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Antenna & Servo leads, how close is TOO close?

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Old 07-18-2004, 05:54 PM
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Capt Jim
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Default Antenna & Servo leads, how close is TOO close?

I am installing servos in the tail in my Giant Aermaster. The servo leads are therefore quite long, as is the case with the aileron servos as well. All servo extension wires are soldered, and the wires are of the twisted variety to help eliminate interference. The aileron servo leads are very far away from the antenna and should not present any potential problem. The fuselage servo leads are led very neatly through a paper tube near the center of the fuse and it runs all the way back to the tail. The crux of the matter is the placement of the antenna. I like to keep it fully internal, and the plane came to me with another small diameter plastic antenna tube, which is also installed right near the center of the fuse. This will place the antenna within a couple of inches of the servo leads, at the wide part of the fuse, and as close as about 1 inch down at the tail. Is this a perfectly safe separation, or should I look for another way to mount the antenna?
Thanks for the advice.
Jim
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Old 07-18-2004, 08:15 PM
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Default RE: Antenna & Servo leads, how close is TOO close?

first, twisted servos are probably more of a myth then anything else. the power wires in the wiring are DC anyway, so twisting won't do much to eliminate noise, twisted wire helps mainly for high frequency (AC) stuff...

anyway, i'm currently running my antenna wire down the inside of the fuse with rear mounted servos on all my planes. absolutely no problems what so ever.
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Old 07-19-2004, 11:38 AM
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Default RE: Antenna & Servo leads, how close is TOO close?

It is best to keep the antenna wire as far from the other wiring as possible. The problem is that the antenna wire and the other wiring act together. The other wiring comprises the other leg of the antenna (all antennas have two sides - think of TV rabbit ears). When the antenna and the other wiring are close together the antenna starts to act like one conductor of a twinlead transmission line rather than like an antenna.

However, many do run their antennas close to other wiring with good results. Their range may be degraded but our RC radio systems give us more range than we usually need.

Just be sure to do a careful range check.
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Old 07-20-2004, 06:30 AM
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Default RE: Antenna & Servo leads, how close is TOO close?

I have found that the Deans (or other brands) of whip antennas work very well. I just went through a session of "glitch busting" with my newest model (A Waco with lots of long leads and lots of metal wires and struts) and found that the whip antenna (vertically mounted) was a big help. I doubled my glitch free range.
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Old 07-20-2004, 10:51 AM
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Default RE: Antenna & Servo leads, how close is TOO close?

Here's a bit of information, for what it's worth.

Close to the ground a vertical antenna will work better than a horizontal antenna. This is because the horizontally polarized RF signal is basically shorted out by the ground, but a vertically polarized signal is not.

Most commercial radio systems that operate close to the ground are designed to use vertical polarization for exacly this reason.

Once you're clear of the ground you may find that the Dean's antenna works about the same as the conventional horizontal wire antenna. Actually, a full length wire antenna should work slightly better than a loaded antenna, provided of course that the wire antenna is well separated from the other wiring.


Jon
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Old 07-22-2004, 02:46 PM
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Default RE: Antenna & Servo leads, how close is TOO close?

ORIGINAL: rkramer

first, twisted servos are probably more of a myth then anything else. the power wires in the wiring are DC anyway, so twisting won't do much to eliminate noise, twisted wire helps mainly for high frequency (AC) stuff...

anyway, i'm currently running my antenna wire down the inside of the fuse with rear mounted servos on all my planes. absolutely no problems what so ever.
I agree with your conclusion, but your reasoning is not correct.

First off, the two wire leads that feed power to the servos may be DC & ground, but the third lead - the singal - is most definitely AC. The purpose of twisting the servo leads is not to keep the servo leads from radiating RF noise, but to keep the leads themselves becoming antennae that pick up RF that might make their way into either the servo or the RX. All 3 wire of the servo lead are capable of doing that.

