A Shortened Centre-Loaded Inverted ‘V’ Antenna for the 160-Meter Band
by Garth Merritt VE5VG
(Pangman, Sask., Canada)
I needed a 160-meter antenna to fit between two existing support structures. A full-sized 160-meter inverted ‘V’ antenna was too long at a length of 120+ ft per leg. Each leg needed to be less than 100 feet in length.
Utilizing EZNEC, I was able to determine the size of the center-loading coils required. EZNEC estimated the input impedance present with no loading at Z=33 – j 404 ohms @ 1.85 MHz, then inductive loads were introduced into the model.
The coil form was made from 4 inch sewer pipe. The sewer pipe has an outside dimension of about 4.2 inches. Using 12 gauge transformer wire, 10 turns created a coil providing 16+/- uH of inductance at 1.85 MHz for each leg (190 ohms induction reactance per leg). The measured inductance varied slightly between coils (+/-.5 uH) when measured with an Autek RF-1. For ease of calculation, coil formulas for reactance were entered into a spreadsheet (Source Handbook RSGB/ARRL).
The antenna height is 50 feet. The ends of the antenna are about 15 feet off the ground. I used 14 gauge wire for the antenna wire which was taken from an existing roll (12#AUG would have been better).
The bandwidth of the predicted model compared to the actual can be seen by comparing the model’s SWR plot with a screenshot of an SWR plot, using a RigExpert-30 antenna analyzer. The bandwidth between the model and the measured are similar (photos attached).
The antenna’s resonant frequency after trimming was a bit lower than the model. Any shortened antenna has performance compromises with respect to a narrower bandwidth, decreased radiation resistance and an increase in R losses.
The model estimates load loss at about 0.6 dB or 12 watts if 100 watts were to be applied. An external antenna tuner allows full band coverage. A dipole at this height is not expected to compete with a full sized 1/4 wave vertical for DX but the antenna preforms well enough for contacts within North America.
(More images illustrating this article can be viewed here)
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