Wednesday, May 29, 2013

Antenna Topics: A simple, indoor multiband dipole antenna. Post #197

Over the past few days, mother Nature has released a torrent of heavy showers over the Hamakua Coast of Hawaii Island, making outdoor antenna work very wet and potentially dangerous.  When the weather turns bad, I usually lower all my verticals and inverted vees to ground level, disconnect antenna feed lines, and unplug my rig from the electrical mains.  I usually run my station off of deep-cycle marine batteries charged by photovoltaic panels, but sometimes, I use the electrical grid to run a bit more power (i.e. more than 50 watts) from my venerable Swan 100-MX.

During today's passing showers, I decided to revive one of my old indoor antennas, a 10, 15, and 20 meter segmented dipole fed by a short piece of 50-ohm cable (RG-58).  This certainly isn't a new idea.  I've used the concept for outdoor antennas as well.  You can choose which of these bands to use by clipping or unclipping dipole segments.  AK7M and NV5I have published versions of this antenna in various ARRL publications.  I will cite the appropriate reference at the end of this post.

I was able to find all of the antenna materials by carefully searching my storage bins in the garage, a place I call the "radio room annex."

So, with the rain falling down, I decided to "homebrew" another indoor dipole.  Hopefully, I could use this simple antenna to make a few contacts before dinner.

MATERIALS:

Approximately 12-feet (3.65 meters) of RG-58 coax for the feed line.  I had a 12-foot (3.65 meters) piece of RG-58 cable with "pigtails" left over from an old Hustler Mobile Antenna.  This coax would be sufficient to reach the Drake MN-4 transmatch ("tuner").

One PL-259 connector.

A 50-foot (15.24 meters) spool of AWG #22 insulated, solid copper wire.  You could use any wire available.  I chose this gauge because I had some stored in the shack.

At least six test leads with alligator clips.

A box of thumbtacks.

A SWR bridge or a transmatch capable of reading SWR.  My old Drake MN-4 was able to handle the SWR chores.

Small pieces of nylon cord to support the ends of the 20 meter segments of the indoor dipole.

ASSEMBLY:

After I attached the PL-259 to the coaxial cable, I wound approximately 8-feet (2.43 meters) of coax nearest the pigtails into a "choke balun" and held the winding together with nylon ties.  Hopefully, the coil would keep rf from running down the shield of the coax into the shack.

Using the general dipole formula ,468/f (MHz)=l(ft), I determined the length of wire needed to make each element of the dipole at 28.400 MHz.  That worked out to be 8.23 feet (2.51 meters) per element.

Next, I cut two wires to this length and attached them to the feed line, one to the center connector and the other to the shield braid.

At this point, the clip leads play their role.  To get the dipole operating at 21.200 MHz (15 meters), I attached a clip lead to the end of each 10 meter segment; calculated the length of wire needed to complete the 15 meter segment (an additional 2.80 feet/0.85 meters for each element); and to complete the 20 meter segment, I added another clip lead to each antenna element and attached an additional 5.34 feet (1.62 meters) to each side to bring the 20 meter elements to a resonant frequency of 14.200 MHz.

Like NV5I, I used thumbtacks to secure the pieces of wire to the ceiling of the radio room.

With all elements connected by clip leads, I had a 20 meter antenna that exhibited a 1:7 to 1 SWR without the Drake MN-4 in line.  For 15 meters, I got a 1.8 SWR without the Drake MN-4 in line.  The 10 meter segment showed a SWR of 1.6 to 1 without the Drake MN-4 in line.  When I used the Drake MN-4 in the antenna system, I was able to get a SWR below 1.3 to 1 on all bands.  With a little trimming, I could use this antenna without a tuner.  In such a case, you may want to cut your antenna elements a little longer than the formula provides.

INITIAL PERFORMANCE:

I was pleased with this "homebrew" antenna.  As NV5I states in his article, "it's inconspicuous, non-hazardous and efficient."  Running 10 watts CW through the old Swan 100-MX, I was getting reports between 559 to 589, depending on propagation.  SSB reports were a bit weaker, ranging from 54 to 57, with approximately 10 watts output.  The antenna works and was easy to build.  Best of all, I didn't have to go out in the rain to adjust the antenna.  This antenna was just right for a bad weather day.

Perhaps this indoor dipole will useful for those amateur radio operators subject to the restrictions of HOAs and CC&Rs.  This could be the answer to some of your antenna problems.  As long as you run low power and use digital modes, you shouldn't have any problems with rfi or nosey neighbors.

REFERENCES:

Barry, Larry A., NV5I.  "An Indoor Dipole Antenna."  Reprinted in "Hinks & Kinks For the Radio Amateur", 13th Edition, ARRL, Newington, CT., 06111, Copyright 1992, pp. 7-26 and 7-27.

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Aloha es 73 de Russ (KH6JRM)

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