F5TNI wrote:
< L aerial on a 18 height meters pylon with another point of inking
has 44 meters more lon and 15 meters height. That will give me 62
meters of wire in L reverses
Didier, this is commonly called a 3/8 wavelength invertedL
(18m vertical and 44m horizontal). It was originally promoted by W1BB
with the main advantage being that it makes a good match for 50 ohm
coax when tuned with a series capacitor to remove the inductive
reactance. However, the vertical pattern is not good for DX because
most energy is radiated at high angles. I recommend a 1/4 wavelength
invertedL instead (18m vertical and 23m horizontal), since it will
radiate most of the energy at low angles. Of course you will need a
UnUn to match the impedance to 50 ohm coax but the pattern is much
better. Here are comparisons of the two antennas modeled in EZNEC
over the same ground (EZNEC Very Good ground):
Antenna Takeoff angle 3 dB Vert beamwidth Impedance
(These are approximate as I did a very quick model only) (R + jX)
1/4 InvL 25 degrees 856 degrees 17.6 +j0
3/8 InvL 90 degrees 22158 degrees 54 +j640
The 1/4 InvL can be fine tuned for j0 (reactance of zero) simply by
adjusting the length of the horizontal wire for minimum SWR through
a UnUn transformer at the feedpoint. The 3/8 InvL needs approximately
135 pF at the feedpoint in series to cancel the +j640 inductance for a
direct match to 50 ohm coax.
I hope you can understand. If not, send me a private Email
and I will ask my XYL to help since she is fluent in French (but
maybe not technical terms)...or perhaps one of the other French
subscribers to the reflector can help translate my message.
73, Bill W4ZV
