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Constructing a dipole for 160 meters with a length of approximately 254 feet and a height of 50 feet or higher will produce a low angle of radiation. This antenna configuration produces a high angle of radiation, consists of a loading coil in each leg resulting in narrow bandwidth, and is only 18 feet at its apex. It will easily fit in most backyards permitting 160-meter operation. WB5UDE describes Near Vertical Incidence Skywave (NVIS) as a radio propagation mode which involves the use of antennas with a very high radiation angle, approaching or reaching 90 degrees (straight up). This propagation path provides usable signals in the range between groundwave and conventional skywave distances — usually 30 to 300 miles. The antenna configuration is shown below. Notice that it resembles an inverted-V with the ends bent back toward the supporting pole. I have previously used this method when building full-size 40 and 80 meter dipoles for tight spaces.
Details
Loading Coil Placement VK8DA mentions that the placement of the loading coil will play a large part in the overall efficiency of the shortened antenna.
The 160-meter loading coils (Hy Power LC1682) are placed at the 70% point following the design by Hy Power for their 82-foot horizontal 160-meter antenna. Since each leg was originally 40 feet long, the loading coils were placed at the 28-foot position. The antenna was tuned for 1840 kHz to maximize FT8 digital mode operation.
Earlier experiments with the 160-meter coil consisted of an end-fed
37-foot
rain gutter antenna. The 160-meter coil plus an 80-meter
mobile antenna were placed at the far end opposite the feed point.
Good grounding was a constant problem, so an Inverted-V using both coils
was eventually constructed. Operating 160-meter NVIS Tuning for lowest SWR was accomplished using the cut-n-try method. An SWR of 2:1 or better was obtained from 1836 kHz to 1853 kHz without the need of an antenna tuner. Bandwidth varies from 15-20 kHz depending upon ground dampness. SWR appears to be lower after sunset and slightly higher after sunrise. Since the antenna is too short for operating CW around 1820 kHz, alligator clips are used to add 3 inches to each end. Most of the CW portion can then be worked with an SWR of 2:1 or better. The efficiency of this antenna configuration is probably only 10-15%, but it does permit operating on 160 meters. Medium range has been 575 miles using FT8 digital mode. The first 100 contacts on 160 meters have ranged from 81 to 1,500 miles depending upon noise and propagation. Contacts beyond 1,000 miles are really DX with this configuration.
Not every call or CQ is answered, but this very
short antenna does permit 160-meter operation. Additional Reading
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