[Mobile-Portable] 10 meter mobile

[John Patterson]

Some additional thoughts....

If the mobile ground system (strap plus connection plus body and frame) were
perfect at the frequency of interest, the 1/4 wave whip could only yield a
best-case base impedance of approx 36 ohms, mostly resistive. Note that
under these perfect conditions, your SWR at the antenna base, measured in a
50 ohm system, would be 1.4:1. Since mobile ground systems never approach
perfection, the actual base impedance of the antenna, especially its
reactive component, can vary widely.

The constant VSWR circle argument is often used to refute the concept of SWR
changing down the line. But it only applies if your installation results in
no RF current flowing on the outside of the coax. In other words, rarely.
Especially in mobile installations, where a normal HF mobile system must use
a "ground" that is oddly shaped and may even intrude into the near field of
the whip itself! Such installations can easily generate outside currents on
the coax. It is possible to choke off most such currents by use of a choke
balun, but not many mobiles use them; perhaps they should. Suffice it to say
that these outside currents foul up the typical ham SWR bridge and cause it
to give different readings at different points along a mismatched feedline.
THEY ALSO FOUL UP YOUR RIG'S BUILT IN SWR BRIDGE, THE ONE THAT TELLS THE RIG
TO REDUCE OUTPUT WHENEVER IT DETECTS UNACCEPTABLE SWR READINGS.

Such conditions do not lend themselves to pristine constant VSWR circle
conditions. Which is to say that RF currents on the outside of the coax are
the norm. In these cases, whatever the base impedance is, it is read as a
different impedance at the distant end of the attached 50 ohm feedline. That
is the impedance the transmitter "sees". Varying the length of that feedline
WILL produce differing impedance (SWR) readings to the TX, even though the
antenna's base impedance, and thus the ant/feedline SWR, have not changed.

So it is true that in a typical mobile installation, where RF currents are
flowing on the outside of the coax, changing the length of the line will
present differing impedances (SWR's) to the rig's SWR detection circuitry
and to any in-line SWR meter the chief op may be using. It is also true that
in an ideal scenario, where the coax feedline is properly terminated to the
radiating element and the "ground" system, and no RF currents flow on the
outside of said line, the measured impedance will show no appreciable change
with line length.

John,  K9OF