The center-fed dipole fed with open wire balanced line may be the second
oldest antenna design, the first being the Marconi-T. Its only real
disadvantage is that today's solid state transceivers are all 50 ohm,
coax oriented and some form of balanced to unbalanced and possibly
impedance conversion is necessary. Solid state gear is also very
unforgiving of mis-matches, unlike an 807 PA with a link coupled tank
circuit. [Note how deftly I avoided using the dreaded word, "balun".
[:-)] Fan dipoles, where one coax transmission line feeds multiple
dipoles on different bands work very well ... if you can get them cut
correctly. There is a lot of interaction between them since all the
dipoles are directly fed.
The lesser known Coupled Resonator antenna is much more forgiving in the
tuning department. It consists of a center-fed dipole for the lowest
frequency normally fed with coax through an unbalanced-to-balanced
network. Close to it [like 1 - 1.5 inches max] are conductors cut for
each of the desired higher frequencies, but which are *not* connected to
the feed point. Trim the lowest frequency to desired resonance first,
then move up to the next in sequence. There is little if any
interaction with the previously trimmed conductors.
A convenient construction method is to use open wire or "window line"
for the conductors with the lowest frequency one also suspending the
assembly. Trim the other one for the next higher frequency. For 3
bands, bond another length to the longest conductor and trim the other
side for the the 3rd band.
I once saw a 5-band C-R with 5 conductors strung through the holes in
the crossed spreaders normally used to create "cage" antenna elements.
The fed conductor was larger wire through the center hole and used for
suspension, with the other four bands at the ends of the spreaders. You
can also feed the C-R with balanced line terminating at a transformer to
get to unbalanced, 50 ohm coax to keep your TX happy.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 12/7/2018 8:39 AM, Don Sanders wrote:
As Don has said, a good dipole as high as possible, even in an attic, fed
with balanced line will out perform most other compromise multiband
antennas.
While being retired in Ecuador I had the advantage to try several antennas.
The best over all was a fan dipole for 40, 30, 20,and 17 fed with RG6 up 80
feet AGL mounted on 30 foot bamboo poles at the top of a hill.
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