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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, 4):

  • 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.

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.

Let's take a dipole that is short with respect to the desire operating frequency. To make it resonant at that frequency, some form of loading is required. By the way: resonant operation is not a requirement - it just makes coupling to a feedline easier. One standard solution is "inductive loading": placing a loading coil somewhere between the feedpoint of the dipole and the tip of each dipole leg. The current-distribution along the radiator is such that the current is highest at the feedpoint. Placing a loading coil here, requires the smallest inductance. The current-distribution tapers off, from maximum at the feedpoint to zero at the tip of the radiator element. So, as the loading coil is placed farther away from the feedpoint, a larger inductance is required. At the tip of the radiator, the current is zero. This would require an infinitely large inductance. See the diagram below. The placement of the coils does affect the shape of the current distribution, but does not change the fact that it is maximum at the feedpoint and zero at the tip.

80 m short vertical

Required loading-coil inductance as function of coil placement


Center-loading (i.e., at the feedpoint) requires the smallest amount of inductance. So why not always use center-loading? This is primarily driven by coil losses, hence, efficiency of the antenna. The coil losses basically depend on the current, coil dimensions, material, construction, and core. Depending on the diameter of the radiator element (tubing, wire) and installation height, the most efficient placement of the loading coil is somewhere between 30 and 60% away from the feedpoint (ref. 1). I.e., around the mid-point. Note that the efficiency-vs-placement curves are fairly flat over a relatively large range around the mid-point. Note that this tends to shift towards the tip of the radiator when "capacitive hat" loading is added. Ref. 4.

My 80 m short (vertical) dipole with center-loading is described on this page

IMPORTANT Note that at my QTH, the installation height of dipoles is basically limited to 2 m (6 ft). The minimum installation height of a horizontal dipole is often stated as about 0.05 λ. For the 80 mtr  band, that is 4-5 m (13-16 ft). Clearly not possible at my QTH. Furthermore, the floor of my terrace is reinforced concrete - "bad" soil. As a consequence, my dipoles will have (much?) higher ground losses, and most radiation straight up ("cloud warmer"). The experiments described below are a nice exercise in building and tuning a dipole, but were doomed from the start from a DX-performance point of view. I am not interested in local QSOs. Caveat emptor! That said, let's have a look at my (futile, hihi) attempts.


2x10 M (2x33 FT) MID-LOADED DIPOLE

February 7 of 2010 was a quiet Sunday. Perfect for building a dipole. The longest dipole that I can fit on my terrace, albeit with a dog leg between the dipole halves, is 2 x 10 m (2x 33 ft), i.e., about ¼ wavelength. I had already stocked up on zip cord (14 AWG household hookup wire), still had half of a polyethylene kitchen cutting board (for insulators), 32 mm PVC pipe (coil cores), and 0.8 mm enameled copper wire (#20 AWG, 0.812 mm Ø). So I have all the necessary ingredients. As always, I use a choke balun in series with the coax.

I took me about 1½ hours to make the insulators, wind the coils and put the whole thing together for initial testing. I determined the ballpark value of the required coil inductance with an on-line calculator (ref. 6A). Note that the calculator is for a straight line, flat top dipole, sufficiently placed in "free space". For the given antenna span, coil placement (mid-point), and wire diameter, the calculator suggest an inductance of 42 μH:


80 m short vertical

K7MEM interactive coil-inductance calculator (ref. 6A)


So far, I have been making all my loading coils with AWG #20 (0.812 mm Ø) enameled copper wire on a PVC core. Based on an on-line calculator for helical coils (ref. 7A), the required 42 μH inductance should take about 50 turns of that wire on a 32 mm (1.25") core. Note that I entered a non-zero turn spacing value. I have measured quite a number of "close-wound" loading coils that I made. With my personal coil winding technique, I obtain a close-wound turn spacing of about 0.12 mm (≈ 4 mils). Note that "Harry's Law of Coils" always applies! As Harry (SMØVPO) says: 1) You cannot wind coils like I, and I cannot wind coils like you; 2) Coil-winding data is a constant that varies from person to person.


80 m short dipole

GUI of the on-line coil calculator that I used (ref. 7A)


Of course, I will start with more than 51 turns, in this case: 56. It is easier to remove turns than to add!


80 m short dipole

The complete dipole, with experimental coils


80 m short dipole

Insulator with experimental coil (56 turns on 32 mm core)


I suspend my loading coils from a narrow strip of polypropylene kitchen cutting board material. My dear friend Rolf, DF7XH, uses a strip that is as wide as the inner diameter of the PVC coil core, and slides the coil over it. In the final configuration, I may also slide the coils over the insulators, rather than dangle them from those insulators. I did measure a 0.1% increase in coil inductance when inserting a strip of the poly material into my coils.


