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 A (late) New Year Quiz - Answers now added !!!! 

True or false: A resonant half-wave dipole .....
Poll ended at Mon Jan 10, 2011 9:36 am
Needs to be exactly a half-wave long to be resonant 7%  7%  [ 7 ]
If resonant at F MHz is also resonant at 3*F MHz; e.g. 7MHz and 21MHz 14%  14%  [ 13 ]
Needs to be shorter when constructed from thicker wire 9%  9%  [ 9 ]
Needs to be shorter when constructed from insulated wire 16%  16%  [ 15 ]
Needs to be much shorter - typically 66% - if constructed with coax, because of the cable's Velocity Factor 7%  7%  [ 7 ]
Is very efficient and typically radiates over 90% of the power fed to it 13%  13%  [ 12 ]
Is more efficient if it is shorter; so a 20m dipole is more efficient than a 40m dipole 1%  1%  [ 1 ]
Feedpoint impedance drops as it is lowered, and very near the ground it's almost zero 9%  9%  [ 9 ]
Has an omnidirectional azimuth pattern - rather than a classic "Figure 8" - when it is low 9%  9%  [ 9 ]
Needs to be a half-wavelength high for best DX performance 15%  15%  [ 14 ]
Total votes : 96

 A (late) New Year Quiz - Answers now added !!!! 
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Post A (late) New Year Quiz - Answers now added !!!!
Just to get the brain cells working well in 2011, here are ten questions about a simple half-wave HF dipole.

Edit: Tick the box if you think the statement is true.

Answers Saturday evening or Sunday morning!

Enjoy!
Steve G3TXQ


Wed Jan 05, 2011 9:36 am
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 A (late) New Year Quiz - Answers now added !!!! 
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Post Re: A (late) New Year Quiz
Steve
thanks for the quiz

Do we need to tick the box if we think it's true and leave it blank for false?

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Wed Jan 05, 2011 10:09 am
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 A (late) New Year Quiz - Answers now added !!!! 
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Post Re: A (late) New Year Quiz
G0BXU wrote:
Steve
thanks for the quiz

Do we need to tick the box if we think it's true and leave it blank for false?

Good question !!

Tick the box if you think the statement is true; I'll edit the Original posting to make it clear.

Thanks,
Steve G3TXQ


Wed Jan 05, 2011 10:25 am
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 A (late) New Year Quiz - Answers now added !!!! 
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Post Re: A (late) New Year Quiz
Selected 2,4,5,7,8....

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Wed Jan 05, 2011 5:30 pm
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 A (late) New Year Quiz - Answers now added !!!! 
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Post Re: A (late) New Year Quiz
M0GVZ wrote:
Selected 2,4,5,7,8....

Conor,

Did you vote yet - I see no votes for No 7.

73,
Steve G3TXQ


Wed Jan 05, 2011 5:46 pm
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Post Re: A (late) New Year Quiz
G3TXQ wrote:
M0GVZ wrote:
Selected 2,4,5,7,8....

Conor,

Did you vote yet - I see no votes for No 7.

73,
Steve G3TXQ


LOL, silly me. Good job it puts a X against what you did vote for....

I'll try that again:

2,4,6,9,10

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Thu Jan 06, 2011 12:40 am
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 A (late) New Year Quiz - Answers now added !!!! 
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Post Re: A (late) New Year Quiz
Don't you hate it when you vote and realise then that one has made a boob :oops:

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Fri Jan 07, 2011 11:29 pm
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 A (late) New Year Quiz - Answers now added !!!! 
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Post Re: A (late) New Year Quiz
Thank you to everyone who took part - there seemed to be quite a bit of interest.

Here are the answers:

1. Needs to be exactly a half-wave long to be resonant

In practice "no", because of something called the "end effect".

If we could make a dipole infinitely thin and somehow suspend it without any end supports, the current flowing at the ends would be zero and it would occupy an exact half-wavelength, calculated using 150/f metres (or 492/f feet). In practice, the thickness of the dipole and the presence of end supports means there is some capacitance at the ends of the dipole which allows a small current to flow; so a practical dipole must be slightly short of an exact half-wavelength for resonance - typically by 5% - leading to the formulas you often see quoted of 142/f metres or 468/f feet.

2. If resonant at F MHz is also resonant at 3*F MHz; e.g. 7MHz and 21MHz

Not exactly. The "end effect" explained in answer 1 is obviously only present once on a wire. So a wire resonant as three half-waves would be:

150/f + 150/f + 142/f long; which is obviously not 3 times 142/f. So a dipole resonant as a half-wave on 7MHz will be "three-half-wave" resonant slightly higher than 21MHz. Folk who build a 7MHz dipole and expect to use it on 15m usually find it is resonant just outside the top of the 15m band.