I do run my RX antenna inside the fuselage. I make an effort NOT to get my antenna wire tangled up with the servo leads, but it is more for neatless than to prevent crosstalk between the RX antenna and the servo leads. I do twist my servo leads, especially the ones with long extensions. RX today are pretty good about noise rejection, but twisting the servo leads cost me exactly zero dollars. I consider that a pretty cheap insurance against a possible source of RF interference.
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Old 07-24-2004, 01:36 PM
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Default RE: Antenna & Servo leads, how close is TOO close?

Actually, twisted wire is longer than the same measured length of straight wire. Now in theory this means your twisted wires cost .001 cent more per wire and your battery time is reduced by .0001 seconds per flight because of the increased resistance <G> But that's only if you wanna be zany about it (Note to serious geeks out there, those numbers were NOT calculated so don't try to prove em wrong)

The signal is AC, but it's at... (you guessed it) the servo refresh rate of 50hz, so you're not going to prevent any RF interference FROM a servo by twisting your wires. And unless you run your antenna perfectly parallel to a servo wire there's not likley to be any measurable crosstalk. Though with the pulsing current of a servo near the antenna, common mode DC shifts in the receivers signal might cause problems with range or general reception depending on the exact electrical setup.

Twisted wires are usefull, that's why UTP cable is so common, but those are only usefully realized if you have a differential signal, and it only usefully cancles out RFI at higher frequencies (which RC'ers do not use)

The problem is if a wire is carrying current, any wire next to it will try to develop a voltage in responce to that current because of the magnet field the orginal creates. Now if you have a servo near an antenna wire they draw a LOT of current (comparativly) something in the neighboorhood of .2 amps to 1amp or more. And when you have a servo under a lot of load in flying conditions (being buffeted or dramatic stick changes) The current going through that wire is varying extremely rapidly. This screws things up if they happen at a frequency that the receiver isn't capaable of dealing with.
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Old 07-25-2004, 05:36 AM
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Default RE: Antenna & Servo leads, how close is TOO close?

Not to be a nitpicker, but isn't the servo signal pulsating dc as opposed to ac? Not that it really matters in this case but I thought I might introduce an irrelevant issue
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Old 07-25-2004, 01:23 PM
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Default RE: Antenna & Servo leads, how close is TOO close?

Some purists might say the same thing, but there is no real difference between pulsating DC and AC. Different people have different requirements for calling it AC, some think AC has to be a sine wave, others think it has to swing between positive and negative voltages
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Old 07-25-2004, 04:11 PM
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Default RE: Antenna & Servo leads, how close is TOO close?

By definition, alternating current is periodic reversals of current flow. The waveform is irrelevant, it may be a sine wave or any other waveform if there are periodic polarity reversals.
In some cases it does make a difference. I was trying to be humorous but got a lecture I really wasn't requesting. I had 30 years in the elecrical & electronics business and am quite familiar with design considerations for ac, dc, pulsating dc, harmonic distortion, etc.
Please spare me
I will now take leave of this thread.
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Old 08-06-2004, 04:28 PM
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Default RE: Antenna & Servo leads, how close is TOO close?

JPMacG;

Some years ago, I bought an antenna designed to go completely internal of any .60 size or larger airplane. I am using it in a Global Skyraider that I built a few years ago. It is just a strip of thin copper maybe 8 or 9 inches long and you connect it by cutting the stock RX ant. and soldering this to it. The one in the Skyraider works great and I am looking for another one to use in my Hangar-9 Corsair but I can't find it. Do you remember these antennas? Can you recall who made it?
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Old 08-09-2004, 04:17 PM
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Default RE: Antenna & Servo leads, how close is TOO close?

Go to a local metal supply store and just pick up the copper ribbon yourself that's all it is. Spray it with a quick coat of poly sealant and you're good to go. Shouldn't cost you more than 50 cents for the copper ribbon.
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Old 08-10-2004, 08:48 AM
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Default RE: Antenna & Servo leads, how close is TOO close?

LYNX

Thanks for the get back.
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