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Designing a High-Performance Inverted L to Fit Your Property

Though this section on inverted L's is embedded in a web site about the FCP, the content of this section applies to any inverted L regardless of it's counterpoise. Much of the "I've got problems" correspondence after installing an FCP had nothing at all to do with the FCP. They had issues to discover with the L chosen for the aerial wire. Removing the loss of a poor counterpoise often exposes other issues with remedies that enhance performance.

If you are designing your 160 L over FCP before you begin permanent things like digging holes and pouring concrete, you are really lucky. Study the

You may be able to avoid a lot of hand-wringing experienced by those of us who discovered these things after we strung up wires. Sometimes there are profitable 160 meter avoidances hard to implement after the fact. You will have to be the arbiter of choices.

Reading end-of-August 160 meter plans and antenna speculations, it's interesting that one reflector's most popular thread concerns 160m four squares. Well, if you have the space, time and money for that, go for it! Crank it up and get it playing for the 160 season.

But most hams seem to lack one or all of property, time and money for 160 meter 4-squares.

So we will describe our best solution for little property, time or money, and explain why it's done that way. Then you could do it like we lay it out, or based those details and the reasons behind them figure a variation that better serves your needs.

Aerial Support

A few fortunately spaced mid to large-size trees often come with the property. Using trees for support has fewer and easier performance issues to discern and solve than support from buildings or towers.

For those who must support an inverted L bend from a tower:

The vertical wire will induce large RF currents in the tower and its cabling. This current is driven into the earth causing significant losses. It travels to earth through the tower base directly or via capacitive coupling from coax shields and insulated cabling between the tower and shack.

For further explanation and countermeasures see and . Depending on the size of your tower, one of these will reduce the induced RF current at the tower base.

Use of Modeling in these Conclusions

Many urban and suburban building lots were filled in and leveled with a polyglot of construction rubble, rocks, concrete, dead clay, etc, and finished off by covering with topsoil. In some parts of the world populated for millennia these properties lay on top of centuries of building ruin. These tend to be the smallest lots, obvious candidates for compact 160 meter antennas. The composition from surface to 10 meters deep makes those locations simply awful "earth" at MF frequencies.

Worse, ham owners almost never possess verified data describing their plot's ground characteristics at RF. Nor do they own the test gear to measure it with useful accuracy. Hams may have "extremely poor" earth for RF purposes with no clue to their ground situation other than the very non-specific tough time working new countries. This creates a need for solutions that maximize performance optimized for "extremely poor" earth without needing ground data, and without penalizing more fortunate situations.

When modeled over "average" or "good" ground characteristics, various aerial wire configurations may show little change between alternatives. "Stress testing" these variations with NEC 4.2 using "extremely poor" or "worst case" ground characteristics frequently exposes significant variation in results. "Sweet spots" in results over extremely poor earth clearly identify the desired solution which maximally serves all sites regardless of ground quality.

Those who only use "average ground" in their modeling sometimes fail to verify our results in their own model runs. That would not be an issue if in fact you live over fairly homogeneous "average" ground measuring (.005,13) with proper instrumentation. Compare your alternatives setting ground charactistics to "extremely poor" (.001,3) or "worst case" (.0005,1). If you do have really good earth, you won't save as much applying our solutions. But if you have the far more common poor-to-awful earth situation, you won't miss the solution that renders a big improvement.

Counterpoise

The FCP's dimensions, wire sizes, using bare wire, number and kinds of spacers, etc, have been refined since 2011 via modeling, experiments and field experience. Be sure to use current specifications. Adjustments to FCP specifications and dimensions, other than increasing bare wire size, will result in increased loss.

Over time the FCP's recommended height above ground has been raised to 8 to 10 feet (2.5-3 m), preferably 10 feet. With experience we discovered a need to keep a 10 foot (3 m) diameter tunnel around the FCP free of trees, bushes, vegetation, or any dielectric material. This cannot always be provided, but do what you can.

There is obviously a tradeoff between FCP height and the possible length of the aerial wire's vertical segment. Thus far, up to ten feet, the tradeoff has favored FCP height.

Vertical Wire

The essential design consideration for the vertical wire is to make the upper end as high as possible. The location specifics will limit this to some maximum height. The bend height will lose a little height from the support height to support the bend far enough out for adequate spacing from an RF-absorbing tree trunk and branches.

The nature of the property likely requires the FCP as counterpoise.

To give ourselves a better grasp on inverted L's we will compare an L over an FCP with a better understood vertical over an FCP. This will expose differences, advantages and disadvantages.

Our comparison L is an FCP at ten feet (3m), with the vertical length a very average 55 feet (17m) over the FCP, setting the L's bend at 65 feet (20m). The horizontal is a predetermined "center of range of best modeled patterns" calculation, 87 feet long (26.5m). A portion of the 87 feet may be supplied by a drooper wire at the end of the horizontal. This serves where a straight pull of 87 feet is not available. 87 feet is always used unless mildly modified in the procedure for tuning an L.

