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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