RF Grounds for MW Antennas

[rfry]

>For a ground-mounted antenna, how about burying a large metal object
>such as an old electric stove and connecting the ground lead to it?
>The stove would have more soil contact than a ground rod (or even
>several). -- JasonW
________________

The purpose and function of buried ground systems is to provide a low resistance path for the RF currents induced in the earth by radiation from the antenna. Those currents flow through whatever conductor is available, back to the ground terminal on the tx. They are a necessary part of an efficient MW antenna system. A high-resistance path for those currents reduces the radiation efficiency of the antenna system.

These induced earth currents are present out to several 1/10s of a wavelength from the radiator. So no matter how much metal you bury under the antenna, the currents it needs to collect from the earth will have to travel through several 1/10s of wavelengths of earth to get there -- and that means the total resistance in the ground system will be very high.

Likewise, even if you can connect to a buried water pipe that is miles long and has a huge surface area in contact with the earth, it still is a very poor MW ground because most of the radiated earth currents it needs to collect will have to travel through a lot of earth before arriving at the pipe.

This is the reason that radials are used. They need to be placed in enough numbers and in the locations around the antenna where they can collect the RF earth currents with only very short paths through the earth, and then conduct them back to the tx.

Here is some data relating the number of symmetrically-placed, buried radials each 0.1-wavelength long, to the current each one collects from a 0.1-wavelength MW vertical radiator, for the same tx power in each case.

No. of Radials > Current (amperes)
8 > 0.3
16 > 1
32 > 1.5
64 > 1.55

So in this example a system with 8 radials has 5 times more ground loss than a system with at least 32 radials. And as radial length increases, the differences in the currents in each one are multiplied. For example, if the radials are all 1/2-wave long, the current in 64 radials will be about 250X greater than the radial currents when only 8 are used.

These increases in current result from the reduced losses that the induced earth currents have as they travel back to the tx. And that higher current flowing in the antenna produces greater useful radiation (for the same tx power).

So that's why a localized "ground" under a MW antenna is not such a good idea. Pardon my rant.
//

 

[am1670acr]

Since such ground radial installations are typically not possible and may be questionable within the "spirit" of Part-15, what results would you expect from a transmitter mounted on large metal objects, say a multi storied metal roofed building or perhaps even side mounted on a small water tower?

>
> The purpose and function of buried ground systems is to
> provide a low resistance path for the RF currents induced in
> the earth by radiation from the antenna. Those currents
> flow through whatever conductor is available, back to the
> ground terminal on the tx. They are a necessary part of an
> efficient MW antenna system. A high-resistance path for
> those currents reduces the radiation efficiency of the
> antenna system.
>
> These induced earth currents are present out to several
> 1/10s of a wavelength from the radiator. So no matter how
> much metal you bury under the antenna, the currents it needs
> to collect from the earth will have to travel through
> several 1/10s of wavelengths of earth to get there -- and
> that means the total resistance in the ground system will be
> very high.
>
> Likewise, even if you can connect to a buried water pipe
> that is miles long and has a huge surface area in contact
> with the earth, it still is a very poor MW ground because
> most of the radiated earth currents it needs to collect will
> have to travel through a lot of earth before arriving at the
> pipe.
>
> This is the reason that radials are used. They need to be
> placed in enough numbers and in the locations around the
> antenna where they can collect the RF earth currents with
> only very short paths through the earth, and then conduct
> them back to the tx.
>
> Here is some data relating the number of
> symmetrically-placed, buried radials each 0.1-wavelength
> long, to the current each one collects from a 0.1-wavelength
> MW vertical radiator, for the same tx power in each case.
>
> No. of Radials > Current (amperes)
> 8 > 0.3
> 16 > 1
> 32 > 1.5
> 64 > 1.55
>
> So in this example a system with 8 radials has 5 times more
> ground loss than a system with at least 32 radials. And as
> radial length increases, the differences in the currents in
> each one are multiplied. For example, if the radials are
> all 1/2-wave long, the current in 64 radials will be about
> 250X greater than the radial currents when only 8 are used.
>
> These increases in current result from the reduced losses
> that the induced earth currents have as they travel back to
> the tx. And that higher current flowing in the antenna
> produces greater useful radiation (for the same tx power).
>
> So that's why a localized "ground" under a MW antenna is not
> such a good idea. Pardon my rant.
> //
>
> >For a ground-mounted antenna, how about burying a large
> metal object
> >such as an old electric stove and connecting the ground
> lead to it?
> >The stove would have more soil contact than a ground rod
> (or even
> >several). -- JasonW
> ________________

