Wednesday, May 21, 2014

Simple Ham Radio Antennas. A basic 40/15 meter dipole. Post #276

My antenna "farm" at the new QTH finally is taking shape. So far, I've built and used successfully a 40-10 meter inverted V fed with ladder line, a 5/8 wavelength vertical for 20, 15, and 10 meters, a 135-ft/41.15 meters "classic doublet" fed with ladder line that works from 80-10 meters, and a 40-10 meter delta loop fed with ladder line.  I've described these antennas in past posts.  All of them work very well with relatively low power (less than 50 watts) with my older rigs (Argosy II, Swan 100 MX, Yaesu FT-7, and a Kenwood TS-520).

Although I prefer multiband HF antennas fed with 450-ohm ladder line, I still use a few single or two-band antennas fed with 50-ohm coaxial cable when I wish to experiment on 40 or 15 meters.  Last Sunday, 18 May 2014, I decided to build a simple two-band dipole covering the 40 and 15 meter bands using a single piece of RG-8X coaxial cable.

The dipole would be designed for the lowest frequency range and then use the third harmonic of  that frequency to operate on 15 meters.  There are all kinds of ways to be sure the SWR is good on both bands. You can use capacity hats, "outrigger" add ons using small clips, or design modifications to use CW on 40 meters and SSB on 15 meters.  One can also use an antenna transmatch or "tuner" to bring both bands into resonance.

For this new antenna, I opted to design the 40 meter segment using a design frequency of 7.088 MHz (the "watering hole" for the daily Hawaii Afternoon Net on Lower Side Band), which would put the third harmonic frequency around 21.264 MHz in the 15 meter phone band.

MATERIALS AND OTHER PROCEDURES.

Before we move forward, it may be best to define what a dipole is.  According to James Healy (NJ2L), "a dipole is a balanced antenna, meaning the 'poles' are symmetrical...they're equal lengths and extend in opposite directions from the feed point."  Or, in other words, a dipole "is an antenna made of wire and fed at the center."  The dipole would be cut to a length of 1/2 wavelength for the design frequency.

The first step is to cut the antenna wire into two equal segments.  Most of the antenna was built on the ground to facilitate construction, soldering, and trimming.  Using the general formula 468/f (MHz)=L (feet) or 143/f (MHz)= L (meters) and a design frequency of 7.088 MHz, I came to total length of 66.02 ft/20.13 meters.  Each segment of the dipole would be 33.01 ft/10.16 meters.  These lengths are only approximate.  Please cut a little extra wire for trimming and adjustment purposes.  For the antenna wire, I chose some #14 AWG house wire left over from another project.

Next, I attached one end of each dipole element to a Budwig Hi Que-1 Center Connector.  You can use ceramic materials, hard plastic, wood, or even glass to make your center and end insulators.  Each connection was soldered and wrapped with several layers of vinyl electrical tape.

A ceramic insulator was attached to the far end of each dipole element.

A 75 ft/22.86 meters piece of nylon rope was attached to each end insulator of the antenna elements.  The other end of each rope was attached to a ceramic insulator.  These ropes would be later shot over two tree limbs approximately 70 ft/21.34 meters apart.  The tree limbs were approximately 35 ft/10.67 meters above ground.

With the dipole elements on the ground, I made an eight turn, 6 inch/15.24 cm diameter "choke balun" out of the RG-8X coaxial cable that would serve as the feed line.  Hopefully, this balun would keep rf out of the shack.  The balun was positioned approximately 18 inches/45.72 cm below the center coax connector.  With the balun wound and taped, I attached the free end of the coax to the Budwig Center Connector.

GETTING THE ANTENNA UP

I used a slingshot to fire each nylon rope over its appropriate tree limb.  There was enough overhang to allow me to adjust the antenna.  With the antenna loosely positioned above the ground, I carefully adjusted each nylon rope so the dipole was mostly horizontal.  If you can get your dipole up to 50 or 60 ft/15.24/18.29 meters above ground, your performance should improve.  Antenna experts recommend that you keep your dipole at least 1/2 wavelength above ground for your desired band of use.

Once the nylon ropes were secured to nearby branches, I ran the RG-8X coax to the shack patch panel.  A 6 ft/1.82 meters piece of RG-8X was run into my trusty Drake MN-4 transmatch ("tuner").  Initially, I bypassed the tuner to get a SWR reading from the antenna system without compensating for the small amount of SWR I expected on the line.  My Ten-Tec Argosy II, a Heathkit dummy load, and a low-pass filter were interconnected to the Drake MN-4.

INITIAL FINDINGS

Without the Drake MN-4 in the system, the SWR ranged between 1.4 to 1 and 1.6 to 1 on 40 meters and between 1.6 to 1 and 2.0 to 1 on 15 meters.  I worked several stations on each band without difficulty and the old Argosy II didn't seem to mind the SWR.  When I put the Drake MN-4 into the system, I was able to get a SWR of 1.1 to 1 across the entire 40 meter band and a SWR of 1.3 to 1 across the entire 15 meter band.  Obviously, some trimming has to be done.  But, for now, the antenna works very well for these two bands.  I'll leave the Drake MN-4 in the system to keep this small amount of SWR contained.

This was a fun project at almost no cost to me.  I had all of the materials on hand, so I just used what I found in the store room.  You can also configure this dipole as an inverted V, a sloper, or even as a parallel fed multi-element "fan dipole" antenna.  All of these variations on the basic dipole design work well and will give your hours of on-air fun.  You can also feed your basic dipole with 450-ohm ladder line connected to a 4:1 balun and an antenna transmatch ("tuner") to give you multi-band coverage from one antenna.

Have fun.  Build a dipole.  It's simple, cheap, and quite versatile.

REFERENCES:

http://www.hamuniverse.com/easydipole.html.

nathan.chantrell.net/old-stuff/radio/radio-scanning/how-to-make-a-simple-dipole-antenna/

http://www/arrl.org/files/file/Technology/tis/info/pdf/9106023.pdf.

rsgb.org/main/get-started-in-amateur-radio/antennas/your-first-antenna-the-half-wave dipole.

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Aloha,

Russ, KH6JRM

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