Wednesday, June 12, 2013

Antenna Topics: A Field Day Vertical with elevated radial system. Post #201

With the ARRL Field Day coming on 22-23 June 2013, there are plenty of antenna ideas floating around cyberspace.  Part of the fun of Field Day is the erecting of familiar antennas such as dipoles, verticals, loops, and hexbeams.  Now and then, a three or four element beam is pressed into service thanks to a generous club member, some fiberglass and metal tower sections always seem to appear, or even a "cherry picker" truck loaned by a service-minded company such as a tree trimming business, the telephone company, or the local cable company makes an appearance.  A good antenna raising team can put a formidable tower and beam into the air in a matter of a few minutes.

But what happens when you operate a Field Day station as a single operator, single transmitter entry with no other team members to help you?  You are the antenna raiser, the cook, the tent builder, and the over taxed operator.  Don't dispair.  With a little planning, you can pre-assemble a simple antenna that can be erected, used, and taken down easily.  If you use a telescoping fiberglass mast from MFJ, Jackite, or any of the surplus outfits selling used military fiberglass or pvc mast sections, most of the battle is won.  All you have to do is make a simple vertical and ground system to attach to the mast and you're ready to go.

During my last few posts, I've described some of the simple Field Day antennas I use for my portable and emergency operations.  Most of these antennas are either inverted vees or loops, since these antennas are easily made and require no ground radial system.  Fed with 450-ohm ladder line and connected to a 4:1 balun and antenna matchbox (tuner), these antennas can cover several bands with low SWR.

Just for the fun of it, I decided to use one of my old vertical antennas from my novice operator days (1977 to 1978).  Since 1/4 wavelength verticals require a substantial ground radial system to work properly, I had visions of laying out 16 to 30 quarterwave radials below the vertical...not an enjoyable experience, since my backyard is quite small.  There must be a better way to get a vertical antenna to perform.

Back in those days, I heard a lot about raised radial systems and how they could be used to replace a large number of buried or on-surface radial wires.  Some writers called these elevated wires a "counterpoise".  According to several experts in the field, including Oliver Duffy (VK1OD) and the late L. B. Cebik (W4RNL), there is a lot of misinformation about what comprises a counterpoise. For VK1OD, a counterpoise "means all things to all people...it is used inconsistently...it doesn't have a really acceptable meaning."  Cebik also notes the "use" and "abuse" of the term "counterpoise."

Cebik, citing the Frank C. Jones "Radio Handbook", 1937, p. 9, says the elevated radial system "acts as a condenser plate with high capacity to earth, with the result of lower loss in the antenna system.  For this reason, the counterpoise should be fairly close to ground...i.e. 6 to 10 feet (1.82 to 3.04 meters) above ground."  An elevated radial system, even a single radial, can improve antenna efficiency.

That's what I was looking for.  Something reasonably efficient and simple that wouldn't require a lot of digging and stringing of wire in a confined space.

As for the definition of "counterpoise", I deferred to the IEEE Standard Definition of Terms.  According to that standard, a counterpoise is "a system of conductors elevated above ground and insulated from ground, forming a lower system of conductors for an antenna."

With that in mind, I built a simple vertical antenna covering 40 through 10 meters using one elevated radial (or counterpoise) in my backyard.  The antenna worked amazingly well for the time and materials invested.

MATERIALS:

A 33-foot/10.06 meters MFJ telescoping fiberglass mast.

Two, five-foot/1.52 meters wooden stakes.  One stake would support the fiberglass mast.  The other stake would support the elevated radial above ground.

One ceramic insulator to tie-off the elevated radial.

One ceramic insulator to attach to the top of the fiberglass mast.  This insulator would support the vertical element of the antenna.

Two equal lengths of #14 AWG house wire.  One piece would be the vertical element.  The other piece would be used for the elevated radial wire.  Using the general formula for a dipole (468/f(MHz)=L(ft) and a chosen frequency of 7.088 MHz (the frequency of the Hawaii Afternoon Net), I cut each wire to a length of 33.01 feet/10.06 meters.

Fifty-feet/15.24 meters of 450-ohm ladder line.

A W9INN 4:1 balun.

