- [2.5 m (8 ft) end-hat loaded]
- [8 m (26 ft) wire radiator, mid & end-hat loaded, fishing pole]
"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.
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".
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...
The hub and spokes of my end-hat
The assembled hub and spokes - no perimeter wire
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
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)
Capacitance of a cylindrical wire
(equation source: ref. 10)
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.
Looking down the spoke-wires of a "Goliath" antenna radiator, to the skirt-wire 34 m (≈100 ft) farther below
(source: ref. 12)
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)
- Ref. 1: antenna loading
- Ref. 1A: "Loading of short antennas", by Doug Flory (WB6BCN) in "antenneX Online", Issue No. 80, December 2003 [pfd]
- Ref. 1B: "Shortened Dipole Study for Conditions On BVARC’s Rag Chew Net", Larry Jacobson (K5LJ), Rick Hiller (W5RH), expanded from same-title article in "Newsletter of the Brazos Valley Radio Club", September 2009
- Ref. 1C: "Element Loading to Achieve Dipole Resonance", part 3 of "Half-Length Dipoles for 40 Meters", L. B. Cebik (W4RNL, SK)
- Ref. 1D: "Signal/noise-ratio performance of loaded wire antennas", P.A. Ramsdale, Proc. IEE, Vol. 124, No. 10, October 1977, pp. 840-844 [pdf]
- Ref. 1E: "Designing a Shortened Antenna", Luiz Duarte Lopes (CT1EOJ), QST Magazine, October 2003, pp. 28-32 [pdf]
- Ref. 1F: "Short Vertical Antennas and Ground Systems - VK1BRH", Ralph Holland (VK1BRH), in "Amateur Radio", Vol. 63, No. 10, October 1995 (non-commercial reproduction allowed) [pdf]
- Ref. 1G: "Vertical antenna with inductive toploading", Rik Strobbe (ON7YD ) [pdf]
- Ref. 2: "Vertical antennas", by Ulli Weiss (DJ2YA), Chapter 9 in "Low-Band-DXing" by John Devoldere (ON4UN), edt.
- Ref. 3: linear-loaded antennas
- Ref. 3A: "The "Cobra" antenna", by Ray Cook (W4JOH), in "73 Magazine", June 1997, pp. 40-41 [pdf]
- Ref. 3B: "Build the "Cobra" antenna", by Raymond Cook (W4JOH) [pdf]
- Ref. 3C: "The mysterious Cobra", by Rick Littlefield (K1BQT) [pdf]
- Ref. 3D: "NEC model of the Cobra antenna", by Owen Duffy (VK1OD) [pdf]
- Ref. 3E: "The K4VX linear loaded dipole for 7 MHz" [an other form of linearly loaded dipole], by Lew Gordon (K4VX), in "QST", July 2002, pp. 41-42 [pdf]
- Ref. 3F: "The Watson 80Plus2 Antenna", by Julian Moss (G4ILO, SK) [pdf]
- Ref. 3G: "How & why I made an aerial for the 14 MHz, 7MHz & 3.5MHz", by Mark Wooton (M5MKW) [pdf]
- Ref. 4: "capacitance" end-loading hats
- Ref. 4A: "A triangle for the Short Vertical Operator" [hatted short dipoles], in "Antennas Tales and Technicals", update of Feb 1999, L.B. Cebik (W4RNL, SK)
- Ref. 4B: "Notes on Hatted Vertical Dipoles for 10 meters", in "Antennas Tales and Technicals", L.B. Cebik (W4RNL, SK)
- Ref. 4C: "End-Hat Loading" section in "Half-Length Dipoles (for 40 Meters) Part 3: Element Loading to Achieve Dipole Resonance", L. B. Cebik (W4RNL)
- Ref. 4D: "Practical antennas for the low bands", Rudy Severns (N6LF), presented at the 2007 Seapac hamfest, Seaside/OR, 116 slides. [pdf]
- Ref. 5: Loaded dipole calculators:
- Ref. 5A: On-line short Off-Center-Loaded dipole calculator by Martin Meserve (K7MEM)
- Ref. 5B: Shortened dipole calculator (.exe) by Alexander Stirling (VE3KSK)
- Ref. 5C: Loaded dipole calculator (.exe) by Al Legary (VE3SQB). Note: the GUI of this calculator shows inductance as "mH" instead of "μH"!
