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Author Topic: Loop Antenna Ground or not?  (Read 2306 times)

Posts: 3

« on: November 24, 2012, 03:16:32 PM »

I am a relatively new ham and finally got around to building a 142′sky loop. So I haven’t hooked up my equipment yet, but the antenna is done. Coax transmission line to the radio room. I have spent countless hours reading about grounding for RF and for lightning. The latest I read was If your house has proper 3 pronged wires in the plug you DO NOT have to ground your radio and tuner etc. My house was built in 1990 and has properly grouded waal outlets, do I still need to ground my radio transceiver and tuner and VFO?? Then I read the loop antenna works without being grounded as long as you use coax and don’t run ladder line to the shack.
My idea:I was going to ground the shield of the coax before it entered the building and do a common ground strip of copper connecting all my equipment together and running a ground cable to all of this to the same ground rod (earth ground rod).

Question:If I ground the Coax shield then am i just shorting my antenna and sending all my signal to ground? As the other end of the coax goes to a feed point and is connected to one end of this loop.With the centre wire of the coax connected to the other end of the loop.

So I guess I should already know this. I have the ARRL antenna book, but between that and all the online ham pages with antenna plans and utube, I still really don’t know what to do, there is so much conflicting information. So I’m looking for a SIMPLE answer. Like YES its ok to run a ground to one side of your lead line or NO don’t do that only the gophers will hear your transmitted signal. If you are still reading thank you. And YES you still need a radio transceiver ground or NO your house wiring will take care of it. Any answers or help much appreciated.

Posts: 1377

« Reply #1 on: November 24, 2012, 03:24:48 PM »

The forum article "station ground question" gives a good reference that you can check. It is about six below your entry in this forum.

The "latest" thing you read about relying upon 3 pin plugs as an adequate ground is just plain wrong. The conductors in your walls are intended to divert the fault current from that particular circuit and the conductors are sized for that purpose only. They will not handle the thousands, tens of thousands (or more) amps of current from a genuine transient event. Depending upon that is a good way to burn your house down with your family inside of it.
« Last Edit: November 24, 2012, 03:31:30 PM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama

Posts: 13016

« Reply #2 on: November 24, 2012, 05:20:35 PM »

Before anyone answers a question about grounding, they should first ask you WHY you want
to add a ground.  There are at least 5 different reasons for grounding, and they have different
(and sometimes conflicting) requirements for an effective ground system.  If someone says
you "need a ground" and can't explain exactly why, then the probably are just repeating
old ham folklore.

1) For lightning protection you have to pay close attention to the details:  effective measures
aren't for the feint of heart.  Go to W8JI's site and read what he says - his towers get hit
several times a year with no damage to the equipment.  The electrical safety ground is
NOT sufficient.

By contrast, here in Oregon we might hear thunder once or twice a year, and strikes in this
area are very rare.  I've never had to install a lightning ground system anyplace that I have
lived in over 40 years as a ham.  But that is a decision that you have to make based on your
local conditions.

2) Electrical safety is handled by the third wire in the power plug.  Older equipment with a
2-wire cord won't be protected.  If you have an external ground connection to the coax
running into the shack, then a GFCI outlet is recommended - this protects you in case
of electrical fault in the equipment which could, for example, cause the chassis of one
piece of equipment to be hot to ground.  You wouldn't want to touch that radio while
holding the grounded coax in your other hand as you prepare to plug it in.

A classic example:  I was operating out of an apartment using the aluminum window frame
as a ground for my antenna.  I noticed a "tingling" as I went in and out of the sliding door
downstairs, and decided that grounding it would eliminate the problem.  I put a clip lead
on the end of a coil of bare wire, clipped it to the doorframe, and took a turn around a
convenient water faucet.  The wire evaporated with a bright arc!

My power supply upstairs had a two-wire cord, and had developed a short from the hot
side of the AC line to the chassis.  If the aluminum window frames had been grounded
properly it would have blown the circuit breaker, but they weren't.  Instead all the window
frames for the whole apartment block were hot to ground, and the only thing that had
prevented a lethal shock to a neighbor was because the pebbled concrete patios made
poor conductors.  With the electrical ground wire bonded to the chassis it would have
prevented the problem by blowing the circuit breaker, while a GFCI would have detected
any leakage current and tripped before anyone was electrocuted.

3) Static discharge requires a DC path to ground, but it can be a high resistance.
A 1M resistance is sufficient, even though that would be totally useless for lightning or
AC power.  I had a problem up in Alaska where the variable capacitor in my antenna
tuner would arc every few seconds due to static build-up on the wire:  I fixed it by
using a metal skewer as a ground rod.

3)  An RF ground system doesn't need to be connected to the earth at all.  For example,
a set of quarter wave radials on a ground plane vertical.  A balanced antenna such as
a dipole or loop doesn't need an RF ground because all of the RF current should be
flowing in the antenna itself.  But an end-fed vertical or wire needs somewhere for
the "other half" of the current to flow:  if you don't provide some other low-impedance
path it often flows back down the outside of the coax to the rig.  So when I use an
end-fed half wave wire antenna I also connect a quarter wave radial to the feedpoint
and string it around the room.

