[TenTec] Vertical Ant. Question / Suggestion
Mon, 2 Sep 2002 09:45:27 -1000
Jim, W7ANF wrote, in part:
> Both the the GAP and Force 12 are center feed vertical dipoles and
> there is no way to attach a ground plane. They both work better over
> water or a very low loss ground. All antennas put out more power
> over a lossless ground.
Yes, and thus the reason for the BIG ground radial fields
under the 1/2 wave vertical radiators at WWVH.
Those interested, can view an excellent photo
of the 15 MHz 1/2 wave verticals at WWVH.
A photo of the pair of phased 15 MHz antennas is in the second
row of pictures. Click on the antenna photo, and you will
have a larger view. Note the elevated center insulator at
the coax feed point. The coax is run up inside the lower
tower section. Note that the lower tower section is grounded;
the actual lower portion of the 1/2 wave antenna is formed
by the several steep down sloping "radials" which terminate
in the large insulators you see. This array of "radials" is
actually forming an HF "sleeve cone", which shields the
coax from RF fields so it can route up to the center feed
point. The half wave lower section is a type of sleeve cone,
formed by these down sloping conductor elements, terminated
at those long, white insulators at the lower ends. Looking closely,
you can see the feed coax going up to the center feed point in the
inside of the right hand tower in the photo (this photo is of one of
the phased array pair out here at WWVH).
You can easily see the center insulators of the towers.
What is not shown, nor discussed is the huge and extensive
ground radial field at each site. This ground "screen" field
had to be installed to recover several dB of signal strength
out in the far Western Pacific from WWVH. Why? Because
of the high E-field at the lower end of the down sloping elements.
The E-field at the ends of these sloping elements is very intense
because of the very high RF voltage at the conductor ends driven
by the 10kW transmitters! These fields induced very high
currents into the ground, but the ground radial field eliminates much
of the ohmic loss of the induced current, thus recovering the
former lost field strength. Again, looking closely, you can see
some of the ground radials passing under the fence in the
near foreground of the photo; appears to be a LOT of ground
If you are curious, there is much other info about these stations
available on the site link above.
73, Jim KH7M
About 18 miles from WWVH here on Kauai.