Tuesday, July 16, 2013

A 3/2 wavelength sloping dipole for 20 meters. Post #213.

One of the easiest and least expensive antennas for the amateur radio operator is the sloping dipole or "sloper."  Ed Noll (W3FQJ) defines the sloper as "a slanted half-wave antenna with one end of the antenna attached to (a) mast top and other end near to ground level."  According to Noll, the sloper shows "a modest directivity in the direction of its slope and considerably less signal pickup from its rear."  A well-designed half-wave sloping dipole can fit in a small city lot using a mast between 20 to 30 feet (6.09 to 9.23 meters), two dipole elements cut to your preferred frequency, a few insulators, a couple of tie-down stakes, and sufficient 50/72 ohm coaxial cable to reach your shack.  Multiband operation is possible using 450 ohm ladder line, a 4:1 balun, and an antenna transmatch.

Since most of my ladder line was being used for other antenna projects, I decided to use 50 feet (15.24 meters) of RG-8X coaxial cable with UHF connectors for my feed line.  The antenna would be a single band affair, concentrating on 20 meters.  I wanted some gain and directivity for the antenna, so I decided to make the antenna 3/2 wave lengths long, with each dipole leg being 3/4 wave length long for my chosen frequency of 14.200 Mhz.


A 33-foot (10.06 meters) telescoping fiberglass mast.  I had a spare MFJ fiberglass mast that would serve as the upper support of the antenna.

Four, 5-foot (1.52 meter) wooden stakes.  The stakes would serve as tie-off points for the sloping dipole.  The stakes were placed NW, NE, SW, and SE of the mast, giving me some directivity in those directions.

A lanyard system using a pulley and approximately 75-feet (22.86 meters) of Dacron rope.  The pulley was attached to the top of the mast and would be used to hoist the sloping dipole into position.

A 5-foot (1.52 meter) support stake for the fiberglass mast.

Using the general formula for a quarter wavelength wire, 234/f (Mhz)=L(ft), I cut a 3/4 wavelength of wire for each element.  This amounted to 50.14 feet (15.28 meters) per leg.

One Budwig HQ-1 center coax connector.

Two ceramic insulators.

Fifty feet (15.24 meters) of RG-8X coaxial cable with UHF connectors.  This would be the feed line.

Antenna transmatch to handle any mismatch in the sloping dipole.  I had a Drake MN-4 available (great "tuner").

Soldering iron, basic tools, nylon ties, vinyl electrical tape, wire cutters, etc.


The antenna was built on the ground.  I hoisted the mast onto its support stake with sufficient rope from the halyard to lift the sloper into position after construction.

Before I attached the coax to the center connector, I wound a 6-turn, 8 inch (20.32 cm) rf choke out the coaxial cable, secured the choke with vinyl electrical tape, and then attached the end of the RG-8X to the center connector.

I soldered the upper portion of the dipole to the + end of the center connector and the lower segment of the dipole to the - end of the center connector.  All connections were covered with vinyl electrical tape.

The upper portion of the sloper was attached to a ceramic insulator, which was then connected to the halyard.  I hoisted the upper half into position and secured the remaining rope at the base of the mast.

The bottom segment of the sloper was attached to a ceramic insulator, which was connected by a short piece of Dacron rope to a 5-foot (1.52 meters) tie-off post.

I ran the remaining RG-8X to the shack and hooked it up to the Drake MN-4.  Short coaxial cable patch cords connected the dummy load, low pass filter, and Yaesu FT-7 QRP transceiver to the transmatch.


Using the Drake MN-4 transmatch I was able to keep the SWR below 1.1 across the 20 meter amateur radio band.  SSB reports have varied between 56 to 59, with cw reports falling between 569 and 599.  All this was done with the old Yaesu FT-7 putting out around 10 watts.

A hidden benefit of this 3/2 wavelength 20 meter dipole was getting resonance at 7.100 MHz in the 40 meter band and finding some matching in the 75 meter band.  According to Noll, "reasonable 75-meter results can be obtained by using a tuner to establish resonance on the 75-meter band."  I was able to get a decent signal out on 40-meters, thanks to the Drake MN-4.  Also, I was successful in getting  good signal reports on the upper reaches of the 75-meter band (above 3.800 MHz).  Although the antenna works best on 20 meters, it can be used on 40/75 meters with very careful tuning.  For my purposes, I'll use the antenna for 20 meters only.

I had fun making this antenna and so will you.  Another good point...this antenna doesn't require a ground radial system.


Noll, Edward M (W3FCJ).  "Easy-Up Antennas for Radio Listeners and Hams."  Limited Edition, 1991.  MFJ Enterprises, Inc.  Mississippi State, MS, 39762.  pp. 110 to 126.





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Aloha es 73 de Russ (KH6JRM).

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.