An antenna transmatch (Drake MN-4).  The combination of the balun, transmatch, and the ladder line would let me use the antenna from 40 to 10 meters.  The old Drake MN-4 had no trouble bringing the SWR to 1.5 to 1 on all bands of interest.

Twenty-five feet/7.62 meters of RG-8X coaxial cable with UHF connectors.  The cable would run from the 4:1 balun to the Drake MN-4.  Simple patch cords would connect the "tuner" to the dummy load, low pass filter, and to the Yaesu FT-7 QRP rig.

A deep-cycle marine battery charged by two solar panels would provide power for the station.

A 5-foot/1.52 meters piece of dacron rope to tie off the end insulator of the elevated radial wire.

A 5-foot/1.52 meter support stake near the operating position.  The stake would support the ladder line off the ground until it reached the 4:1 balun near the portable table I used for a desk.

Small table and chair to serve as the operating site.

Small tool kit, logging materials, food, and water.

ASSEMBLY:

I built the antenna on the ground.  I taped the vertical element (33.1 feet/10.06 meters) to the fiberglass mast, attached a ceramic insulator to the antenna, and secured the insulator to the top of the mast with nylon ties.  One wire of the ladder line was attached to the bottom of the vertical element.  The other part of the ladder line was attached to the elevated radial wire (also 33.1 feet/10.06 meters).  All connections were soldered and wrapped with vinyl electrical wire to protect the system from the weather.

The elevated radial wire was led off from the base of the antenna at a slight angle to a 5-foot/1.52 meters wooden stake, approximately 38 feet/11.58 meters from the base of the antenna.

Once I assembled the antenna, I hoisted the mast onto its support stake.

I led the ladder line to the the support stake.  I attached the W9INN 4:1 balun to the stake.

I connected the 25-foot/7.62 meters RG-8X coaxial cable to the balun and ran the cable to the Drake MN-4.  Small patch cords connected the Yaesu FT-7 to the dummy load, the "tuner", and a low pass filter.

RESULTS:

Considering that this hastily made antenna used only one elevated radial (or "counterpoise", depending on your definition), I was satisfied with the contacts I made last Saturday (08 June 2013).  Like my initial work with this antenna back in 1977 and 1978, I was able to keep SWR below 1.5 to 1 on 40, 20, 15, and 10 meters, thanks to the ladder line, the balun, and a good transmatch.  Of course, I couldn't use 20 meters as a novice operator in those days.  Once I passed my general license exam, this antenna gave me good service in the 20 meter band.

Will this antenna match the performance of a beam on a 50-foot/15.24 meters tower?  No.  But it will get you on the air quickly with low SWR.  I've received CW reports ranging from 559 to 599 and SSB reports of 54 to 57, depending on band selection and propagation.  Not bad for 10 watts output from an old rig.

The addition of several elevated radials would undoubtedly improve the efficiency of this antenna.  But, for now, this simple vertical with one elevated radial works fairly well.  The antenna can be erected and taken down quickly.  The antenna can be packed in my van with no problems.  Most of the materials can be obtained at the nearest hardware or home improvement center.  You can also use pvc pipe for the mast and ordinary #14 AWG house wire for the antenna elements.  450-ohm ladder line can be ordered from the major amateur radio supply houses (HRO, AES, Ham Station, etc.).  There are a variety of suppliers for RG-8X coaxial cable.  In an emergency, I've even used RG-58 from Radio Shack, although there are quality issues with some of the coax sold by the "Shack".

REFERENCES:

http://www.w0uce.net/K2AVantennas.html.
http://www.sgcworld.com/radialtechnote.html.
VK1OD.net/antenna/misc/CounterPoise.html.
IEEE. 1993. IEEE standard definition of terms-IEEE std 145-1993. New York:  IEEE.
http://www.antennex.com/shack/Dec06/cpshtml.
Straw, Dean, ed. 2003. The ARRL Antenna Book, 20th ed. Newington, CT. ARRL. 27.25-27.32.
Also, check out articles by L. B. Cebik (SK), W4RNL.

Have fun this Field Day!

You can follow our blog community with a free email subscription or by tapping into the blog RSS feed.

You can leave comments on the form at the bottom of this post.

For the latest amateur radio news, please visit my news site--http://kh6jrm.com.  I've added a few news stories at the bottom of this post.

Aloha es 73 de Russ (KH6JRM)

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