Topband: 3/8 Wavelength Inverted-L

[Bill Tippett]

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 inverted-L
(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
inverted-L 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 Inv-L       25 degrees      8-56 degrees              17.6 +j0
3/8 Inv-L       90 degrees      22-158 degrees            54 +j640

The 1/4 Inv-L 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 Inv-L 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 E-mail
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