Pattern, feed, etc all depend on the overall length of the inverted L
compared to the wavelength of the frequency in use.
For instance, an inverted L 65 feet up and 65 feet out can be used:
a) on 160m as ~1/4 wave inv el
b) on 80m as ~1/2 wave inv el
c) on 40m as ~ full wave inv el
d) on 30m as ~3/2 wave inv el
Whether you are at an exact length (exact 1/4, exact 1/2) is not
critical.
This is because NONE of these are 50 ohm antennas. Tuning/matching of
one sort or another is required, unless you are using the 1/4 wave
mode and have a BAD (LOSSY) ground system. In the latter case you have
a great SWR to an ugly antenna losing more power in the dirt than
radiating from the wire.
** Feed impedances for the four modes above:
a) 20-25 ohms
b,c,d) 450-1500 ohms, varies considerably, highest right at exact
halfwave lengths for frequency in use.
** Radiated patterns for the four modes above:
a) Mostly like the pattern of a 1/4 wave vertical, good low angle
radiator. Has a minor amount of high angle radiation from the
horizontal part. Most of the antenna current is in the vertical
portion, however.
b) The radiation pattern is omnidirectional, and hemispherical. Gets
high angle radiation from the horizontal part, and low angle from the
vertical part. This is an excellent all-direction compromise antenna.
c) Has the same pattern as a half wave dipole strung between the same
supports as the horizontal part. Mostly low angle broadside to the
wire. This is not intuitive but true nonetheless.
d) Has a "lumpy" low-angle omnidirectional pattern.
** Ground considerations for the four modes above:
a) Good radial field is critical. Because of the lower radiation
resistance, is even more sensitive to radials than a regular 1/4 wave
vertical.
b,c,d) Radial field can be minimal, can be moderately successful with
nothing but a ground rod, not that I recommend that. Four or five
miscellaneous buried radials and a ground rod at the center will do
quite well.
** Feed/matching considerations:
a) 1) Tuner at the base 2) Matching section consisting of two
electrical 1/4 wave sections of 75 ohm coax in parallel 3) two to one
impedance reduction unun
b,c,d) 1) Parallel tuned circuit between wire end and ground. Tap and
capacitor varied for match. 2) 3:1 or 4:1 turns ratio unun. Provision
for high voltage required. 3) Tuner at the base may work or may not
have enough range to match. Random wire tuners will work.
----- Original Message -----
From: "Jim Johnson" <jim@kc4hw.net>
To: <towertalk@contesting.com>
Sent: Thursday, August 29, 2002 3:03 PM
Subject: [Towertalk] Inverted L
> What is the formula for figuring the appropriate length of an
inverted L?
> Unfortunately, my Antenna Handbook does not contain that
information.
> Nor was I able to find it in ON4UN's Low-Band DXing.
>
> What is the radiating direct of the horizontal portion of the L? Is
it
> boardside or off the end?
>
> Originally, I sent this to the wrong address, my apologies!!
>
>
>
> Jim Johnson, Melbourne, Florida
> jim@kc4hw.net - http://www.kc4hw.net
> DXCluster RF Connection: 144.95
> Telnet://kc4hw.homelinux.net:8000
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