Tom,
Your question is an excellent one! I've begun going through my concrete
design standards and design guides to find the answer. I may have to do a lot
of digging, but give me a few days. I may have to call the ASCE or ACI to fnd
out. If that doesn't work, I'll call some of my civil engineering professor
associates and see if they know. It seems to me there is an excellent chance
this could work at amateur power levels where the current in the concrete is
small and the resulting heating would be minimal and easily disipated. This may
not be true for 50KW broadcast stations though.
One thing that may be more important is the resistance between the end
of the tower that sticks out the bottom of the concrete base into the soil
below. If so, this might be avoided by using three PVC pipes secured to the
lower end of each tower leg to let the water drain below the bottom of the
base. This would allow the water to pass through the base while insulating the
tower from the soil under the base. This is a real "out of the box" idea that
may work just great. It certainly deserves careful scientific investigation
and verification by testing.
As a matter of fact there are several people doing this now in another
form. W7XU has four elevated radials with his grounded 160M quarter wave
vertical. Radials are 15 ft up at the tower and 5 ft high at the ends. He hooks
the shield of his coax to the tower legs at 15 ft and uses an L network to
match to the radial ends at 15 ft which are insulated from the tower. So he
feeds the radials. For this to work his concrete base must be acting as an
insulator to some degree. Either that or there is minimal coupling between the
ground and radials such that the majority of current flows up the tower and not
though the ground. Not exactly what you're asking about, but shows it may work.
Your question could easily be answered by someone with a base insulated
tower that uses ground radials. All they have to do is take a field strength
reading "as is" and record the forward and reflected power at the antenna feed
point. Then, jumper the base insulator so the tower is now hooked to the
concrete base. Again, take a field strength reading where the difference
between the forward and reflected power is set to be the same as the first
test. You've then got the answer to the efficicncy part of the question. It
would be good to see guys with bare and insulated ground radials do this test.
The next question is about the effect of tower grounding for lightning
protection, but there's a way around this. I sure hope some of you guys with
base insulated verticals try this and let us know how it works.
I'll get back to you in a few days or so.
73,
Dick, K5IU
PE
_____________________________________________________________________
Does anyone know the conductivity of concrete?
Is it sufficiently low to use a concrete base as a base insulator
for a quarter wave vertical fed against ground based radials?
In an experiment conducted by NN4T/WA4CTA, he measured the
feed impedance of his 160M elevated GP with the tower base
connected to several ground rods.
Then he measured the feed impedance with the ground wires
removed. There was a noticable change.
Then he measured the feed impedance when the concrete was
wet and saw no difference from when it was dry (with ground wires
removed).
>From this data, I concluded that the concrete base was an adequate
insulator.
With a quarter wave vertical, the base impedance is low (<35 ohms)
and voltages are below 250 VRMS. Presumably, a leakage path of
350 Ohms or greater will have little effect on efficiency.
Your thoughts and comments are solicited.
Thanks and 73, Tom - N4KG
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FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
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