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Author Topic: Stray RF  (Read 3425 times)
KF7LPU
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Posts: 15




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« on: July 11, 2012, 11:01:24 PM »

Over the last two weeks I have been messing with my FT-107m to address some SWR issues. I noticed tonight that when I transmitted I was getting stray RF. The touch lamps were turning on and off, the wireless internet went out, and my signal was heard through my home stereo speakers. Like I mentioned earlier, I am using an FT-107m with matching power supply, Vectronics VC-300M tuner, RG-8 running to a new OCF dipole. The dipole's balun is 40 - 50 ft off the ground, and the legs run over the house. The roof does have a tin roof, now I don't know if that is the problem, but I sure could use a second opinion as to what may be the case for the RF issues. Any help would be helpful, I am still learning how to properly setup a station.

Joseph
KF7LPU
73s
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K8AC
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Posts: 1471




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« Reply #1 on: July 12, 2012, 06:08:49 AM »

The problem here is with the devices that are being affected by the RF.  This isn't "stray RF", it's your fundamental signal being radiated by your antenna so close to the house and these devices  (probably exacerbated by the OCF dipole, improperly choked, but that's another subject).  Think about it - none of the devices you mentioned were designed to receive radio signals and so they should no be reacting to an RF field, but they are.  One approach would be to get the antenna away from the house wiring and the devices.  Another approach would be to address each of the devices separately and solve the problem with that device before moving on to the next.  Touch lamps are a well known problem - you may never be able to solve that one.  But, chokes on the AC line to the amp may  help.  For the wireless internet router, try chokes on the power supply lead near the router and on any other cables connected to it.  For the speakers, you should first determine how the RF causing the problem is getting into the amplifier or receiver being used.  Lot's of possibilities there ranging from poorly shielded cables to disconnected ground shields, or the RF could be entering through the speaker leads themselves.  Chokes on the offending leads may solve the problem. 

There used to be a good booklet from the FCC that did a good job of addressing how to deal with RFI problems - see if you can find that.  I believe the ARRL distributed it at one time.

73, K8AC
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K5LXP
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« Reply #2 on: July 12, 2012, 06:41:39 AM »

There's a systematic approach to dealing with any kind of fundamental overload like this.  First step is to characterize the frequencies and power levels that affect all the susceptible devices.  Then make a change - clamp on ferrites, bypassing, dress connecting cables, add common mode chokes, what have you, then re-sweep to determine if the change had a positive effect.  Given the number of problem devices you have, you have your work cut out for you.

While having your antenna right over your house isn't ideal it's also what most hams typically have, and most hams operate this way without any RFI problems at all.  So it's possible, don't give up.


Mark K5LXP
Albuquerque, NM
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W5FYI
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« Reply #3 on: July 12, 2012, 07:24:55 AM »

Explain your balun--if it's a choke balun (unun), that's good; but if its a voltage or current balun, then it shouldn't be used with an off-center fed antenna. A choke unun will keep (most of) the common mode currents off the coax so that only the antenna radiates.

The other suggestions are valid; you probably could use chokes on the WiFi Cat5, speaker, and touch lamp cords to minimize stray RF. (Touch lamps are especially sensitive to oddball RF and static problems). Radio Shack and other outlets sell clamp-on chokes (RS273-104 is their best, at around $10 each, and is what I'd recommend for the speaker cables; but 273-105, 273-069 and 273-067 are possible substitutes). GL in solving the problem.
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K4SAV
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« Reply #4 on: July 12, 2012, 07:55:07 AM »

Your first problem is that you are using an off-center-fed dipole.  RFI is very common with those antennas because they have a large amount of common mode current on the feedline.  If you want to stay with that antenna, you need to add some more chokes on the feedline.  A commercial choke will work or you can make your own.  A large toroid with several turns of coax thru it will be best.  You may need more than one. 

The 4 to 1 balun at the antenna feedpoint should be a current mode balun, NOT an un-un and NOT a voltage mode balun.   Those do nothing for common mode currents.  Do you have a commercial OCF?  Which one? There is one manufacturer of OCFs that uses a voltage mode balun and apparently allows feedline currents on purpose.  That would be the worst one to use.  I would recommend replacing the balun on that antenna.

Once you get the antenna fixed, then you can tackle the things that still have a problem,  Those will probably require some ferrite chokes too.  If you can use toroids that will work best.  Multiple turns thru a core is much better than a single turn thru a clamp-on.

Legs of the antenna running over the house can put a lot of RF into the house by direct radiation from the antenna.  If you can't move the antenna, and choking the feedline doesn't help, then you have to address the problem at the device that is having the problem.

Jerry, K4SAV
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KF7LPU
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Posts: 15




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« Reply #5 on: July 12, 2012, 10:13:02 AM »

(probably exacerbated by the OCF dipole, improperly choked, but that's another subject).

