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INTRODUCTION

"Loading" is a way to lower the (primary) resonant frequency of an antenna radiator. This technique is typically used in antennas that have radiating elements that are too short for the desired resonance frequency. There are several standard ways to load a radiator (ref. 1, 2, 3):

  • Inductive loading: placing a loading coil (inductance) somewhere between the feedpoint of the antenna and the tip of the radiator. This compensates for the capacitive feedpoint reactance of the short radiator. Ref. 1, 9.
  • Helical loading, by winding the radiator into the form of a linear spiral. I.e., a distributed inductor.
  • Linear loading, by folding a long radiator wire in a zig-zag form onto itself. The result is a radiating element that is three or four times shorter than the overall wire length. The folded wires are parallel and closely spaced. The interaction between the parallel wires is complex, and introduces sub-band resonances. Ref. 3.
  • End-hat loading with a "capacitive hat", typically installed at (or near) the tip of the radiating element(s). This counteracts the tapering off of the current distribution between the feedpoint and the tip of the radiator. It also raises the radiation resistance of the antenna, i.e., makes the antenna a more effective radiator. Ref. 4.

The diagrams below show the effect on the current distribution (green lines) of a 6-spoke end-hat with a perimeter wire that interconnects the tips of the spokes. This clearly shows the advantage of the end-hats: the current does not taper to zero at the tip of the radiator. Instead, it is non-zero at the tip. At the "hub" of the hat, the current splits equally into six branches (in "my" hat, there are six spokes in a symmetrical configuration). Due to the perimeter wire, the six current distributions in the "hat", terminate at the mid-points between the six spokes. So, the total area under the green current-distribution lines is significantly larger than without the hat. That's good! This area is equivalent to the antenna's radiation resistance. Short antennas have a small radiation resistance, hence are less efficient. The end-hats (especially with a perimeter wire) are very effective way to increase the radiation resistance. Note that the radiation resistance cannot be measured directly, and is not the same as feed-point impedance. Note that the effect of the "hat" has not much to do with "capacitance". So calling it a "capacitive hat" or "capacity hat" is actually (common) misnomer - even though the hat does have some capacitance. Ref. 4.

80 m short vertical

Current distribution along a radiating element, without and with end-hat loading

(common-mode suppression at feedpoint assumed)

My end-hat and center-loaded, very short (but quite effective) 80 mtr vertical dipole is described on this page. A variation on this technique is the "umbrella" vertical antenna. The one I built is on this page. I also included end-hat loading in my 8 mtr (26 ft) mid-loaded wire vertical.


END-HAT ADDED TO MY 2.5 M (8 FT) BASE LOADED VERTICAL

In June of 2010,  I decided to add an end-loading "hat" to my 2.5 m (8 ft) base-loaded vertical, and see what happens. That antenna has a fine-tuning bolt that is screwed into the threaded tip of the radiator. The radiator is made of aluminum tubing with an outer diameter (OD) of 7.5 mm (≈5/16"). The bolt is easily removed, and replaced with a screw-on "hat".

80 m short vertical

Bolt in the tip of the 2.5 m aluminum radiator, for fine-tuning the resonance frequency


First, I made the "hat". This is straightforward. I got two M3 ( = 3 mm, 1/8") threaded rods of 1 m (≈3 ft) in length, and cut each rod into three pieces of 33 cm (≈1 ft). I then took an M6 screw with hexagonal head (unlike the round-head M6 tuning screw described above), drilled holes through all sides, and tapped an M3 thread into each hole. Then I screwed the rods into these holes, and fixed them in place with an M3 nut. That's all! Note that I did not add a perimeter ("skirt") wire to the tips of the spokes. A hat with such a wire is about twice as effective as a hat without it...

80 m end-hat loaded short vertical

The hub and spokes of my end-hat


80 m end-hat loaded short vertical

The assembled hub and spokes - no perimeter wire


80 m end-hat loaded short vertical

The hat atop the vertical antenna


Results of adding this hat: the resonance frequency went down from 3574 to 3176 kHz - a whopping 400 kHz! SWR was reduced slightly: from 1.42 to 1.37.

This hat weighs 95 grams (≈ 3.4 oz). This is (too) much for my rather flexible aluminum radiator. The slightest breeze will make this antenna sway a lot. On a more rigid antenna, this would be no problem.


