Adding a bit to K9YC’s discussion, a ground plane, or any “ground” in a piece
of equipment may indeed be at 0 volts as far as DC power is concerned, but it
is also an excellent carrier for other signals. It is very common for RF to be
conducted on that ground plane, and even emitted to the surroundings. I had a
problem once at a large radio commercial vendor I worked for where the 250th
harmonic of a 7 MHz microcontroller oscillator was deceasing the receiver - the
rf was being conducted specifically on the ground plane trace of the radio,
along with DC and a few other signals. The discovery astounded many excellent
RF engineers, and reminded us of the premise of this discussion, at RF, there
is no such thing as ground.
- Jack B, W6FB
> On Jan 23, 2015, at 6:15 PM, Jim Brown <jim@audiosystemsgroup.com> wrote:
>
> On Fri,1/23/2015 3:20 PM, Steve Maki wrote:
>> Can't one specify a perfect ground (even though impossible in the real
>> world) when modeling antennas? Is that not a useful exercise as an
>> educational tool?
>
> "Specify?" I think you mean "assume." But few antennas depend upon a
> connection to earth for their operation (some RX antennas do).
>
> One can ASSUME a homogenous earth of known properties for the purpose of
> modeling an antenna, but that's not what this discussion is about, nor does
> in involve a CONNECTION to the earth, nor is the fairy tale that is the
> subject of this email useful. Rather, modeling programs consider the
> interaction of electric, magnetic, and electromagnetic fields (and
> wavefronts) with the earth. The coupling of the fields results in loss (for
> real earth) and the interaction of the wavefronts produces the vertical
> pattern.
>
> The only good reason for assuming homogenous (uniform) earth is that it makes
> the math simple enough that we can write the equations and solve them on
> computers that we can afford to have on our desks, and within a reasonable
> length of time.
>
> There's another major point about this. Circuit common is NOT a single point.
> When we draw schematics, we show only half of the circuit -- the signal flow
> through components. But current flows in loops, the return path -- what EMC
> guru Henry Ott calls "the hidden schematic lurking the ground symbol." Those
> return paths can be well controlled -- a transmission line -- or they can be
> random and uncontrolled.
>
> When we build our circuits on multi-layer boards with a continuous layer
> forming a ground plane under a layer with traces on it, the trace and ground
> layer form an unbalanced transmission line, and the current on the trace
> returns in a narrow area directly under the trace, minimizing crosstalk and
> making the circuit more stable. If there is no ground layer, or if the ground
> layer is broken under a trace, the return current finds whatever random path
> it finds, depending on the whim of the equipment designer, and the circuit is
> subject to crosstalk, noise pickup, noise radiation, and circuit instability.
>
> If you look inside a good power amp or antenna tuner, you will see coax
> running between the antenna switches and relays. The signal returns on the
> coax shield. Poorly designed units omit the coax, using single wires, and the
> current returns on the chassis. The otherwise very nice Ten Tec antenna
> tuners have that design defect -- try measuring a 6M antenna through that
> tuner. It's a mess. By contrast, Ten Tec HF power amps use coax for antenna
> switches, and 6M goes through just fine (with the amp inactive, of course).
>
> 73, Jim K9YC
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