Should the elevated radials on a ground plane antenna be DC grounded to earth?

[misfit138]

Just as the title asks.
30 years ago as a kid I had a few Shakespeare "Big sticks" as well as an A-99 with a ground plane kit for CB tranceiving.
One of the HAMs in my area strongly admonished me on the importance of grounding my antenna.
I took his advice, and as I had schooling in the electrical trade, I used some 2/0 cable available to me and I believe about 6 copper clad 8 foot rods.
I connected the antenna base end of the cable to one of the U bolts.
This, I was told, was a good start, but if I wanted to achieve perfection, I would need 15 to 120 ground radials (not elevated radials) at the base of the mast.
Needless to say, I did not go that far.
So to come back around to my question..
Since the elevated radials on a ground plane antenna are cut to, say 1/4 wave, and connect to the coax shield, is there not also conductivity between them, the mounting base, the U bolts, and conductive mast?
If so, then why must they be 1/4 wave? Does not the extra length and capacitance of the mast, ground cable and earth disrupt their length, and therefore, the SWR?
In fact, without even connecting a grounded conductor to the elevated radials, does not the very existence of the conductive mast (which may very well be coming out of the ground) disrupt the performance of these antenna? If not, why not?
I'd appreciate the insight of you guys, if any.

Sent from my phone.

 

[prcguy]

I'm not an expert on this but generally speaking, some amount of tuned 1/4 wavelength ground radials will decouple the vertical antenna from its mast sufficiently where no RF currents will be flowing on the mast or coax and at that point the elevated antenna is its own entity and further grounding to earth with ground rods or radials below will have little or no effect on its performance.

There are some antennas like the Kreco "stacked co-axial", which is a coaxial dipole with several tuned but grounded 1/4 wavelength skirts spaced every 1/2 wavelength below the main top dipole with a final set of tuned ground radials at the bottom. These antennas rely on the top coaxial dipole not being very well decoupled from the mast and they are making use of that stray RF current on the mast by lighting up other grounded elements as "sympathetic radiators" in a colinear array.

Some other antennas that make use of poor decoupling on the main antenna and sympathetic radiators along the mast are the old AEA Isopole which used a second coaxial cone at a critical tuned location and Cushcraft Ringo Ranger II which uses a tuned length of grounded coax and ground radials below the main antenna to sort of add a grounded but colinear coax element to the original and poorly decoupled antenna.

Many CB antennas and especially the A99 and other 1/2 wave, 5/8 wave and similar antennas with no ground radials have very poor decoupling of the antenna to mast and do light up the mast and coax with RF currents. If you vary the length of mast and/or coax on these antennas you will change the SWR, radiation pattern and performance some. Its just part of that design and properly adding radials near the feed point or 1/4 wavelength down from the feedpoint should improve performance and better decouple the RF off the mast and coax. I suspect you can add some noticeable gain to these antennas by taking the Kreco approach with a series of sympathetic radiators along the mast below the antenna, then decouple with some ground radials.

The reason for 1/4 wavelength ground radials is complicated and is related to the RF making a 1/2 wavelength path from the feedpoint to the end of the 1/4 wave radial and back, therefore replicating the characteristic low impedance of the feedpoint. Every 1/2 wavelength in coax or on a ground radial, etc, the RF will mimic whatever impedance it started with 1/2 wavelength back. I can picture what the RF is doing in my brain but I'm not so good at describing it.

On most antennas the grounded side of the coax is DC grounded to the mast, ground radials, U-bolts, etc, but if the antenna is sufficiently decoupled via ground radials or similar, everything below the ground radials should be mostly out of the circuit and will have little influence on the antenna. For most mast mounted antennas with adequate decoupling, the addition of 0000 welding cable attaching to a dozen ground rods below will have zero effect on performance. The antenna will not even know it exists.
prcguy

Just as the title asks.
30 years ago as a kid I had a few Shakespeare "Big sticks" as well as an A-99 with a ground plane kit for CB tranceiving.
One of the HAMs in my area strongly admonished me on the importance of grounding my antenna.
I took his advice, and as I had schooling in the electrical trade, I used some 2/0 cable available to me and I believe about 6 copper clad 8 foot rods.
I connected the antenna base end of the cable to one of the U bolts.
This, I was told, was a good start, but if I wanted to achieve perfection, I would need 15 to 120 ground radials (not elevated radials) at the base of the mast.
Needless to say, I did not go that far.
So to come back around to my question..
Since the elevated radials on a ground plane antenna are cut to, say 1/4 wave, and connect to the coax shield, is there not also conductivity between them, the mounting base, the U bolts, and conductive mast?
If so, then why must they be 1/4 wave? Does not the extra length and capacitance of the mast, ground cable and earth disrupt their length, and therefore, the SWR?
In fact, without even connecting a grounded conductor to the elevated radials, does not the very existence of the conductive mast (which may very well be coming out of the ground) disrupt the performance of these antenna? If not, why not?
I'd appreciate the insight of you guys, if any.

