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Author Topic: Strange interference on VX-150  (Read 2488 times)
KE2KB
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Posts: 219




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« on: February 21, 2013, 12:06:49 PM »

Hi;
I have my VX-150 connected to a Ringo Ranger ARX-2B (without the radials for now).
I am experiencing interference from an AM broadcast station under specific conditions.
When the radio is just lying on a surface (pretty much any surface) and I am not touching it, the interference is not present.
If I move my hand towards the front of the radio, from when my hand is about 1/4" from the front of the radio (speaker area to top of keypad) and when my hand is actually in contact with the radio, I can hear a very distorted AM broadcast station. The volume of the station increases or decreases as I move my hand around slightly. It also varies slightly over the 2m band (from 144 to 161Mhz). The interference seems to be the strongest in the "extended" receive coverage area of the 2m band (from about 150Mhz to 162Mhz) but I have found it between 144 and 148Mhz as well.

I have identified the AM broadcast station that I am hearing by placing an AM radio next to the HT while hearing the interference and tuning through the AM band.

The antenna system is as follows:
Ringo Ranger ARX-2B (without radials) about 30ft high, mounted to side of house at peak.
ICE #302 lightning arrestor mounted on the mast, about 6ft below the base of the antenna.
ICE and mast grounded through approx 40ft #4 wire to electrical service ground electrode.
50ft LMR400 coax running into the attic, then down to the radio on the 2nd floor of the house.
The house is wood frame.

Thinking that my grounding system may be faulty, I moved the ICE arrestor down to the electrical service grounding electrode and bonded it there with only a few inches of wire. I connected the LMR400 from the antenna (which is now running down the outside of the house) to the ICE arrestor. I then connected the radio directly to the other side of the ICE. I am still hearing the AM station under the same conditions as before.

Next, I disconnected the antenna system from the electrical ground, so there was no ground at all. The interference got worse.
This leads me to think that I may not have a good enough ground to earth.
Perhaps adding a second grounding electrode to the electrical system, and bonding the coax shield (through the ICE arrestor) to this second electrode located directly below the antenna (about 20ft closer than the existing one) would help, but I am hesitant to make any changes to the electrical system that will require not only a permit from the township, but for installing the ground rod, I believe I also need to notify the untlities so they can mark off any underground lines. Of course I am sure there are no underground lines, but the law says you've got to get lines marked before digging or putting anything that deep into the ground.

Any ideas?

Thanks

KE2KB
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KA4POL
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Posts: 2125




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« Reply #1 on: February 21, 2013, 09:53:42 PM »

I think you are looking in the right direction as far as grounding is concerned. How is your electrical wiring? Do you have a safety ground or is it just two wires? The latter would be a possible reason.
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K1CJS
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Posts: 6061




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« Reply #2 on: February 22, 2013, 05:13:08 AM »

One thing you didn't specify is how close the AM station is to your house.  If very close, what you may be experiencing is simple front end overloading of the HT.  Even though the Ringo Ranger antenna doesn't have any appreciable gain, it is many times superior to the rubber ducky antenna that your HT is designed to work with, and the receiver may simply be getting too much signal from the AM station which comes through your FM HT as distorted reception. 

The fact that it is only present when you move your hand closer to the HT would seem to confirm that--your body is providing more of an antenna to the HT case--the other half of the antenna the HT is made to work with through capacitance coupling.
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KE2KB
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Posts: 219




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« Reply #3 on: February 22, 2013, 05:47:21 AM »

Hi;
Thanks for your replies;
The electrical system in the house is OK. 3-wire. But that is really irrelevant with the HT, since it is not connected to the electrical system, except at the grounding electrode outside the house.
The offending AM broadcast station transmitter is located about 2 miles from me with a power of 5,000 watts.
Knowing the frequency of the AM station, 1330Khz, and the IF of my radio, I find that it is just about 3x the 450Khz of the IF.

Note that the interference is still present if I remove the antenna connection from the HT, and then just touch the shield of the connector to the HT antenna jack.
It appears that the VX-150 may be somewhat lacking in RF shielding, but of course it had to pass FCC testing. I was thinking that perhaps I could improve the shielding in the radio in some way, but I don't want to start making that type of mod, as it would probably make things worse, not better.

Since the AM broadcast signal appears to be traveling along the shield (and probably the center conductor as well) of the coax (indicated by the test I mentioned earlier in this post), adding a filter inline is not going to help.
I believe what I need is a better RF ground of the antenna, mast, and coax.
Optimally, I should probably run heavy copper braid from the mast to a ground electrode located directly below the antenna. But with the price of copper, such a braid would cost me hundreds. I could run the galvanized EMT tubing that is the mast all the way down to the ground and then bond it to the ground electrode. That would seemingly accomplish the same thing as running braid, at a fraction of the cost. It might look a bit unsightly though, having the metal pipe running up the side of the house, and I would need to anchor it to the ground so it wouldn't just slip out and fall on someone.

I am trying to find some good reference material on antenna grounding, but so far haven't found much. I have an old (2002) copy of the Amateur's Handbook, but it's on pdf and a very old version, so it's kind of awkward to work with. I may purchase the 2013 edition soon. In the meantime, I keep Googling and reading. When the weather warms up, I'll be taking the antenna down again to re-install the radials and perhaps improve the whole installation.

