Saturday, May 4, 2013

Antenna Topics: A simple multi-band inverted V dipole for 40 and 15 meters, post # 186

KH6JRM's Amateur Radio Antenna Topics

A simple multiband inverted V dipole for 40 and 15 meters.

If you have a small backyard and only have room for a few antennas, a segmented inverted V dipole covering two to three bands of your choice is an option worth considering.

It is quite easy to build an inverted V dipole antenna for two-band operation, with each leg of the dipole separated by a ceramic or plastic insulator.  A jumper clip associated with  each of the two insulators is closed for 15 meter operation and left open for 40 meter operation.  While 15 meter operation is possible with just a 40 meter inverted V (using the third harmonic of your 40 meter frequency), you could risk high SWR on 15 meters, depending on what 40 meter frequency you use.

For example, a few years ago, I used my coax fed 40 meter inverted V on 15 meters with few problems since my prime 40 meter frequency was 7.088 Mhz ( the frequency of the Hawaii Afternoon Net).  With the antenna cut for 7.088 Mhz, I had only a small mismatch on my 15 meter frequency of choice--21.264 Mhz.).  I was able to use much of the 15 meter phone frequencies without cutting any antenna elements.  When I got lazy, I just used the Drake MN-4 match box ("tuner") to reduce the SWR.

Although this plan allowed me some frequency selection, I wasn't happy with constant adjustments to the Drake MN-4.  There had to be a better way.

An obvious solution was to use 450-ohm ladder line and a 4:1 balun in combination with a tuner to provide all band coverage from 40 to 10 meters.  I built a few of these ladder line fed antennas, and they worked very well.

However, when I wanted to revisit the old 40/15 meter inverted V, I had no 450-ohm ladder line or 300-ohm TV twin lead at my disposal.  I did have a spare 33-foot (10.06 meters) MFJ telescoping fiberglass mast, a 50-foot (15.24 meters) piece of RG-8X coax with UHF connectors, and a 100-foot (30.48 meters) spool of #14 AWG house wire.  Coupled with my Drake MN-4, a static discharge system near the shack window, and my trusty Swan 100-MX transceiver, I had the makings of a simple, cheap, and easy to erect inverted V that would give me great contacts with low SWR.

MATERIALS:

A 33-foot (10.06 meters) mast.

Three wooden stakes.  One to support the mast and two to support each inverted V element.  I used three, 6-foot (1.82 meters) wooden stakes for this purpose.  The choice of stakes will depend on what's available in your area.  I had a few stakes left over from an old vegetable garden.

One Budwig center coax connector (available from Fair Radio Sales, Lima, Ohio).

Four ceramic insulators--two for each antenna element.

50 to 75 feet (15.24 meters to 22.86 meters) of #14 AWG housewire for the antenna elements.

A few feet (a few meters) of dacron rope to tie off each antenna element.

ASSEMBLY:

The inverted V was built on the ground.

I first determined the length of the 40 meter elements using the general formula 468/f(Mhz)=L(ft).  Since my preferred 40 meter spot was 7.088 Mhz I made each antenna element 33.01 feet (10,06 meters).  I attached a clip lead to the end of each 40 meter segment before I attached an end insulator.  My chosen 15 meter frequency was 21.050 Mhz ( I like CW).  To make the 40 meter elements into a 3/2 wavelength 15 meter antenna, I added a wire segment of .33 feet or 3.96 inches (10.05 cm) to each side.  A clip lead was attached to the upper end of this very small segment, with the remaining portion attached to an end insulator.
To run a 40 meter schedule, I used only the 40 meter portions of the antenna.  To use 15 meters, I attached the small outrigger wires to the 40 meter clip leads.  With all the leads connected, the inverted V would run as a 3/2 wave length antenna with low SWR.

Before I attached the RG-8X feed line to the Budwig center connector, I wound several turns of the feed line into an 8-inch (20.32 cm) diameter "choke balun" to prevent rf from entering the shack by the feedline.

All connections were soldered and covered with several layers of plastic vinyl electrical tape.

I then hoisted the mast and secured it to the wooden support stake.  I tied off the ends of the inverted V to two pre-positioned wooden stakes.  The antenna elements also helped support the light fiberglass mast.

I ran the RG-8X to my static discharge system and used a 10-foot (3.04 meters) piece of RG-8X to connect the antenna to the Drake MN-4.  Short  coaxial patch cords connected the rig, low pass filter, and dummy load to the Drake MN-4.

PRELIMINARY RESULTS:

Without the tuner in the antenna system, I was able to get a 1.5 to 1.7 SWR across the 40 meter band and a SWR of 1.7 to 2.0 across the 15 meter band.  Obviously, I have a bit of trimming to do.  With the Drake MN-4 in the system, I was able to reduce SWR across both bands below 1.5 to 1.  Results have been good, considering propagation conditions.  Using 50 watts CW into the antenna, I'm  getting 569 to 599 reports on 40 meters and 549 to 579 reports on 15 meters.  Not terribly outstanding, but the antenna works and the equipment stays cool.  I've been able to get satisfactory results on the SSB portions of each band by adjusting the Drake MN-4 for minimal SWR.

Most of the materials can be found at the nearest hardware or home improvement outlet.  The same design principal can be use to create segmented dipoles for 80/40 meters, 20/15 meters, and 15/10 meters.  I'll investigate those varieties later. But, for now, I'm having fun with an inexpensive antenna I built myself.

REFERENCES:

The ARRL Antenna Book,22nd Edition, ARRL, Newington, CT, 06111.

Noll, Edward  M., W3FQJ.  Easy-Up Antennas for Radio Listeners and Hams.  Limited Edition, 1991. MFJ Enterprises, Inc., Mississippi State, MS, 39762.  pp.117-118.

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Aloha es 73 de Russ, KH6JRM
BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.