On Sat, 12 Sep 2009 13:07:48 +0200
Björn Mohr <bm@broadcast.se> wrote:
>...I decided to make a 19 meter / 63 feet vertical that >I can also use for 80
>meter. Initially it will be an >inverted L...
> If anyone has a good idea of how to connect/disconnect
>the horizontal loading wire for 80 meter operation
Bjorn,
The inverted L you are proposing will work. For your
information, however, I have done some comparison testing
using my antenna, just before I rebuilt it. When my
vertical broke the last time, I temporarily converted it
to an Inverted L very similar to what you are propsing (23
meter vertical section and a "horizontal" wire sloping at
20 degrees). Prior to rebuilding the antenna (to what was
described in my earlier e-mail), I asked Carlos, N4IS, who
lives about 40 km north of my QTH, to accurately measure
my signal, with the goal of comparing it with the 30m tall
center loaded vertical configuration. His measurement
showed a 1.8 dB improvement from the Inverted L. This is a
bigger improvement than EZNEC indicates, but it is hard to
argue with a hardware measurement.
On the issue of isolating the horizontal wire for 80 meter
operation, I don't know of any easy way, especially if the
vertical is not grounded. Traps are bulky and a relay is
difficult to feed and would have to have 20kV++ isolation
(for 1500 W).
An Inverted L sized for 160 meters, is hard to feed on 80,
but more importantly, it radiates straight up. I have come
up with an easy solution for the VK9GMW DXpedtion. The
horizontal wire is bent down at about 12 meters and the
rest of the wire drops vertically to the ground, where a
remotely controlled grounding (vacuum) relay is installed.
On 160 the relay is open. The fact that part of the
"loading" wire is now vertical has only a small effect on
the antenna's performance on 160. For 80, the grounding
relay is turned on, grounding the lower end of the loading
wire (you'll need a good ground!) Now the atenna will have
excellent low angle radiation on 80. (The feed point
impedance will be very high, so you will need an L network
with high voltage components. On the other hand, losses in
the inductor will be small.) You can see details of this
arrangement at the VK9GMW website at
http://vk9gmw.com/documents/VK9GMW_ANTENNA.pdf .
One additional point here: if instead of the horizontal
wire of an inverted L, you would add a section of spider
pole and a loading coil (easy to wind on the spider pole
section), you would have an antenna that would radiate
well on both 80 and 160 meters. You would still need some
network to feed it on 80 meters, but...
GL,
George
AA7JV
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