AM Broadcast station coming in on shortwave

KC1LML

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May 23, 2019
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Peterborough, NH
This week I put up a new random/long wire antenna. It's 107 feet long and 18 feet high. It has a 1:1 balun at the end that comes into the house on 30 feet of RG8X coax. I only use it for receiving MW and shortwave. The antenna works fine and is definitely an improvement over my old antenna. I'm out away from any large city AM radio stations. Boston is the closest at 75 miles. There are several low powered AM stations about 20 miles away.

But I have noticed something odd. My Tecsun PL-880 is picking up Boston's WBZ 1030 AM station at 22630 MHz. And it's coming in almost as well as it does at 1030 KHz. This only happens when I connect the Tecsun to my new antenna. That frequency is totally dead using just the whip antenna, and I'm fairly certain the Tecsun didn't pickup WBZ at that frequency on my old 58 foot long wire. I restore antique radios and have several shortwave ones. None of them pickup WBZ at 22630 MHz with my new antenna, so its something peculiar to the Tecsun.

I don't see how it can have anything to do with harmonics, because 22630 is not divisible by 1030. What do you think? A bug in the Tecsun's firmware?
 

mastr

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May 7, 2005
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That is a symptom of inadequate image rejection, reviews of that receiver indicate that others have experienced similar results.
 
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RFI-EMI-GUY

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You should put an extenal preselector ahead of that receiver if you intend to use that antenna, or buy a better receiver like an ICOM R71A or R75A

 

RFI-EMI-GUY

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It is an odd spurious response with 30 KHz or 1000 KHz as an IM mix factor.

22630/1030 = 21.97087

21*1030 = 21630, 22630-21630= 1000 KHz
22*1030 = 22660, 22630-22660= 30 KHz

The mix could be happening inside the radio or externally. Are you hearing any OTHER audio in the background?

A preselector is liklely the only fix.
 

KC1LML

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Peterborough, NH
I'm not hearing any other audio in the background at 22620. It's getting close to 7 PM here. My WBZ reception always gets very weak for me in the evening, even though it's a 50K station. So, right now it's weak at 1030 KHz, but coming in quite well at 22620 with my new antenna.

I always pickup up a lot of AM stations in the evening. So, now in the evening I'm discovering I'm receiving other AM stations up in the shortwave bands. Obviously a shortcoming with this Tecsun, at least with such a long wire antenna. I mainly use the Tecsun as a piece of test equipment when I'm repairing and aligning tube radios, so using it with my outside antenna is not really necessary.

I have thought about getting that MFJ preselector in the past to help eliminate some of the RF noise around here. Does it have a good reputation in that regard or is it mainly to block closeby stations?
 

WA8ZTZ

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Feb 23, 2014
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My guess is front end overload. The portable is designed to work primarily with the whip. The longer new antenna is picking up more signal than the old antenna or the whip and the portable cannot handle it. There are a number of ways that a strong signal can produce undesired internal mixing in a receiver. For example, looking at the schematic of the 880, it appears there are back to back diodes at the antenna input (probably for static protection). These could be driven into conduction by a strong signal producing mixing products. The antenna itself could be introducing external mixing but my bet would be on something in the receiver. Try turning down the gain switch. Otherwise, something as simple as a MFJ-956 may help.
 

RFI-EMI-GUY

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Dec 22, 2013
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I'm not hearing any other audio in the background at 22620. It's getting close to 7 PM here. My WBZ reception always gets very weak for me in the evening, even though it's a 50K station. So, right now it's weak at 1030 KHz, but coming in quite well at 22620 with my new antenna.

I always pickup up a lot of AM stations in the evening. So, now in the evening I'm discovering I'm receiving other AM stations up in the shortwave bands. Obviously a shortcoming with this Tecsun, at least with such a long wire antenna. I mainly use the Tecsun as a piece of test equipment when I'm repairing and aligning tube radios, so using it with my outside antenna is not really necessary.

I have thought about getting that MFJ preselector in the past to help eliminate some of the RF noise around here. Does it have a good reputation in that regard or is it mainly to block closeby stations?
I don't have any specific advice on the performance of that preselector. There is an older Yaesu external preselector that shows up on e-bay from time to time for around same price as the MFJ new. I read reports from a lot of newbies who connect an SDR receiver to a long wire and wonder why there are odd noises all up and down the spectrum. But your hollow state receivers are probably well equipped with internal preselectors. It seems receiver design is becoming a lost art.
 

ka3jjz

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Putting that much wire on a portable in an urban setting is just asking for trouble. Portables simply can't handle the amount of RF that will receive, and yes, I agree it's likely a mixing product due to overload. There are other *passive* preselectors out there, and we have a small list of them here...see sections 5 and 6 of...


I would expect an Icom or Drake to laugh at the overload, but they too can be overloaded if there's enough RF in the air.

Mike
 

ka3jjz

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Hopefully not by clipping it on the whip. That's so dangerous - any slight static discharge might render the portable as deaf as a post. There's likely an amp connection close to or actually on where the whip connects to the circuit board...Mike
 

KC1LML

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May 23, 2019
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Peterborough, NH
The Tecsun PL-880 has a jack for connecting an external shortwave. The schematic shows a pair of 1N4148's connected to the whip but not the jack. I keep a shorting plug on my external antenna when it's not connected to a radio to keep static from building up due to wind, etc.
Capture.JPG
I confess I do clip my other ext antenna (VHF/UFH) to the whip for FM. But I also keep it shorted out when not connected to a radio. I know that's not fool-proof.
 

Boombox

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Sounds like front end overload of some type. An antenna tuner would reduce it. A simple antenna tuner reduced overload on my Realistic DX-160, a 70's era, single conversion superhet receiver.
 

jim202

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Mar 7, 2002
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I had an issue many years ago when I was working on Millstone Hill in Westford MA. They had a high powered UHF ionospheric sounder on the hill there that I had to drive right by it to get to the Westford Observatory. I had in my truck an old GE Master II on VHF. That sounder cleaned out the front end of the receiver 2 times before I figured out what was happening.

Putting a pair of the 1N4148 diodes back to back reduced the amount of voltage getting into the front end of the receiver and never had a problem after that. So those diodes will work well to protect the front end of your receiver I am surprised there are not 2 more and the caps to go with them on the switching jack that is showing on the diagram of your receiver.
 

dlwtrunked

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...My Tecsun PL-880 is picking up Boston's WBZ 1030 AM station at 22630 MHz. And it's coming in almost as well as it does at 1030 KHz. This only happens when I connect the Tecsun to my new antenna. That frequency is totally dead using just the whip antenna, and I'm fairly certain the Tecsun didn't pickup WBZ at that frequency on my old 58 foot long wire. I restore antique radios and have several shortwave ones. None of them pickup WBZ at 22630 MHz with my new antenna, so its something peculiar to the Tecsun.

I don't see how it can have anything to do with harmonics, because 22630 is not divisible by 1030. What do you think? A bug in the Tecsun's firmware?
Of course you mean 22630 kHz not "22630 MHz". Intermodulation ("intermod') could cause such a problem. If you were to get very strong signals from World Harvest Radio (on 21600) , I would suspect it were the cause as 21600+1030=22630. (Sometimes in intermod cases, one does not immetiately notice the second station.) But I suspect you do not have such a strong signal from them on 21600 if any signal at all.
 
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