RE: FOLDED DIPOLES

REF: Parallel Multi-band Folded Dipoles
 
In the early 1970's, Ray Vincent & his SRI crew found that the
infamous technique of feeding multiple dipoles with the same
identical feedpoint will always yield undesired coupling problems
and nasty input impedance. They greatly improved performance
by using 8 to 12" of vertical spacing between the dipole feed
points and "fanning out" the dipoles (NOT running them parallel)
with about 15 degrees or more between bands.
 
We recently designed a 3 band dipole for an ionospheric probe
running at 5, 11 and 16 MHz, using the SRI approach. The NEC
and MININEC models agreed very nicely with the measured
results. We also measured the ground conductivity and
permittivity under the antennas, which really helped with the
correlation btwn the numerical models and the antenna as
installed.
 
I assume part of the problem cited with the folded multiband
dipoles is due to the close coupling of feedpoints and dipole
arms.
 
Dick Adler, Naval PG School
Received on Thu Jul 27 1995 - 19:58:00 EDT