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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.
  • End-hat loading with a "capacitive hat", typically installed at 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.
  • 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. 3C).
  • Helical loading, by winding the radiator into the form of a linear spiral. I.e., a distributed inductor.

Obviously these loading methods can be combined. Linear-loading and end-hat loading, by themselves, will not sufficiently reduce the resonant frequency of a radiator that is really short. It will have to be combined with inductive loading.

My linear-loaded short multi-band "Cobra" dipole is described on this page.


3 M RADIATOR MADE OF 3-CONDUCTOR "COBRA" WIRE

Mid-March 2010 I built yet another short vertical for the 80 m band. This time I used a 3 m (10 ft) telescopic fiberglass fishing pole from another project.

One way to make a radiator with parallel wires, is to fold a long wire and keep the folded wires separate and parallel with spreaders, such as used in cage dipoles. It is more convenient to use multi-conductor wire (not to be confused with single-conductor multi-strand wire). In this case, the wires are very close together: they are only separated by the wire insulation. An example of this is ribbon cable, and flat antenna rotor control cable.

The radiator of this antenna consists of 2.7 m (9 ft) of 3-conductor antenna rotor control cable (ref. 4). The conductors are AWG #20 (0.8 mm Ø) multi-strand copper wire. One of the three conductors is tinned, which is not great for RF. The wires of this cable are connected into a zigzag configuration, like I used for my "Cobra" dipole.

80 m short vertical

3-conductor wire in "Cobra" configuration


cobra dipole

Flat 3-conductor antenna rotor control cable


In my experience, the "Cobra"-style linear loading makes the radiator appear about 10-20% longer than its span. Obviously nowhere nearly as long as the total wire length. This means that for the same radiator length, a smaller loading coil is required. Conversely, it means that a radiator can be used that is 10-20% shorter than a regular wire radiator for the same primary resonance frequency.

Like my other short vertical antennas for the 80 mtr band, this one is also base-loaded. The loading coil is wound on an 18 cm section of 32 mm OD PVC. The coil core is slid all the way down on the fishing pole, until it rests on the end stop of the pole. This is then pressed into a 32-to-40 mm PVC adapter piece. The adapter is mounted onto my 2 m "tall" antenna mast of 40 mm OD PVC tube. To prevent the pole from falling through the adapter piece, I cut the top of a 32 mm PVC end cap, and glued that into the adapter piece.


80 m short vertical

The bottom end of the fishing pole and PVC pieces of the holder

(18 cm PVC with 32 mm OD, 32-to-40 adapter with 40 mm female-to-female sleeve, and 32 mm end cap)


80 m short vertical

PVC disk sliced off the end cap


The antenna was hooked up in my standard configuration: 11 m coax, a current choke, and a single 7 m (23 ft) horizontal radial. I.e., an L-antenna configuration (ref. 7, 8). Antenna and radial are elevated on a 2 m (6 ft) tall PVC "mast". Tuning was done with the help of my miniVNA antenna analyzer. Coil tuning data is captured in the table below. SWR=2 bandwidth is about 45 kHz.

80 m short linearly loaded vertical

Coil tuning data


80 m short linearly loaded vertical

During the evening of March 17, 2010, I ran a transmission test around 21:00 local time. Transmissions were with 50 watt on 3579 kHz. Testing was done in Hellschreiber mode by repeatedly transmitting the characters "1 2", and using a Web-SDR receiver to check my signals. The remote receiver that I used is located at 935 km (580 mi) north of my QTH in the south of France. The signal processing delays in the SDR plus the internet delays added up to 2 sec. This allows me to listen to almost 2 sec worth of my own signals after completion of a transmission, and switching back to "receive". The screenshot below represents a period of about 2 minutes, to see fading, if any. I consider this to be qualitative but a reasonably objective test. The tests shows that all three antennas get a decent signal out (at my location), especially when considering its size!

80 m short vertical

Transmission of "1 2" via 3 m vertical, and reception via remote Web-SDR


During a period of twenty minutes, I actually did these transmission test with this antenna, and also with two of my 80 mtr short base-loaded verticals: one with a 2.5 m (8 ft) radiator made of aluminum tubing, and one with a 6 m wire strung along a telescopic fishing pole. These antennas and the comparative testing are described on this page.

REFERENCES


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.