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 Technical Puzzle #30 - "Can lower be better?" 

What happens when you lower an 80m dipole from 50ft to 15 ft?
Poll ended at Mon May 17, 2010 10:53 am
DX performance and the NVIS performance are both significantly worse 42%  42%  [ 5 ]
DX performance is significantly worse, but the NVIS performance is better 50%  50%  [ 6 ]
DX performance and NVIS performance stay the same 8%  8%  [ 1 ]
DX performance is about the same, but the NVIS performance is significantly worse 0%  0%  [ 0 ]
Total votes : 12

 Technical Puzzle #30 - "Can lower be better?" 
Silent Key

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Post Technical Puzzle #30 - "Can lower be better?"
It's been a while since we had a 'teaser', so here's one for the weekend:

Nick manages to erect a half-wave dipole for 80m at a height of 50ft using various scaffold poles and fibreglass tubes for support. It works well, but pretty soon he is getting complaints from his neighbours about the "eyesore". To keep the peace Nick decides to lower the dipole to 15ft where it is much less noticeable. He realises this will damage the DX performance of the dipole, but consoles himself with the thought that at least the NVIS performance will be better.

What do you think?

73,
Steve G3TXQ


Fri May 14, 2010 10:53 am
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 Technical Puzzle #30 - "Can lower be better?" 
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Post Re: Technical Puzzle #30 - "Can lower be better?"
Hi Steve,

Welcome back...miss yer teasers. Nice relatively easy one(might regret saying that :roll: ) to break us back into 'em.

On HF\80m operation, Inter G was my bag. So this was the way to go...

Dave

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Fri May 14, 2010 11:09 am
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 Technical Puzzle #30 - "Can lower be better?" 
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Post Re: Technical Puzzle #30 - "Can lower be better?"
I'll go for both worse, as I imagine ground loss will be more significant at 15ft than at 50ft, and this will affect things more than the change in radiation pattern itself.


I await to be corrected. :D

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Fri May 14, 2010 11:52 am
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 Technical Puzzle #30 - "Can lower be better?" 
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Post Re: Technical Puzzle #30 - "Can lower be better?"
I have gone for the option of no change. Partly because I think that the antennas height is too small as a function of wavelength to make any real difference, and because I just threw some values into EZNEC and it says so too.

And now I await teacher telling me that I have done it wrong...

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Fri May 14, 2010 1:11 pm
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 Technical Puzzle #30 - "Can lower be better?" 
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Post Re: Technical Puzzle #30 - "Can lower be better?"
Sorry about my ignorance but what is NVIS? :oops:


Fri May 14, 2010 5:01 pm
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 Technical Puzzle #30 - "Can lower be better?" 
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Post Re: Technical Puzzle #30 - "Can lower be better?"
G4FUT wrote:
Sorry about my ignorance but what is NVIS? :oops:


Near Vertical Incidental Skywave....

The military use it for getting HF transmissions over mountainous regions.

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Fri May 14, 2010 5:12 pm
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 Technical Puzzle #30 - "Can lower be better?" 
Silent Key

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Post Re: Technical Puzzle #30 - "Can lower be better?"
Apologies - I should not have assumed everyone would understand the term 'NVIS'.

As has already been said it's "Near Vertical Incidence Skywave". It's what you would typically be using when working local G stations on 80m or 40m. An antenna which is good for NVIS needs to put out a strong signal at high take-off angles.

Hope that helps.

Steve G3TXQ


Fri May 14, 2010 6:06 pm
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 Technical Puzzle #30 - "Can lower be better?" 
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Post Re: Technical Puzzle #30 - "Can lower be better?"
Quote:
The military use it for getting HF transmissions over mountainous regions.


:D :D I spent years in the Army, some as a Wireless Op, and never heard of that! :D
Never too old to learn. :D
Anyway many thanks for the explanation.


Fri May 14, 2010 6:18 pm
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 Technical Puzzle #30 - "Can lower be better?" 
Silent Key

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Post Re: Technical Puzzle #30 - "Can lower be better?"
I'll post the answer Sunday evening, so there's still plenty of time if anyone else wants to take part in the poll.

