Along those same lines ...
When I was a young product engineer for a semiconductor company, I was
responsible for Tuning Diodes and made a trip to one of the top
manufacturers of mechanical UHF TV tuners to try to get them to switch
to electronic tuning. I had a very interesting discussion with two of
their RF engineers (a couple of very smart old timers) who explained the
various reasons why they were reluctant to do that (the ability of
strong levels to pull the frequency of the oscillators, the higher noise
level of tuning diodes versus mechanical capacitors, the higher cost,
etc), but the most startling reason they gave me was that with
mechanical tuners they could adjust the coupling between stages by
routing "ground" currents where they wanted them to be in the completely
shielded case by cutting slits in it.
It seems to me that an "RF ground" in any environment can be viewed in a
similar manner. It is NOT "an infinite sink or source of carriers" and
should not be thought as such. Currents whirl and swirl even in a
lossless conductor and the fields they generate influence where they
travel (skin effect anyone?). A theoretically infinite pool of
carriers may be interesting to speculate about, but in spite of Mr.
Duffy's post it serves no useful purpose for understanding circuits or
antennas because nothing ever acts like that, and only bad conclusions
can result by pretending one exists.
Dave AB7E
On 1/23/2015 7:54 PM, Jack Brindle wrote:
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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