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 Technical Puzzle #25 - Linear Loading 

Which of these comments about linear loading are true?
Poll ended at Mon Feb 08, 2010 3:51 pm
Height: 14ft 8%  8%  [ 1 ]
Height: 34ft 8%  8%  [ 1 ]
Height: between 14ft and 34 ft 38%  38%  [ 5 ]
Impedance: 35 ohms 0%  0%  [ 0 ]
Impedance: less than 35 ohms 8%  8%  [ 1 ]
Impedance: more than 35 ohms 38%  38%  [ 5 ]
Total votes : 13

 Technical Puzzle #25 - Linear Loading 
Silent Key

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Post Technical Puzzle #25 - Linear Loading
Folks,

John, G7SSE, recently drew our attention to a dipole which used "linear loading" to achieve significant physical shortening; so I thought it might be of interest to look at linear loading in a bit more depth:

Lenny is interested in building a 40m quarter-wave vertical, but he knows a full-size one would be about 34ft tall as shown at A below. He doesn't want to go that high, so he's thinking about "linear loading" it as shown at B:

Image

He intends to make the folded section 10ft tall. He's wondering what the overall height will need to be for resonance on 40m, and whether he will have to match it differently from the full-sized quarter-wave. As ever, he asks for advice from the local gurus and gets these answers:

* You still need a total of 34 ft of wire for resonance - you've used 20ft (2x10ft) in the folded sections, so you need the final section to be 14ft
* The radiation from the two folded wires cancel one another, so the third wire still needs to be 34ft for resonance.
* Something between 34 ft and 14ft
* It's still acting like a quarter-wave vertical so the feedpoint impedance will be the same - about 35 ohms
* It's a loaded vertical, so its feedpoint impedance will be lower than for the full-size quarter-wave
* The feedpoint impedance will be higher than 35 ohms because the total length of wire is more than 34ft.


Enjoy!
Steve G3TXQ

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Fri Feb 05, 2010 3:51 pm
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 Technical Puzzle #25 - Linear Loading 
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Post Re: Technical Puzzle #25 - Linear Loading
i went with option 3 and 6

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Fri Feb 05, 2010 6:15 pm
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 Technical Puzzle #25 - Linear Loading 
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Post Re: Technical Puzzle #25 - Linear Loading
iv gone 3 and 5.

i have only one experience of linear loading but that was horizontal so cant give an explination to my choices as there was nothing mathamatical about the way i did it ,






. i do have a vertial antenna book which covers linear loading but havn't read it yet.

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Sat Feb 06, 2010 10:03 am
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 Technical Puzzle #25 - Linear Loading 
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Post Re: Technical Puzzle #25 - Linear Loading
Likewise, I believe the feedpoint impedance will greater 35 ohms. So its 3 & 6 for me.

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Sun Feb 07, 2010 9:33 am
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 Technical Puzzle #25 - Linear Loading 
Silent Key

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Post Re: Technical Puzzle #25 - Linear Loading
Thankyou to everyone who took part again!

1. Resonant length

We can quickly rule out the 34ft option - that would make the linear-loaded vertical no shorter than a full-sized quarter-wave vertical, so there would be no point ever using linear-loading!

Nor can we simply say the total amount of wire, including the folds, must equal a quarter-wave - there are complex interactions taking place between the elements of the folded section, and it certainly isn't behaving like one continuous straight piece of wire. I know of no way to predict accurately the loading effect of the folded section other than to use a modelling programme. EZNEC predicts resonance when the height is 25ft - making #3 the correct answer.

2. Resonant Impedance

We might well expect that the impedance of the linear-loaded vertical will be different from the 35 ohms of the full quarter-wave vertical - after all they are different heights and different construction. Many of you opted for the "higher than 35 ohms option". Let's look carefully at the currents flowing around that folded section in this magnified drawing:

Image

Notice the currents at the top of the 10ft folded section - we have 1A in wire A and 0.65A in wire C flowing in one direction, and 1A in wire B flowing in the opposite direction. Those wires are so close together that the radiation from each wire will either cancel or reinforce the radiation in the others. So the net radiation will be proportional to 1A+0.65A-1A=0.65A. Similarly at the bottom of the folded section the radiation will be proportional to 1A+0.9A-0.9A=1A.

What we have here is a quite high current flowing through 3x10ft of wire, but because of the cancellation much less RF is being radiated than if the same current was flowing through a 30ft straight piece of wire. That doesn't mean the antenna can't radiate effectively - it simply means we will have to supply more current to get the same power radiated than we would have had to supply into the quarter-wave.

That gives us our answer: If Power=Current*Current*Resistance, and we know that with the linear-loaded vertical we have to provide more current to get the same power radiated, then its resistance must be lower; that makes Answer #5 correct.

Don't be misled by the fact that this antenna is made up of 45ft of wire - it still behaves like an electrically short antenna that is 25ft tall. When we make verticals shorter than a quarter-wave their radiation resistance is always lower, no matter how we load them. To emphasise the point, if we looked at a vertical for 40m that was 6ft long with a large base loading coil, we wouldn't expect the length of wire making up the loading coil to cause the radiation resistance to be high; rather, experience tells us that a 6ft vertical will have a very low radiation resistance on 40m.

EZNEC predicts the radiation resistance of our linear-loaded vertical will be 15 ohms compared to 35 ohms for a full-size quarter-wave. That means we have to flow 50% more current into it than into a full-sized quarter-wave to radiate the same power. In turn that makes it potentially less efficient if we have a poor ground system; for example 50% efficient compared to 70% if the ground losses were 15 ohms. We could have achieved at least the same performance by base-loading a simple 25ft vertical with an inductor, and we could certainly have done better with a 25ft vertical mid-loaded with an inductor.

In summary: linear-loading is one way of making an antenna shorter, but it can't be treated as if it were simply a full-sized antenna with part of the wire folded up; it may be a convenient way to load a short antenna, but it's not the most efficient way.

73,
Steve G3TXQ

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Sun Feb 07, 2010 4:47 pm
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 Technical Puzzle #25 - Linear Loading 
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Post Re: Technical Puzzle #25 - Linear Loading
Another good teaser, and reasonably straight forward too, that said I still got it wrong :oops: I would have gone for answer three, but I thought 14ft was a little short so went for answer two, I had also read somewhere that folded elements cancelled out each element, it was probaly from one of the antenna gurus down at the club :lol:

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Mon Feb 08, 2010 11:46 am
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