Why Ground a Vert?

[flintstone mop]

Hello everyone,
Nice WX here in Western Pa. Very Nice!!
My tech qustion is why ground your Coax with a ground rod at the vert, when the Earth is lossy for RF anyway?
I'm thinking of isolating the coax ground from the Ground Rod and just connect it to the radial system and using a spark gap from the vert. go to ground with that for static(?) or a near-by lightning strike(?)
Does it matter? Would there be better RF performance keeping the shield from contacting the ground?
Thanks

Fred

[WA1GFZ]

Your ground rod will be in parallel with the radials. The better the radials the less RF current into the ground rod. Might be different if your antenna is in the lightning RX mode if you float the radials.

[k4kyv]

If you have plenty of radials I wouldn't worry too much about a ground rod. I think it would be a waste of time. The  radials will conduct rf and lightning surges equally well.  Think about your total set of radials, if they were bundled into a single cable.  How would they compare to the ground rod?  The rod would be even less effective than an equivalent cross-sectional area of wire radials, due to the skin effect.

Lightning pulses are so steep that they can be considered more akin to rf than to dc or to 60~ ac.  I think I may have put down some rods at the base of my tower, but don't recall for sure and if I did, they are totally buried.  If I did, it would have been only for lightning protection until I got the radials installed. Before I installed my radial system, I cut the copper wire to length in 120 pieces, and laid all the radials out on the ground in a bundle.  As I recall, that bundle was about 3 inches in diameter.  That would have made one hell of a lightning rod.

If you have both, I don't see any problem at all with grounding the coax to radials, ground rods and all.  Each rod will just act like an additional short radial that goes into the ground vertically instead of horizontally.  The more the merrier.

[Steve - WB3HUZ]

I think it's the other way around - more radials, more current into the ground rod. After all, the radials collect the current that would otherwise be dissipated in the lossy ground.

You'll want a ground rod, unless you want a ton of current on your coax shield.

Your ground rod will be in parallel with the radials. The better the radials the less RF current into the ground rod. Might be different if your antenna is in the lightning RX mode if you float the radials.

[k4kyv]

I think it's the other way around - more radials, more current into the ground rod. After all, the radials collect the current that would otherwise be dissipated in the lossy ground.

You'll want a ground rod, unless you want a ton of current on your coax shield.

How's that? 

Think of the circuit for the rf path to be from the shielded central conductor inside the coax, through the radiating antenna, and from all the points along the length of the radiator as displacement current through the capacitance that exists from each point on the radiator to ground, and through the ground back to the common point where the shield of the coax is connected.  Without radials, most of the energy is wasted by the ohmic resistance of the path through the lossy earth back to the common point.  With a ground plane, the radials carry the current back to the common point with little loss, most of the "loss" appearing as radiation as the current passes through the radiation resistance of the antenna. 

There would be a minimum of current on the outside of the coax shield under the above conditions.  Actually, the shield of the coax acts like one additional radial, since the shield and all the radials are connected together at the common point, along with any ground rods, assuming the vertical radiator is insulated and series fed by a matching network right at the base.  If the  run of coax is at least as  long as the radials, there is no reason why the outer surface of the shield would carry any more or any less current than any one of the radials. If you have 29 radials, the coax shield would carry 1/30 of the total base current.

Rf doesn't penetrate very deeply into the soil, so a ground rod would act like one short radial whose total effective length is the maximum depth into the soil with any significant rf penetration.  Without radials, the ground rod would serve as a very lossy common point terminal connecting the lossy earth to the shield of the coax and collecting what current makes it through the soil back to the common point.  Unless the transmitter is located right at the base of the antenna, the coax shield would serve the one and only ground radial in the system, and would carry a substantial amount of the base current.

Coax cable is actually a three-conductor line as  far as rf is concerned.  You  have (1) the central shielded conductor, (2) the inner surface of the shield and (3) the outer surface of the shield.  The active feedline carrying the rf from the transmitter to the matching network at the base of the antenna is a two-conductor line, consisting only of the central conductor and the inner surface of the shield.  The currents on each should be equal but opposite in polarity, with zero current appearing at the outer surface of the shield.  If there is current at the outer surface of the shield, it is called "common mode" current; except for the current that results from the shield's function as a ground radial, ideally there should be zero current flowing through the outer surface of the coax, in other words, no common mode current.

