From: dnewkirk@arrl.org (Dave Newkirk)
Newsgroups: rec.radio.shortwave
Subject: Re: antenna and ground
Date: 27 Jan 93 08:52:18 est
Organization: American Radio Relay League

In rec.radio.shortwave, cb@tamarack13.timbuk (Chris Brewster) writes:
>Kenneth V Zichi writes:
>
>   >Why is often recommended to put our receiver (portable or not) to ground
>   >potential in order to get the best operating conditions ?
>
>   The way it has been explained to me is that the RF ground is 
>   helpful because like all electric signals, radio waves need to
>   flow FROM something (like and antenna) TO something else (like the
>   Ground).  Putting a good ground into the picture acts like connecting
>   the second wire from the battery to the light bulb.  Since there is
>   a bit of "leakage" anyway, even without a ground, there is some
>   signal flow, but a good ground improves things.
>
>That isn't the way it was explained to *me*, but I was hoping an EE type would
>answer this one.  What I've been told is that the ground is necessary to provide
>a stable reference voltage against which the signal is read.  With absolutely no
>ground, what you have is any capacitance that's part of the system.  Harry Helms
>argues that most current sets are adequately grounded through AC, and that a
>ground wire can create problems like spurious signals.  Any experts out there?
>
>Chris Brewster
>cb@cray.com

Although I'm not an EE, I *am* an expert in this. :-)

Voltage implies *differential* -- a difference in electrical potential. Our
radios receive radio signals as tiny, varying potentials at -- "across" -- 
their antenna inputs. It doesn't matter whether that potential exists 
between the end of a single wire and a ground wire, or between the two 
identical halves of a *dipole* antenna -- as long as the system is arranged
to produce a *potential difference* across the radio's antenna input, it
will work.

Next comes the question of whether the potential difference is enough to
be usable. If you're able to to adequately understand the message sent by
the radio station you're listening to, your radio is getting enough signal.
This may sound facile, but it's true: *The whole purpose is getting the
message through."

If you make a radio sensitive enough, *very little* potential difference
is needed across its antenna input for the radio to produce readable
audio. I cite my little old Sony ICR-4800 pocket-sized receiver as an
example. On shortwave, it uses a whip that's about a foot long -- and it
produces usable audio from medium to strong signals whether or not I'm
holding it, *with no grounding whatsover.* How can this be? What is "the
other side of the circuit", the "ground" or other conductor that, in
conjunction with whip, produces enough radio-frequency (RF) potential
across the '4800's antenna input?

The answer is: The radio's "common" circuit board foil. Sleazily
graphified, the system would look something like this:


-------------------------[]----  
        ^                ^   ^ "common" circuit-board foil
        ^                ^
      whip          antenna input
                     circuitry

This arrangement produces enough potential difference across the ICR-4800's
input circuit to produce a detectable, usable signal, and usable audio.
Holding the radio in my hand, placing it on a metal surface, or installing
headphones imprve the situation by electrically extending the circuit-board
foil, and thereby increasing the potential difference across the input
circuitry. If the set had an option AC cord, that cord (and the house 
wiring) could also electrically extend the "common" (AKA "ground") side of
this arrangement, and therefore might improve signals. Connecting an AC
cord sometimes also introduces electrical noise if the noise produces
sufficient potential difference across the receiver's input circuitry.

Regards/WJ1Z



David Newkirk, Senior Asst Tech Editor  |  voice: 203-666-1541 X280
American Radio Relay League             |  fax: 203-665-7531
225 Main St, Newington CT 06111 USA     |  net: dnewkirk@arrl.org

   "Wait a minute--cancel that. It says *HELF*." -- Gary Larson