[SFDXA] What Take Off Angles Do DXers Really Need from Vertical Antennas? Gain vs Height

Tue Nov 19 10:58:05 EST 2013

A summary of gain versus height for horizontal polarization (H-pol) and vertical 
polarization (V-pol) for various ground condition from poor to salt water is 
summarized on the SFDXA web site.  From the home page  http://sfdxa.com  click 
in the "[Download SFDXA presentations here] "
and then select "Oct 2013 - QRP-Quarterly: Ionospherica-II" for download, then 
see Figure 2 which shows the field strength at your location versus height. See 
the text for info about how the Figure scales for different arrival angles.

Height make a BIG difference, and the optimum height for vertical pol is 
different than for horizontal pol in agreement with the transmit patterns of ac6la,
and H-pol is much less sensitive to ground parameters than V-pol.

73
Kai, KE4PT

On 11/19/2013 10:30 AM, Bill wrote:
> An Antenna Expert Tests it:
>
>>>> Are you really measuring what DXers are really after when comparing 
>>>> vertical antennas at different heights above ground and ground mounted.?
>
> Putting aside for the moment the distinction between measuring and modeling, I 
> wanted to investigate what VK3QI said about "DX angles" (in this case 10°) vs 
> "zero" angle.  First thing I did was create an adjustable vertical dipole 
> model that also has ground conditions that can be set via a variable.  Here's 
> the AutoEZ "Variables" sheet showing (to start) the base (variable B) at 20 
> ft, the length (variable D) at 35 ft, and the ground type (variable G) as 
> "Average".
>
> http://ac6la.com/adhoc/VertDip1.png
>
> With those starting conditions I used the Resonate button to reset the length 
> (variable D).  After that the length was not changed.
>
> http://ac6la.com/adhoc/VertDip2.png
>
> Then I set up a series of test cases with the base at 0.5 ft and with the 
> ground characteristics varying through all the non-water choices, from 
> Extremely Poor to Very Good, per the EZNEC definitions of such.
>
> http://ac6la.com/adhoc/VertDip3.png
>
> Here's an animated gif showing the results.  The value for "G" (Ground type 
> index) may be seen in the lower right corner.
>
> http://ac6la.com/adhoc/VertDipB05.gif
>
> As the ground gets better the gain at 10° gets better (pretty much).  Then I 
> ran a similar series of test cases with the base of the dipole at 40 ft.  
> Again, look in the lower right corner to see the "G" value for any given frame 
> of the animation.
>
> http://ac6la.com/adhoc/VertDipB40.gif
>
> That was interesting.  With the base at 40 ft the gain at 10° gets_worse_  as 
> the ground gets better.  More of the energy is going into the second (higher) 
> lobe.  However, note that the outer ring is the same for both these animations 
> so you can compare magnitudes as well as pattern shapes.  For any given ground 
> type the "DX angle" (10°) gain is always higher for the higher dipole.  For 
> example, here's a comparison for Average ground.
>
> http://ac6la.com/adhoc/VertDip4.png
>
> So that seems to address the first point that VK3QI made.  Gain at "DX angles" 
> (in this case 10°) gets better as the dipole is raised and it doesn't matter 
> what the ground conditions are.  Now for his point about actual measurements:
>
>>>> Measuring a vertical at another location 5 miles away, but at the same 
>>>> relative height is really measuring the ability of the vertical antenna to 
>>>> couple to the ground to produce a vertically polarised ground wave.
>
> And that seems to be in regards to page 76 from the K9YC presentation:
>
> http://ac6la.com/adhoc/VertDip5.png
>
> K9YC measured an increase in gain of 9.5 dB as the dipole base was raised from 
> 0.5 ft to 40 ft.  If both he and the 3 watt transmitter at W6GJB had been 
> located on the wheat fields of Kansas that would have been one thing, but they 
> are both in the Santa Cruz mountains and I'm betting it's not exactly 5 miles 
> of flat ground (as NEC assumes) between them.  Hence I'm not sure how to use 
> NEC to verify the K9YC measurements since there is no Far Field at an 
> elevation of 0°.  So I fudged a bit.
>
> Here's an animation with the green dot marker at 1° elevation, not exactly the 
> same as equal heights but close.  The ground type is fixed at Very Poor per 
> K9YC's comments.  The dipole base ranges through 0.5, 10, 20, 30, and 40 ft.
>
> http://ac6la.com/adhoc/VertDipFF1.gif
>
> As the base height is raised the gain at 1° increases by about 6.6 dB (from 
> -19.34 dBi to -12.72 dBi).  That's not identical to the measured increase of 
> 9.5 dB but it seems to verify that the gain at "ground level" (almost) should 
> increase as the dipole height is increased, which seems to corroborate what 
> K9YC measured.
>
> Not sure what any of this proved or didn't prove, just thought I'd share it.  
> Here's the model file I used if anyone else would like to play with it.  (Note 
> that you can't just click on the link to open the model.  Save it to your 
> computer then open it from within AutoEZ, as explained in Step 3 of the AutoEZ 
> Quick Start guide.)
>
> http://ac6la.com/adhoc/VertDipole.weq
>
> This model will work just fine with the free demo version of AutoEZ which you 
> can download from here.
>
> http://ac6la.com/autoez.html
>
> Dan, AC6LA
> http://ac6la.com
>
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