Thursday, July 11, 2013

An 80-Meter Vertical Helix

Like many amateur radio operators, I live on a small lot surrounded by neighbors, utility lines, and civic-minded citizens concerned about the "attractiveness" of my community.  Whether by design or outright fear, I've adopted the "stealth" approach to ham radio antennas.  It's the old "out of sight, out of mind" idea applied to amateur radio antennas.

The amateur radio press is full of articles describing the struggle of amateur radio operators to pursue their hobby under the burdensome regulations of CC & Rs, HOAs, and other civic minded citizens who object to antenna farms.  So far, my modest verticals, loops, and inverted vees have blended well with the vegetation and trees bordering my small backyard.  Vertical antennas have always been a problem because of the limited space for a radial system.  There are times, however, where a shortened vertical for the lower HF bands (such as 80/75 meters) is necessary where horizontal space is lacking.

In one of my recent posts, I described an inverted "L" antenna I built for  80 meters--a band I find difficult to use because of my restricted space.  The antenna was a joy to build and I got many contacts despite a mediocre ground system.

The one drawback I had was in quickly deploying and taking down the antenna when I was done operating for the day.  Call me extremely cautious, but I believe in lowering my antennas before I "call it a day", partly to prevent evening thunderstorms and lightning from affecting my antennas and partly because I wanted to reduce the visual impact of any antenna I build.

So, enter the helically-wound vertical for 80 meters.  My antenna was based on a design originally presented in QST by Gary L. Ellingson (WA0WHE).  The article was reprinted in "The ARRL Antenna Anthology" back in 1978.

From my initial research in various ARRL publications, I discovered that a half-wavelength of wire for a chosen frequency wound in a helix on an insulating form such as a wooden pole, a fiberglass pole, or even pvc pipe could duplicate the performance of a standard 1/4 wavelength vertical antenna.  As WA0WHE states, "by using a helical configuration, the overall antenna height is reduced, thus eliminating the need for guying."  He continues by noting that "with the helically wound antenna, more equal current and voltage distribution is accomplished (as compared with a lumped impedance from a loading coil)."  In fact, the elongated coil is the antenna.

The are some tradeoffs with this antenna:  narrow bandwidth and the requirement for an effective radial system.  Even though my yard lacked space, I managed to squeeze in a number of non-resonant radials for the helix.

Here is the materials list for the 80 meter helix:

A 33-foot (10.06 meters) telescoping fiberglass mast.  I had a spare MFJ mast for this project.

Enough #14 AWG housewire for the helix.  Using the general dipole formula for a half-wavelength antenna, 468/f (MHz)=L(ft), and the chosen frequency of 3.750 MHz, I calculated a total wire length of 124.8 ft/38.04 meters.

A five-foot/1.52 meter wooden stake to support the lightweight fiberglass mast.  I have a homemade pivot system to lower the mast when I'm done for the day.  DX Engineering offers a professional metal pivot system at a modest price.

An 8-foot/2.43 meter copper ground rod at the base of the mast.  The radial system would be connected to the ground rod.

A Budwig HQ-1 coax connector.  The + lead would be attached to the vertical helix element, while the - lead would be attached to the copper ground rod with some copper braid.  The radial wires would be attached to the copper ground rod.

Four 62 ft/18.90 meters radial wires.  I used some #22 gauge wire for this part of the antenna.  More wire will be added later.  I used four radial wires to test the antenna system.

A 4-foot/1.21 meters antenna "stinger" from an old mobile antenna.  The metal whip would serve as a "top hat" for the helix.

Twenty-five feet/7.62 meters) of RG-8U coaxial cable with UHF connectors on both ends.

Vinyl electrical tape and nylon ties to secure the helix to the mast.

A clip lead to attach the helix to the top-loading "stinger".

An antenna transmatch to take care of possible mismatches in the antenna system.  My old Drake MN-4 was available.

Station equipment, including a transmatch, low pass filter, dummy load, and my faithful Swan 100 MX.

MAKING THE ANTENNA:

I built the helix on the ground.  With a chosen frequency of 3.750 MHz and 124.8 feet/38.04 meters of #14 AWG available, I began winding the helix on the MFJ fiberglass mast.  I used vinyl electrical tape to secure the coil as I proceeded up the mast.  At the top of the mast, I used a clip lead to attach the antenna to the "stinger."  That connection was covered with clear fingernail polish and several layers of vinyl electrical tape.

The bottom of the helix was soldered to the + lead of the Budwig connector.  The - lead of the Budwig connector was soldered to a small piece of copper braid.  The braid was then soldered to the copper ground post.

Each radial was in turn soldered to the copper ground rod and spread out as evenly as I could.  Because my backyard is fairly small, I had to adjust the position of each radial to conform to the "lay of the land".  Not all radials were straight--some were bent to pass through the garden along the edge of my property, while others ran under the house (my house is built on a post and pier system) and over to the driveway.  The last step in the process was attaching the clip lead from the helix to the top-loading "stinger."

It only takes a few minutes tor wrap up the radials when it comes to mowing the lawn.  Later on, I will use my pizza cutter to entrench the radials into the lawn.

Once everything was attached and protected from the weather, I hoisted the mast onto its wooden support stake.  No guy lines were needed.  I will add some later this week to give the mast more stability.

INITIAL RESULTS:

With the Drake MN-4 in line, I was able to keep the swr below 1.5 to 1 from 3.725 MHz to 3.775 MHz.  The bandwidth is quite narrow, but, with careful tuning, I could use most of the 80 meter band without surpassing a SWR of 2.3 to 1.

Much remains to be done with this shortened helical vertical.  As the late Jerry Sevick (W2FMI) suggests, improvement in the ground radial system will help performance.  Four decades ago, Sevick conducted a series of experiments with shortened verticals on 40-meters and demonstrated that short verticals can produce excellent results with a large number of radials and some "top loading".

I've made some good contacts on 80-meters, despite the noisy conditions found in the summer time.  Although local contacts can be made, the vertical helix does a much better job on DX.  In the past, I've used low-mounted dipoles to cover Hawaii on 80-meters.  That tradition will probably continue.  But for now, the vertical helix and its soon-to-be-improved radial system will give me many hours of fun on 80 meters.  I may even feed the "beast" with 450-ohm ladder line and a 4:1 balun to see what happens.

When I finished working through a few contacts, I disconnected the feed line and lowered the mast.

With every project, one learns a little more about about antennas and the skills necessary to communicate with our fellow amateurs around the world.

REFERENCES:

Ellingson, Gary L (WA0WHE).  "A Helically Wound Vertical Antenna for the 75-Meter Band."  Contained in the ARRL Antenna Anthology.  ARRL. Newington, CT. 06111.  Copyright 1978. pp. 20-21.

The ARRL Antenna Book.  14th Edition.  ARRL.  Newington, CT. 06111.  Copyright 1982. pp.10-11 and 10-12.

Sevick, Jerry (W2FMI) (SK).  "The Ground-Image Vertical Antenna/The W2FMI Ground-Mounted Short Vertical."  Contained in the ARRL Antenna Anthology.  ARRL.  Newington, CT. 06111.  Copyright 1978.  pp. 22-29.

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Aloha es 73 de Russ (KH6JRM).

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