Sunday, January 19, 2014

Simple Ham Radio Antennas: Antennas without "tuners", part 3. Post #256

In some of my previous posts, I've investigated, built, and used multiband dipole and inverted vees using a single 50 ohm coaxial feed line.  With a telescoping fiberglass mast (33-ft/10.06 meters extended), a simple halyard and pulley system, and separate antennas cut for each band of use (40, 20, 15, and 10 meters), I've been able to change bands fairly quickly by lowering the dipole/inverted vee elements, substituting those of another band, and raising the new antenna.  With each band element cut for the desired frequency, the efficiency is high with low swr for that band.  Since each antenna element is trimmed for the lowest swr at my mast location, an antenna transmatch isn't necessary.  Sometimes, as in the case with the 40 meter dipole/inverted vee, an antenna cut for the cw portion of the 7 MHz band will often work well on the phone portion of 15 meters (21 MHz band). The 15 meter band will operate on the third harmonic of the 40 meter band.

In order to save me a trip outside to change bands, I then made a "fan dipole/inverted vee" with separate dipole elements for each band attached to a common point on the feed line located at the apex of the mast.  The principal band of use (in my case, the 40 meter band) was made as a nearly horizontal dipole with each element attached to tree branches approximately 33-ft/10.06 above ground.  Inverted vee elements were led from the common connection point at the apex of the mast and "fanned out" below the main 40 meter dipole and attached to lower branches, with each end of the antenna element approximately 5-ft/1.82 meters away from the elements above it.

In both of the above antenna projects, a "choke" balun was attached to the mast just below the feed point of the coax connector.  I used an extra Budwig center connector for the common connection point.  You could also use a "homebrew" center connector made from lucite, fiberglass, or teflon.

Both of these antennas work very well with a swr of less than 2:1 across each band.  However, you may want to cut your dipole/inverted vee antenna elements a bit longer than calculated to allow for trimming and swr adjustment.

SEGMENTED DIPOLES--another approach to coax-fed multiband antennas.

Fan Dipoles/Inverted vees are great antennas, but they often are a bit tedious to tune because of the coupling between elements.

A way to avoid the extra work and still get good performance is to design and build a segmented dipole/inverted vee, with the main elements used for your lowest frequency band and segmented dipoles with clip leads to provide coverage on other bands.

When I first started refurbishing my permanent home in the Puna District of Hawaii Island, the first antenna I built (and still use) was a combination 40, 20, and 15 meter segmented dipole using a single telescoping fiberglass mast, a halyard and pulley raising and lowering system, and a single RG-8X coaxial cable with UHF connectors for the feed line.  This antenna didn't require an antenna transmatch or "tuner".

I chose the inverted vee configuration because this antenna used only one tall support and could be raised and lowered by one person.  This antenna didn't require a ground radial system.

MATERIALS:

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

Three, 5-ft/1.82 meters wooden support stakes.  Two stakes would be used to tie off the ends of the segmented inverted vee.  One wooden stake would be used to support the lightweight fiberglass mast.  The antenna elements would also help support the mast and would function as "guy wires".

A total of 125-ft/38.10 meters of #14 AWG house wire for the antenna elements and segments.  I didn't require this much wire, but a little extra would be handy if I miscalculated the length of any element.

Seventy-five feet/22.86 meters of RG-8X coaxial cable with UHF connectors.  This would be the common feed line for the multiband antenna.

Basic tools, including wire cutters, soldering iron, nylon ties, vinyl electrical tape, and assorted items.

Transceiver (Swan 100 MX), low pass filter, Drake MN-4 transmatch--I would use the SWR meter on the transmatch to help trim the antenna for the lowest swr, and the trusty Heathkit Cantenna dummy load.  The antenna transmatch would always be available if I chose to use it.  But in this case, I bypassed the "tuner" and just used the SWR meter.

Four ceramic insulators.  These would be used to tie off the ends of each antenna segment.

Four antenna clips to join the main 40 meter inverted vee elements to the 20 meter segments.  Small pieces of dacron rope would be used to tie off the final segments to the end support stakes.

ASSEMBLY:

The antenna was built on the ground.  The antenna elements were cut and attached to the Budwig center coax connector in the garage where I could solder without interference from the wind.

The 40 meter antenna would be cut using the standard dipole formula, 468/f (MHz)=L (feet).  I used a design frequency of 7.088 MHz (the frequency of the Hawaii Afternoon Net).  Wire measuring 66.02 ft/20.13 meters was cut for the 40 meter portion of the antenna.  Although my calculations were fairly accurate for my location, you may want to cut your elements a bit longer to allow for trimming and swr adjustment.  I then divided the dipole into two equal segments, measuring 33.01-ft/10.06 meters each.

Each element was connected and soldered to the center coax connector.  The connection was wrapped with several layers of vinyl electrical tape.  The end of each element not connected to the center connector was attached to a ceramic insulator.  After securing the wire to the insulator, an inch (2.54 cm) of wire insulation was stripped off and soldered to a small battery clip.

