[TowerTalk] RF Ground is not a Myth

[Don]

Ground is only a reference point relative to a measurement, whatever type it 
is.

Don W7WLL

-----Original Message----- 
From: Jack Brindle
Sent: Friday, January 23, 2015 6:54 PM
To: Tower Talk
Subject: Re: [TowerTalk] RF Ground is not a Myth

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
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk

_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk