Help me understand the mechanism causing BC AM to be heard on 6m...

[K8DI]

I have an iCom IC-7200 radio. My antenna is similar to a fan dipole with 80m and 40m sections. I've found it tunes well on 6m.

There is a local (8 miles away off the end of my antenna) AM BC station on 1260kHz. It is a 10kW day/1kW night station. For reference, there's also a 1kW daytimer on 1450kHz, half as far away, also off the end of the antenna.

If I tune around the lower end of 6m, like near 50.4, where one might hear AM, one does....the 1260 program. It is weak, it is broadband, it is rumbly/warbly/distorted.  By broadband I mean it isn't a harmonic you "tune in" it is present equally as one tunes across a hundred kilohertz.  The iCom has IF at 64.455 MHz, 455 MHz (not kHz), and 15.625 kHz (essentially audio, that is the IF the DSP processes so they can claim IF DSP).  Since none of those IF's are harmonically related to 1260kHz, how do you suppose the signal is getting through the radio?  Any ideas??  Note that the 1450 station is not heard, at all, even when the 1260 is dead carrier (badly run religious station, happens often).

Ed

[W1ITT]

Can you hear the problem station on any other radio connected to the same antenna?  The first step would be to isolate the problem to something in your shack, or out in the wide world, or back at the AM transmitter site.

73 de Norm W1ITT

[K8DI]

Can you hear the problem station on any other radio connected to the same antenna?  The first step would be to isolate the problem to something in your shack, or out in the wide world, or back at the AM transmitter site.

73 de Norm W1ITT

The only other radio I have that tunes 6m does not hear it, but it is a handheld Yaesu VX5R. It is not present on any other band on the iCom (except, well, on 1260kHz, where it is of course heard, with the S meter pinned). 

Ed

[KB2WIG]

A bad conection acting as a diode, somewhere. Could be your ant, bad connection on a power/ phone pole somewhere..... could be 1/4 mile from your house,could be . 8 miles is kinda far for front end overload of yer Icom.

Sounds like you have some searching  2du.
Gud luck,
KLC

[KA2DZT]

Probably overloading the front end of the Icom.  Try a series tuned wave trap on the input of the Icom.  A BC tuning cap along with a BC antenna (something from an old BC radio).  Try tuning the trap to 1260klhz to weaken the signal as shown on the S meter.  Check 6 meters for the BC image with the wave trap in line.

I agree with Kevin, could be a bad connection somewhere on your wires creating the diode affect creating a harmonic on 6 meters.  You would need a VHF field strength meter to find the bad connection.  Doesn't have to be your antenna.  I've seen rain gutters do this, bad connections on BX ac wiring system.  The strong BC signal could even enter the Icom via the AC line.

Fred

[W1ITT]

Ed..  Another useful technique is to place an attenuator in the receiver line (or the antenna connection in the case of a transceiver).  If you put in a 3 db pad and the spur goes down 3 db, it's happening outside of the receiver  But if you put a 3 db pad and the signal goes down significantly more than 3 db, you may suspect intermodulation distortion in the front end, as internal IMD is a square law function.
One trouble is that your S-meter (and mine as well) is not well calibrated.   But you can see what a given attenuator does to the reading on a different signal and take that as an indication of what your S-meter thinks 3 db is. And if the rig has switchable attenuators, they may or may not be exactly what they claim they are.  But, as a general test, it's worth a try.
And don't forget to take the attenuator back out of line so that you don't burn it up when you transmit. 
If you have a ham friend in the general area, have them look for the spur at their location(s) as well. 
And external "dirty diode" rectification problems don't always produce nice mathematical harmonics and mixing products.  Years ago, I installed an FM broadcast antenna for WHOM on Mt. Washington in NH.  Subsequently, there appeared spurs in the 2 ghz microwave STL receiver., with no respect for arithmetic.   After much travail, I determined that the ladder that went up the inside of the tower was loose enough to make crummy galvanized diodes that were excited by the strong RF field.  I went up the tower and put hose clamps and straps across everything that seemed the least bit wiggly.  Finally Marty, the transmitter engineer, came out of the building and shouted up to me "That's the one."  These problems don't always act in accord with what the textbook says.
Divide and conquer.

73 de Norm W1ITT

[Pete, WA2CWA]

quote author=K8DI link=topic=46642.msg333573#msg333573 date=1614530858]
The iCom has IF at 64.455 MHz, 455 MHz (not kHz), and 15.625 kHz (essentially audio, that is the IF the DSP processes so they can claim IF DSP).  
[/quote]

Never happen. It should read  .455MHz not 455 MHz This typo has been on Icom's IC-7200 web cover page since they first introduced the IC-7200

See below page 92, Specifications,  of the Icom IC-7200 Instruction manual

[DMOD]

I have an iCom IC-7200 radio. My antenna is similar to a fan dipole with 80m and 40m sections. I've found it tunes well on 6m.

There is a local (8 miles away off the end of my antenna) AM BC station on 1260kHz. It is a 10kW day/1kW night station. For reference, there's also a 1kW daytimer on 1450kHz, half as far away, also off the end of the antenna.

If I tune around the lower end of 6m, like near 50.4, where one might hear AM, one does....the 1260 program. It is weak, it is broadband, it is rumbly/warbly/distorted.  By broadband I mean it isn't a harmonic you "tune in" it is present equally as one tunes across a hundred kilohertz.  The iCom has IF at 64.455 MHz, 455 MHz (not kHz), and 15.625 kHz (essentially audio, that is the IF the DSP processes so they can claim IF DSP).  Since none of those IF's are harmonically related to 1260kHz, how do you suppose the signal is getting through the radio?  Any ideas??  Note that the 1450 station is not heard, at all, even when the 1260 is dead carrier (badly run religious station, happens often).

Ed

Hi Ed,

I too have an Icom-7200 as a backup DC powered radio and I live about 2 miles from a 5kW AM on 600 kHz, and have a 46' vertical with 15 buried radials.

On all of my car radios I can hear them on 1200 kHz as well.

On the Icom-7200 I can hear the Ghosts lightly on various bands. I am quite sure it is due to mixing of the internal mixer frequencies with the strong AM baseband and harmonics of the local station.

Phil - AC0OB

[K8DI]

Pete, that being a typo makes much more sense than an IF at 455MHz on an HF rig. Thanks for noticing it.

Phil, now that I think more about it, I can't definitively say I don't hear the BC station across other bands -- background noise is high here, to the point that 80 is mostly unusable (I can't even hear the big guns when the noise is S9+10 most days). Six meters is quiet, under S1. The BC bleed could be all over the place.

I recently did a search-and-destroy for local RF noise sources (invariably cheap Chinese ac adapters/chargers); less noise may be why I heard it.

I'm going to try a trap inline to see what happens (I have a handful of NOS BC radio receiver coils and variable caps, so easy to experiment with).

Ed

[WA2SQQ]

I bet if you connected a dedicated 6m antenna the problem would go away. You’ve got a lot of wire capturing a lot of RF. I live 2 miles away from a 50kw station on 770 kHz and had similar problems.