The Invertel 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.


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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