80 m short dipole

Dipole loading coil made by Rolf, DF7XH


80 m short dipole

Center-insulator with temporary BNC adapter


The antenna is installed horizontally. I tried two configurations, both only 1-1½ m (3-5 feet) above the terrace floor. Not great, but no choice. The L-configuration (90 degree dog leg) is shown in red in the diagram below. The end-insulators and the center-insulator are tied off with bungee cords. One leg runs parallel to the steel railing of my terrace. The other leg will run close to a steel-reinforced concrete walls & terrace floor, and partially underneath a steel pergola. So there is capacitive loading in all directions. An alternative installation (purple line in the diagram) is almost straight, but is suspended a concrete wall, and between a concrete flat roof and the concrete floor of the wrap-around terrace.

The antenna tunes beautifully, but ..... I don't seem to be getting much signal in our out! To a great extent, this is caused by the fact that the antenna is only installed about 1+ meters above the terrace. This antenna height is very low, compared to the 80 mtrs wavelength. So most of the transmitted energy will go straight up or be lost in the ground. Not a fault of the antenna!


80 m short dipole

Top view of the installations on my terrace


80 m short dipole

Views from the feedpoint along the dipole legs

80 m short dipole

I hooked my miniVNA antenna analyzer up and obtained the plot below.

80 m short dipole

Sweep from 2 to 4 MHz with my miniVNA antenna analyzer - initial coil


The resonance frequency is too low, so obviously the coil has too many turns. No problem! So I removed turns in two steps to arrive exactly at my target frequency: 3578 kHz. Sometimes you get lucky (rare when dealing with antennas, hihi)! The bandwidth between the SWR=2 points is 110 kHz.

80 m short dipole

Coil tuning data (40 ft coax + current choke)


The next plot shows the SWR and Rs curves for the final coils.

80 m short dipole

Sweep from 3.52 to 3.65 MHz - final coils


2x6.5 M (2x21 FT) MID-LOADED DIPOLE

Three weeks later.....  Clearly, the 2x10 mtr dipole described above did not perform well - as installed. Not to be blamed on the antenna itself (and definitely not on its creator, hihi)! My "Cobra" dipole (2x6.3 mtr, ≈2x20 ft) is also installed on my terrace, and is not "deaf". It actually performs quite well on 40 and up. It is also installed horizontally, but at an angle with respect to the walls. Maybe that will do the trick, or at least improve things. So, why not try convert the 2x10 mtr to a 2x6.5 mtr mid--loaded dipole, put it where the "Cobra" is (see the green antenna in the diagram below), and see what happens? .

80 m short dipole

Per the K7MEM on-line calculator (ref. 6A) I will need now need significantly more loading coil inductance: about 72 μH instead of 42 μH.

80 m short dipole

Estimated loading coil inductance when the 2x10 mtr dipole is reduced to 2x6.5 mtr


I used the same dipole calculator (ref. 6A) to estimate the required inductance for all lengths of this dipole. A similarly shaped curve applies to all dipoles! Obviously the required loading coil inductance is zero for a full-size (0.5 λ) dipole, and infinite for a zero-length dipole:

80 m short dipole

For the required 72 μH coil, I used a PVC core with a diameter of 50 mm (2") instead of 32 mm (1.5"). The estimated required number of turns is 43: 

80 m short dipole

I re-used some zip wire from my antenna components junk pile. However, with 2m88+2m94 per side (2x5m76 total), the antenna ended up a little shorter than intended. The resulting resonance frequency was about 85 kHz too high. No worries! Just replace the "outboard" wire sections with longer wire, and "prune & tune" down from there. The table below shows how I did that, after changing to 2m88+3m46=6m34 per side. The resonance frequency ended up at 3581 kHz, close enough to my 3579 kHz target.


80 m short dipole

80 m short dipole

Tuning data



80 m short dipole

3.5 - 3.7 MHz sweep of the dipole with 11m coax and choke


Tested the antenna, but found that like the 10 m version it is "deaf" (other than for local siganls). I had hoped that the slightly different installation location (45 deg angle to the walls, slightly higher) would have helped... Again, not a fault of the antenna.

As the above table shows, I also checked the antenna impedance characteristics with 11.6 m (38 ft) of 300 Ω twinlead as feedline.


80 m short dipole

1 - 30 MHz sweep of the dipole with 300 Ω twinlead



REFERENCES


External links last checked: October 2015


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