Re: Technical Puzzle #14 - Spring loaded!

Folks,

Thanks for taking part in the poll and for your comments.

The thing that makes an antenna efficient or not is the amount of current you need to flow into it for a given radiated power. If we can reduce the current flow, the losses in things like the ground resistance and wire resistance will also be reduced. The parameter that determines this current flow is the antenna's radiation resistance (Rrad); so, generally speaking, the higher the radiation resistance the higher the efficiency, if we have fixed loss resistances.

Let's look at Rrad for the various loading options for the short vertical and see what the efficiency is, assuming a Ground system resistance of 15 ohms:

Capacitive top loading: Rrad=19ohms Efficiency=55%

Mid inductive loading: Rrad=12ohms Efficiency=41%

Helical loading: Rrad=10ohms Efficiency=37%

Bottom inductive loading: Rrad=6ohms Efficiency=27%

Linear loading: Rrad=5ohms Efficiency=23%

That's about a 3.8dB difference between best and worst. If I'd assumed a really poor Ground system - like a single earth rod - the difference could have been as high as 5dB. Just for the record, the helical was 136 turns of wire on a 2" form spread evenly over the 17ft. The linear loading was three 17ft vertical lengths of wire spaced 3" apart.

You can get a really good "feel" for how efficient an antenna will be by picturing the current distribution along it - shown in red on these sketches of a very short vertical with various types of loading:

Top-loading is always the most efficient - the current is maintained along the whole length of the antenna and so the current can be relatively low for a given amount of power radiated.

Mid-loading is quite efficient because the current below the coil is fairly constant and it doesn't tail-off until above the coil

Helical-loading has an intermediate efficiency - the current falls-off all along the antenna, but not as quickly as for bottom loading

Bottom-loading suffers because the current falls-off linearly all along the length, and therefore has to be quite high at the base to achieve the same radiated power

Linear-loading is a complex case. I've tried to show in red the current in the "end" vertical element, and in blue the current in the middle element. Those current flows are in the opposite directions and so the radiation from them will tend to cancel out. The effective radiating area is therefore the blue hatched area but with the red hatched arera subtracted. You can see intuitively that you will need a very high current at the base to radiate the same power as the other antennas.

Let's go through each of the Puzzle answers in turn:

1. The fact that a helical requires about a half-wavelength of wire is pretty much a coincidence - it certainly doesn't make it a high impedance feedpoint as would be the case with a half-wavelength of straight wire end fed. It's still quarter-wave resonant and therefore low impedance; the extra wire doesn't convey any efficiency advantage.

2. In fact the helical

does contain a lossy loading coil - it's a long thin one occupying the whole length of the antenna! Unfortunately, long and thin is a poor shape for a high Q inductor. Using a discrete loading coil has the advantage that we can design its form factor for high Q and can probably wind it with thicker wire than we could the helical. If the loading coil is placed at the centre of the vertical, the inductive loading option is more efficient than the helical option.

3. This one is just plain silly. The helical is still a 17ft antenna and behaves like one, no matter how much wire it contains.

4. Actually the helical loading is more efficient than the linear loading. As explained already a significant proportion of each wire in the linear loading is carrying a similar amplitude, but opposite-phase, current to its neighbour 3" away. That means the radiation from the adjacent wires tends to cancel; and in turn that means you have to supply more current in order to radiate a given power.

So the correct answer was Number 5 - none of the above!

Can I wish you all a very happy Christmas and a peaceful New Year,

Steve G3TXQ