[TowerTalk] Modeling vs Experimenting Crowds

David Gilbert xdavid at cis-broadband.com
Thu Sep 12 00:45:39 EDT 2019


I think most of that is fairly misguided advice when it comes to antennas.

Here's what I don't understand.  Of all the things we as hams have under 
our control to improve our ability to communicate, antennas are at or 
near the top of the list.  Up to a certain point (as was discussed here 
recently), the effectiveness of an antenna has more bang for the buck 
than pretty much anything else we have a handle on.  So why on earth do 
some hams relegate it to "just put something up and see if it works"?  
That can certainly be fun, and I have personally built multiple dozens 
of different antennas just for grins, especially over the several 
decades I've operated Field Day or gone camping.  But if you aren't 
trying to optimize your results just admit that you don't really care.  
Which is fine, of course. There are lots of aspects of ham radio I have 
zero interest in, but antennas isn't one of them and I would have 
thought that anyone subscribed to this list was here to learn how to 
make better ones.

If you are indeed trying to optimize your antenna(s) , trial and error 
is a terrible way to learn how to do it because you can't control enough 
of the variables and some variables are even pretty hard to measure.  
Multiple iterations are slow, as well as being imprecise (because of the 
variables) if you're trying to improve things like gain or pattern or 
the ability to be heard in general.

It used to be that we were either stuck experimenting with antennas or 
doing laborious mathematical calculations by hand, but even basic 
modeling programs have turned that completely around.  It isn't that the 
models are infallible (they are not) .... it's our ability to quickly 
and easily learn from them that is important.  As I said above, I have 
built many, many antennas over the years and I played around with most 
of them to see what would happen, but I've learned far more from looking 
at the current distributions along the elements in EZNEC and observing 
the effects on gain bandwidth, pattern, and SWR than I ever did with 
various experiments.  The rest of what I've learned came from smarter 
folks than I here on TowerTalk who either understand antenna theory 
better than I do or were better at modeling that I am.  I can honestly 
say I haven't learned much of anything worthwhile from somebody who put 
up something and claimed "it worked."

You say no one doubts the results of the experimenter.  That maybe true 
of the chemist or circuit designer who can control his environment, but 
I ALWAYS doubt the results of the antenna experimenter who can't explain 
his results with either theory or a suitable model, or both, because I 
know in most cases he wasn't able to control or allow for important 
variables (propagation, proximity effects, noise variability, etc) and 
probably didn't even understand enough to do so if he relied only on his 
experimentation to teach himself about antennas.  I have a few of my own 
examples of this, the most notable being an antenna I tried for Field 
Day one year.  I wasn't able to do my normal preparation so I modeled a 
flat elongated vertical wire rectangular loop fed in the middle of one 
of the short vertical segments.  The model said it would perform 
marginally at best, but it was easy to put up and could be fed on both 
20m and 40m, so I went with it anyway.  The night before Field Day I 
gave it a try and worked an FR5 (FR5DN, I think) first try using 5 watts 
from here in Arizona with Q5 CW signals both ways (somewhere I have the 
QSL card to prove it).  I thought hey, maybe this thing is better than I 
thought. It wasn't.  I didn't work a single other DX station that night 
and the antenna turned out to be one of the worst I ever built for Field 
Day as well.

The bottom line is NOT that antenna modeling will always give you the 
right answer, but it will make you a heck of a lot smarter more quickly 
than simple experimentation will, and in most cases you will at least 
know in which direction to make changes if you're trying to make 
improvements.

One last example.  I recently built a 5 element 6m yagi using dimensions 
posted by DK7ZB on his website, and he came up with those dimensions 
using modeling software.  Most 5 element yagis have a feedpoint 
impedance down between 12 and 15 ohms, but his design (others have done 
the same) natively gives a 50 ohm feedpoint at a very small sacrifice of 
forward gain.  It does so by making the first director actually longer 
than the driven element.  How much time and how many iterations do you 
think it would take an experimenter to stumble across that?

By the way, modeling has given me FAR more "eureka" moments than any 
antenna I ever built.

73,
Dave   AB7E




On 9/11/2019 6:45 PM, Don wrote:
> Well put by Mr Fox, Shawn. Reminds me of a little picture frame with 
> the following which hung on the wall of one of our top engineers in 
> the test and instruments company I worked for. Smart, learned, well 
> educated and a dedicated experimenter, determined to defy the 'it has 
> to be so' crowd. Quite successful.
>
> "No one believes the results of the computational modeler except the 
> modeler, for only he understands the premises. No one doubts the 
> experimenter's results except the experimenter, for only he knows his 
> mistakes'. Beneath was two handwrittenlines on a strip of paper. 
> "Modeling is not as exciting as experimenting where the outcome can be 
> an Eureka moment".
>
> I had copied that and tucked it away in my old company history files I 
> left with.
>
> Don T W7WLL
>
> On 9/11/2019 2:08 PM, Shawn Donley wrote:
>> I occasionally teach a class on modeling certain mechanical systems 
>> using Simulink.  The second slide is a quote from the British 
>> mathematician George E.P Box.  I think it may apply to this 
>> discussion as well.
>>
>>
>> "All models are approximations. Essentially, all models are wrong, 
>> but some are useful."
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