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Signal Rectification Problem




 
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WE1X
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« on: May 11, 2018, 08:11:59 AM »

I have an issue that's been plaguing me for the past several months.

On 3.710 there is a broadcast AM signal present 90% of the time.  Sometimes the signal is modest while at other times it's rather bodacious.  Using an SDR I tracked the original signal to 1.490.  However, the image on 3.710 appears to be a mix of multiple (at least 2) broadcast stations.  With the SDR I can see the the intensity of the image change from time to time.  Adding to the problem is the fact that at times I will have various strong AM broadcast images across the 75m band. However, when this happens (a) it's as if someone is switching it on and off, (b) the duration can last from a few seconds to a minute and (c) all the images are incredibly strong.  The presence of these multiple images doesn't happen all the time, but rather infrequently. When they do happen, however, the band is not usable.

My antenna is an OCF dipole for 80m to 10m up around 50 feet fed with LMR-240. It has served me well for many years.

The shack consists of 2 solid state rigs and a collection of 9 boat anchor receivers. The image(s) appears on all.

All rigs are connect via copper braid to a common copper buss. The buss is connected to an 8' ground rod outside the shack.  The antenna via a lightning arrestor is connected to the ground rod. That ground rod is connected to the ground rod at the entrance panel. 

What I tried so far:

(a) I cleaned all connections on the shack ground rod to ensure there are good connections and no corrosion. No improvement. (I still need to do the same to the rod at the entrance panel)
(b) I removed the connection from the shack ground buss to the ground rod. No improvement.
(c) I have taken several rigs off the shack ground buss. No improvement.
(d) I have 2 SDRplay receivers I use from time to time. Knowing these can be susceptible to overload I removed them. No improvement.
(e) All internal connections in the shack (ground buss, coax, switches, etc.) are good.

What still needs to be done:

(a) Clean connections at the entrance panel ground rod.
(b) Check coax from coax entrance window panel to arrestor on the ground rod.
(c) Re-check coax to the antenna (bit difficult to do as lowering the antenna will be a chore given it's up in the trees and among branches).

Any other thoughts or suggestions would be greatly appreciated.

Thanks...Harry
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WE1X
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« Reply #1 on: May 11, 2018, 08:18:08 AM »

Forgot to mention that I can use an antenna tuner as a preselector and it helps quite a bit...for awhile. Then the image(s) return.
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PA0NVD
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Nico


« Reply #2 on: May 11, 2018, 09:08:42 AM »

Sounds like it is coming from an other source nearby re-radiating the sh..., not from your system. Do you have bigger electronic systems like TV antenna systems or centralized telephone/internet systems? Electronics that rely on signals over cables? If so try to disconnect antenna like cables and switch it off. Power a receiver with a battery and switch off the mains to find if the origin is in your house.
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kb3ouk
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The Voice of Fulton County


« Reply #3 on: May 11, 2018, 10:02:35 AM »

If you have a decent portable receiver, take a drive around the area and see if you can narrow down where the source is. How does the 3710 signal relate in strength to 1490? Its not unheard of a rusty joint on something out in the area acting as a diode and rectifying and mixing two signals. 3710-1490=2220 which is the 2nd harmonic of 1110 and 3rd harmonic of 740, check for a signal there and see which of the two stations it belongs to, if there is a harmonic there then you will probably want to try contacting the engineer at either of those two stations to let them know.
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N1BCG
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« Reply #4 on: May 11, 2018, 10:12:53 AM »

I had a similar issue with two very strong b'cast signals mixing and resolved it with a parallel L/C circuit inline with the receiver's antenna terminals. Tuning the "trap" to either of the two signals did the trick. This solution is easier when using a separate receiver, though.

OCFs can get tricky that way as they are unbalanced by design and the lead-in becomes an integral part of the antenna.
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KA2DZT
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« Reply #5 on: May 11, 2018, 01:44:32 PM »

Do the drive around with a potable receiver.  Could be a bad connection somewhere along the power lines  Most likely a corroded connection.  Can be anything where different metals are touching including fences, BX cables, heating ducks, rain gutters.  Check how your antenna lines are entering the house.  If they are near any other metal structures, the BC signal may be getting induced onto those other metal systems and poor connection there can be radiating the interference.

