TopBand: Elevated Radials

[Tom Rauch]

> Date:          Wed, 18 Mar 1998 22:41:37 -0500

Hi John,

Thank you for being civil and non-personal in your  comments. That 
prevents it from being a peeing contest.

> 1. Radiator portion electrical length and configuration. Obviously, 127 feet
> or so perfectly straight up would be better ; )  Less vertical length means
> more horizontal component, less low angle radiation. 

Not unless the radials,  feedline, or other conductors other than the 
vertical element are radiating. The far field pattern that contraols 
wave angle is mainly due to ground effects a long distance from the 
radiator.  That's very well documented in engineering books and 
actual measurements.

> 4. Spacing and length of radials/counterpoise.  Like a dipole close to
> earth, the coupling makes the resonant point lower. 

If you are talking about the resonant frequency of the antenna 
system, a better ground often makes the resonant frequency higher
and more stable with moisture changes in the soil. 

For the radial itself, moving it close to earth makes the resonant 
frequency lower. But resonance has little to do with loss or 
efficiency.  

> Improving the
> conductive ground screen under a vertical usually results in narrowing the
> bandwidth.

That's a popular rumor, but factually bandwidth can increase, 
decrease, or stay the same when system efficiency is improved. I can 
site countless examples of that, I'll give two.

If I construct a groundplane near earth using four short loaded 
radials as a ground, I can adjust the radials until the base 
impedance is very low (at or below theoretical for the size radiator 
employed). BW will also be very narrow.

If I move that same radiator over a copper plate 1/2 wl in diameter , 
BW will increase along with efficiency and feedpoint impedance.

In this example, BW and feedpoint impedance increased while 
efficiency also increased. 

I have two 160 meter mobile antennas, the most efficient antenna has 
the highest feedpoint impedance and greatest BW! So much for general 
rules that tie unrelated parameters into a result. It's the system 
that matters, not anything you measure at the feedpoint.

> Less of the RF is going into lossy ground and more is
> reinforcing the radiating element.  Lengthening elevated radials beyond
> resonance doesn't make much sense, because the current distribution in the
> radial system should be balanced, leaving the vertical portion as prime
> radiator. 

Current in the radials will unbalance due to coupling to the 
surroundings as well as antenna pattern. You can actually force the 
system to have LESS efficiency by forcing equal currents in a 
system.

Balancing the currents does NOT guarantee minimum radiation from 
the radials (unless the are in free space and equal lengths and 
spacings), and it almost certainly will not improve efficiency 
since it has little to no effect on nearfield coupling... and that is 
the main source of loss. 

>Probably the ideal would be pairs of resonant 1/4 wave elevated
> radials, up to eight or so.  More than that, and it seems hard to avoid some
> imbalance between pairs.  This is the crux of the elevated radial issue, I
> believe.  

I believe the crux of the issue is:

1.) There are NO comprehensive tests that show the NEC models of 
low horizontal wires (like radials) are accurate.

2.) There are real world direct measurements from multiple sources 
that show small elevated system, when close to earth, are less 
efficient than a modest sized conventional system.

3.) A lot of stuff gets published without being reviewed or checked 
out, and that starts rumors that live forever.

4.) We all like to think our systems are best because we all can 
make contacts. We stick 15 antennas within 300 feet of each other, 
test them over the air where QSB alone can make 20 dB of change from 
minute to minute against people hundreds or thousands of miles away, 
postulate some very strange theories that are one step above pure 
voo-doo, and reach a cast-in-stone conclusion.

But hey, that's why it's so much fun! Think how boring it would be if 
we all had the same antennas, noise level, propagation, and skill 
levels. Just don't take any of the rumors, articles, or folklore 
seriously until they are proven with direct measurements.

73, Tom W8JI
w8ji.tom@MCIONE.com

--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com