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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.
  • 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.

20 m short vertical

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!

20 m end-hat loaded short vertical

L-configuration: top half of the vertical dipole with a (slightly) elevated radial


20 m  loaded short vertical

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.

20 m loaded short vertical

Top half of the mid-loaded vertical dipole - with Slink coil replacing the bottom half


20 m loaded short vertical

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).


20 m short vertical

SWR plots with "free hanging" Slinky coil (left) and with coil length adjusted for resonance in the 20 mtr band



REFERENCES


External links last checked: October 2015


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