4)  A ground-mounted vertical requires a particularly good RF ground system (at least
for the common types that are an electrical quarter wavelength.)  That reduces the
amount of RF current flowing through the dirt (it flows along the radial wires instead)
and improves the system efficiency.  Such a system might not be effective for
lightning, as there is no need for the wires to actually be grounded - they can be
made of insulated wire and run over the top of the ground.

So there are 4 examples of different types of "ground", all of which have different
requirements, and what works well for one may be inadequate for another.  That's
why you find a lot of misinformation (to use a polite phrase) about grounding when
someone doesn't realize the differences among them.

In your case, if your coax is properly decoupled from the antenna (by using a balun
or similar) then grounding the shield should make no difference in the operation.
(Or, from a different perspective, if grounding the coax shield changes the SWR or
the radiation characteristics, it isn't properly decoupled.)  That will provide a static
drain ground.  With a loop you shouldn't need an RF ground of either type because
the antenna is complete all by itself.  Whether it is sufficient for lightning protection
depends on how you accomplish it:  if you bond the shield outdoors where the coax
reaches the ground and use an adequate surge suppressor at that point, and the
local ground rods are bonded to the house electrical system as required by code,
then that may be adequate to protect the rig in case of a direct strike (even if
the antenna is evaporated.)  But if you just run a #12 wire out the window to an
inadequate ground rod that isn't properly bonded, that could place several thousand
volts of potential difference between the antenna ground and the electrical system,
which can fry your equipment (not to mention the lightning currents coming in
via the coax and taking various paths through the house to ground.)

Does that get you started in the right direction?

Posts: 854

« Reply #3 on: November 24, 2012, 08:28:12 PM »

Depending on what type of equipment you are using (3 pin type with ground, or 2 pin type), you may or may not need to bond it all together.
This is mainly a matter of electrical safety rather than RF considerations (although it does have some effect).
Most ham radio manuals for common equipment recommends bonding it all together electrically and taking it to a good external ground.
Only you know the state of your gear, and its type, so we can only make assumptions.
For example, only you know whether you are working with an incorrectly wired transformerless 2 wire mains plug, giving a potential hot chassis.
So the tentative answer for this part of your question without knowing the details of your gear is YES - bond it together.

As for the antenna/coax - it is a roughly balanced antenna, and so feeding it with coax is going to give some common mode current on the outside of the coax.
It does not matter whether you ground the coax or not, current will still flow back towards the shack.
The amount may be trivial or major(causing hot lips on the microphone).
Take it back all the way to your antenna tuner or rig without grounding it anyway on the way.
A UNUN or even ferrite choke on the coax would be beneficial however.
My recommendation would be NO - do not ground the coax at the entry to the shack.

As regards lightning - it will take the easiest target to earth.
Traditional methods of protecting buildings is a sharp copper spike with a THICK conductor to ground.
If your antenna is a better target than anything else in the area - the lightning will come to it.
I would suggest putting a lightning arrestor spike on something higher and away from the antenna which will get preferentially hit.
If your antenna gets hit by lightning - all bets are off - millions or thousands of kilovolts at huge currents is going to toast anything.
Do not operate when a lightning storm is nearby, and disconnect any equipment when not in use - that is the best protection.

So in summary:

Tentative recommendations without knowing your exact equipment configuration:

1. Electrically bond it all together (providing they share a common earth).
2. Use an external earth if you want and connect the electrically bonded equipment to it.
3. Take the coax directly to the rig/atu.
4. A UNUN or choke is going to help reduce common mode RF currents coming back to the shack.
5. If you are concerned about lightning - install a lightning arrestor spike high and away from the antenna.
    Disconnect gear from the antenna during lightning storms.

Good luck,

73 - Rob


Posts: 3

« Reply #4 on: November 25, 2012, 10:24:18 AM »

Just wanted to say thank you to:AA4HA , WB6BYU and STAYVERTICAL-Rob for your feedback. This is excellent info. I really appreciate it. I am going to take a little more time to plan everything out and do it right,based on this feedback.

In case you are interested I will be using a  (3 pronged plug equipment) Vintage Hybrid solid state with 3 tubes.. driver and 6146B finals 1980's Kenwood TS-520 and Kenwood:AT230, external VFO-520 and external speaker SP-520. Some of which I purchased a couple of years ago on e_overpricedBay. I love the old hallicrafters boat anchors and have the 75 pounders: HT-37 and SX-101A, plus a couple of the smaller Hallicrafters tube receivers in my collection, and will be sure to take the given advice and never plug any of it in (two prong plugs) without proper grounding first, but I won't be using these until I get some HF experience. With the price of scrap metal  going up the Hallicrafters are increasing in value. (this is what I keep telling my wife).  Smiley


Posts: 12669

« Reply #5 on: November 25, 2012, 10:45:10 AM »

Most lightning damage is not cause by a bolt of lightning directly striking a typical amateur antenna. It is caused by inductive coupling (like a transformer) between the feed line and other lightning paths like a large tree, etc. Lightning current, like any other current, follows Ohm's law and divides between all the parallel paths to Earth ground with the lowest impedance taking the largest amount of current. The goal is to provide a very low impedance path outside of the house so that the majority of the current flows in that path. Note that I said inductance rather than resistance. That is because lightning current contains a good deal of high frequency energy - it is not pure DC.
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