73, K8AC

Can you tell me a little more about this topic.

Joseph
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K1CJS
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Posts: 6034




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« Reply #6 on: July 12, 2012, 10:16:03 AM »

Although you've done OK here, this really should be in the RFI forum to get the attention it may deserve.  73!
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KF7LPU
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Posts: 15




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« Reply #7 on: July 12, 2012, 10:18:47 AM »

Explain your balun--if it's a choke balun (unun), that's good; but if its a voltage or current balun, then it shouldn't be used with an off-center fed antenna. A choke unun will keep (most of) the common mode currents off the coax so that only the antenna radiates.

The other suggestions are valid; you probably could use chokes on the WiFi Cat5, speaker, and touch lamp cords to minimize stray RF. (Touch lamps are especially sensitive to oddball RF and static problems). Radio Shack and other outlets sell clamp-on chokes (RS273-104 is their best, at around $10 each, and is what I'd recommend for the speaker cables; but 273-105, 273-069 and 273-067 are possible substitutes). GL in solving the problem.

I have a current balun at the top where the wires legs hook in, but a choke balun "ugly balun" near the shack.

Joseph
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KF7LPU
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Posts: 15




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« Reply #8 on: July 12, 2012, 10:23:06 AM »



The 4 to 1 balun at the antenna feedpoint should be a current mode balun, NOT an un-un and NOT a voltage mode balun.   Those do nothing for common mode currents.  Do you have a commercial OCF?  Which one? There is one manufacturer of OCFs that uses a voltage mode balun and apparently allows feedline currents on purpose.  That would be the worst one to use.  I would recommend replacing the balun on that antenna.


Jerry, K4SAV


I built my own OCP dipole using the design from the following website.

http://www.packetradio.com/catalog/index.php?main_page=index&cPath=49&zenid=fdjs0fq1hqrki5vseqr29mr114


Joseph, KF7LPU
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K4SAV
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Posts: 1840




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« Reply #9 on: July 12, 2012, 01:20:06 PM »

I built my own OCP dipole using the design from the following website.
http://www.packetradio.com/catalog/index.php?main_page=index&cPath=49&zenid=fdjs0fq1hqrki5vseqr29mr114

Great!  I applaud you for building your own.  It's too bad that most newer hams don't do that.

I have a current balun at the top where the wires legs hook in, but a choke balun "ugly balun" near the shack.

The current balun is the correct type.  The "ugly" balun (choke) can sometimes cause problems.  It could help or it might make common mode currents worse, depending on several parameters.  The "ugly' balun looks essentially like an inductor.  That can possibly resonate with the feedline and antenna and increase the common mode currents, or it may not, it's a crap shoot.  Try removing that and see what happens.  It would be best to replace that with a ferrite choke.  Use a ferrite with a high loss tangent (high resistance).  Those look primarily like a resistor to common mode currents.  That won't resonate with anything.

Jerry, K4SAV
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AC7ZN
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Posts: 43




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« Reply #10 on: July 12, 2012, 02:29:10 PM »

Hello Joseph,

Just a couple of ideas to try:

My experience with my own OCF dipole indicated that the choke at the antenna was not enough at some frequencies.  Also, your metal roof may be reintroducing some current on the coax shield (my aluminum siding did).  You may have to add another choke further down the line ('down' being toward the transmitter)...try 1/4  wavelength at the frequency that seems worst, or just after the coax leaves your roof on the way to the transmitter.  I used seven snap-on beads (about 1" d.) and that really helped on the 40 m band.

So many devices getting RFI may indicate you are somehow inducing a lot of RF current into your AC house wiring.  Check to make sure  you are not running your coax near house wiring anywhere.  This would include making sure it is not induced through your shack AC wiring, but I think you mentioned you had a choke at shack entry.  Some snap-on ferrites on, for example, your touch-lamp AC cord will tell you if the AC wiring is a factor in your RFI.  The cords are usually small enough that your can make the choke multiturn which is more effective.

The auction sites are usually good sources for snap-on ferrites.  At HF, usually multiple ferrites (or turns) are necessary.

73 and GL,

Glenn AC7ZN




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KF7LPU
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Posts: 15




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« Reply #11 on: July 13, 2012, 01:20:58 PM »

Thanks to everyone who replied to my post, thank you for everything. I think we can close this post now.  Smiley

Blessings
Joseph, KF7LPU
73s
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KD2CJJ
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Posts: 369




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« Reply #12 on: July 18, 2012, 10:30:11 AM »

I am following this post - Which answer fixed the issue if at all?  It would be good for someone like me who is a newbie general..

Thanks
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73

Mike
KD2CJJ
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