Date: Thu, 2 Jun 2016 10:27:34 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@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
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