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.
Incidentally the reason I eventually put the radials in the ground was
in part the difficulty of keeping the radials stable through the
season. Deer would whack them in the summer, ice would pull at them in
the winter, and the winds are ever-present here. I used bungees to
tension them and those needed to be replaced. Guys ask me which is
better - and I don't have a good reason. But it's definitely a pay me
now or pay me later situation. If you put the radial field in the
ground, it's a lot of work as you need a lot of radials to get the same
performance - so it's a pay up front thing. The benefit is that once in
the ground, they tend to be stable vs. time and weather. Elevated
radials on the other hand are quick, cheaper, but have on ongoing
maintenance obligation which varies with your local circumstances and
implementation method. Think of elevated radials as a subscription
antenna - regular payments of maintenance is needed. No free lunch with
either choice of radial systems.
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
On 8/30/20 9:15 AM, Dennis W0JX via TowerTalk wrote:
To All:I apologize for not providing the reference to the Dick Weber K5IU
article on the subject of shortened elevated radials.
Here are the references:
"Optimal Elevated Radial Vertical Antennas" by Dick Weber, K5IU. COMMUNICATIONS
QUARTERLY, Spring 1997, pp 9-27.
"Modifying a 160 Meter Elevated Radial Vertical" by Duane Walker, KE7BT,
COMMUNICATIONS QUARTERLY, Summer, 1998, pp 19-28.
Also, Les Moxon, G6XN wrote about this topic in Chapter 4, pp 43-45, of his very useful book,
"HF Antennas for All Locations." (RSGB 1982)
In summary, 1/4 wave radials may not be the optimal arrangement for your
particular antenna system depending upon many factors. Dick Weber's work
suggests that for shortened radials, 60 degree radials (approximately 67% the
length of a 1/4 wavelwength) is an optimum length. Shorter than that and
efficiency drops and bandwidth narrows significantly.
Measuring your radial currents with either an MFJ current meter or a homebuilt
test set is a good way to check your radial currents. Incidentally, using a
current meter on your ground radials is also worthwhile to check their
effectiveness.
If any one wants a copy of the above Communications Quarterly articles, I will
be happy to email them to you.
73, Dennis W0JXMilan, OH
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