Friday, December 20, 2013

Simple Ham Radio Antennas: A multiband Inverted-V Antenna. Post #249

One of the most popular amateur radio antennas is the Inverted-V.  This antenna is a first cousin to the half wavelength horizontal dipole whose antenna elements are drooped down so that the included angle between them is between 90 and 120 degrees.  According to William I. Orr (W6SAI) and Stuart D. Cowan (W2LX), the "bandwidth is somewhat lower than for a conventional dipole...Because the wires of the Inverted-V do not lie along one axis, the physical length is somewhat longer than that of a dipole cut for the same frequency."  For general design purposes and allowing for some trimming of antenna elements, you can use the general dipole formula, 468/f (MHz)=L (feet) to compute the total length of the Inverted-V dipole.  Some antenna experts believe the drooping halves of the Inverted-V change the resonant frequency, and, therefore, recommend a slightly different formula be used to calculate the length, such as 464/f (MHz)=L (feet).  I use the 468/f (MHz)=L (feet) formula to establish a general length parameter and use the old "cut and trim" method to bring the antenna to resonance on my chosen frequency.  More often than not, I cut a compromise length and use my trusty Drake MN-4 transmatch to take care of the small SWR found on the feed line.

Allowing for some performance shortfalls, the Inverted-V has some definite advantages:

Only one support structure is needed.

No ground radial system is required.

The Inverted-V can be fed with 50 ohm coaxial cable for single band use or fed with 300 ohm television twin lead or 450 ohm ladder line for multiband use.

The Inverted-V is simple to build, inexpensive, and portable.

I've used a variety of single and multiple band Inverted-Vs in my amateur radio "career" with excellent results. Now that I'm moving to a new home site with plenty of room for antennas, I thought a few antenna experiments would be in order.

In previous posts, I've related my adventures with homebrewed delta loops, vertical ground planes, doublets, and even double extended zepp antennas.  All of these antennas have worked very well, even at power levels below 10 watts.

My recent foray into antenna territory has been the construction of a multiband Inverted-V covering the 80, 40, and 20 meter bands.  Unlike some of my other antennas, I didn't use tuned feeders because my stock of 450 ohm ladder line was exhausted.  I did, however, have several 50-foot/15.24 meters lengths of RG-8X coaxial cable with UHF connectors.  Properly designed, a coaxial feed line can be used to cover several bands if "outrigger" segments for each additional band are attached to the main Inverted-V dipole.  Since I would be covering 80 meters as the lowest band, the principal V would be cut for 3.800 MHz.  Subsequent segments would be cut for 40 and 20 meters and connected  by ceramic insulators and alligator clips to constitute a workable antenna for each respective band.

A design attributed to Ed Noll (W3FQJ) provides a series of clip on jumpers for operation of the 80 meter Inverted-V as a 5/2 wavelength antenna on 20 meters and as a 3/2 wavelength antenna  on 40 meters.  The 80 meter antenna is cut as a 1/2 wavelength antenna with various segments added to bring bring each band into resonance.  A single 50 ohm feed line is connected to the top of the mast where the 80 meter elements are attached.

MATERIALS:

The antenna was built on the ground and later hoisted to the top of a mast by means of a pulley and lanyard system.

One 33-foot/10.06 meter MFJ telescoping fiberglass mast.

One 5-foot/1.82 meters wooden support stake for the mast.

Approximately 225 feet/68.59 meters of #14 AWG housewire.  The wire will serve as 80 meter antenna elements and as the 20 and 40 meter outrigger segments.

One Budwig HQ-1 center insulator.

Six ceramic insulators to tie off and join outrigger segments to the main 80 meter antenna.

Four, 5-foot/1.82 meter wooden stakes to support the outrigger segments.

Fifty feet/15.24 meters of RG-8X.  This will be the antennas feed line.

One Drake MN-4 antenna transmatch to handle the small amount of SWR on the feedline.