73,
Steve G3TXQ


Sat May 15, 2010 7:28 pm
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 Technical Puzzle #30 - "Can lower be better?" 
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Post Re: Technical Puzzle #30 - "Can lower be better?"
Oooooooops clicked the wrong box.
:oops: :oops:


Sat May 15, 2010 7:56 pm
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 Technical Puzzle #30 - "Can lower be better?" 
Silent Key

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Post Re: Technical Puzzle #30 - "Can lower be better?"
Folks,

Thanks to everyone who took part - an interesting spread of answers!

Let's overlay the elevation responses of the dipole at different heights so that we can see what is going on:

Image

I've shown in black the elevation response of an 80m dipole at 100ft - wouldn't that be nice ;) We see that the best take-off angle is around 40 degrees; but there is a useful amount of low-angle energy for DX work, and also significant overhead energy for NVIS.

Now lower the dipole from 100ft to 50ft - the blue trace. We get the expected changes in performance - the low-angle DX performance gets much worse, but the NVIS performance improves.

Finally, do what Nick did and lower the dipole from 50ft to 15ft - the green trace. The low-angle DX performance was already very poor at 50ft and doesn't change much at 15ft - in fact it's marginally better. But the antenna is now so close to earth that ground losses begin to have a marked effect on the NVIS performance and the overhead response is 5dB worse.

So, the correct answer was #4: DX performance is about the same, but the NVIS performance is significantly worse.

The message is that on the LF bands, very low horizontal antennas are bad news because of ground losses - Nick was kidding himself if he thought the NVIS performance would be better at 15ft than 50ft. Over average ground, the best height for NVIS with an 80m dipole is around 50ft-70ft.

73,
Steve G3TXQ


Sun May 16, 2010 4:15 pm
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 Technical Puzzle #30 - "Can lower be better?" 
Silent Key

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Post Re: Technical Puzzle #30 - "Can lower be better?"
Rob,

A dipole is a good choice for 80m Inter-G: a height of about 60ft should do fine, then move it up to 300ft when you want to work DX ;)

Steve G3TXQ


Thu May 20, 2010 8:37 pm
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 Technical Puzzle #30 - "Can lower be better?" 
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Post Re: Technical Puzzle #30 - "Can lower be better?"
G3TXQ wrote:
Rob,

a height of about 60ft should do fine

Steve G3TXQ


LOL... I wish, My 80m dipole is just at 25ft.

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Thu May 20, 2010 9:08 pm
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 Technical Puzzle #30 - "Can lower be better?" 
Silent Key

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Post Re: Technical Puzzle #30 - "Can lower be better?"
Jason,

Yes, the 60ft was said 'tongue in cheek' :)

But my point is that anything much lower than 50ft-70ft will be weaker. QST magazine had a 'crusade' a few years ago to dispel the myth that very low dipoles made excellent NVIS antennas.

73,
Steve G3TXQ


Thu May 20, 2010 9:37 pm
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 Technical Puzzle #30 - "Can lower be better?" 
Silent Key

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Post Re: Technical Puzzle #30 - "Can lower be better?"
Rob,

The figures vary a bit depending on the quality of the ground under the antenna, but here are the Gain figures for a signal launched vertically from an 80m dipole over 'average ground':

Height Gain
5ft -4.8dBi
10ft -1.1dBi
15ft +1.5dBi
20ft +3.3dBi
25ft +4.5dBi
30ft +5.3dBi
35ft +5.9dBi
40ft +6.2dBi
45ft +6.4dBi
50ft +6.5dBi
55ft +6.5dBi
60ft +6.4dBi

Optimum height is around 50ft which is 0.178 wavelength
At one eighth wavelength - 35ft - it's still pretty good; just 0.6dB down on optimum
One twentieth of a wavelength - 14ft - it's way too low; it's 5dB down on optimum.

Check those EZNEC figures against values measured by the US military during the Vietnam war, shown on Tom's web site:
http://www.w8ji.com/nvis_n_v_i_s_antenna.htm

73,
Steve G3TXQ


Fri May 21, 2010 1:14 pm
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