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All-Band Sloping Inverted "L"
Antenna
by
LA1IC
Put that 4:1 balun, a couple
of insulators and some extra wire to good
use!
If your major
antenna problem is deciding whether your new rotary beam will be a yagi or
a quad, this article is not for you. I am aiming at two groups of
amateurs. Those in one group consider themselves lucky to have an antenna
at all. Any antenna they put up must be simple, inexpensive, easy to
erect, and hopefully a good radiator on a number of bands.
Those in the other group are enthusiastic
participants in amateur radio's favorite outdoor sport -
experimenting
with antennas. Here are two SLOPERS for your sporting
enjoyment!
Please note....the article
below is in two sections: Metric and non-metric! Take your
pick.
Note: In picture above,
30cm = 11.8 inches (12
inches)
7 - 8 meters = (average 7.5 meters = 24.6 feet)
1 -2 meters
= (average 5.00 feet)
The
Inverted "L"
HOW IT WORKS:
Longwires fed against earth which are 5, 7, 9, 11, 13 or 15
electrical quarterwaves long will have feedpoint impedances somewhere
between 150 and 250 Ohms. Consequently we can feed such antennas with 50
Ohms cable via a 1:4 balun. At the same time the need for good earth
becomes less important. A galvanized iron tube driven into the ground with
a few short radials around, will do.
"MAGIC" WIRE LENGTHS
Are
there any lengths of wire where odd numbers of quarterwaves correspond on
several amateur bands? Yes, indeed. Take a look at these two alternatives
presented in Metric and non-metric. Odd quarterwaves in
BOLD.
NON-METRIC
CALCULATIONS
(See
metric calculations below)
Alternative 1:
15M: 246 x 10.95
/ 21.050MHz = 128
ft.
12M: 246 x 12.95
/ 24.900MHz = 128
ft.
10M: 246 x 14.95
/ 28.750MHz = 128 ft.
Alternative 2:
30M: 246
x 4.95 / 10.030MHz = 121.4
ft.
20M: 246
x 6.95 / 14.085MHz = 121.4 ft.
17M: 246 x
8.95
/ 18.140MHz = 121.4 ft.
=======================================
METRIC
CALCULATIONS
Alternative 1:
15M: 75 x 10.95/21.055(MHz) = 39
meters =127.95 Feet (128 Feet)
12M: 75 x
12.95/24.900(MHz) = 39
meters
10M: 75 x 14.95/28.750(MHz) = 39
meters
Alternative 2:
30M: 75 x 4.95/10.035(MHz) = 37 meters = 121.39
Feet (121.4 Feet)
20M: 75 x 6.95/14.085(MHz) = 37
meters
17M: 75 x 8.95/18.140(MHz) = 37
meters
The numbers in
BOLD above are the number of odd quarterwaves on the antenna on bands in
question, including the velocity factor(0.05%) in free space by which one
must shorten the wire. This factor differs a little from bare wire to
PVC-insulated wire but this is of no consequence in this relation. Other
minor shortening factors are surroundings, distance to electrical earth,
quality of earth and so on. So our 128 ft. of wire will always have to be
pruned a little in order to achieve resonance.
By
the way, I always tell newcomers that it is easier to cut a wire than to
lengthen it!
TUNING PROCEDURE for the INVERTED SLOPING L
ANTENNA
START
WITH 39 METERS
With 39 meters of wire, (128.0 feet of #12 to 16 ), in
operative height you start trimming on 15M. Cutting 18cm of wire (7.00
inches) will raise the frequency about 100Kc. Continue little by little
until you get minimum SWR somewhere around 21.055MHz. Resonances on 12 and
10M will be found around 24.9 and 28.750MHz, respectively.
You have
now optimized your antenna for these three DX-bands.
(10, 12, and 15
meters)
10, 12, 15M with one length and 17, 20 & 30M with another
length ALL WITHOUT AN ANTENNA TUNER ! THAT'S 6 BANDS WITH 2
ANTENNAS!
With the
"meager" years of propagation ahead you may prefer to optimize your
antenna for 17, 20 and 30M operation. But wait a little before cutting to
much! If you trim it up to about 21.450MHz you will still have a SWR of
1:1.2 on 21.000MHz. Resonances on 12 and 10M are now around 25.3 and
29.350MHz. But still the SWR is only 1:1.2 on 24.9 and 28.7MHz.
Let us
see what happens to the other bands. Resonance on 17M will be around
17.850MHz with a flat SWR curve up to 1:1.2 on 18.240MHz. On 20M you will
find SWR 1:1 on 13.9MHz, with a flat curve up to 1:1.2 on 14.320MHz. And
finally, with a 30M resonance on 9.9MHz we still have a SWR of 1:1.2 over
the entire amateur band.
As you have seen, like all asymetrically fed
antennas this one is also very broadbanded. In other words, there is no
urgent need to go on with Alternative 2 until the 10, 12 and 15 meter
bands become hopelessly quiet in a couple of years!
40 AND 80
METERS
Although this antenna is not resonant on 40 and 80 it will,
using your ATU, do a fair job on these bands as well.
160
METERS
On 160M our antenna is a quarterwave fed against poor earth,
which in turn raises the feedpoint impedance somewhat. Nevertheless, even
on this band the SLOPING INVERTED L antenna will get you on the air!
73! Rolf, LA1IC
Submitted by Rolf Brevig,
LA1IC
(Age 73 - licensed radio amateur since 1949), - and
originator
of the
"Coaxfeed 6
meter Extended Double Zepp Zip Cord Antenna on
Hamuniverse.com!
73
See also
Rolf's Slopping Longwire plans here
ARRL Antenna Book! Get your copy here!
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author
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