[TowerTalk] FCP vs On ground.txt

[Jeff Blaine]

Dave,

There are a number of articles that reference non-1/4 WL radials and the 
benefits.  That made complete  sense to me and the verts and 4sq that I 
built were made purposely with lengths not on quarter wave marks for the 
reason you mention.  I forget where I saw this mentioned the first time, 
but I want to say it was an article from the 70s.  So it's not a new 
idea.  Never measured currents as you did but I did "tune" all over the 
radials to the same Fc with the assumption that they would distribute 
the currents well enough.

The funny part is that I must have had 20 guys ask me about the non-1/4 
WL choice.  And I think 19 of them went away unconvinced. The lore 
driving a quarter wave elevated radial length is just impossible to 
overcome.  Yet a guy can see the cost/benefit for various lengths in the 
N6LF article.  Fig 15.

Congrats on your build.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com


On 1/21/22 5:52 PM, Dave Sublette wrote:
> I have followed this thread with interest.  I am surprised that nobody
> seems to have considered the following:
>
> Several months ago I was given a link to a couple of papers by K5IU and
> someone else, who I forget, about RF currents in radial systems.
>
> This information is available in the "Lowband DXing" manual.
>
> Briefly, with full sized quarter wave radials, measured RF currents were
> found to be unequal in each radial.  I have had a full quarter wave 160
> meter vertical made with Rohn 45 and a broadcast insulator section 27 feet
> above ground operating for more than 30 years. I had 8, full quarter wave
> radials suspended 27 feet above ground, sloping to 10 feet at the ends.
> The whole antenna footprint is over an acre.
>
> So, having read the paper, I climbed to 27 feet and measured the RF current
> in each radial.  Much to my surprise, only two of the radials carried the
> bulk of the current.  The rest hardly had any at all.
>
> The article recommended using one eighth long radials and tuning the
> reactance out with a coil in series with the shield of the feedline and the
> connection of the radials.   So, I took down my full sized radials and
> replaced them with four, one eighth wavelength radials and a matching
> coil.  Modeling shows capacitive reactance of 160 ohms for these shortened
> radials, 14 uh at 160 meters.  That's what I wound and it works perfectly.
> The RF currents in each of the radials is now equal and the sum of the four
> currents is equal to the current flowing in the driven element. I don't see
> any improvement or degradation since changing the radials.
>
> Here is the bottom line to me.  I have this vision of everyone going to all
> the expense and labor (considerable labor!) to either bury or suspend
> quarter wave radials and the result would seem to be less than optimum.
> The efficiency and pattern should be affected by unequal currents flowing
> in the radials.
>
> In light of this info and my experience,  this discussion has missed the
> mark.
>
> I might add that in neither case , short or long radials, the angle of
> slope of each radial with respect to the others was not equal.  Nor did I
> go to any trouble to try and tune them to exactly the same frequency.
> These differences might contribute to the unequal currents, but didn't seem
> to affect the shortened radials at all.
>
> Offered in a spirit of curiosity and hoping to learn.
>
> 73,
>
> Dave., K4TO
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