[TowerTalk] RF Ground is not a Myth

[David Gilbert]

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 at 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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