14 PM »

Hi Bandmax,

The problem may be caused by an open 1N4148 diode (D17 or D18 connected back to back from the external antenna terminal to ground to prevent front end ESD damage).

With one diode open, the other would act as a multiplier for the strong 1030 kHz AM signal from the transmitter nearby.

The result would be multiples of the 1030 kHz signal causing interference across the spectrum.

Should that be the cause, replacement of the damaged diode should solve your problem.

Regards,

Nandu.



Hi Bandmax,

The 1N4148 diode reference numbers in the PL-880 schematic are D1 & D2 and not D17 & D18 as wrongly mentioned earlier. The error is regretted.

Regards,

Nandu.
Logged

AA5MT
Member

Posts: 253




Ignore
« Reply #13 on: January 08, 2019, 07:32:43 AM »

I had this problem with 1030 AM also.  I used a swl radio.  It had a metal case, but the station was clearly heard in the radio even without an antenna.  My ham radios had no problem on any band.  I could not use the swl radio again until I moved.  It still works fine.

Tom
Logged
KD0REQ
Member

Posts: 2388




Ignore
« Reply #14 on: January 08, 2019, 09:42:28 AM »

here's how to notch that station out

http://vk3il.net/projects/broadcast-band-filter/
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!