[ARC5] AN/ARC-5 on MF 630 Meters

[David Stinson]

On 3/19/2022 1:45 PM, Hubert Miller wrote:
> ...The feed is higher off the ground,
> but you wouldn't think this smallish difference mattered.
> -Hue

I think I see what he means now, feeding it using an
auto-transformer configuration, but raising the bottom
of the loading coil and coax feed higher
above ground.  It's possible that could increase radiation,
under specific circumstances.

With the loading coil sitting on ground level, the coil has
a lot of capacitance to ground relative to the rest of the
antenna, so there is a large circulating current at a low
elevation.   Radiation resistance- the "imaginary" resistance
in which current flows and radiates signal- depends on
currents flowing at elevations above ground.  Currents
flowing to ground from a coil on the ground radiates
very little.  The top of the vertical element has a small
capacitance to ground and a smaller current, but
a much higher elevation, so the radiation resistance at
that point and the signal launch from it are much better.

The vertical parts of an electrically-short antenna-
meaning less  than 10 degrees long- is in fact
the only part that creates significant radiation.
The horizontal "top hat" is a capacitor plate, just like
the ground below it.  Each plate is in the opposite phase
of the other, as one would see with between the elements
of a horizontal dipole.  Since the
length is so short, the fields of the two plates are
very nearly 180 degrees out-of-phase from and parallel
to  each other, so they cancel radiation from each other,
leaving only the effective height of the vertical elements
to calculate radiation resistance.

The horizontal "top hat" wires are a capacitor plate,
large relative to the rest and the vertical
element, which is in series with it.  So the "top hat"
capacitor causes a larger current to flow through
the vertical element at it's very top, and that
greatly increases the height/current flow calculation.
A loaded mobile whip antenna on your car
works the same way- the "tuning stinger" is both
part of the vertical element (in this case,
radiating) and a capacitor plate, your car being the
other plate.  If you want to demonstrate this,
connect a clip lead to the top of your mobile whip
at the base of the stinger, and pull the other end
of it out horizontal, then measure the antenna's
lower resonant frequency.  The horizontal clip lead
is a capacitor plate, the other side of which is your car.
More cap means lower resonance.

So- If we move the loading coil up in height, and
the so-called "ground leg" is also electrically-short
so that there's no significant phase shift from the
bottom of the coil to the actual ground connection,
this will greatly reduce the coil's capacitance to ground,
reducing the currents flowing at low level, and cause more
current to flow up high, where it can do some good.

But there is an even bigger issue: raising the loading coil
above ground reduces it's self-capacitance to ground,
which is part of the equation in resonating the
antenna (same as a variable capacitor to ground
across the coil).  If one raises the coil, one must add
more inductance to bring the antenna's resonant point
back down.  This introduces more coil turns and thus
increases IR losses- experimenters in the 20s and 30s
concluded any increase in radiation from lifting the
coil is soon swamped by the added IR losses. I have
a copy of the IEEE paper on this somewhere and,
if anyone is interested, I'll dig it up.

We actually did try something like this back in the 1980s
on 1750M, as part of debate and experiments with
moving all or most of the inductive loading up
away from the ground plain.   I built one with a part of
the inductive loading at the top of the vertical, connected
to the "top hat" capacitor plate and leaving the remaining
needed inductance at the bottom.  Many configurations
were tried.  The general consensus (and there was other
opinion) was that the mechanical considerations of a
loading coil suspended either in the middle or at the top
of a top-hatted wire vertical did not provide enough
improvement to compensate for the difficulty and expense
of making the structure mechanically stable, and instability
also introduced tuning problems.   Most of us decided that
improving ground resistance and IR losses of loading networks
while leaving the coil stable near the ground
would bring better returns than such configurations.

I can hear the objections about "well my mobile antenna
has a center coil and it doesn't have these problems."
Respectfully- your mobile antenna is a rigid structure
6 or 8 feet tall.  A wire element 50 feet tall connected to
a horizontal loading element, as one would use on
630M, 1750M or 2200M is another story.

Other ideas and opinions welcome.
GL OM ES 73 DE Dave AB5S






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