We are not comparing an unrestricted best possible vertical against an unrestricted best possible inverted L. We are constrained by the best possible vertical height on the specific property where construction will take place. We compare two antennas with identical vertical wires in the same place and position, over the same FCP counterpoise. We compare a pure vertical versus the same wire with an attached horizontal, an inverted L.

The superimposed NEC 4.2 plots upper left show comparison results. Four curves are shown. Two are over "average" ground with "AvgGnd" in the name and two are over "extremely poor" ground, with "ExPoorGnd".

There is an urban myth that the inverted L loses low angle performance because the horizontal uses up power radiating straight up. This is clearly not true if both antennas are limited by the same height maximum.

The L's upper takeoff angles are filled in, eliminating the doughnut hole in the vertical pattern. The inverted L wins at any angle toward the bend. The L radiates more power up and more power toward the bend, while radiating the same power toward the end. Adding the long horizontal wire has added power to the pattern, not redistributed power as in the myth. How is that possible?

The vertical top-loading effect of the horizontal increases the RF current density at the top of the vertical, launching RF less subject to ground and tree losses. The top-load also raises the feed R, lowering current at the feedpoint and in the FCP, lowering fields at the ground and reducing induction and dielectric loss from the low parts of the antenna. Reducing these losses increases total power in the pattern. Figures middle left and lower left illustrate the differences in RF current distribution between pure vertical and inverted L with identical vertical wires.

The currents on the vertical wires are highlighted, and the RF current display scale is set the same on both. The pure vertical's concentration of RF current close to the ground creates a disadvantage, as seen middle left: 25% of RF current density in the upper half of the vertical wire and 75% in the lower half. RF current close to the ground induces current and dielectric loss in the ground. The L, lower left, has 45% of its vertical wire RF current density in the top half and only 55% in the bottom. The power density is proportional to the square of the field density, so the inverted L will launch 3 or 4 times more power than the pure vertical from the more efficient upper half of the vertical wire.

The FCP current is higher in the pure vertical because the vertical's feed R is much lower than the L's at the base of the vertical wire.

Adding a "T" top to the pure vertical will decrease these differences, but will also force the vertical farther away from the support by half the length of the T top, shortening the vertical length for the same tension on the support rope(s).

Horizontal Wire with Vertical Wire Length 49 feet (15m) and Shorter

This may involve extending the horizontal plus possible drooper beyond a total of 87 feet (26.5m). This depends on information supplied in the horizontal and drooper wire segments below, and will be discussed following those.

Horizontal Wire, All Vertical Lengths

With sufficient vertical length, the major function of the horizontal wire is improving antenna efficiency being a topload for the vertical wire. This increases RF current density on the top half of the vertical wire, as discussed above. 87 feet (26.5m) is at the peak of a broad data curve generated by perhaps 500 NEC 4.2 model runs of horizontal length versus efficiency at various heights. The broad nature of the curve allows smallish adjustments to the horizontal length to adjust feed resistance and reactance. This excursion should be limited to +/- three feet (1m) to stay in the efficiency sweet spot, but can be more if you accept the reduction in efficiency wandering off the sweet spot.

Drooper Wire

What if there's not enough clear air space available between potential L support points to support 87 feet? Other sections on this site warn against losses incurred running aerial wire through or on top of trees to gain length.

To deal with small support spaces, erect the L with the bend in the clear per text in prior paragraphs and the far end with 3 feet (1m) of space between the horizontal wire far end and leaves/branches of the far support tree. Drop the horizontal and measure it. 44 feet (13.5m) is the practical minimum for the horizontal wire while retaining reasonable efficiency. There are some 45' + 45' + 45' deployments which could be called a 1/4 wave inverted U over FCP. But the web site material still applies. The weakness in the horizontal is deeper and the feed R is lower in this case.

Subtract the horizontal's length from 87 feet and use the result to construct a "drooper" wire. Attach one end of the drooper wire to the far end of the horizontal wire. Add an insulator and support rope to the remaining end of the drooper wire. Horizontal plus drooper will provide the desired top load for the vertical.

In deploying the drooper, avoid pulling the drooper back toward the vertical if at all possible. Particularly for longer droopers, better results follow if the drooper is pulled to either side off vertical, and/or further away from the bend. In some current successful deployments, the drooper has miscellaneous bends in the needed drooper length to somehow get the needed vertical top load while avoiding various lossy dielectric items in the area

Larger droopers will increase the L's pattern weakness in the horizontal direction.

L's with drooper wires should normally be adjusted for R and X at the far end of the drooper.

Horizontal plus Drooper for Vertical
Wire Length 49 feet (15m) and Shorter

While the pattern and gain are well served by horizontal plus possible drooper = 87 feet (26.5m), a vertical shorter than 49 feet (15m) may present a tuning issue. It may be necessary to extend the horizontal wire (or drooper wire if you have it) to get a feed R+X within tuning range. This is usually preferable to using a coil at the feedpoint.

A larger drooper is sometimes not possible and the tuning method must accomodate the larger matching task.

Methods for best placement, and tuning your inverted L over FCP with the many issues involved are discussed in sections and .

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