Saturday, May 11, 2013

Antenna Topics: A simple inverted vee dipole that doesn't require a tuner, post #189

KH6JRM's Amateur Radio Antenna Topics:

A simple inverted vee dipole that doesn't require a tuner.

Hawaii Island has been enjoying some excellent, if somewhat voggy, weather the past few days--a perfect time to design and build a simple antenna for my favorite amateur radio bands.  Although I enjoy working 10 and 20 meters, I've found plenty of contacts and even some decent DX on the 40 and 15 meter bands.  During the day, there are plenty of Hawaii hams to chat with on a variety of nets.  Early afternoon chats on 15 meters range from locals to the exotic realms of the southwest Pacific and Japan.  If I decide to work 10 or 20 meters on a whim, I can always switch to my 40 meter vertical with tuned counterpoise to capture a few elusive contacts on 10 meters.  This antenna uses tuned feeders (450-ohm ladder line) and can work amateurs from 40 through 10 meters with the help of a 4:1 balun and a good transmatch, such as my Drake MN-4.

However, this week, I've had the Drake MN-4 apart for its semi-annual maintenance check-up.  The tropical air in Hawaii sometimes leaves a small layer of salt and other contaminants on metal surfaces, particularly on antenna wires and other connections. So, every few months, I clean and lubricate all the contacts, switches, and dials on my equipment--a necessary task in the tropics.  While I have the Drake MN-4 on the workbench, I have no readily available "tuner" or transmatch available.  I have an old MFJ 941-E in storage, but I didn't want to unpack it this weekend.  I guess I'm just lazy.

Anyway, I had a spare MFJ 33-foot (10.06 meters) telescoping fiberglass mast in the garage just waiting to support another homebrew "special".  This antenna would be used on 40 and 15 meters.  The 15 meter operation would run as the third harmonic of 40 meters and would give me a low SWR if I cut the antenna elements properly.  If I cut the 40 meter portion for the lower part of the band (i.e. around 7.088 Mhz--the Hawaii Afternoon Net frequency), I should get decent performance at 21.264 Mhz in the 15 meter band.  That would place me in the region for SSB contacts on 15 meters.

Of course, with the Drake MN-4 in line, I would just cut the antenna elements for my favorite 40 meter frequency and use the old Drake to minimize SWR.  Since I didn't have the MN-4 available, I reached a compromise--CW on 40 meters and SSB on 15 meters.

Since I was using 50-ohm coaxial cable for the feed line (RG-8X), adventures into 20-meter land would be out of the question because of high SWR and loss of power in the coax.

With the sun shinning early this morning, I began the project.

MATERIALS:

One 33-foot (10.06 meters) MFJ fiberglass mast.

Three, 5-foot (1.52 meters) wooden posts.  One post would serve as the mast support.  The other two posts would be used as tie-off points for the inverted vee.  I chose the inverted vee format because of limited space in my backyard.  A regular flat-top dipole using two masts would be preferrable, but I used what I had on hand.

I chose a 40 meter operating frequency of 7.088 Mhz--a favorite Hawaii net frequency.  The antenna would also be usable on the third harmonic, 21.264 Mhz.  In this case, the 40 meter antenna would serve as a 3/2 wavelength antenna on 15 meters with a fairly small SWR.  I cut two pieces of #14 AWG house wire for the 40 meter elements.  Using the general formula 468/f(Mhz)=l(ft), the total length of the inverted vee dipole worked out to be 66.02 feet (20.13 meter).  Each antenna element was then cut to 33.02 feet (10.06 meters).

The antenna was assembled with the MFJ mast on the ground.

One Budwig HQ-1 center coax connector (available from Fair Radio Sales in Lima, Ohio, USA).

Two ceramic or plastic insulators attached to the bottom end of each antenna element.

Two wooden posts to tie-off the antenna elements.  I used some dacron rope I had stored in the garage.

A 50-foot (15.24 meters) length of 50-ohm coaxial cable with UHF connectors.  You can use RG-58, RG-8, or RG-8X.  I used a spare reel of RG-8X in the storage room for the feed line.

ASSEMBLY:

With the MFJ mast on the ground, I attached the top portion of each antenna element to the Budwig HQ-1 center coax connector.  I soldered each connecction and wrapped several layers of vinyl plastic electrical tape around each connecction.

I then attached the bottom portion of each antenna element to a ceramic insulator.

Before I attached the coax feed line, I made a "choke balun" out of several turns of the coax, secured the coils with vinyl plastic electrical tape, and attached the "choke balun" to the mast with nylon ties.

I attached the feed line to the Budwig HQ-1 center connector.

I ran the coax feed line down to the mid-point of the fiberglass mast (about 16 feet/4.87 meters) and secured  that portion with nylon ties.

I then hoisted the MFJ mast onto its wooden support stake.  I tied off each antenna element to a pre-positioned stake.

The coax feed line was led to the garage and secured to a plastic hook.  The coax was allowed to sag a little,   with the line approximately 15-feet (4.57 meters) above ground from the mast to the garage.

The feed line was connected to my static discharge unit near the shack window.  A 15-foot (4.57 meters) piece of RG-8X coaxial cable ran through a window opening to the Swan 100- MX.  Separate cables  ran to a dummy load and a SWR meter.  If the Drake MN-4 was in service, the cable would have been connected to the MN-4 first and then to the station equipment.

PRELIMINARY RESULTS:

Initial tests were good.  The SWR across the lower 100 Khz of the 40 meter amateur band ranged from 1.5 to 1.7 to 1 without the "tuner" in the line.  On the upper portion of 15 meters, the SWR ranged between 1.6 to 2.0 to 1.  Obviously, some adjustments must be made, but, for now, the equipment seems to work without overheating.  Once the old Drake MN-4 is put into its normal place and a bit of trimming is done, I should have lower SWR readings.  Running 50 watts out of the trusty Swan 100-MX, I get CW reports ranging from 559 to 599 and SSB reports varying from 55 to 59, depending on the band selected and the time of day chosen for operations.

This antenna won't bust a DX pileup, but it will get you contacts at moderately low power levels.  Since I had the materials on hand, my costs were minimal.  Most of the materials for this antenna can be found at your nearest hardware or home improvement outlet.  Mast materials can be made of pvc pipe, wood, long fishing poles, or even expandable aluminum poles used to clean swimming pools.

I had fun building this simple antenna.  It works.

REFERENCES:

http://www.youtube.com/watch?v=8KHzsKYsZL0.
http://hamuniverse.com/easydipole.html.
http://arrl.org/file/Technology/tis/info/pdf/9106023.pdf.
http://ehow.com/how_4528486_that-doesnt-require-antenna-tuner.html.

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Aloha es 73 de KH6JRM
BK29jx15
Along the beautiful Hamakua Coast of Hawaii Island.