Also do an outside walk around your house with the portable receiver.  Lots of possibilities but I'm sure you'll find the problem.

Fred
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Steve - K4HX
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« Reply #6 on: May 11, 2018, 01:56:47 PM »

Take some potable water too.
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Carl WA1KPD
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WWW
« Reply #7 on: May 11, 2018, 06:21:35 PM »


Using Voodoo magic against  RFI spells influencing your life is a good way to get things back on track and to eliminate the images that someone has brought on you.

You will need

Large handful of dirt from directly below the feed point of your antenna (possibly several)
At least 5 chicken feathers from the nearest chicken farm
5 black peppercorns- Only from McCaffery's
5 whole mustard seeds- Must come from the Amish market
5 pieces of myrrh resin - any source
1 black candle
You need a medium sized bowl, and put the feathers, pepper, mustard and myrrh in the bottom. Then fill up the rest of the bowl with dirt. Pack it down tightly. Push the candle down in the center, using the dirt to hold it up like a candle holder.

Light it, and hold your hands up over your head. Order the negative energy to leave you alone, and to return to its source.

You will find the RFI gone and your signal will increase by 10 db in most locations
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"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
Carl WA1KPD
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« Reply #8 on: May 11, 2018, 06:35:44 PM »

Just thought of something. You have not by any chance added a Hallicrafters SX-16 to your collection recently? Even unplugged, just sitting on a shelf they can cause all sorts of problems. If you have gotten one your best bet is to send it to your sister in law in Connecticut. She knows someone who can deal with it.
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"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
KB2WIG
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« Reply #9 on: May 11, 2018, 06:55:53 PM »

Carl is trying to be helpful, but his mojo is somewhat weak. It could be that someone, maybe from you deep, deep past, is seeking revenge from your TVI.

You really need to put some salt on your doorstep.

Squeezing some lemon juice at the corners of your house may help. Organic lemons if you can get them.

If things don't get better for you, I have some special chicken bones I can send you.


klc
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What? Me worry?
PA0NVD
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Nico


« Reply #10 on: May 11, 2018, 09:06:02 PM »

Harry
When I did live in Spain, there was a Spanish MW station close and an FM station. They were on top of a mountain. The mod signals were sent via internet multiplexed and at the site de-muxed, but badly filtered. That resulted in the same kind of problems, harmonics every approx 200 kHz deep into the shortwave. 80 and 40 were not usable. The signals were not constant and seem to depend at adjustments and over modulation. The higher the frequency of the EMI the worse the modulation became. I did hear the modulation of both stations mixed  at every frequency but above 5 MHz it was that distorted that it became non-readable.
Is it possible that you have the same problem? Does the transmitter site have more than 1 transmitters?
If so, also cross modulation from one transmitter to the output of the other one gives this problem, well known if the transmitter site has transmitters in close proximity. I solved various cases by re-alocating antennas
Good luck with the search, I know how frustrating this is from the above experience
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Carl WA1KPD
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« Reply #11 on: May 11, 2018, 09:40:32 PM »

Harry,
In all seriousness, aren't there several other hams in the neighborhood? You might want to connect with one or 2 and see if they observe the same issue when you do
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"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
WE1X
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« Reply #12 on: May 11, 2018, 10:20:09 PM »

Guys, and Carl, thanks for the suggestions all of which are constructive. I was listening to 75m this morning and there were no AM broadcast signals with the exception of a very weak image on 3.710. That was the situation for about an hour then all hell broke out with 3.710 going ape and at times other images appearing (unrelated to 3.710) driving the signal level way up. BTW the image on 3.710 was over S9+40 then would quiet down the come roaring back. This went off and on for a while at which time  I packed it in and did something more constructive.

Next week I will start checking things in the house as well as the feedline and the ground rod at the entrance panel.
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