From: jcarroll@gath.cray.com (Jeff Carroll)
Subject: Re: antenna and ground
Originator: jcarroll@gath
Organization: Cray Research, Inc.
Date: 25 Jan 93 21:47:45 CST


In article <CB.93Jan25095843@tamarack13.timbuk>, 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?

Friendly neighborhood EE type stepping up to bat.

Engineers usually mean one or both of two things when they talk about "ground":

a) something that is in good electrical contact with the earth, at the
   potential that we choose to call 0 volts DC

b) something that is capable of sourcing or sinking a relatively large amount
   of current, at a given frequency or band, without a significant change in
   its own potential

When we wish to differentiate between these concepts, we refer to (a) as
"DC ground", and (b) as "AC ground" or "signal ground".

The goal of a radio receiver antenna is to be a structure in which enough
current is induced at the frequencies of interest by the EM field set up 
by the transmitter to present an intelligible signal at the receiver front
end. At shortwave frequencies our antennas are usually never big enough, and
(for portable receivers, anyway) are often monopoles.

Remember that when you use a monopole antenna, the other side of your
receiver front end is connected to ground. This is important for the
reason Chris gives: the input transformer needs to have one end tied
to a good voltage reference (equivalently, it needs to be connected
to something that's going to be able to source and sink plenty of current
through the input transformer. On the other side of the input transformer,
good signal ground paths to the power supply are important for the proper
operation of the radio, but this is usually a design consideration rather
than anything the user can do anything about.

Since most portable radios today are made mostly of plastic and move around
a few feet above the ground, they are obviously not at DC ground and
also for the most part provide pretty crummy signal grounding (generally,
the more metal something has in it, the better a ground it will make,
because of all those loose electrons running around). Although the AC 
adapters you buy for portable radios don't directly connect your radio
chassis "ground" to DC ground (i.e. the earth), they do connect it to the
electric company's power grid, which has a lot of metal in it and a lot of
loose electrons that don't mind being jiggled around at MHz frequencies 
(ask any designer of switching power supplies). This is why the performance
of about any portable SW receiver improves when you plug it into the wall. 

If your tabletop receiver is well designed, you shouldn't have to worry
about whether it's grounded or not. If it needs to be DC grounded, the
manufacturer will have provided you with a three-prong plug on the power
cord (NOTE: people outside the USA, disregard this paragraph completely).
If your antenna is a monopole in any of its various forms, you will need
to provide a good ground for the other side of the line, and a balun if
you use balanced antenna feed. If both your receiver and your antenna are
grounded, you run the danger of having currents which belong inside your
radio coupled directly to the front end; this is called a "ground loop",
and can be a Bad Thing in many ways.

People who have old tube receivers may need to worry about DC grounding
their receivers due to the high voltages running around inside the
chassis; in those cases one might want to think about using dipoles
with balanced feeds. (I'm not old enough to know :^).)

Jeff Carroll
carroll@herndon.cray.com
Newsgroups: rec.radio.shortwave,soc.culture.canada