Don,
You are quite right about how easy it is to put up a Half Square, and it is
quite a forgiving antenna as far as dimensions are concerned, although
symmetry seems to be important. When they were up I fed each of my 40m Half
Squares at its corner using the outside braid of the feeder as one leg of
each antenna, along with a high impedance L - C tuned trap a 1/4 wave down
from the feedpoint using the braid of coiled feeder as the L - solenoid
wound not scrambled. I used two Half Squares at right angles because the
nulls off the ends of a Half Square are quite deep at low angles.
I agree with your comment about the choking impedance offered by a simple
coil of coax, which is why I use high impedance tuned L-C traps in these
applications with the braid of the coiled feeder coax acting as the L. A
simple coil of coax could be used if its interwinding capacitance resonated
with the inductance of the coil at or very close to the operating frequency
thus increasing the impedance, but not a good idea. One penalty gained when
using the braid of the feed coax wound as a coil for a tuned L - C trap is
that the jacket of the coax has to be opened at the start and end of the
coax coil to gain access to the braid for connecting the capacitor, so
weatherproofing is very important at those points. The function of the trap
is simply to establish the length of the outside of the feeder's braid that
acts as the bottom 1/4 wave of the dipole, or leg in the case of a Half
Square, and prevents the real antenna current going any further. The trap
has no effect on the feeder itself, other than to increase its length, since
the feed currents are flowing inside the coax.
Hope that this clarifies some.
73,
Geoff
GM4ESD
Don W3FPR wrote:
A half square antenna is also a phased pair of 'upside-down' verticals -
easy to get up if you have 2 supports just a bit more than 1/4 wave high
and
a bit more than a half wavlength apart. Feed at the upper corner directly
with coax or with a parallel resonant tank at the lower end of the
vertical
section.
I have one observation about the 'coax dipole' that you described - the
choking impedance must be very high for it to work properly - consider
that
the impedance at the end of a halfwave dipole is quite high (4000 ohms or
so), to effectively de-couple the remaining coax shield at the 1/4 wave
point, you would need a choking impedance at least 5 times the impedance
at
that point - more like 20,000 ohms. The impedance of a few turns of coax
is
not going to make an effective choke at this high impedance point. I
might
consider it to be a 3/4 wave dipole fed at the 1/4 wave point with the
choke
placed at a point 1/2 wavelength down the coax. I guess I have to try one
just to see how the feed impedance works out, several folks report success
with these coax antennas, but I do not understand how the choke could work
at 1/4 wavelength down the coax.
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