Sunday, June 9, 2013

Antenna Topics: A modified inverted v antenna for Field Day. Post #200.

The ARRL's Field Day is coming Saturday and Sunday, 22-23 June 2013.  Thousands of amateur radio operators in the USA and Canada will be trying to establish contact with fellow amateurs around the world in one of the oldest emergency communications exercises in existence.

Although many amateur radio operators will be joining their clubs at public parks, schools, and public safety buildings for the exercise, many amateurs, including yours truly, will be operating from our homes and even our work sites.  For those of us running in the single operator, single station category, our Field Day group belongs to us only...we are responsible for everything in our portable or home stations.  That means all antenna, cooking, and sleeping responsibilities fall on us.  Although I enjoy working with my amateur radio club (The Big Island Amateur Radio Club), there are times when I can't  drive out to the Field Day site and must work alone.

With that in mind, it pays to "pre-arrange" your portable station ahead of time, so you can spend the maximum time allowed pursuing those elusive Field Day contacts.  I have a small portable station packed in my van, consisting of a Yaesu FT-7 QRP rig (10 watts), a J-38 hand key from my military service days, a collapsible 33-foot (10.06 meters) MFJ fiberglass mast, 100-feet (30.48 meters) of #14 AWG housewire, 50-feet (15.24 meters) of RG-6 coaxial cable with "F" to "UHF" connectors, a Drake MN-4 antenna transmatch ("tuner"), various tools, wooden stakes, tape, nylon ties, ceramic insulators, and various center connectors (I prefer the Budwig HQ-1 center connector).  To power my low-profile station, I use a deep-cycle marine battery charged by a set of small solar panels.  I also have 100-feet (30.48 meters) of 450-ohm ladder line and a W9INN 4:1 balun if I want to use one antenna for several bands.

I've used this portable station many times, using a dipole antenna configured in either an inverted v or sloper arrangement.

Although I prefer the sloper for portable operations, I decided to try out another design that I once used back in 1980 when I was a fairly new operator (I got my novice license in 1977 and was a general class operator by 1980).  This antenna would be an inverted vee with separate antennas cut for 40, 20, 15, and 10 meters.  A pulley was attached to the top of the mast and was used to raise and lower each separate antenna as the band conditions changed.

The antenna consisted of four separate dipoles cut for 7.088 MHz (the frequency of the Hawaii Afternoon Net), 14.200 MHz, 21.250 MHz, and 28.400 MHz (in the middle of the novice/technician class 10 meter phone band).  I used #14 AWG house wire for the antenna elements.

Each antenna was attached to a separate Budwig HQ-1 center connector and tied off to a ceramic insulator.

Using the general formula, 468/f (MHz)=L (ft), the lengths of each dipole element became:

7.088 MHz--66.02 feet/20.13 meters.  Each dipole element was cut to 33.1 feet/10.06 meters.

14.200 MHz--32.95 feet/10.04 meters.  Each dipole element was cut to 16.475 feet/5.022 meters.

21.250 MHz--22.02 feet/6.71 meters.  Each dipole element was cut to 11.01 feet/3.35 meters.

28.400 MHz--16.47 feet/5.02 meters. Each dipole element was cut to 8.23 feet/2.50  meters.

Each separate antenna had its own Budwig HQ-1 center coax connector and end insulator for tie-off purposes.

I used three 5-foot (1.52 meter) wooden stakes for the antenna system.  One stake would support the fiberglass mast.  The other stakes would serve as tie-off points for the inverted v antenna elements.

I attached a brass, marine pulley to the top of the mast.  I ran a 50-foot/15.24 meters piece of dacron rope from ground level through the pully and back down the mast.  The rope would be tied to each antenna segment as I needed it.  Each segment could be raised or lowered depending on the band of choice.

Two additional pieces of dacron rope (50 feet each/15.24 meters each) were used to tie off insulators belonging to each antenna element.  The rope length could be adjusted depending on which antenna segment was chosen.

ERECTING THE ANTENNA:

I assembled each antenna segment on the ground--two segments for each band (40, 20, 15, and 10 meters).

Antenna elements were soldered to each  Budwig HQ-1 center connector.  All connections were wrapped with vinyl electrical tape to protect the connections from the weather.  In the end, I had four separate dipole elements ready to be used.

I first attached the 40 meter dipole elements to the "pull-up" rope, connected the RG-6 coax (with a choke balun), and raised the antenna segment to the top of the mast via a pulley and dacron rope.  The rope was secured to the bottom of the mast to keep the antenna element fixed at the top of the mast.

I hoisted the fiberglass mast on its wooden support stake and secured the inverted vee elements to their respective stakes.

To change bands, I lowered the 40 meter antenna and disconnected it from its wooden tie-off stakes.  I then connected the 20 meter element, attached the coax, raised the antenna to the tip of the mast with the rope and pulley combination and tied-off the antenna elements to the wooden stakes.

A similar procedure was used for the 15 and 10 meter bands.

In most cases, the antennas exhibited a SWR between 1.6 and 1.8 to 1 without the Drake MN-4 in the feed line. With the Drake MN-4 in the system, I was able to keep SWR below 1.3 to 1 on each band.

As I found out back in 1980, a bit of trimming was necessary to reduce the SWR.  I decided to leave the antenna elements unaltered and used the Drake MN-4 to manage the small mismatch.

This antenna works very well for an inverted v.  Raising, lowering, and changing bands are quite easy with the rope and pulley system.  Although I'm partial to tuned feeders, I found this approach workable and simple to use.  Although the RG-6 has a nominal impedance of 75 ohms, it seemed to match each antenna segment without problems.  The Drake MN-4 handled the small mismatch without causing any difficulties.  I used RG-6 because that's what I had stored in the shack.  "F" to "UHF" connectors are available at Radio Shack.

This antenna is simple, efficient, and requires no ground radial system.  Most of the materials are available at the nearest hardware store or home improvement outlet.  Have fun!

REFERENCES:

http://www.hamuniverse.com/dipival.html (antenna calculator).
http://www.youtube.com/watch?v=fyOWRTWdDKM.
http://www.k7mem.com/Electronic_Notebook/antennas/inverted_vee.html.
http://www.arrl.org/files/Technology/tis/info/pdf/0683033.pdf.

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Your comments are always welcome.

Aloha es 73 de Russ (KH6JRM)

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