The more complete the ground plane (the more radials and the longer each one is), the closer the antenna should come to the ideal situation as described above.

[KD6VXI]

I think it's the other way around - more radials, more current into the ground rod. After all, the radials collect the current that would otherwise be dissipated in the lossy ground.

You'll want a ground rod, unless you want a ton of current on your coax shield.

Your ground rod will be in parallel with the radials. The better the radials the less RF current into the ground rod. Might be different if your antenna is in the lightning RX mode if you float the radials.

Radials BETTER have a lower R to ground than the (as we all know) lossy ground rod.

The ground rod might have 10 ohms, relative to "ground", whereas a blanket of radials might have 3 or less (optimum sized, etc).

Or, to look at it another way, there is a LOT more capacitive coupling coming off even 50 foot of radial wire than there is on a 4-10 foot ground rod.

--Shane

[Steve - WB3HUZ]

Yep. You are correct. This is only true if the vertical is perfectly unbalanced, which it is not. Hint, there are other paths to ground.

I think it's the other way around - more radials, more current into the ground rod. After all, the radials collect the current that would otherwise be dissipated in the lossy ground.

You'll want a ground rod, unless you want a ton of current on your coax shield.

How's that? 

Think of the circuit for the rf path to be from the shielded central conductor inside the coax, through the radiating antenna, and from all the points along the length of the radiator as displacement current through the capacitance that exists from each point on the radiator to ground, and through the ground back to the common point where the shield of the coax is connected.  Without radials, most of the energy is wasted by the ohmic resistance of the path through the lossy earth back to the common point.  With a ground plane, the radials carry the current back to the common point with little loss, most of the "loss" appearing as radiation as the current passes through the radiation resistance of the antenna. 

There would be a minimum of current on the outside of the coax shield under the above conditions.  Actually, the shield of the coax acts like one additional radial, since the shield and all the radials are connected together at the common point, along with any ground rods, assuming the vertical radiator is insulated and series fed by a matching network right at the base.  If the  run of coax is at least as  long as the radials, there is no reason why the outer surface of the shield would carry any more or any less current than any one of the radials. If you have 29 radials, the coax shield would carry 1/30 of the total base current.

Rf doesn't penetrate very deeply into the soil, so a ground rod would act like one short radial whose total effective length is the maximum depth into the soil with any significant rf penetration.  Without radials, the ground rod would serve as a very lossy common point terminal connecting the lossy earth to the shield of the coax and collecting what current makes it through the soil back to the common point.  Unless the transmitter is located right at the base of the antenna, the coax shield would serve the one and only ground radial in the system, and would carry a substantial amount of the base current.

Coax cable is actually a three-conductor line as  far as rf is concerned.  You  have (1) the central shielded conductor, (2) the inner surface of the shield and (3) the outer surface of the shield.  The active feedline carrying the rf from the transmitter to the matching network at the base of the antenna is a two-conductor line, consisting only of the central conductor and the inner surface of the shield.  The currents on each should be equal but opposite in polarity, with zero current appearing at the outer surface of the shield.  If there is current at the outer surface of the shield, it is called "common mode" current; except for the current that results from the shield's function as a ground radial, ideally there should be zero current flowing through the outer surface of the coax, in other words, no common mode current.

The more complete the ground plane (the more radials and the longer each one is), the closer the antenna should come to the ideal situation as described above.

[WA1GFZ]

I assumed you have a common mode choke on the coax. If not I agree with Steve.
A common mode choke would put both grounds in parallel. There will be plenty of shield current without the ground rod and a nice lightning path into the house.

[flintstone mop]

I think I'm getting a picture of sorts. The ground rod would provide a safety ground for the house and the addition of radials to that ground make the antenna more effecient for the HF frequency.
And yes an isolation balun choke will be in the line also.

Fred

[k4kyv]

I assumed you have a common mode choke on the coax. If not I agree with Steve.
A common mode choke would put both grounds in parallel. There will be plenty of shield current without the ground rod and a nice lightning path into the house.

