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Kevin Muenzler wrote:
>
> Gang,
> I've seen the inverted-L described a couple of times but haven't
> paid much attention. (sorry) Would someone please
> describe to me the inverted-L antenna.
Dave Henderson Wrote:
>>Kevin
Simply put an inverted L is a vertical antenna in which some portion of
the upper end of the vertical is bent over 90 degreees parrallel with
the ground. They are most ofetn seen on 160 meters where the mechanical
challenges of putting up a free standing 130' structure are either
mechanically challenging or prohibitive expensive for most people's
budgets.
For exaample an inveretd L for 160 meters which is 50' high but 85'long
on the vertical leg will work within a db or so of full sized vertical.
Another trick with inverted L's is to make there total length about 3/8
of a wavelength, then tune out the inductive reactance with a simple
series capacitor. At this length the Inverted L the "real" portion of the
feed point impeadance is very close to 50 ohms. The radition pattern is
pretty much vertically polarized and omni-directional at low take-off
angles, At higher angles(above 30 degrees) there is a slight pattern skew
in favor of the direction of the end of the horizontal wire (not broadside!).
I have used this antenna on 160 meters for over 10 years (175 countries worked
so far), In fact in recent years I have used two of them in phase. One final
note
to be really effective as with any vertical antenna you need a really good
ground
system made up of tens of radials.
Dave
soon to be the ham "formerly known as N0DH"
Couple of additional comments:
Either of the Hygain options of base loading coil or
inverted L trap at 24 feet are not attractive.
Several hams have beefed up the 80 meter portion of a
Hytower to take horizontal load of wire. Then, with guy
lines, one can put the trap only a few feet below the top of
the 80 or 75 meter element.
If possible strip the stubs for bands you don't need.
Best trap construction is a B&W or home made coil about 3
inches diam by 4 inches winding length shunted by 170 to 200
pfd of ceramic door knob capacitance. Dont try to make the
capacitor from one component. Put two 100's or 85's in
parallel to handle the current. Depending on value of
capacitors, the number of turns is 20 to 25, using 10 or 12
gauge wire. It will also give inductive loading to the
inverted L wire of 10 - 15 feet. This means if you go up 50
feet and were going to go out 80, the trap provides loading
so the horizontal wire is 65 -70 feet actual length.
Another way to gain height is to find a straight section
from an old TV tower from the Rohn-Spaulding AX series, and
then guy the top of the Hytower as well as the point the
trap and loading wire attach to the vertical tubing above
the tower portion.
Good luck George, K8GG
PS: depending on ground conditions, somewhere between 160 - 180 feet,
somewhat less than 3/8 wavelength overall length may be better as
there is less chance for making a 1/4 wave horizontal radiator out of
the horizontal part of the inverted L.
One of the older articles said a 3/8 wave inverted L only about 50
feet high would come in with an impedance of Z ant = 50 + 400j ohms
and needed a vacuum variable set to about 250 pfd to tune out the
reactance.
A 5/16 inverted L about 80 ft up and 80 ft horizontal comes in at
Z ant ~= 50 + 200j, requiring a series capacitor about 500 pfd to tune
out the ractance.
73 agn George
--IMA.Boundary.340167348--
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