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Author Topic: Grounding a loop antenna???  (Read 473 times)
KD7ZFH
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Posts: 79




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« on: October 03, 2005, 11:39:00 AM »

Hello to all,
  I have a 530ft loop antenna fed with ladder line from a dentron MT-3000a Tuner.I was told to ground the antenna to get rid of any static build up on the antenna.
  What is the best way to do this? My tuner is grounded.So would I cut one leg of the feed line and splice in another ground wire straight to the grounding rod?Or just go off one leg of the tuner to my grounding buss. I was told I might need some sort of resistor or diode to go inline also.
   Thanks for any help .
73
Tom
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WW5AA
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Posts: 2088




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« Reply #1 on: October 03, 2005, 12:15:46 PM »

Tom,

I think someone was confused when they gave you that advice. If your loop is a closed loop it will not build up static, an open loop would, and require grounding. The ground on the tuner is all you need. I have had my loop up for years, but someone my correct me if I'm wrong.

de Lindy
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WB2WIK
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Posts: 20543




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« Reply #2 on: October 03, 2005, 01:14:35 PM »

Depends on how your tuner's set up.  The loop may or may not be "grounded" (for static), depending upon what's inside your tuner connecting to the ladder line.

In any event, you could certainly add an open-wire lightning arrestor to your system, and you probably should if you don't have one.  That's something that would install *OUTDOORS* (never in the ham shack!), routing your ladder line through it and providing discharge protection to a solid ground that should be installed right next to the arrestor.  Some of those use gas tubes (line to ground) that flash over to become a momentary short circuit at 1000V or so, and some become a permanent short if they arc; in that case, the protector element must be replaced after some lightning events.  

WB2WIK/6
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KD7ZFH
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Posts: 79




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« Reply #3 on: October 03, 2005, 01:35:20 PM »

ok ...Thanks for the help. The question came about when I told people that I had S7 static when every body else had S3 static( white noise)it was suggested that I may have some static build up of some kind.I will put an lightning arrester in line for safe measure.Thanks again.
73
Tom
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W3JJH
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WWW

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« Reply #4 on: October 03, 2005, 01:42:55 PM »

Another method of bleeding static charge off of a balanced feedline is to connect a pair of 10-to-100 kohm wire-wound power resistors from line to ground, one on each side of the transmission line.  The resistors should have sufficiently high impedance to the RF on the line that they won't change the SWR, but they will provide a discharge path for static charge.

Note that this scheme is not an lightning arrestor.  A lightning strike will probably destroy the resistors.  Also, the power rating of the resistors should be sufficient to withstand the RF leakage current that will flow through them.
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WB6BYU
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« Reply #5 on: October 03, 2005, 01:59:49 PM »

If you have a standard balun on the output of the tuner,
that will provide a DC path to the tuner chassis from each
of the incoming wires.  As long as the tuner chassis is
grounded, that will drain off any remaining static charges.

This is more commonly a problem with single wire antennas
which go directly to the coax fitting on a "T"-match
tuner with a series capacitor in the output.


However, I doubt that your problem is caused by static
build-up on the loop.  There is a big difference between
"static build-up", and "static" noise.  They aren't the
same at all.  You want to drain static build-up from your
antenna because otherwise it can develop a lot of voltage.

But if you have a high noise level, that likely is some
other problem: a noisy lamp balast, air purifier, motor,
or other source of electrical interference.
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W5EUT
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Posts: 19




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« Reply #6 on: October 03, 2005, 02:00:27 PM »

There are lightning arrestors that will keep the antenna grounded and also give some protection to your equipment.  Even though it is a closed loop, if it is not grounded static charge can build up.  You should see the sparks come off my loop feedline when it is disconnected.

http://thewireman.com/openwirearrestors.html  for open wire arrestors.  There are several brands of coax arrestors that will work for coax.

73 Jim
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KA0GKT
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Posts: 555




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« Reply #7 on: October 03, 2005, 02:01:31 PM »

An old trick for short wave receivers back in the day was to run a 2-Meg or higher carbon composite resistor from the antenna connection to ground to drain static.  Using a pair of 2-Meg resistors, one from each side of the open wire to ground ought to do the trick.  Another answer would  be to redesign your balun.  Since, by definition, the output of a balun is balanced, the center of the output winding should be at zero potential, RF-wise...depending upon the design of the balun.  This point could be grounded to a good low impedance earth ground without affecting the balance of the feed.  A similar system is used to apply phantom power on balanced line microphones.

A heavy transmitting RF choke from each side of the feeder to ground might also work to eliminate static, if that indeed is your problem.

The wire wound idea bothers me because WW resistors can have significant X sub L and the additional inductive reactance might cause problems on certain frequencies.

73 DE KAØGKT/7

--Steve
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W8JI
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« Reply #8 on: October 03, 2005, 04:11:18 PM »

Tom,

Virtually ANY balun will provide that ground path for a loop.

Don't expect it to reduce noise, as a general rule it makes no difference at all in noise.

The only thing grounding does is prevent the antenna (and even a loop can charge up) from charging the capacitors in the tuner and the feedline until something flashes over. The result is a "tic" or "pop" in the receiver, and the transient might have enough energy to damage something.

Steady noise, as opposed to a charging and flash over producing a loud occasional pop, is not affected by a dc path to ground. That's a myth.

To confirm you have a dc path to ground just measure each antenna terminal of the balun to ground with the loop connected. You will find with a loop antenna connected both terminals of ANY balun have a reasonably low resistance path.

73 Tom
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W6HB
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Posts: 38




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« Reply #9 on: October 03, 2005, 09:18:44 PM »

...another route is to use a link coupled tuner as I do. I use old BC-610 plug in tank coils. I ground the center tap that went to the B+, tap the feeders onto the coil and tune the entire coil with a variable capacitor. The 50 ohm output from the rig goes to the internal swinging link.
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N6AJR
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Posts: 9890




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« Reply #10 on: October 03, 2005, 11:47:17 PM »

I don't think actually grounding a wire antenna is a really good way to get better signals, seems to me that grounding a wire antenna is going to kill signals.  

Now I know I have beams and verts that are electrically grounded, and rf isolated, but thats hard to do with a wire.  you don't gama or delta match a wire loop.  

I think a lighting/ spark arrester in the line is a great idea, from a polyphaser to the old  used sparkplug trick, but to actually GROUND the loop seems counterintuitive to me, I may be wrong..
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