Older editions of the Antenna Book took this into account by quoting a resonant length formula of:

492*(N-0.05)/f feet

where N is the number of half-wavelengths


3. Needs to be shorter when constructed from thicker wire

Yes - slightly. The thicker the wire, the more the end capacitance and the shorter the dipole needs to be for resonance. On an 80m dipole the difference in length might be around 6" between one made with #12 gauge wire and one made with #18 gauge wire.


4. Needs to be shorter when constructed from insulated wire

Yes. The velocity of propagation of a signal along an insulated wire is slower because of the dielectric properties of the insulation; so a resonant dipole ends up being shorter. The thicker the insulation, the slower the velocity. Typically the shortening effect is in the range 0% - 5%.

5. Needs to be much shorter - typically 66% - if constructed with coax, because of the cable's Velocity Factor

No - this is a common "myth". In fact on another radio discussion site someone recently put it forward as a way of making a 6m vertical much shorter!

Velocity Factors like 66% only apply when the signal is totally contained within the dielectric material of the coax cable; that's what happens when the cable is operating as a transmission line. But when you use it as an antenna, the current all flows as Common-Mode current on the outside surface of the braid; then, only a small proportion of the signal is affected by the dielectric constant of the outer jacket, and again the shortening effect will be just a few percent.

6. Is very efficient and typically radiates over 90% of the power fed to it

Yes. The only loss mechanism in a half-wave dipole is the resistance of the wire, and even with something as thin as #18 gauge the resistance is very small compared to the Radiation Resistance.


7. Is more efficient if it is shorter; so a 20m dipole is more efficient than a 40m dipole

Yes - this one seemed to surprise most folk. Assuming both the dipoles are constructed from the same size wire, even though the efficiency of both is high, the 20m dipole is slightly more efficient. The RF resistance per unit length on 20m will be 1.4 times that on 40m because of the "skin effect"; however the 20m dipole is only half as long. So the length "wins" over the skin effect, and the effective loss resistance of the 40m dipole is greater than that of the 20m dipole - making it less efficient.

8. Feedpoint impedance drops as it is lowered, and very near the ground it's almost zero

No - that's what many text books show, but it only applies over a perfectly conducting ground. Over real soil, the impedance begins to climb again as the dipole gets very near to the ground. Over average ground, a dipole's impedance never falls below about 45 Ohms, no matter how low it is.


9. Has an omnidirectional azimuth pattern - rather than a classic "Figure 8" - when it is low

It depends! A low dipole transmits its strongest signal vertically, and in that direction it is pretty much omnidirectional. However it still transmits plenty of signal at lower elevation angles, and in those directions the pattern still approximates a "figure 8". So if you were trying to work DX with a low(ish) dipole, it still matters which way it is orientated.

10. Needs to be a half-wavelength high for best DX performance

No - it needs to be higher! The best height for DX performance is the height at which the dipole transmits most signal at a take-off angle that matches the DX path. Typically that will be much higher than a half-wavelength. For example, over a long-haul DX path, a 20m dipole at 100ft will be almost 8dB stronger than one at 30ft (half-wavelength).

But give yourself a point if you interpreted the question as "at least a half-wavelength high" ;)

Cheers,
Steve G3TXQ


Sat Jan 08, 2011 6:09 pm
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 A (late) New Year Quiz - Answers now added !!!! 
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Post Re: A (late) New Year Quiz - Answers now added !!!!
Thanks for the quiz and thanks for the explanations, always an opportunity to learn, happy new year.

Stephen

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Sun Jan 09, 2011 7:05 pm
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Post Re: A (late) New Year Quiz - Answers now added !!!!
M0TNX wrote:
Steve..

Apart from what is posted on your own website, could you do a post on arrival/take off angles? I for one would find that of great interest..


I'd be happy to do something. Is anyone else interested?

Steve G3TXQ


Sun Jan 09, 2011 7:11 pm
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 A (late) New Year Quiz - Answers now added !!!! 
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Post Re: A (late) New Year Quiz - Answers now added !!!!
+1, specifically the kinds of ranges and paths one would expect from a 10-30 degree take-off.

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Sun Jan 09, 2011 7:16 pm
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Post Re: A (late) New Year Quiz - Answers now added !!!!
Most definitely - Antenna theory is always good (if i can understand it!!)
:)

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Leigh - M0NEX


Sun Jan 09, 2011 7:19 pm
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Post Re: A (late) New Year Quiz - Answers now added !!!!
OK - I'll work on something!

If nothing appears within a couple of weeks, feel free to "remind me" ;)

Steve G3TXQ


Sun Jan 09, 2011 7:23 pm
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