[TowerTalk] 15 meter yagi height for 6000 miles

[Pete Smith N4ZR]

Frank, all that any good current book on antenna design will do is refer 
you back to the same propagation realities that are used by HFTA and 
other modeling software.  HFTA produces nomograms that show what the 
best takeoff angle will be for a particular target area, as share of 100 
percent.  Those are then matched with the antenna patterns generated 
over real ground profiles, and a figure of merit is produced that tells 
you, on *average*, how well a given antenna system at your QTH will 
perform relative to the target area.  Of course, conditions will vary 
from day to day, or year to year, that is exactly what you want to 
know.  It doesn't absolve you from the need to design and build a good 
antenna, but even the best antenna will be (on average) inferior if 
placed at the wrong height.

73, Pete N4ZR
The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000 and
arcluster.reversebeacon.net, port 7000


On 3/2/2012 4:21 PM, Frank wrote:
> David Gilbert wrote:
>
>> That says more about what some people read into computer programs than
>> it does about about "what some computer says is optimum".  VOACAP is
>> based upon empirical data and statistical analysis, and as such it
>> displays predicted (not absolute) results for a representative (not
>> absolute) time period within certain accuracy limits .  HFTA used VOACAP
>> to determine the predicted best takeoff angles for various paths **as an
>> average over an entire eleven year sunspot cycle**.
>>
>> As a planning tool HFTA is the best we have to work with.  It is based
>> upon sound science and was written by a competent software engineer.
>> It does pretty much exactly what it was intended to do ... give you the
>> best chance of optimizing your results over a long period of time.
>> Anybody who thinks that it will hold true in every instance, or that
>> actual results won't often be even better than predicted, doesn't
>> understand the tool that they are using or the physics behind it.
>>
>>
> This is the best argument I have yet heard on why the best available may
> not be good enough.
>
> If it requires an exceptional understanding of the physics behind it to
> understand why it is not providing useful results, then it is pretty
> much worthless as a tool for those seeking practical realtime applications.
>
> Most hams are more interested in practical antenna designs and not so
> much in the physics that explains why a certain computer model can't
> seem to provide useful answers.
>
> Figures don't lie but it is very simple to come to false conclusions
> using statistics and averages.  Statistical averages over time can be
> paticularly misleading.
>
> In my opinion you can get better results in less time by simply reading
> a good book on antenna design.
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>