Assuming the length of coax is within the ballpark of that of the length of the radials, the coax should not have any more exterior shield current than any one of the radials.  If the  coax is buried or lying on top of the ground its exterior will simply act as one additional radial, and a dangerous surge on its surface will have the possibility of dissipating to the ground before it reaches the house.  If the coax is elevated above ground, and the vertical has an ineffective ground plane or nothing more than a ground rod or two, the coax could conduct a dangerous surge into the house in the event of a direct or near-direct hit.

A high voltage lightning surge could easily jump across or fry a common mode choke, depending on its construction.

[WU2D]

Now I am worried. I never grounded any of my 75M ground planes!

Mike WU2D

[N2DTS]

I just took my butternut vert out of the ground and stuck it on the roof.
Its the 80 and 40 meter version, 40 feet tall.
Over 20 years ago I put it in the ground with about 6 radials, and the trees had grown around it, it was 10 or more feet from the trees when I put it in!

It always made a good dummy load in the ground, although it would sometimes help on receive.

Instead of tossing it in the trash, I put it on a pipe on the roof, and figure it might make a good diversity antenna for the flex.

I did not get a chance to run coax to it yet, or run some radials, I hope to do that this week.
It looks a little wobbly, as its been battered by the trees I guess, I wonder how long it will stay up, after all, its about 25 years old and un supported except at the base.

I have the 160 meter coil for it but dont think I will bother, but I do wonder if it will work any better at all.
I think these type antenna's work better for DX on higher bands, just not big enough for 80 meters?

One good thing, it does seem to handle full AM power without problems..

Brett
N2DTS

[KM1H]

Dont confuse the function of the rod with the radials. A rod 8' deep in good soil will have very low DC and low AC frequency resistance. Radials on the surface are capacitively coupled to the soil at RF frequencies and only provide incidental coupling at low AC frequencies. One good lightning strike can vaporize the radials, while conducting the pulse to earth thru the rod.

If its a long coax run on the ground you can place another rod at midpoint as well as at the house entrance and thru an arrester.

Carl
KM1H

[K5UJ]

I have a dx engineering radial plate out there at the f/p of the 75 m. vertical/160 m. inv. L.  It has 101 radials made with no. 14 solid that were stapled down so they are by now just a hair below grade.  I started out with a coax isolation choke but eventually found I didn't need it because the return current was divided amongst so many paths there wasn't enough on the outside of the feedline to be a problem.  so firstly, if you use enough radials (I don't know how many that is except that 101 seems to be enough) you can toss your isolation choke. 

I have a ground rod bonded to the radial plate and feedline shield but to me its main use is to serve as a stake to hold everything in place.  I read somewhere that the current from a lightning strike radiates out from the strike point on the surface of whatever it is it hits (ground, water, whatever) so I figured the radials would facilitate that and disperse the energy.  For RF, the most effective "grounding" is supposed to be some copper of large surface area not necessarily all that deep.  The point of depth i.e. long ground rods, is to simply stay in contact with moist dirt; dry dirt is of no use.  If you have a location that is swampy or in some way the ground stays wet only a few feet down all the time then you might be better off burying wide strap in a trench at the necessary depth--6 inch wide stuff 20 feet long for example.   

With lightning, the current pulse is huge but brief and flows on the surface of the conductor.  Ground is the big resistor; you want it used for that by avoiding any potential difference between two points that can be equalized via one of your conductors that has something delicate (any piece of equipment) attached to it.   Anything out there at the base of the vertical that helps the strike current get into dirt instead of on ur feedline is what you want-- it can be radials, or a star of ground rods all around the base or both.  Another trick is to do something that makes the feedline look like a resistor to the strike surge so it prefers something else.  This is done with those gas discharge gadgets; some medium wave bc stations put a loop in the copper pipe that runs from the tuning house to the tower.  That's a single turn inductor intended to give some Xl to the strike current so it goes off somewhere else. 

You can also try dissipating the charge before it builds by placing a static discharge inductor between your vertical and ground.  High Xl at operating frequency but it drains off static electricity.  I have one here from a bc ATU.  It's about 2 feet long and 2 inches o.d.  and wound with no. 20 enamel wire.

73 rob k5uj

[WA1GFZ]

You don't need the choke if the wire is in the ground. we put 2000 amps of WF5A through #12 wire all the time. Wire can take a lot of current but I agree with carl a mid point ground can't hurt. My brother had a near ground strike that came into the house and took a bunch of stuff out. It also went along a metal sill plate and shot out the side of the house.