Floating L over FCP

Vertical Wire Horizontal Short to Tower

(Experimental)

**If you decide to attempt this experimental scheme please get in touch with K2AV at k2av [dot] guy [at] gmail [dot] com. Hearing details from the first implementations is very important, especially so if there are issues discovered that need to be resolved, or needed adaptations to the design. We need to communicate improved and tested details as soon as possible.**

**This design reduces RF losses in the tower base and close dirt surroundings by shorting the Inverted L's vertical wire to the tower. The short supplies a direct counter-voltage to the induced voltage, producing a minimized current everywhere in the tower, including the tower base. The vertical wire is supported three feet (1 m) from the tower for its entire length. The FCP is at a minimum 10 feet (3 m) above ground.**

The NEC 4.2 model to the left contains a 70 foot tower plus enough top-loading to simulate a tribander on a mast. The tower goes to ground with a "buried" 20 foot section in red, length picked to approximate all a tower's ground connections. The amount of RF current driven into the ground is the loss issue, so to model the problem literally requires NEC 4. A ** partly** correct NEC 2 modeling can done using perfect ground and a series load in the second segment from ground. Changing the R value will demonstrate the loss potential. However it will poorly demonstrate the effects of nearby real ground, and how the tower base losses increase with poorer ground around the base.

The relationship of the tower to ground is fixed because the base is buried in the ground and further connected to ground with lightning grounding, etc. To allow the horizontal short to force the needed relationship between the L/FCP current and the tower current, the L/FCP has to ** float** and the feed coax

The NEC 4 wire and RF current illustration to left and below represent the same wires and tower as above left. The viewpoint has been rotated around the Z (vertical) axis to maximize the model's display of the RF current in the vertical wire and tower.

The illustration to the immediate left displays the tower current ** without** the short between the L vertical wire and the tower. The tower current is highlighted. Note that there is far more total (integrated) current in the tower than the L. At the bottom of the tower the RF current is 4.5 amps at 1500 watts, the highest current anywhere in the antenna wires and tower, and those 4.5 amps are being driven into lossy ground. This is true even though there is no direct RF drive to the tower.

The wire and current illustration below left displays the tower current ** with** the short between the L vertical wire and the tower. The tower current again is highlighted. Note that the amount of overall tower current is significantly reduced. Especially reduced is the current at the base of the tower being driven into the ground. The tower current at the ground is 1.7 amps at 1500 watts. The absolute value of the loss at the base of the tower will depend on the effective RF series loss at the base.

The RF current on the tower above the short is in-phase with the current on the vertical wire. Below the short it's out of phase. Compared to loop detuning of a tower and the high current within the loop, the current everywhere in the tower is considerably less than the vertical wire's 6 amps average. The vector current sum of wire plus tower is about same above and below the short. The L/FCP/tower combination has less of a weakside null than a free L. The combo at this height consistently models around 25 ohms feed Z. It will take some experience to see if that holds up in practice.

**By design, the L and FCP must be allowed to float against the tower's RF potential at the horizontal short to produce the desired minimal tower current and ground losses. At QRO this can assert thousands of volts RF between lower antenna system wires and the tower/earth. This produces the following requirements:**

2) ** The junction of vertical wire and FCP must be fed with the isolation transformer described in

5) ** The base of the tower must be grounded. Normal tower lightning ground and/or Ufer ground is sufficient. **

At 1500w NEC 4.2 models demonstrate that substituting a ferrite balun or block for the isolation transformer in this application can have disastrous consequences. Even an otherwise excellent 5000 ohms resistive ferrite block on 160 meters can have a thousand volts RF across its blocking resistance, dissipate hundreds of watts, and destroy the device, possibly setting it and surroundings on fire. A number of times in pre-publish FCP history K2AV and W0UCE did exactly that, once setting a utility building shelf on fire. If we had not looked out the window and seen smoke, it probably would have burned down.

In modeling, this design appears to support the 80/160 dual banding method for the standard L. Adding to the dual band circuitry, an inductor is substituted for the short between vertical wire and tower while on 80 meters.

This table presumes the tower is topped by a medium sized 14-28 MHz multi-band beam antenna. Data below shows the shorting wire heights for various horizontal wire and tower heights. At lower heights in the table, data for the slanting vertical method demonstrates where the two methods overlap and deteriorate in the other's range. The mechanisms creating this crossover point are not clearly understood.

The horizontal (or horizontal wire plus drooper wire) is always 88 feet in this table. This technique optimizes low angle results by keeping RF current density high on the vertical wire. See for further details.

Figures are from NEC 4.2 modeling over "average" ground. For reference, a 1/4 wave vertical over USA FCC specified commercial radial system (120 times buried AWG 4 copper 0.4 wavelength buried radials) in "average" ground produces 1.18 dBi gain.

Tower |
L Horiz |
Shorting |
Shorting |
Slanting |

Height |
Wire Ht |
Wire Ht |
Max Gain |
Max Gain |

--Ft-(m)-- | --Ft-(m)-- | --Ft-(m)-- | dBi@Elev | dBi@Elev |

120 (36.6) | 100 (30.5) | 74 (22.6) | +0.16@24 | ---------- |

110 (33.5) | 90 (27.4) | 62 (18.9) | +0.03@25 | ---------- |

100 (30.5) | 80 (21.3) | 57 (17.4) | -0.11@25 | ---------- |

90 (27.4) | 70 (21.3) | 51 (15.5)) | -0.18@26 | ---------- |

80 (24.4) | 70 (21.3) | 44 (13.4) | -0.39@27 | ---------- |

70 (21.3) | 70 (21.3) | 39 (11.9) | -0.40@29 | -0.92@24 |

65 (19.8) | 65 (19.8) | 32 (9.8) | -0.56@29 | -0.76@25 |

60 (18.3) | 60 (18.3) | 28 (8.5) | -0.89@29 |
-0.75@26 |

55 (16.8) | 55 (16.8) | 23 (7.0) | -1.20@29 |
-0.86@27 |

50 (15.2) | 50 (15.2) | ---------- | ---------- | -1.05@28 |

Note how results at heights 60 and 55 feet are quickly dropping off. The short's placement in modeling does not appear critical. Moving the short up or down a foot or two varies results very little. The positions posted above are the center of the usable range. For heights between listings, scale the setting between the listing above and below to better approximate the center of range.