Station equipment, including a Swan 100 MX transceiver, dummy load, and low-pass filter.

Various 3-foot/0.91 meters lengths of RG-8X cable for connecting equipment to the Drake MN-4 transmatch.

ASSEMBLY:

First, I cut the 80 meter dipole according to Ed Noll's (W3FQJ) instructions. Each dipole element measured 62 feet, 2.5 inches/18.98 meters.  

I attached and soldered the top end of each 80 meter element to the Budwig HQ-1 center connector.

I attached a clip lead to the bottom end of each 80 meter element and secured that connection to a ceramic insulator.

Next, I cut each element of the 20 meter segment to a length of 21 feet, 3 inches/6.48 meters.  I attached a clip lead to each end of the 20 meter segments and threaded them through the end insulators of the 80 meter elements.  I then attached  clip leads to the other end of the 20 meter segments and threaded them through ceramic insulators.

I then cut each element of the 40 meter segment to a length of 16 feet, 0 inches/4.87 meters.  A clip lead was attached to one end of each segment and threaded through the end insulators of the 20 meter segments.

To use 80 meters, I left all clip leads unattached.  To pursue DX on 20 meters, I connected each 20 meter segment to each 80 meter element.  This connection would serve as a 5/2 wavelength antenna for 20 meters.  To chase local contacts and occasional DX on 40 meters, I connected all of the segments together. The arrangement would serve as 3/2 wavelength antenna on 40 meters.

With the 80 meter antenna and all of its outrigger segments joined by clips and ceramic insulators, I attached the Budwig HQ-1 coax connector to the apex of the mast.  I then hoisted the mast onto its wooden support stake.

Five-foot/1.82 meters wooden support stakes were attached to the insulators joining the 80 and 20 meter segments and the 20 and 40 meter segments.  The wooden stakes kept the outrigger segments off the ground and limited antenna sag.

INITIAL RESULTS:

Without the Drake MN-4  transmatch in the antenna system, I was able to get a SWR of less than 2:1 across all portions of 20 and 40 meters.  Although I was able to get a decent SWR in the neighborhood of 3.800 MHz, it was not possible to cover the entire 80 meter band without the aid of the Drake MN-4.  With the transmatch in the line, I was able to get an acceptable match on most of the 80 meter band.  The transmatch kept the SWR below 1.3 to 1 on the 20 and 40 meter bands. 

As an experimental multiband antenna, the segmented Inverted-V performed well at my home site in the Puna District.  To change bands, all I have to do is lower the Inverted-V and change the clip leads.  Since I had all the antenna materials on hand, my cost was minimal.  You can use this outrigger design for any bands you desire.  Have fun!

REFERENCES:

Orr, William I. (W6SAI) and Cowan, Stuart D. (W2LX).  The Radio Amateur Antenna Handbook.  Radio Publications, Inc.  Lake Bluff, Illinois, 18044.  Seventh Printing, 1988. pp. 131-133.

Orr, William I. (W6SAI) and Cowan, Stuart D. (W2LX).  Simple, Low-Cost Wire Antennas.  Radio Publications, Inc.  Winton, Connecticut, 06897.  Fifth Printing, 1979.  pp.70-86.

McCoy, Lew (W1ICP).  Lew McCoy On Antennas--Pull Up A Chair and Learn From the Master.  CQ Communications, Inc.  Hicksville, New York, 11801.  Second Printing, 1977.  pp. 51-52.

Turner, Rufus P.  The Antenna Construction Handbook for Ham, CB & SWL.  Tab Books, Inc.  Blue Ridge Summit, Pennsylvania, 17214.  Second Printing, 1981.  pp. 91-92.

The ARRL Antenna Book.  American Radio Relay League, Inc.  Newington, Connecticut, 06111.  Fourteenth Edition, Second Printing, Copyright 1983.  pp. 8-9 and 8-10.

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

Thanks for being with us today!

Aloha de Russ (KH6JRM)

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