Sent from my phone.

 

[misfit138]

Ah, thank you. So, the existence, size, position and proximity to the vertical of the elevated radials themselves should ideally complete the RF circuit...and the vertical whip radiation should "ignore" everything below them.
Fascinating.
The continuity of the radials to earth ground is inconsequential (theoretically) because the mast and coax shield are below the point of vertical insertion.
Makes sense.

I had many times picked the brains of some HAMs I knew growing up about building antennas, and they pretty much dissuaded me with phrases like, "You'll just pull your hair out and waste lots of money and time."

I did try once, with some 1/2" copper tubing, with meager success; the best I could do was 1:9:1 SWR.

Still, many decades later I've got the itch and maybe I will embark on another attempt in my old age.

Thanks again.

 

[jonwienke]

One of the HAMs in my area strongly admonished me on the importance of grounding my antenna.
I took his advice, and as I had schooling in the electrical trade, I used some 2/0 cable available to me and I believe about 6 copper clad 8 foot rods.
I connected the antenna base end of the cable to one of the U bolts.
This, I was told, was a good start, but if I wanted to achieve perfection, I would need 15 to 120 ground radials (not elevated radials) at the base of the mast.

Your HAM is an idiot.

First off, he's confusing RF ground (needed for proper antenna performance) and static grounding (needed for lightning protection).

The theoretical ideal antenna ground plane is an infinitely large flat metal plane at the base of the radiating element. This is obviously impractical, but it can be reasonably approximated with a number of horizontal elements extending out from the base of the radiating element approximately 1/4-wavelength. Most antennas use 3 or 4 horizontal ground radials to get a "good enough" ground plane.

What you did to your antenna was a better-than-average job of static grounding. The only detail you may have missed was that the ground rod network for the antenna tower should be tied to the ground network for your house electrical system.

Where your HAM totally went off the rails was conflating these. The RF ground plane must be located at the base of the radiating element to work properly. For a CB antenna, that is going to be 20-40 feet in the air in most cases. Creating a ground plane at ground level will do nothing whatsoever to improve antenna performance when the base of the antenna is above ground level.

There are certain HF antenna designs in which the entire tower/mast is the radiating element, and a ground plane must be constructed at ground level because the base of the radiating element is at ground level. The ground plane network your HAM buddy described is something that might be used by a commercial AM radio station antenna, not a CB antenna.

 

[prcguy]

Ground radials are a little different than an infinite sheet metal plane attached to the same point. If you first imagine an inverted V 1/2 wavelength dipole on its side, you have what looks like a 1/4 wave ground plane with only one 1/4 wave radial. But its not because both sides of the dipole will radiate equally and the pattern from an inverted V on its side will have both vertical and horizontal polarization.

Now if you add just one more 1/4 tuned radial or several to the inverted V on its side, the RF currents will be instantaneously traveling in opposite directions on all the radials out to the tips and reflecting back to the feed point over and over again, canceling any radiation. This leaves only the vertical element to radiate.

If you change the length of the radials from 1/4 wavelength they will no longer replicate the low feed point impedance and will transform that to something higher and cause an overall matching problem. There should be longer tuned radial lengths that will provide the same impedance as 1/4 lengths ones but I have not explored that yet.

With a 1/4 vertical element over an infinite ground plane things are a little different and less critical where tuned radials will really mess up the match if not tuned and a large or infinite sheet metal plane not so much. The characteristic impedance of a 1/4 wave antenna over infinite sheet metal ground is about 36 ohms where a 1/4 ground plane with 3 radials is closer to 50 ohms, partially because there is less capacitance between the vertical element and ground plane. Add more radials and you have more capacitance between whip and plane, impedance goes down some. You still have RF currents traveling all over the surface of an infinite ground plane, bouncing between the feed point and edges and back to the feed point and it does not radiate.

If you add too many radials (not really possible) and the impedance is pulled down to the 36 ohm range you can bend them down and away from the top whip and raise the impedance up closer to 50 ohms. Bend them too far where they are parallel but in opposite direction of the whip and you have made a coaxial dipole with about 70 ohm impedance. There are complications with this where a true coaxial dipole needs some dialectic inside the coaxial sleeve to make the inside appear slightly longer than the outside, but that's another topic.
prcguy

Your HAM is an idiot.

First off, he's confusing RF ground (needed for proper antenna performance) and static grounding (needed for lightning protection).

The theoretical ideal antenna ground plane is an infinitely large flat metal plane at the base of the radiating element. This is obviously impractical, but it can be reasonably approximated with a number of horizontal elements extending out from the base of the radiating element approximately 1/4-wavelength. Most antennas use 3 or 4 horizontal ground radials to get a "good enough" ground plane.

What you did to your antenna was a better-than-average job of static grounding. The only detail you may have missed was that the ground rod network for the antenna tower should be tied to the ground network for your house electrical system.

