At 19:19 17/12/2001 -0500, you wrote:
Hi Jim I am planning my transmitting station . Could you send me details
of your
ant?
Is it end Fed/ Is it a vertical? What is ground system
I could set up a long sloping wire 4 meters high at fed end 13
mtr high
at far end es 80 meters long. How does that sound?
Dear John,
In the daytime, my ant is a simple inverted L about 8.5m high on average
and with a 40m long single top wire. At night, when the neighbors can't
see, I prop it up in the middle with a 13m pole to produce an inverted V
13m high in the middle, 10m at the ends. This increases the radiated signal
by about 4dB. The downlead is still connected to the end of the V; I don't
think it matters terribly much where it is fed. The loading coil lives in a
shelter with a timber frame covered in polythene sheeting at the bottom end
of the downlead. After some experiments, I found that having the loading
coil outside several yards from the house reduced the losses by a dB or
two. The ground system is currently about 8 x I metre long ground rods
scattered around the feed end, plus the house wiring and plumbing. More
than about 6 rods seems to make very little difference - the ground here is
wet clay, and seems to be fairly easy to make a good connection to;
probably the same would not be true of sand or rocky soils.
The desirable objective with LF antennas is to get the high current parts
as high up as possible - a concern with your proposed wire the part that is
4m high will effectively rob current from the higher part of the wire, and
will increase losses while reducing the radiated signal. A couple of
suggestions are:
-Use some kind of temporary support at the 4m end - even using a simple
length of timber as a temporary way to increase the height by a few meters
will help a lot.
-Have the downlead in the middle of the span, replace the 4m high end of
the wire with rope, and have a shorter top loading wire section from the
middle of the span to the higher support. This will increase the effective
height, and in theory should give about 2-3dB more radiated signal,
although you will need a bigger loading coil to make up for the reduced
capacitance, and the feed point will be in a less convenient place, so you
would need to put the loading coil in a box with some sort of remote tuning.
Or do both of course - Having said all that, my first LF antenna was only
about 5m high on average, and using 100W I was able to get around Europe on
QRSS fairly easily, so it would definitely be worth your going ahead if you
end up using the wire as originally suggested. Good luck, I'm sure you will
have plenty of people looking for your signal.
Cheers, Jim Moritz
73 de M0BMU
|
