- [2x3 m (2x10 ft) mid-loaded vertical dipole]
- [3 m (10 ft) L-antenna]
- [3 m (10 ft) monopole with Slinky® coil "counterpoise"]
"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.
- 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.
My really short 20 mtr dipole (2x 0.7 m, 2x2.3 ft !!!) ) that can easily be installed vertically is described here.
20 MTR VERTICAL MID-LOADED 2x3 MTR (2x10 FT) DIPOLE
In May of 2008, I dug up my my 20 meter mid-loaded short dipole from a dark corner of my "antenna cemetery". This time I installed in a vertical position. Note that each of the two fishing poles that make up the arms of the dipole, is almost 3 meters long. I used a 3 m section of 40 mm (1.5") diameter PVC pipe from the Do-It-Yourself store as a mast. The base of a large room fan served as a temporary stand.
My vertical mini-dipole and its creator
I only tested the antenna very briefly, as there was too much wind for the antenna to stay upright by itself. Received signals seemed similar to those of my multi-band linearly-loaded "Cobra" dipole. The footprint is definitely much smaller! One of these days, i will have to do more extensive testing, preferable with a constant test signal. To be continued...
3 MTR (10 FT) MID-LOADED MONOPOLE WITH ONE ELEVATED RADIAL
July 2008. Just for the fun of it, I did some experiments with half of the 2x3 m dipole describe above. I installed it as a 3 m monopole, about 1 meter off the ground.
As a single radial, I used 3½ m (11.5 ft) of installation wire - just because I had it handy. The radial runs horizontally from the base of the antenna. Basically an L-antenna configuration (ref. 7, 8). Tunes/loads easily. Marc, DL5NA, gave me an SSTV RSV of 585. Some stations come in slight weaker than with my Cobra dipole, others same strength. Below is the plot from my miniVNA antenna analyzer (antenna + coax + choke balun). The resonance peak has moved up a couple of 100 kHz with respect to the mini-dipole. More to experiment with!
L-configuration: top half of the vertical dipole with a (slightly) elevated radial
20 MTR VERTICAL 3 MTR (10 FT) MONOPOLE WITH SLINKY® COIL "COUNTERPOISE"
14-July-08. I changed the antenna configuration again - just for fun and out of curiosity. This antenna is simply the top half of the center-loaded vertical mini-dipole shown above, but this time loaded with a Slinky® coil counterpoise below it.
Top half of the mid-loaded vertical dipole - with Slink coil replacing the bottom half
Close up of the feedpoint and (adjustable) bottom of the Slinky coil
First I tried to tune the antenna by shorting-out a number of windings at the bottom of the Slinky coil. Managed to tune from 8-12 MHz and above 15 MHz, but not in the 20 meter band!? Gave up on that, and tried to tune simply by raising and lowering the bottom of the Slinky coil. With a free-hanging Slinky coil, the resonance frequency was about 12320 kHz, resistance very close to 50 ohm, and an SWR of 1.2. See the he antenna analyzer plot below (left). Not bad, just the wrong frequency, hi! Raising the bottom of the coil to 90 cm (35") below the top of the coil did the trick: tuned close to the SSTV calling frequency (14230 kHz), SWR of 1.4, and a bandwidth of about 400 kHz between the SWR=2 frequencies. Just taped the bottom of the coil to the PVC mast. Obviously when the wind moves the coil, the SWR moves around a bit (1.2-1.5).
SWR plots with "free hanging" Slinky coil (left) and with coil length adjusted for resonance in the 20 mtr band
- 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. 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, SK)
- 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: ground systems:
- Ref. 10A: "An experimental look at ground systems for HF verticals", Rudy Severns (N6LF), in "QST", March 2010, pp. 30-33.
- Ref. 10B: "Experimental Determination of Ground System Performance for HF Verticals, Part 1-7", Rudy Severns (N6LF), in "QEX", Jan/Feb 2009 - Jan/Feb 2010 [pdf]
External links last checked: October 2015
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