Where your HAM totally went off the rails was conflating these. The RF ground plane must be located at the base of the radiating element to work properly. For a CB antenna, that is going to be 20-40 feet in the air in most cases. Creating a ground plane at ground level will do nothing whatsoever to improve antenna performance when the base of the antenna is above ground level.

There are certain HF antenna designs in which the entire tower/mast is the radiating element, and a ground plane must be constructed at ground level because the base of the radiating element is at ground level. The ground plane network your HAM buddy described is something that might be used by a commercial AM radio station antenna, not a CB antenna.

 

[misfit138]

Your HAM is an idiot.

Heh. This was a guy who was middle aged 30 years ago who was probably just trying to show me something and possibly prove he was smart.

First off, he's confusing RF ground (needed for proper antenna performance) and static grounding (needed for lightning protection).

I think you may have hit it on the head.

What you did to your antenna was a better-than-average job of static grounding. The only detail you may have missed was that the ground rod network for the antenna tower should be tied to the ground network for your house electrical system.

Eventually, I did just that. I understand the importance of bonding all grounds together to prevent voltage potential, for safety.

Where your HAM totally went off the rails was conflating these. The RF ground plane must be located at the base of the radiating element to work properly. For a CB antenna, that is going to be 20-40 feet in the air in most cases. Creating a ground plane at ground level will do nothing whatsoever to improve antenna performance when the base of the antenna is above ground level.

If I recall, he indicated that with "Big Stick" antennas, the mast (and by extension, earth ground) was an integral part of the antenna's intended radiation and function. Later, he seemed adamant that purchasing a ground plane kit for the A99 was a complete waste of money for this very reason.
This was in the mid 1980s, when there was no worldwide web for me to fact check any of this and any books I skimmed through were vague and did not address a lot of my curiosity.

There are certain HF antenna designs in which the entire tower/mast is the radiating element, and a ground plane must be constructed at ground level because the base of the radiating element is at ground level. The ground plane network your HAM buddy described is something that might be used by a commercial AM radio station antenna, not a CB antenna.

I see. So, it was based on something..but not applicable to my particular setup.

 

[misfit138]

....
If you add too many radials (not really possible) and the impedance is pulled down to the 36 ohm range you can bend them down and away from the top whip and raise the impedance up closer to 50 ohms. Bend them too far where they are parallel but in opposite direction of the whip and you have made a coaxial dipole with about 70 ohm impedance. There are complications with this....

So, with this in mind, would it be practical to build a 1/4 wave 11 meter ground plane antenna matched to 75 ohm coax?
I always wondered about this, since the available dual, tri, and quad shielding on rg 56/rg 11 is so vastly superior to all the rg 58/8 I have seen over the years.
I am sure of my ignorance here, but curious nonetheless.
Thanks in advance.

 

[prcguy]

If you bend the ground radials down at a very steep angle you should get close to 60-70ohms but you may not reach 75 ohms if that is your goal. In free space a dipole is around 72 ohms and that is two 1/4 wave elements pointing opposite directions.

Shielding is not a big deal at HF unless you have a bunch of cables tightly bundled and are getting some unwanted signals bleeding through the braid. As you go higher in frequency the problem gets a little worse and at 1-2GHz where RG-6 is used for satellite downlink signals you can get noticeable bleed through between cables. Otherwise there is absolutely no reason to use quad shield cable, the loss specs are exactly the same as that same mfrs version of single shield cable.
prcguy

So, with this in mind, would it be practical to build a 1/4 wave 11 meter ground plane antenna matched to 75 ohm coax?
I always wondered about this, since the available dual, tri, and quad shielding on rg 56/rg 11 is so vastly superior to all the rg 58/8 I have seen over the years.
I am sure of my ignorance here, but curious nonetheless.
Thanks in advance.

 

[misfit138]

Thanks for your insight, prcguy.
This actually leads me to another question about the DC connection of coax shield that ties back to your explanation of the coax and mast being below the point of insertion on a vertical ground plane antenna.
In the case of a coaxial vertical antenna, if I understand correctly, the coax feeds through a hollow, conductive section which can serve as the lower radial and is electrically connected to the coax shield. The top vertical element would be connected to the center conductor.
Now...Since the coax shield is, (just as in the case of the ground plane vertical antenna) connected to an opposing element, why does the entire length of coax shield not create a mismatch in length? The lower (mast) section is simply a hollow conductor housing the coax shield within, so, as far as the RF or antenna "behavior" is concerned, my intuition is to conclude that this would essentially be "half an antenna", because I see no real distinction between coax shield and hollow mast...
The point of insertion analogy does not seem to make sense here as it does with the vertical ground plane antenna design.
In that antenna design, it makes sense to me that the coax shield is, ideally, "ignored" due to its position below the elevated ground plane radials..But a coaxial vertical design seems a little puzzling to me.
What am I missing?