[TowerTalk] Fwd: Change in Frequency As Antenna Height

[Jim Thomson]

Date: Thu, 2 Jun 2016 10:27:34 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Fwd: Change in Frequency As Antenna Height

On Thu,6/2/2016 10:08 AM, Jim Thomson wrote:
> But I dont know if scaling or extrapolating the 10M data is valid.

Gee, we've only been building and testing scaled models to help us 
understand and predict complex systems for 100 years, in fields I'm 
familiar with as widely varied as antennas and room acoustics. Maybe YOU 
don't know, but those who have taken the time to study do.

The degree to which scaling works depends strongly on how elements of 
the model accurately describe conditions at the scaled frequency -- in 
room acoustics, it's things like the reflections and diffusion 
characteristics of the wall materials, the attenuation of air, etc.; 
with this particular antenna example, it's the difference between how 
soil acts at 7 MHz as compared to at 30 MHz, and the diameter of the 
antenna conductors as a fraction of a wavelength.


73, Jim K9YC

##   Some stuff doesnt scale too well, like Q.   A  4 uh coil with an ESR of say 1 ohm
will have an  XL = 175.84 ohms..and   a Q of   175.84   @  7 mhz. 

##  Same  4 uh coil on 28 mhz  will only have an ESR  of 2 ohms....(due to skin effect.   sq rt of 4 =2 )
XL =   703.36 ohms.   Meanwhile  Q is now   351.68 ohms  @ 28 mhz. 

##  so we quadrupled the freq..and the same 4 uh  coil  has now doubled.  its Q. 

##  scaling yagis  etc  from  28 mhz  down to 7 mhz is not as easy as it 1st appears. 
BW is typ a small percentage like 3%.   3%  of  28 mhz =  840 khz.   3%  of  7 mhz =  210 khz. 
Sure the BW..% wise scales,  but the end result may well not be what we want.   The FB  +  gain +
swr  might not scale like where you want.   If we quadruple the tubing diam....say from 1 inch OD on 28 mhz...
to  4 inch OD  on 7 mhz... our  length  to diam ratio  remains the same, but who is using 4 inch tubing on
7 mhz ? 

##  The ground on 28 mhz is not the same as  7 mhz.  Try a ground  mounted 10M vert some time, with loads of 
radials, and tell us how it works out.   At one time,  EZNEC  would model any coil as having zero length, which is nonsense 
of course.  I have seen simple  wire ants  designed using  EZNEC, that spat out absurd amounts of gain.  160M  GPs mounted close
to the ground don’t fare to well, yet eznec says otherwise.  perhaps  nec-4 would fare better..which is what the ant makers  use.  

##  None of what has been said  by anyone so far explains to me why a 40m dipole, a real one, full size, not a software model, doesnt  
shift 200+  khz higher in freq,  when the same real (full sized) dipole is changed in height from  40 ft to almost 80 ft.   Ditto with
increasing the height of a loaded 80m rotary  dipole...from 40 ft to 100 ft.   So is my 40M wire dipole screwed up..cuz it
doesnt jive with eznec ?   Just cuz steve managed to see a 820 khz increase in res freq on 10m..when increasing height from
5-25 ft.... doesn’t appear to scale to a 40m  full size dipole...that is being elevated from 20- 100 ft. 

##  at just 5 ft above the dirt, the C could be significant enough to skew his results....on any band.

##  I dont buy into any theory about capacitance from the yagi / dipole to the dirt below it.  Its so miniscule, its not in the
equation...not at  40-100 ft range.     Now stray  C  from one yagi to another yagi  either  above , and /or below, might come into play, depending 
on vert spacing between arrays...and even if the  yagis are identical, they still have the els  bunched up at one end.    

Jim   VE7RF