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 to do with the FCP. They had issues to discover with the L chosen for the aerial wire. A poorly done L can cancel all the benefit of an FCP.
If you are designing your 160 antenna layout before you begin permanent things like dig holes and pour concrete bases for towers and guy anchors, you are really lucky. Particularly after you study the you may be able to avoid a lot of hand-wringing experienced by those of us who thought about 160 meters last and only recently, finally goaded by high bands diminishing at the solar minimum.
There are profitable and sneaky 160 meter avoidances to work into the plan which might not be possible after the tower goes up in the wrong place. Note that all the inverted L considerations may not work in each other's favor on your property. You will have to be the arbiter of choices.
The best placement for an L is between and supported from two mid to large-size trees on opposite sides of a clearing with at least 50 feet of clear air between the trees' branches. 90 plus is ideal for simple installation of the 88' horizontal plus maximum vertical wire. Shoot the support rope over the tree to support the bend as high as possible. Allow enough rope beyond the canopy so the wire stays five feet outside the tree canopy or no closer than 15 feet to the trunk, whichever is farthest.
Some will not have this space available, period. Do what you can, get as close to the goal above as possible, to minimize loss. An alternative is to minimize the horizontal to as little as 44 feet and a far end drop wire of up to 44 feet. This turns into an end-fed inverted U with significant variance in feed Z and from an omnidirectional pattern. This is discussed in
We frequently use terms like "inside the bend". Confusion in early correspondence clearly exposed conflicting ways to describe an Inverted L's directions and dimensions. We had to pick one set of terms to use and explain. So with apologies to those with other perfectly reasonable verbal schemes, here are a few pictorial definitions for the inverted L terms used in this document.
The low angle azimuth pattern of an inverted L is best described as an off-center circle favoring the direction looking toward the bend. On flat land it has a mild disadvantage looking toward the end.
In the inverted L plots, the azimuth elevation (takeoff angle) is 20 degrees. The inverted L with "average" ground underneath has a front-to-back of 1.5 dB. With very poor ground underneath, the front-to-back can be as much as 3.5 dB.
Because of the front-to-back, we need to decide what 90 degree slice of the horizon we are least interested in for low angle or DX propagation. Then, starting from the bend, stretch out the horizontal wire in that direction, so looking toward the end is somewhere in that least desired 90 degrees. K2AV looks toward the end at 185 degrees azimuth. 270 through north to 45 are the most important directions for contests from there. 185 is not a precision direction for his least desired, but it's a possible direction given the trees and property lines at K2AV. It has the most important directions in the best half of the pattern.
This exercise will vary considerably per individual, per QTH. There is no one-size-fits-all.
See drawing above for a pictoral definition of "inside the bend". Picking clear space for inside the bend, or clearing out the space inside the bend is one of the most profitable loss remedies in the
In addition to surprising RBN improvements after removing trees inside the bend of an L, NEC4 Near Field tables show a very high RF field concentration inside the bend of the L. The graph on the left shows RF field values along a line parallel to and directly underneath the horizontal wire at 40 feet above ground, extending either side of the vertical wire. The y axis values are volts per meter vertically polarized RF with 1500 watts to the antenna. The x axis values are feet from the vertical wire. Minus values are beneath the horizontal, and positive values are on the other side of the vertical wire.
The field values inside the bend are 6 to 8 times higher than the values outside. This makes the space inside the bend of an inverted L extraordinarily more sensitive to dielectric material than space outside.
Suppose you had a decent sized tree and its root ball in a dump truck, and you could drive it around while keeping it vertical. You drive it 40 feet from the vertical wire inside the bend. Then you drive it in front of the bend 40 feet from the vertical wire. The times 6 or times 8 ratio of the fields inside versus outside the bend means that the loss in watts caused by the tree inside the bend would be 36 to 64 times as much as the loss from the same tree in front of the bend.
Trees inside the bend of an L are sneaky silent killers of RF. At K2AV in the space of 15 minutes, the tree man cut down three juicy, healthy sweet gum trees inside the bend of the L down by the creek. The primary reason why it was done was because the sweet gums were fast growing and starting to tangle the wire in a wind storm. There was also a suspicion that the trees caused loss, so the RBN at W4KAZ (7.5 miles or 12 km distant) was checked immediately before and after the felling. The weather was and had been dry for the better part of a week, and the time was a little after noon local time.
Felling the trees immediately and permanently raised the W4KAZ RBN 2 or 3 dB. That's a lot of loss. A third to half the power on the antenna was warming up the sweet gums. The best trick is to avoid trees inside the bend to start. Lacking that, it's your problem to decide for yourself what to do or not. At least you know that the loss factor is there.
Depending on the genus of tree and time of year, being too close to the tree's trunk will induce some degree of lossy current or dielectric loss in the trunk itself. Looking at the near field strength graph, on the positive number side of the graph, 20 feet to the right is down to about a third of the NEC4 maximum near field strength. However, increasing the distance on the support rope starts to severely lower the height of the bend. 15-20 feet (4.5 to 6 m) is a compromise between reducing loss at the trunk and losing height of the vertical wire.
The strongest near field RF fields are right at the wire. That means that stringing aerial wire through the tree, or dropping the vertical wire down through the tree, puts the tree in very high RF near fields. There have been some number of cases where stringing an L through the trees has invoked both inside-the-bend losses and too-close-canopy losses, producing significant total loss and defeating the project.
If the horizontal direction is also up a hill, the horizontal direction weakness will be further weakened vs. level ground. If the direction of the bend is up a hill, DX angles in the direction of the bend will be reduced vs. level ground.
Model the tower plus boom and longest elements of the top HF antenna on the tower. Model the L and FCP. In EZNEC NEC2 versions, use perfect ground. Place a series load in the second to bottom segment at the base of the tower and set the R value to values from 25 to 500 ohms to see the effect of the tower driving RF into ground. The pattern will allow you to see if the arrangement diminishes a required direction.
In this case it is imperative to take steps to minimize the RF current at the base of the tower. This current is being driven into the ground, and will be dissipated as pure loss.
For towers 65 feet (20m) and above, consider the experimental floating L/FCP plus short to tower method in This scheme reduces RF current in the tower overall, and in particular reduces the RF current driven into the ground at the base of the tower. This method starts to drop off at 60 feet (18m) outperformed by the 60- method.
For towers 60 feet (18m) or less consider the slanting vertical wire method found in
The 60- method is already in use in the field. With the bend supported 3 feet (1m) from the tower, the vertical wire slants down to the FCP with its center on a pole separated from tower by 13 to 15 feet (4 to 5m)
This method is outperformed at a tower height of 65 feet (18m) and above, by the 65+ method above.