- Ref. 5D: MIDLOAD calculator for the design of very short, center-loaded dipoles above lossy ground by Reg Edwards (G4FGQ, SK)
- Ref. 6: Coil calculators:
- Ref. 6A: "Helical coil calculator" on pages of the Tesla Coil web-ring
- Ref. 6B: "K1QW Inductor Calculators"
- Ref. 6C: "ON4AA Single-layer Helical Round Wire Coil Inductor Calculator"
- Ref. 7: "The L-Antenna", L.B. Cebik (W4RNL, SK) [pdf]
- Ref. 8: the "Up & Outer" antenna:
- Ref. 8A: "The "Up-and-Outer", a golden-goodie", C.F. Rockey (W9SCH, SK), in "SPRAT", Journal of the G QRP Club, Issue 67, Summer 1991, page 18p. 18
- Ref. 8B: "A four-band "Up and Outer" antenna", C.F. Rockey (W9SCH, SK), in "SPRAT", Journal of the G QRP Club, Issue 69, Winter 1991-1992, p. 16
- Ref.8C: "The "Up and Outer" Antenna" by Craig LaBarge (WB3GCK) [pdf]
- Ref. 9: Loading coils discussions:
- Ref. 9A: "How Does an Inductor or Loading Coil Work?" by Tom Rauch, W8JI
- Ref. 9B: "Current distribution in the Antenna Loading Coils", Yuri Blanarovich (K3BU)
- Ref. 10: eq. 3-1, 3-2, 3-2 in "The calculation of electrical capacitance", Y. Iossel, E. Kochanov, M. Strunskiy, U.S. Air Force Systems Command, Foreign Technology Division, FTD-MT-24-269-70, 1969, 295 pp.
- Ref. 11: "Experimental Determination of Ground System Performance for HF Verticals Part I - VII", Rudy Severns (N6LF), part 1, 2, 3, 4, 5, 6, 7, in "QEX", Jan/Feb 2009 - Jan/Feb 2010, [pdf 1-7]
- Ref. 11B: "Ground Systems With Missing Sectors", Part 7 of "Experimental Determination of Ground System Performance for HF Verticals", Rudy Severns (N6LF), in "QEX", January/February 2010, pp. 18-19 [pdf]
- Ref. 12: "Der Goliath" page of the "Geschichten über Kalbe (Milde)" website (in German) by Henning Krüger, 2009 [pdf]
- Ref. 13: "Conductors for HF Antennas", Rudy Severns (N6LF), originally published in "QEX", November/December 2000 [pdf]
- Ref. 14: ground systems, radials, counterpoises:
- Ref. 14A: "An experimental look at ground systems for HF verticals", Rudy Severns (N6LF), in "QST", March 2010, pp. 30-33.
- Ref. 14B: "Experimental Determination of Ground System Performance for HF Verticals, Part 1-7", Rudy Severns (N6LF), in "QEX", Jan/Feb 2009 - Jan/Feb 2010 [pdf]
- Ref. 14C: "Elevated radial wire systems for ground-plane type antennas, Part 1-3", Jack Belrose (VE2CV), in "Communications Quarterly", Winter 1998, Spring 1998, Spring 1999 [ 1-3]
- Ref. 14D: "Reducing an antenna's footprint using perimeter loaded radials" [tuned "counterpoise"], Tony Preedy (GL3LNP), RadCom (RSGB), Vol. 92, No. 2, Feb. 2016, pp. 40, 41 See note 1 at bottom of page. This is actually only an expansion with a perimeter wire, of the "umbrella antenna with tuned counterpoise" described in ref. 14E.
- Ref. 14E: "Reducing mast height at MF" [umbrella antenna with tuned counterpoise], in "Technical Topics", Part Hawker () column in "Electronics & Wireless World", February 1990, p. 190.
- Ref. 14F: "Optimal elevated radial vertical antennas", Dick Weber (K5IU), in "Communications Quarterly", Spring 1997, pp. 9-27
- Ref. 14G: "Counterpoise? On the Use and Abuse of a Word" [incl. elevated radials with perimeter wire], L. B. Cebik (W4RNL, SK). [pdf]
- Ref. 15: "Radiated Power From Vertical Antennas" [radiation resistance, efficiency], Rudy Severns (N6LF), February 2015 [pdf]
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External links last checked: October 2015
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