[Elecraft] grounding K2

[Don Wilhelm]

Rick (long answer),

You are most likely mixing RF ground with a 'Mother Earth' ground.  They 
are quite different.  The main reason is that RF does not behave like 
low frequency AC or DC.  You are not alone - that fact seems to mystify 
and confuse many, many hams.  I will try to give a conceptual 
explanation.  First understand that I use RF Ground as any point where 
the RF voltage is zero (note that the RF current will be a maximum at 
that same point).   No other concept of RF ground has ever made any 
sense to me.

With a balanced dipole antenna (center fed doublet), a point of RF 
ground will occur at the midpoint of the center feed.  That is where the 
RF voltage crosses zero.  If the radiator is fed with a parallel 
transmission line, the currents on the two conductors will be equal an 
opposite all along the feedline, so that effective RF Ground point is 
carried all the way back to the shack along the transmission line.

If one uses coax to feed the antenna, a similar situation exists 
*inside* the coax - due to skin effect, the inside of the coax shield 
can be considered as a separate wire than the outside of the shield.  
The currents on the center conductor and the inside of the shield will 
be balanced (just like in the parallel transmission line).  The problem 
with coax is at the junction of the radiator and the shield.  The RF 
voltage (and current) on the inside of the shield sees two conductors at 
that junction 1) the antenna wire, and 2) the outside of the coax 
shield.  A current balun installed at the antenna feedpoint will place a 
high impedance on the outside of the coax shield while not interfering 
with the balanced currents between the center conductor and the inside 
of the shield.  So, RF ground is maintained inside the coax - it is a 
zero potential point that can be visualized as being midway between 
those conductors.  The item that seems to cause the most confusion is 
that the coax shield can be at ground potential for DC and low frequency 
AC, but it is *not* at ground potential for RF.

So, you do not need to create an RF ground for your dipole - it already 
exists.

For a vertical with an elevated ground plane, a similar RF ground exists 
midway between the vertical radiator and the ground plane (yes, the 
ground wires radiate and are part of the antenna system - cancellation 
of the ground wire radiation must be by the physical orientation so the 
radiation cancels due to phase relationships).  If a vertical radiator 
is ground mounted, the earth takes the place of the ground plane - and 
actual earth is not a really good conductor of RF, so we usually augment 
that conduction by burying a lot of radials, the lengths are not 
critical in those installations because we just want to improve the RF 
characteristics of the real earth.

Bottom line is that I make the statement that an actual RF ground is a 
property of the antenna system and is not to be equated to a Mother 
Earth ground.

73,
Don W3FPR


Rick Dettinger wrote:
> If I remember correctly, the base K2 came without the ground screw.  
> Its been a long time.  I think the ground screw came with the ATU.  
> The K1 doesn't have a ground screw, even with the ATU.  I am confused 
> about the use of an RF ground with a balanced antenna.  One of the 
> advantages of a balanced antenna, verses an antenna that works with a 
> ground such as a vertical, is efficiency.  If I use an RF ground with 
> a balanced antenna, am I losing efficiency.  Especially if the ground 
> is mediocre?  I don't want to put down 60 radials for my center fed 
> doublet.  When I used my K1 or K2 with a battery and a balanced  
> antenna, I had confidence that most of  the power was getting radiated.