8 MTR (26 FT) VERTICAL MONOPOLE WITH MID-LOADING AND END-HAT LOADING

under construction

End-hat loading is the most efficient way of loading an antenna radiator. Ref. 4. However, if the radiator is really short, either enormous end-hats are needed, or a combination of inductive loading and end-hat loading.

The plan for my next "new and improved" short vertical antenna for 80 mtrs is as follows:

  • The tallest vertical that I have built so far, measured 6 m (20 ft). This time I will use a  9.5 m (31 ft) telescopic fishing pole. Only the bottom 8 m (26 ft) will be used: the top section is too thin and flexible. With 8 mtrs in length, the radiator will now be close to 0.1 λ, compared to 0.07 λ of my 6 mtr tall vertical, a 30% increase! Maybe I'll even motorize the extension/retraction of the fishing pole!
  • An end-hat that is as large a practicable (weight, wind load, shape retention). Probably with 6 spokes. Definitely with a perimeter wire. I would like the hat to have a diameter of at least 1.5 m (5 ft). The bigger the hat radius, the less important the diameter of the radials network becomes.
  • Inductive loading with the coil placed at least half way up the radiator. With the end-hat, the current distribution along the radiator is more uniform than without the hat. It does not taper off to zero at the tip of the radiator. So a smaller inductance (less loss) can be used than would be required at the same placement but without the hat. The more uniform the distribution, the less effect coil placement position has, and the less the required inductance varies with that position. It may end up looking like an extreme case of Off-Center-Fed vertical dipole, with unequal end-hats. Not unlike a hatted ground-plane vertical.
  • I may dimension the loading coil for resonance between the 80 and 40 mtr band (to make it a little easier for my ATU), or in the 40 mtr band (and possibly add a switchable loading coil at the base, for 80 mtrs)
  • Instead of a single elevated radial, I will use a small radial network: 3 radials with a perimeter wire (ref. 14D), covering a 45 deg sector in my preferred direction. The radials will be 5m (16 ft) long. Possibly I'll add one or more short (2.5 m, 8 ft) radials in the opposite direction. See the installation diagram below. Also see ref. 11 for asymmetrical radial networks.
  • A thick radiator reduces losses (that increase when radiator length is reduced) and increases the bandwidth, but does not necessarily affects gain. Obviously, a thicker radiator is also heavier. I'll probably use the braid (shielding) of a relatively thin coax cable, such as the RG58/U that I have lying around. The outside diameter of the jacket sleeve is about 5 mm (0.2"), but that of the braid is only about 3.3 mm (0.13"). But this cable is lighter than insulated 3.3 mm multi-strand copper wire... Note that when the diameter d of the radiator is increased ( = λ/d ratio decreased), the capacitance of the radiator is increased (see graph below). Hence, to keep the resonance frequency unchanged, the geometric/mechanical length of the radiator must be reduced. Conversely, with a thicker radiator, a short radiator appears less short. So, for the same length radiator, the inductance of an additional loading coil can be reduced ( = less coil losses)

wire capacitance per meter

Capacitance of a cylindrical wire

(equation source: ref. 10)


80 m short vertical

Top view of the planned antenna system


I hope to build this design summer/fall of 2016.

Of course, end-hat loaded "umbrella" antennas are nothing new - they have been used at least since the early 1900s. My favorite umbrella antenna system was the one built for the German navy in WW2, for their "Goliath" 1 megawatt VLF (15-60 kHz) transmitter. That is a wavelength of 5-20km (3-12 miles)! The vertical radiators, despite being 204 m tall (≈670 ft), were still very short: only about 1% of the wavelength! However, the efficiency of the complete antenna system was an impressive 47% at 15 kHz and 90% at 60 kHz.

Goliath umbrella antenna

Looking down the spoke-wires of a "Goliath" antenna radiator, to the skirt-wire 34 m (≈100 ft) farther below

(source: ref. 12)


Radial ring

December-2015 - work in progress: a ring that fits on the PVC mast, for attaching radial wires

(made out of a kitchen cutting board; radials plugged-in with banana plugs)


REFERENCES


Note 1: due to copyright reasons, this file is in a password-protected directory. Contact me if you need access for research or personal study purposes.

External links last checked: October 2015


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©1999-2016 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.