[TowerTalk] Shortened Elevated Radial Articles

[jimlux]

On 8/30/20 8:10 AM, Jeff Blaine wrote:
> There is a chart in the N6LF articles which shows the impact of radial 
> length on field strength.  Figure 19.  60% would be 0.15 WL which is on 
> his chart.  Rudy's numbers say there would be roughly a 0.4 dB drop in 
> overall gain vs. 0.3 WL which is about the optimal length in the case of 
> 4 radials.
> 
> Note this assumes a certain soil, consistent soil, 8' height and 80m 
> frequency - some other charts show the interaction of these variables 
> with gain so some interpolation of the various charts may be needed.  In 
> my builds I had a bit of slope, drainage varied across the site and in 
> one antenna, there were two bands involved.
> 
> You can literally look at these things until your eyes are crossed 
> trying to nail down what they are saying to the 1/10 of a dB but that's 
> probably not necessary.  I think a reasonable case can be made that more 
> radials is better (which N6LF states outright) - more meaning 8+, and 
> that current balance can be improved by moving away from exact 1/4 WL - 
> either longer or shorter.  I've never seen any other data on measuring 
> radials individually for the same X0 - so that may or may not have value.
> 
> I say longer or shorter - both push the radial off it's minimum Z which 
> is really what you are going for.  For a higher count of radials (8+), 
> the optimal length is a bit longer (around 0.35 WL).  For my 80/160 
> elevated, I think those were about 100' long which was a compromise 
> value on the two bands.
> 



This is an interesting idea - the variation in X with length is MUCH 
bigger than the variation in R - and length for "minimum X" is highly 
dependent on surroundings.

So what you're doing is pushing the radials to be deliberately 
non-resonant, so they're reactive, and the large reactive impedance 
swamps the real impedance.  Then, you tune out the cumumulative 
reactance at the feedpoint.

I'd have to think about what this does to the phase of the current in 
the radials. In a resonant circuit, adding extra reactance (shortening 
or lengthening the radials) is generally bad, because the reactive 
current flow has IR losses. (why small mag loops are inefficient).  And, 
of course, the pattern might be affected if the current in the radials 
is seriously "wrong" vs the current the vertical.