[TowerTalk] Re: [Antennas] Ground System suggested for QRO Amps

[Tom Rauch]

Hi Jim,

> Drive one  ground rod in.  Then measure straight up about 5 feet,  and put
> in another rod.  From that rod, go towards the upper right,  at a 45
> degree angle a distance of 3 feet,  in goes another rod.  From that rod,
> go left 4 feet,  and plant another one.  Now you have a pattern something
> as follows:            __
>                                      /
>                                      |              
> 
> Lengths in that are not to scale,  but the shape is close to correct,  hi.

Why drive a rod in at an angle??  What is the magic in the odd 
spacings?

> Of course the upper ends of the rods are connected together with wide
> strapping or copper tubing.  This was N6VLV's recommended ground set up.
> 
> Now,  I also recv'd another input which takes a different approach to the
> RF "ground" situation in the shack,  from Chuck Constantine  KR6C,
> 
> "What you're interested in is a equi-potential ground reference.  That
> means ALL equipment is at the same potential.  You do NOT need a (earth)
> connection for a RF ground reference.  People seem to think the earth is a
> magical RF sink where spent electrons go.  They want to get back to their
> source...not necessarily earth (Kirchhoff's first law). Aircraft do not
> drag ground wires and ones I work on run several kW transmitters and
> things are not hot! We make sure things have low resistance/Impedance
> connections and EVERYTHING is bonded together (multi-point).  You can make
> a great RF ground reference by putting a patch (4' sq) piece of copper
> sheet on your floor and use wide straps from your ground bus on the back
> of your desk to this plate and be guaranteed better RF performance than
> any rod driven into the earth...especially on a Volcanic island. (think of
> it as a big RF bypass capacitor if you like.  Major computer installations
> tie all the flooring webs together for low inductance equi-potential
> groundplanes).

Such a small area ground could be a fair ground on 28 MHz and 
higher, but it would be a useless ground on 1.8 MHz. It would be 
useless for lightning also.

While it does keep all the equipment at the same potential, it does 
nothing for potential between the equipment and everything else in 
the world. 

This ground would never help with RFI or lightning, except for stuff 
in the room with the ground plate.

> There are three basic types of RF grounds, Single point (where everything
> connects to a single point best for eliminating possible ground loops)
> Mulit-point (where grounds are made to a common reference like different
> points on a copper plate or different points on a ground bus) and Hybrid
> (combination of both).  Most Amateurs use multi-point (like whats shown in
> ARRL handbook) because of simplicity and in order for a single point to
> work properly you would have to separate all your equipment several feet
> from each other to prevent stray capacitive coupling between boxes..

Capacitance has nothing to do with not using a single point ground. 
Convenience does.

.not
> worth the effort. The ARRL handbook or antenna book method is acceptable
> and works fine if done correctly.
> 
> The two most important things to minimize are Resistance and INDUCTANCE!

It is not only an inductance problem, it is a transmission line 
problem. At frequencies above a few hundred kHz the ground lead 
becomes so long it acts like a transmission line. A wider conductor 
forms a lower Z line, and so the impedance transformation is not as 
great along the line.
 
> I use the 1/2" 6' length of copper waterpipe attached to the back of my
> desk then every piece of equipment is connected with a braided strap to
> several points on this ground buss then the end of the bus connects with a
> large fat tinned copper braid strap to the ground rod.  The attachment to
> the ground rod only provides electrical SAFETY ground and if you install
> separate ground rods you MUST make sure they are ALL connected together. 
> This means connected to the main ground rod at your power service entrance
> to your house so everything is at the same potential. If not your shack
> ground might have a lower resistance than your house electrical ground and
> fault currents could use your station and equipment as ground for
> electrical shorts making things very dangerous!
> 
> Electrons want to return to their source via a LOW INDUCTANCE path. 

LOW IMPEDANCE would be a better term.

> length of wire at 28mHz will act like a 1/4 wave inverter making a high
> impedance connection at one end.  Someone once told me the copper pipe
> makes a good antenna element because it can be resonated. .I say B.S. with
> all the equipment connected to it and the parasitic capacitive coupling
> between equipment its very hard to "resonant" the ground.  You can use an
> antenna bridge connoted between your ground rod and station multi-point
> ground and try to find a resonance if your really concerned and if by
> chance you do...bet its because you used wire between the rod and station
> bus bar and thats whats resonating.

Copper tubing will resonate JUST like copper wire. Think about the 
elements in a yagi, or a cage antenna.

The only time a conductor really becomes broad-banded is when 
its diameter or width is very large compared to its length...or when 
it is loaded with losses.
 
A "ground plate under the rug" isn't a bad RF idea for eliminating in 
shack RF, especially at the higher frequencies, but it is NOT a 
good RF ground or lightning ground. It will not prevent common 
mode current from getting back in the power or telephone lines on 
lower bands. It does little to improve lightning protection.

If feedlines are properly treated outside the shack, and the 
equipment is anywhere reasonable in design, it won't be necessary 
to use a shack ground for RF. RF in the shack generally means 
common mode current is coming back on the antenna cables 
because the antenna is exciting the cable.

RF grounding, unless you are feeding a single wire antenna FROM 
THE SHACK,  is a band-aid for the real problems.



73, Tom W8JI
w8ji@contesting.com


--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm