by Howie, W9NHM
Purpose:
This article was written to discuss the use of an 80 meter dipole efficiently on 160 meters. For use on 160, the 80 meter dipole becomes a T-shaped (top loaded) vertical radiator, fed against ground. The circuit shown has been used with a center fed (open wire) 80 meter antenna having the following configuration. This is a balanced 80 meter dipole antenna system and a robust ground system is not essential in the configuration shown below. The drooping horizontal dipole is intended to show that one may bend the antenna if necessary to accommodate obstacles or fit in a smaller city lot.
Notice: the above graphic does not show the grounding system
needed when configured for use on 160 meters.
However, on 160 a robust ground system is imperative. The ground bears 1/2
of the effective antenna system.
Principle for 160:
The vertical section (the "feeders", ladder line, window line, or twin lead for 80, 40 or 20) function as the principle radiator. The top sections (the horizontal part) serve primarily as a "capacity hat", effectively lengthening the antenna. The theoretical height for 160 meters is 1/4 wave length or about 120 feet. In the antenna shown above, the electrical length is about 116 feet (measure only one side from tuner to end of one leg). Experience shows that 50 feet of vertical feed line component and a good grounding system will work quite well on 160 meters.
The easiest feed system is series tuning of the antenna against
ground (see note below). Tie your dipole feed
lines together in the hamshack. Use an antenna tuner to "tune"
your antenna system. This arrangement will feed your antenna "against
ground". The tied together lines attach to one terminal of the tuner,
and the other terminal of the tuner goes to an efficient ground system. If
your tuner has a terminal for "long wire" then use this connection
for your antenna and then you MUST ground the tuner to your ground system.
A good ground is essential for this configuration. The ground should be the best RF ground available. This will include ground radials, buried water distribution system (your well or cold water line) or a long counterpoise. A single driven ground rod is no good, no matter how deep you drive it! Notes on grounding
Coupling to the antenna is accomplished by a suitable "tuned link" fed with coax from your transceiver. Notice the use of "antenna system". This is an important point: we are tuning the antenna, feed line, and ground system. All 3 items are equally important in obtaining an efficient antenna system for 160 meter operation. With this system in place, you will very likely NOT have RF in the shack! In fact, it is the best way to prevent this problem - that being, the use of a inductively coupled link tuner. Notice the diagram below:
The Parts
L1 - 7 turns #14 insulated house wire wound outside L2 (close wound)
C1 - 365 pf broadcast variable
L2 - 17 turns #14 bare, 8 turns per inch, 2.5
inch diameter
C2 - 365 pf. broadcast variable
C3 - 200 pf fixed mica (may need more for difficult tuning situations)
Switch may be used for convenience
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