At this point, the antenna could be used as a 40/15 meter antenna, with the 40 meter segment cut for the cw part of the band, while the third harmonic of 7.088 MHz could be used in the phone portion of the 15 meter band.  In Hawaii, amateur radio operators can use 7.088 MHz for LSB purposes.  So, this works out rather well for those of us in the Central Pacific.

To use the antenna on 20 meters, an "outrigger" segment must be added to each 40 meter element via a clip lead.  Fortunately, the addition of a 1/4 wavelength element for 20 meters to each 40 meter element will do the trick.  This makes the new 20 meter dipole/inverted vee a 3/4 wavelength antenna for each side of the dipole/inverted vee.  With all segments connected, the 20 meter antenna will function as a 3/2 wavelength antenna on 20 meters.  So, using the general formula 468/f (MHz)=L (feet) with a chosen frequency of 14.200 MHz, the length of the 20 meter segment will be 32.95- ft/10.04 meters or 16.47- ft/5.02 meters for each dipole/inverted vee element.

The total length of the combined 40/15/20 meter inverted vee will be 49.48-ft/15.08 meters per side or a total length of 98.96-ft/30.16 meters for the entire antenna.

Once the 40 meter dipole/inverted vee is made, the 16.47-ft/5.02 meters segments for 20 meters are attached.  The 20 meter segment is threaded through the end insulator of each 40 meter element and connected to a small battery clip.  The connection to the clip is soldered.

With the telescoping mast on the ground, I attached the Budwig center coax connector to the halyard and pulley system.  Just below the center connector, I connected a "choke balun" comprising 8 turns of RG-8X coaxial cable, 8-inches/20.32 cm in diameter to the Budwig center connector.  The "choke balun" is then secured to the mast with nylon ties and vinyl electrical tape.  The coaxial cable feed line is run down the mast to a point 16-ft/4.87 meters above ground level.

I hoist the fiberglass mast onto its wooden support stake and slowly pull up the antenna center connector with the halyard and pulley arrangement.  Once the center connector is at the tip of the mast, I secure the halyard and tie off the ends of the 20 meter segments to their respective wooden stakes.  I adjust the antenna to a uniform and balanced appearance.  The clip leads are easily attached by using a small ladder to connect the segments.  To use the inverted vee as a 40 meter antenna, leave the clip leads disconnected.  To use the antenna as a 3/2 wavelength on 20 meters, connected all clip leads together.

I run the remaining length of RG-8X to a hook on the garage door about 10-ft/3.04 meters above ground.  There is sufficient clearance for a vehicle to pass under the antenna without snagging the feed line.

I then run the feed line through my "homebrewed" patch panel in the shack window into the old Drake MN-4 transmatch.  The "tuner" is bypassed so only the swr meter is used.  A series of short patch cables (RG-8X measuring 18-inches/45.72 cm) connects the Swan 100-MX transceiver to the bypassed "tuner", dummy load, and low pass filter.

Since the terminal point of the inverted vee is around 5-ft/1.82 meter above ground, connecting the segments together for 20 meter operation is easy.  I was lucky to have the 20 meter segment read a swr below 2:1 across the band.  The 40 meter segment was better at 1.6 to 1.  With the Drake MN-4 in line, I was able to get the swr to read 1:1 across both 40 and 20 meters.  On 15 meters, the swr measured 1.7 to 1 with the Drake MN-4 bypassed.  With the transmatch in the line, I was able to get a 1:1 swr.  Obviously, some adjustments must be made.  But, for now, the antenna works very well without the "tuner".  The old Swan 100- MX stays cool running 20 to 30 watts cw and ssb.

Contacts are being made both locally and throughout the Pacific and mainland U.S.A.  This antenna doesn't outperform a yagi on a 50-ft/15.24 meters tower, but it does do well for what it is.  Because the current maximum is at the tip of the mast, it provides an acceptable match for 50 ohm coaxial cable.  Best of all , I had most of the material on hand and I didn't have to make an extensive ground system.

When I'm done for the day, I lower the antenna with the halyard and pulley system, disconnect the antenna feed line, and ground the antenna system to a ground rod in back of the garage.  Because the antenna is behind my house and is surrounded by trees, it is nearly invisible.  So far, there have been no complaints from my neighbors.

Next time, I'll look at "tuner-less" antennas that use a combination of 450 ohm ladder line, a balun, and coax to attain multiband capability.

RESOURCES:

Noll, Edward M. (W3FQJ).  "Easy-Up Antennas for Radio Listeners and Hams.  MFJ Enterprises, Inc., Mississippi State, MS, 39762.  Limited Edition, 1991. pp. 111-112 and pp. 117-118.

Noll, Edward M. (W3FQJ).  "73 Vertical, Beam, and Triangle Antennas."  Editors and Engineers, Indianapolis, IN, 46266.  Seventh Printing, 1979.  pp. 70-74.

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Aloha de Russ (KH6JRM).

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

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