This is an end-fed half-wave antenna. That is, it is a piece of wire one-half wavelength long with the feedline connected at one end.
The very common dipole antenna is a half-wave wire that is broken at the center for connection to the feedline. Think of this as a dipole that is fed from one end instead of the center.
Like a dipole, it might be an odd multiple of half wavelengths: λ/2, 3λ/2, 5λ/2, and so on.
Unlike a center-fed dipole, an end-fed antenna has a very high input impedance.
Its name comes from its first common application — a resonant antenna consisting of a half-wave wire dangling from a Zeppelin.
Dipole Half-wave Zepp
<----------λ/2----------> <----------λ/2---------->
x----------F F----------x xF F-----------------------x
| | | |
| | | |
| | | |
feedline feedline
F = feed point
x = insulated endpoint
Vertical end-fed half-wave antennas are commonly used on VHF and UHF FM, as they are easy to build and perform well. In that setting they are often called J-pole antennas because of their shape, especially when made from copper pipe.
A copper pipe J-pole is certainly sturdy, but it's even easier to make one from television twin-lead transmission line. That is, as long as terrestrial television broadcast continues, and twin-lead is commonly available...
The general design for a twinlead Zepp is:
---------+ +----------------------+ +-------------------------------------------+
feedline +--+--+ λ/4 matching section +---+ λ/2 radiating section |
---------+ | +----------------------+ +-------------------------------------------+
+-+-+
| S |
| t |
| u | <-- for matching feedline to antenna input impedance
| b |
+---+
The practical design seems to be as follows.
<-L3-> <--------L2---------> <----------------------L1---------------------->
+----[C]----------------------------------------------------------------------
|
+----[S]-------------------- ------------------------------------------------
^ ^ ^
| | |
Short this end Cut a 5mm notch Drill or cut a hole in the
dielectric at this end and
suspend it from a string
C = connect center conductor of 50Ω coaxial cable here.
S = connect shield of 50Ω coaxial cable here.
Lengths, where F = frequency in MHz
L1 = 135/F meters = 1350/F cm = 13500/F mm = 442.914/F feet = 5314.96/F inches
L2 = 57/F meters = 570/F cm = 5700/F mm = 187.008/F feet = 2244.09/F inches
L3 = 5/F meters = 50/F cm = 500/F mm = 16.404/F feet = 196.85/F inches
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