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 Technical Puzzle #14 - Spring loaded! 

Good things about helical HF antennas
Poll ended at Thu Dec 24, 2009 7:07 pm
High feedpoint impedance 0%  0%  [ 0 ]
More efficient than loading coils 44%  44%  [ 4 ]
Lower take-off angle 0%  0%  [ 0 ]
A bit worse than linear loading 11%  11%  [ 1 ]
None of the above 44%  44%  [ 4 ]
Total votes : 9

 Technical Puzzle #14 - Spring loaded! 
Silent Key

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Post Technical Puzzle #14 - Spring loaded!
Folks,

Here's your last homework before the Christmas break :) This one completes the current series on electrically-short antennas:

Sam is keen to build a vertical for 40m, but he is limited to a maximum height of 17ft - about half what he needs for a full-size quarter-wave. He has heard that there are various ways to make this short antenna resonant by "loading" it: base inductive loading, mid inductive loading, top capacity loading, linear loading, and helical winding. He's attracted to the helically wound option because of various encouraging comments made by the local antenna gurus, including:

1. It has a high feedpoint impedance because it is wound with a half-wavelength of wire; that improves efficiency when you don't have a perfect ground system
2. Because it contains no lossy loading coils it will be more efficient than any of the inductive loading options
3. Because it is effectively a half-wavelength long it will have a lower take-off angle than any of the other loading options
4. It will not be quite as efficient as the linear loading option which contains less wire and therefore has less copper loss

Which, if any, of these statements are true?

Enjoy,
Steve G3TXQ

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Tue Dec 22, 2009 7:07 pm
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 Technical Puzzle #14 - Spring loaded! 
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not sure re the reference to half wave steve.. is he going or been told to use a halfwave piece of wire in the construction of the antenna ?? or has the "guru" got confused between half the physical length of the antenna available for 40m and a half wavelength ? or does the "guru" say its made with halfwave of wire?

billy

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Tue Dec 22, 2009 10:59 pm
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 Technical Puzzle #14 - Spring loaded! 
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Post Re: Technical Puzzle #14 - Spring loaded!
I went with option 2

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Wed Dec 23, 2009 12:28 pm
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 Technical Puzzle #14 - Spring loaded! 
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ill take it he has been told the helically wound " 1/4 " wave consists of a halfwave length piece of wire then...

1, true/false.. although it will have a high feed Z a less than good ground will decrease efficiency but will improve the swr ..


2, although some of the inductive loading options are more efficient than others , the losses may well be higher as in effect its just one big coil , the Q of the coil would make a difference to losses but im not sure if it could be made efficient enough to compete with say center loading or even base loading..

3, not sure

4, i think the fashion in which an antenna is linear loaded will determine the amount of wire used , as as from what i remember it relies on self /mutual coupling if im thinking striaght ( probably nonsense that one )

im going to go with none of the above

billy

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Wed Dec 23, 2009 3:08 pm
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 Technical Puzzle #14 - Spring loaded! 
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Post Re: Technical Puzzle #14 - Spring loaded!
Not as comfortable with the old hf vertical stuff. Good groundplane and leave the rest to the ATU :lol:

Was leaning towards 2. But went with 4, as I was not too sure.

Looking forward to this explaination head is spinning with all the options of loading :?

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Wed Dec 23, 2009 7:58 pm
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 Technical Puzzle #14 - Spring loaded! 
Silent Key

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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:

Image
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

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Thu Dec 24, 2009 2:45 pm
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 Technical Puzzle #14 - Spring loaded! 
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Post Re: Technical Puzzle #14 - Spring loaded!
nice i got one right at last :lol:


Thu Dec 24, 2009 9:40 pm
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 Technical Puzzle #14 - Spring loaded! 
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Post Re: Technical Puzzle #14 - Spring loaded!
3 on the trot :mrgreen: but it took me 10 goes to get one right :lol: But I kinda guessed this one :wink:

It appears I have trouble relating the offered answers. I ticked 5 and stated option 4 and not for the first time! :oops:

Steve, can't you number or use alphabet for the possible answer for those with attention deficiency :roll:

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Sat Dec 26, 2009 1:11 pm
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