Topband: Unique Short Radial Challenge

W7TMT w7tmt at
Sun Oct 21 12:02:22 EDT 2007

Subject: Re: Topband: Unique Short Radial Challenge

TNX to all for the multitude of direct emails, suggestions, etc.

I hope any resident TopBand "Physicists" can either confirm or deny the
validity of this info I just ran across in my research:

That Fact of physics is called RF Depth of current penertration or skin
depth. What this basically means is that as you sink RF current deeper into
any material the loss increases. There is a transitional zone where the loss
is considered acceptable equating to 37% of RF current loss this is the skin
Depth. The SKin depth of Salt water is fairly uniform and is about 7 inches
at 1.8 mhz and 2 inches and 30 mhz. Once you go below 7 or 2 inches its like
installing your ground system under a copper plate the losses are so great
and none of your RF current will return to the antenna feedpoint to be
radiated. Its that simple.

If this is true, I'm open to ideas for how to handle affixing some radials
into the salt water with a 12 foot tide fluctuation, so that I stay within
the 7 inch depth zone.  This may have to be done in the rain with very high
winds.  Umh, I'm now having second thoughts about this trip, but it's all
booked so I gotta grind-it-out :-)

Tnx & 73...

Rick, K6VVA

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When I was looking for advice on radials for my balloon supported vertical
deployed from my old wooden sailboat Tom, W8IJ mentioned the skin depth
issue and I found similar data as you. Over the last couple of years I was
able to run a few very simple experiments using three different ground
configurations as described below. All of them less than ideal but what was
possible under the circumstances.

1) A single 1/4 w elevated radial and a shorter one about 50' long one bent
to fit the floating wooden dock & elevated 8' above the water. This
configuration has its problems because the boat is located in a privately
owned marina and given our mild NW winters there are still a lot of folks
coming and going around the docks even in December. The management is very
tolerant of my annual antenna deployments but I also know there are limits.
While the elevated radials seemed attractive from a technical standpoint the
logistics seemed risky and I most likely I didn't really get them high
enough again due to the dock limitations. See performance note below.

2) Just the underwater hardware of my boat (prop, prop shaft and its
supporting strut, several bronze thru-hull fittings all bonded together
including a 3X6 foot bronze plate installed outside the hull in a
centerboard trunk. The plate was part of the original marine radio system
installed by the builder when the boat was built almost 40 years ago and is
positioned so that the edge of its long dimension is at water level. In
other words has a lot of surface area within the acceptable skin depth

3) My own attempt to deal with the skin effect and still put out radials. I
use two radials one about 90 feet long and the other 40 foot long that run
along under the edge of the dock that I "float" on the surface. The lengths
are less than ideal but that's what fits without getting in anyone's way. I
"float" the radials using short pieces of foam pipe insulation. The small
stuff for 1/2" pipe that has a slit in the side that allows it to snap
around a pipe. I just cut short sections and then use plastic ties to hold
it in place. I run the assembly along the edge of the dock and keep it from
drifting away with a few short strings placed strategically here and there.
I could support the entire thing from the dock but found the "float"
approach easier to install.


Trying to measure the difference between the three configurations was again
a huge compromise given it's not my property and I needed to use a
neighboring marinas dock to locate the test device which was nothing more
than the simplest type of field strength meter located about 500 feet away.
Probably not far enough but that's what I had access to. I found no
difference between the admittedly compromised elevated radials and any of
the other two systems measured independently. I was able to see slight
improvement using the "floating" radials and the underwater hardware tied
together. Given the ease and low risk of that installation that's what I now

No doubt folks far more knowledgeable than I will have comments that will
bring my approach into question but based on my testing (albeit with limited
equipment and setup) I've found that this works pretty well given the
numerous limitations. 

There was a short article in the March/April 2005 NCJ describing my first
efforts at this installation. I have experimented with the ground options
described above since then and made several other changes including
increasing the size of wire I use and refining the support configuration.
All in all it allows me to get on 160 for the contest season with a far
better antenna than I could ever manage from my tiny city lot.

Perhaps the floating radial approach might work for you. Run the floating
radial(s) out from the shore and anchor the far end unless this is a
location with boat traffic close to shore of course.

Patrick, W7TMT 

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