[Antennas] Feedlines, Tuners, and myths

[Pat W]

 I am writing this only to try to dispel some myths out
there in Ham Radio land about feedlines and tuners.
Unfortunately some wrong beliefs continue to get
propagated and it seems necessary to examine what
happens when we connect our transmitter(and receiver)
to our feedline.
 
I am not an engineer but have managed over time to
finally grasp the concepts necessary to build effient
antenna systems.  Please don't flame me for my simple
analysis.  These ideas finally gelled after reading
Reflections II" by Walt Maxwell.  I recommend this book
for others who have been confused in the past. 
    
Here are some of my ideas:

 All feedlines are transformers. Any feedline must
transform the load impedance to an input impedance,
depending on the line length. If the line length
happens to be exactly 1/4 wavelength, the
load is transformed by the equation Zi=Z0**2/ZL, where
Zi is the input impedance, ZL is the load impedance,
and Z0 is the feedline impedance.
 From the equation, note that when the feedline
impedance equals the load impedance, then the
transformation = 1, or Zi=ZL=Z0.  That is why
we can use a 50 ohm coax on a resonant antenna(assuming
50 ohms of load).
Taking advantage of this equation, we can feed an off
resonant antenna by choosing the correct feedline
length. 
Example, an 80 meter dipole on 40 meters with a ZL of
5000 ohms, and feedline Z0=500 ohms will provide about
a 50 ohm input impedance! A perfect match.
   
If the line length is exactly 1/2 wavelength long, then
the Zi = ZL regardless of the feedline impedance Z0.
(this is only true when the load is not reactive, ie,
ZL=R + j0).

If the line length is neither of the above then all
bets are off and the Zi is transformed by the
"Transmission Line Equation". See the ARRL handbook for
the definition but do not expect to understand it.

Note that the equations apply to all elements making up
the feedline, including anything grounded between the
transmitter and the antenna!!!

2). The SWR on the feedline is constant and cannot
change by changing the feedline length.  If the SWR
does change, then there is a problem with the feedline
caused by what are called 'common mode currents'. 
In other words, the feedline has become part of the
antenna and  changing the feedline length is changing
the antenna  length. When this situation occurs, a
balun is mandatory to prevent the common mode
currents.  However, changing the feeline length may
allow a tuner to find a better match but that does not
mean that the SWR has changed, only that the
transformation that occurs is creating a different r +j
for the tuner.

3). Tuners do not "fool" the transmitter into seeing a
different load.
 A tuner provides what is called a 'Conjugate Match'
between the transmitter and the antenna. The conjugate
match, in a simplified way, takes reflected power and
adds it to the outgoing forward power to be delivered
to the load. For example, if 1/3 of the power is 
reflected at the load, then 2/3 of the power is
absorbed by the load. The tuner takes the reflected 1/3
power and adds it to the outgoing forward power to be
delivered to the load.  A 100 watt transmitter in this
case, would have 150 watts outgoing, 50 watts
reflected, and 100 watts(or 2/3 of 150) delivered to
the antenna.
Tuners can have losses and are not meant to match
anything we can think of.  The reflected power adds to
the loss in the line, which can be quite high when
using coax.  Open wire lines are much less lossy which
allows high SWR on the line with minor signal
degradation.

4). "A low SWR tells me what the feedline is doing,not
what the antenna is doing"(From LB Cebik). We seem to
be obsessed over SWR and forget that is not the primary
motive here.  A lossy coax feedline dampens the
SWR curve as the load SWR increases which can fool us
into thinking our antenna is efficient, when is it is
not.
The G5RV antenna is NOT an all band antenna. The longer
the coax feed, the higher the losses and corresponding
dampened SWR curve. A G5RV with 66' of RG58 can have
SWR over 100:1 at the antenna, but will show 15:1 at
the transmitter and have 10 dB of loss! A very nice
dummy load. The best G5RV is one with no coax!!.
 The typical 33' of ladder line is meant to provide 1/2
wavelength of feedline on 20 meters to make the load
impedance transparent thru the feedline.
On other bands, it's different of course.

 Pat W0OPW

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