What would you define RF ground as? There seem to be a lot of different
ideas.

"David" nospam@nospam wrote in :

What would you define RF ground as? There seem to be a lot of different
ideas.

Except in the case of ground-mounted antennas, there is really no such
thing. There is RF neutral, though.


--
Dave Oldridge+
ICQ 1800667

"David" wrote
What would you define RF ground as? There seem to be
a lot of different ideas.
______________

A good "r-f ground" has a very small impedance to the flow of r-f current at
the frequency of interest.

A good r-f ground is especially important when using a ground-mounted
vertical monopole radiator, because the path to "ground" is in series with
the r-f current flowing on the monopole. Power consumed by the ground
system is wasted (not radiated as EM energy).

At medium wave broadcast frequencies and in the 160 meter and low HF bands,
a system of ~120 buried radials each about 1/4-wave long provides a
reasonably low-Z ground connection -- probably 2 ohms or less, regardless of
the ground conductivity at the site. This was determined experimentally by
Brown, Lewis and Epstein of RCA in 1937.

Copper water pipes in the home, and even buried ground rods typically are
not good, low-Z r-f grounds.

But many antenna types - such as a dipole - do not need or use such an r-f
ground for efficient radiation.

RF

"David" nospam@nospam wrote in message
...
What would you define RF ground as? There seem to be a lot of different
ideas.


1. One needs to define properties or "regular" ground.

2. Illuminate it with RF of particular frequency and particular antenna.

3. Study the effect of 1 on 2 and you will get some idea. Amount of
reflection or absorption would be the indicator of how good "RF ground" it
is.

As far as suitability of "ground" to "work" with antennas it is somewhere
from horrible (rocky ground) to "perfect" (sea water, copper mine, etc.)

Other than that, if you can walk or swim on/in it and you expose it to RF,
then I suppose you could call it RF ground.
If there is no RF, the RF ground disappears :-)

Somethinglikethat?!?

73 Yuri, K3BU
da salt water muddy RF ground lover
join the Tesla Sparks at N2EE www.TeslaRadio.org

rf ground is where ever you connect your meter/scope/analyzer ground lead.

"David" nospam@nospam wrote in message
...
What would you define RF ground as? There seem to be a lot of different
ideas.

David wrote:
What would you define RF ground as? There seem to be a lot of different
ideas.



David:

Ground can be a relative thing. What I have always found to be good
advice is that EVERY ground, at some point, be allowed to reach a low
ohmic earth ground (best possible if it all occurs at the exact same
earth ground point--or no current flows and there is no voltage
potential between such grounds.) For example, although a dipole needs
no rf ground directly at the point it connects to the feed-line, the rig
hooked to such an antenna and feed-line should be given a good earth ground.

If the above is coupled with a good understanding, awareness, and
practice of avoiding "ground loops", I think one can claim to have a
good and adequate grounding system. Ground loops are well discussed in
books, mags, and internet pages, etc.

In planes and outer space the earth ground can be ignored, your crafts
metallic shell will serve.

Regards,
JS

On Sat, 11 Nov 2006 12:06:16 -0000, "David" nospam@nospam wrote:

What would you define RF ground as? There seem to be a lot of different
ideas.

Hi David,

This is another instance of scale matters. Ground is meant to imply
an infinite resource of charge with no impedance to its flow.

Of course, "infinite" and "no impedance" are factors that are the
first casualties when RF is added as a requirment. This is from the
simple consequence of scale, because distance causes phase shifts and
brings impedance. A long wire that is perfectly capable as a ground
lead for 60 Hz can become a liability to short wavelength RF. That
wire (that we call ground) may connect to an infinite resource of
charge (the proper ground), but that charge can't get to the other end
because of possibly infinite impedances [again, infinite is in the eye
of the creator].

If you can contrive to make that lead to ground half a wavelength long
to the RF of your interest, that wire ceases to offer impedance and
acts much as you would demand of a ground wire. Curiosly enough, it
will never be zero impedance because it qualifies as a radiator (the
paradox of ground) which adds the loss of radiation. For most
purposes, however, it may be your only choice and you live with it.

Now, when we get to the actual mud of ground, and how well it performs
as an infinite resource of charge, RF still brings problems of scale
because that mud will also exhibit impedances correlated to wavelength
(corrected for velocity factors) that disconnect it from the total
earth's resource for infinite charge. This is why you lay down
radials (which simply increases coverage, but never completely escapes
the scale of wavelenght problem).

Moral to this tale:
Put your source (transmitter) as close to the mud as possible;
lay out as much copper as you can from that point to suit
a broader range of frequencies.

Alternative moral:
Put your source (transmitter) as far from the mud as possible;
Use a dipole and insure there are no connections to ground
whatever. Any violation of this last rule brings grief.
Such violations are legion and few escape.

73's
Richard Clark, KB7QHC

"John Smith" wrote in message
...
David wrote:
What would you define RF ground as? There seem to be a lot of different
ideas.

David:

Ground can be a relative thing. What I have always found to be good
advice is that EVERY ground, at some point, be allowed to reach a low
ohmic earth ground (best possible if it all occurs at the exact same earth
ground point--or no current flows and there is no voltage potential
between such grounds.) For example, although a dipole needs no rf ground
directly at the point it connects to the feed-line, the rig hooked to such
an antenna and feed-line should be given a good earth ground.
snip


Regards,
JS

When you refer to hooking the rig/dipole to a good earth ground, are you
still talking about an rf ground, or a safety ground?
I see no requirement to connect a nicely matched dipole to an earth ground
for rf purposes. For example, a battery operated transmitter feeding a
dummy load wouldn't need one either.

Wayne wrote:
"John Smith" wrote in message
...
David wrote:
What would you define RF ground as? There seem to be a lot of different
ideas.
David:

Ground can be a relative thing. What I have always found to be good
advice is that EVERY ground, at some point, be allowed to reach a low
ohmic earth ground (best possible if it all occurs at the exact same earth
ground point--or no current flows and there is no voltage potential
between such grounds.) For example, although a dipole needs no rf ground
directly at the point it connects to the feed-line, the rig hooked to such
an antenna and feed-line should be given a good earth ground.
snip

Regards,
JS

When you refer to hooking the rig/dipole to a good earth ground, are you
still talking about an rf ground, or a safety ground?
I see no requirement to connect a nicely matched dipole to an earth ground
for rf purposes. For example, a battery operated transmitter feeding a
dummy load wouldn't need one either.



Wayne:

Both. Only a dummy would think he needed an rf ground for a watt burner
of proper impedance.

However, there always is that "special case;" if the nitwit was running
a kw off a forklift battery, he just might want that rf ground.

JS

An RF ground is where RF energy flows into the earth. A lot of folks
misuse the word "ground" when they really mean a common connection
point, which may or may not have anything to do with earth. Be careful
to define just what you mean.

Bill, W6WRT


------------ ORIGINAL MESSAGE ------------

On Sat, 11 Nov 2006 12:06:16 -0000, "David" nospam@nospam wrote:

What would you define RF ground as? There seem to be a lot of different
ideas.