KE2KB
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KE2KB
Member

Posts: 219




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« Reply #4 on: February 22, 2013, 06:33:32 AM »

Hey guys;
Looks like I left out a very useful piece of information here, but I only did this test a few minutes ago:
I said that I was hearing the AM radio interference when only the shield of the antenna were making contact with the shield of the radio's connector.
Now I learn that if I touch a wire that is connected to a nearby electrical outlet ground to the shield of the radio's connector, then place my hand in the same position on or near the radio, I get the same interference.
So the problem is not that the signal is coming down my antenna transmission line; it is using that line as a path to ground. I am the active element of the antenna.
So it doesn't look as though there is anything I can do about this, except get a better radio that was designed for use with an external antenna. It appears that an HT is designed to be held in the hand of its operator, with no external connections. I guess I should have known this.
So now, I need to look into a VHF transceiver for use as a fixed station, and also that will double as my scanner for railroad monitoring.

KE2KB
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WB5ITT
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Posts: 100




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« Reply #5 on: February 22, 2013, 03:05:56 PM »

HTs have poor selectivity due to the size/Q of the front end coils and the case is usually plastic (Lexan) so RF can easily penetrate that and bypass the antenna and front end. However, as I mentioned in another post, the railroads are going digital..any ham rig will not be able to pick them up once they do. Look for a digital scanner that decodes Kenwood/ICOMs digital mode (NXDN/ICAS, NOT P25) and also does trunking (LTR format as the railroads intend to go to trunking on various channels later)

Chris
WB5ITT
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AA4PB
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Posts: 13032




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« Reply #6 on: February 22, 2013, 03:23:45 PM »

I'll bet that the radio station is being picked up by the power lines and winds up on the grounding connections in the house wiring. The electrical system ground rod isn't very effective at 1330KHz. Since your coax shield is connected to that same ground, the shield pipes it to the HT.

Try disconnecting the shield from the electical system ground and see if it goes away. It's against code to leave it that way but it will confirm the problem for you and is an indication that you probably need to add some additional ground rods to the electrical system.

Inside the HT it might be getting into the IF or it could even just get coupled into the audio circuits.
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KE2KB
Member

Posts: 219




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« Reply #7 on: February 22, 2013, 03:44:50 PM »

I'll bet that the radio station is being picked up by the power lines and winds up on the grounding connections in the house wiring. The electrical system ground rod isn't very effective at 1330KHz. Since your coax shield is connected to that same ground, the shield pipes it to the HT.

Try disconnecting the shield from the electical system ground and see if it goes away. It's against code to leave it that way but it will confirm the problem for you and is an indication that you probably need to add some additional ground rods to the electrical system.

Inside the HT it might be getting into the IF or it could even just get coupled into the audio circuits.

I did temporarily remove the bond between the antenna system and the electrical ground electrode and the interference became worse.
I also tried touching just a wire from an electric outlet ground to the shield of the HT's SMA connector, and it produced the same result as when the coax is connected (or just the coax shield is touching the SMA shield) to the HT.

I am planning to install a 2nd ground electrode directly beneath the antenna anyway, just so that the route of the grounding conductor is straight. Then I will bond the 2nd GE to the main GE where the electrical service is tied, and where my antenna ground wire is currently connected. I'm not sure whether I will gain anything by relocating the ICE arrestor to the new ground electrode, or if I should just leave it up on the mast. The coax enters the house just a foot from the mast, so that location should be OK, except for the longer wire between it and the ground electrode.

KE2KB
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K1CJS
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Posts: 6061




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« Reply #8 on: February 23, 2013, 08:11:02 AM »

Being that close to the AM station transmitter is going to be problematic, but you can try to install a better ground.  If the soil is always dry around your home, it could be that your grounding system is not deep enough to be really effective.  If you can manage it and want to try, drive a longer ground rod (there are some ground rods made that connect together) to see if you can get a better ground system.

Your plan to get a better transceiver for your home and just use the HT as a portable radio would be best--and simplest--to do.
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KE2KB
Member

Posts: 219




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« Reply #9 on: February 24, 2013, 07:39:06 AM »

Being that close to the AM station transmitter is going to be problematic, but you can try to install a better ground.  If the soil is always dry around your home, it could be that your grounding system is not deep enough to be really effective.  If you can manage it and want to try, drive a longer ground rod (there are some ground rods made that connect together) to see if you can get a better ground system.

Your plan to get a better transceiver for your home and just use the HT as a portable radio would be best--and simplest--to do.
I'll see what the 2nd ground electrode will do. If nothing else, it helps to meet NEC on having the most direct route for the ground conductor from antenna mast to ground electrode. Currently I have it routed to the foundation of the house, then it turns sharply (not good) and runs another 20ft to the GE at the electrical service.
I will also be moving the ICE arrestor to the location of the GE, and thus a much lower impedance for spikes caused by lightning.
Only bad thing about this is that I will have a bit more LMR400 to go back up to my "shack" from the point of entrance which will now be at the basement. It really doesn't amount to much though, and I think the added safety and compliance with NEC is worth the trouble.

KE2KB
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KD5FPO
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Posts: 28




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« Reply #10 on: March 03, 2013, 06:18:47 PM »

Don't use the house electrical ground to ground your HT, if the AM station signal IS coming in through the house wiring then your putting the HT in the same Circuit as the offending noise, as well as the blender, the stereo, the vacuum cleaner, etc., it is not an earth ground, don't confuse the two. With the HT connected to your outdoor antenna touch a piece of wire from an EARTH ground, (copper cold water pipe maybe) to the ground side (shield) of the antenna connector on the HT, see if your noise disappears, I'll guess that it will.

The ARX-2B has plenty of gain, almost 8db, if you live close to a hospital you're likely to hear IMD from medical pagers. It tends to be a noisy "city" antenna best suited to rural locations.

Also get those antenna "radials" reconnected, they arent radials really, they're decoupling stubs, (big difference) without them your radial ring is vibrating around against the mast causing distortion as well as causing wild swings in VSWR, which can add to your IMD problem. Good luck & 73.
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