 

[radiopilot]

> >For a ground-mounted antenna, how about burying a large
> metal object
> >such as an old electric stove and connecting the ground
> lead to it?
> >The stove would have more soil contact than a ground rod
> (or even
> >several). -- JasonW
> ________________
>
> The purpose and function of buried ground systems is to
> provide a low resistance path for the RF currents induced in
> the earth by radiation from the antenna. Those currents
> flow through whatever conductor is available, back to the
> ground terminal on the tx. They are a necessary part of an
> efficient MW antenna system. A high-resistance path for
> those currents reduces the radiation efficiency of the
> antenna system.
>
> These induced earth currents are present out to several
> 1/10s of a wavelength from the radiator. So no matter how
> much metal you bury under the antenna, the currents it needs
> to collect from the earth will have to travel through
> several 1/10s of wavelengths of earth to get there -- and
> that means the total resistance in the ground system will be
> very high.
>
> Likewise, even if you can connect to a buried water pipe
> that is miles long and has a huge surface area in contact
> with the earth, it still is a very poor MW ground because
> most of the radiated earth currents it needs to collect will
> have to travel through a lot of earth before arriving at the
> pipe.
>
> This is the reason that radials are used. They need to be
> placed in enough numbers and in the locations around the
> antenna where they can collect the RF earth currents with
> only very short paths through the earth, and then conduct
> them back to the tx.
>
> Here is some data relating the number of
> symmetrically-placed, buried radials each 0.1-wavelength
> long, to the current each one collects from a 0.1-wavelength
> MW vertical radiator, for the same tx power in each case.
>
> No. of Radials > Current (amperes)
> 8 > 0.3
> 16 > 1
> 32 > 1.5
> 64 > 1.55
>
> So in this example a system with 8 radials has 5 times more
> ground loss than a system with at least 32 radials. And as
> radial length increases, the differences in the currents in
> each one are multiplied. For example, if the radials are
> all 1/2-wave long, the current in 64 radials will be about
> 250X greater than the radial currents when only 8 are used.
>
> These increases in current result from the reduced losses
> that the induced earth currents have as they travel back to
> the tx. And that higher current flowing in the antenna
> produces greater useful radiation (for the same tx power).
>
> So that's why a localized "ground" under a MW antenna is not
> such a good idea. Pardon my rant.
> //
>

Rfry,

This is all good stuff and I believe alot of us know this already, radials are the best to have, IF you can have them.

I believe the topic being addressed was for the unlucky ones who live in apartments, condos, townhouses, homeowner association challenged residences, etc., these people need to be more creative, and laying out 5/8, 1/2, 1/4, 1/8 wave or even 3 feet of radials just won't do!

These are the people trying to install a simple 4-8 foot ground rod into the soil, some places not even soil, but a mixture of concrete and tar! How do these potential hobbyist even try to get a decent ground anywhere?

The suggestions in the below posts, wacky as they may seem offer some ideas of what one can do to get that ground, in lieu of that perfect ground system.

Trust that we've all 'done this, done that' routine, some of us are blessed with the perfect soil conditions, by the sea, large acrege, high up on a hill, living on the 50th floor with a metal roof, etc. and we get excited about our great signal and we think others will get the same if they follow our course, but each transmitter site will be different...

Before I started my radio station, I believe I spent 3 years researching all I could get my hands on before I bought a single transmitter, years later I can proudly say the reading and experimentation has paid off even if others may believe differently.

Radiopilot

 

[rfry]

> Since such ground radial installations are typically not
> possible and may be questionable within the "spirit" of
> Part-15, what results would you expect from a transmitter
> mounted on large metal objects, say a multi storied metal
> roofed building or perhaps even side mounted on a small
> water tower?
_____________

If a good radial ground system is impossible/impractical, an alternate would be to use 3 or 4 short, symmetrically-placed radials elevated a foot or two above the ground, and connect the Part 15 ground terminal to the radial connection point just under the 3-meter antenna. Elevated radials behave a lot differently than buried ones, and just a few elevated ones will perform as well as lot more of them when buried.

Some AM broadcast stations have done this when they couldn't use buried radials due to the rocky ground, "wetlands" issues, building top locations, etc and they work fine. Best to use 1/4-wave radials, though, and shorter ones will not be as good.

Side-mounting a Part 15 AM antenna off a metallic water tower will have some serious affects on its impedance and radiation patterns.
//