Collapsible 3-element, 2-meter Yagi
I decided to give myself a challenge to see how far I could communicate on 2-meter FM with just my Wouxun KG-UVD1P and a better antenna sitting on a portable mast. I already knew that I could reach a number of repeaters in the area with my roll-up J-pole. I wondered what I could do if I had a Yagi to reach more distant repeaters and operating under simplex, for that matter.
For my first go, I decided to build a Yagi with two elements. I thought that this would be relatively simple and I found a lot of plans available on the Internet. Being one to tinker, I wanted to do something a bit different. I wanted an antenna that was easy to transport and store when not in use. I considered a tape-measure Yagi, but I thought that constant use would eventually result in broken tape measures. I also found designs that stowed the aerials in the boom. I didn't like these either, as they all seemed to require a balun or some other fixture to obtain tuning. I had access to some very inexpensive 24-inch whip antennas. I wanted to use these, as they would certainly provide enough length for operation on 2-meters, but would collapse down to only five inches for transport and storage.
Design
I downloaded the Yagi-Uda antenna design utility (yagi.exe) written by VE3SQB to design my Yagi. A screen capture of the design resulting from my parameters is shown below. Notice that I entered parameters for tapering elements. This is, of course, as coarse approximation for the stepped diameter of the whips.
To get an idea of the predicted antenna pattern, I used NEC2++ 1.3.1 to analyze the antenna pattern of the antenna and Qantenna 0.2.1 to visualize the results. I probably went a bit overboard and entered the length and diameter of each section of the whips into the NEC file. The Yagi is oriented for horizontal polarization, no ground plane, and a frequency of 146 MHz.
CM NEC Input File for two-element, 2-meter collapsible Yagi
CM PT control card suppresses printing of element currents
CM TL control card specs transmission line in terms of Z,length,and shunt Y
CM Whip antenna specifications (sections fully extended):
CM Segment 1 is 0.122 meters long and 0.00300 meters in radius
CM Segment 2 is 0.101 meters long and 0.00250 meters in radius
CM Segment 3 is 0.099 meters long and 0.00200 meters in radius
CM Segment 4 is 0.097 meters long and 0.00150 meters in radius
CM Segment 5 is 0.102 meters long and 0.00100 meters in radius
CM Segment 6 is 0.004 meters long and 0.00300 meters in radius
CM Reflector center hex stand-off specifications:
CM 0.02540 meters long and approximately 0.00367 meters in radius
CM Driven element hex stand-off specifications:
CM 0.01111 meters long and approximately 0.00367 meters in radius
CM The reflector should be 1.0056 meters long
CM The driven element should be 0.95607 meters long
CM The reflector should be 0.40834 behind the driven element
CM The Yagi is suspended 7.62 meters (25 feet) in the air
CE
GW 1 2 -0.40834 0.50280 7.62000 -0.40834 0.49880 7.62000 0.00300
GW 2 4 -0.40834 0.49880 7.62000 -0.40834 0.43170 7.62000 0.00100
GW 3 4 -0.40834 0.43170 7.62000 -0.40834 0.33470 7.62000 0.00150
GW 4 4 -0.40834 0.33470 7.62000 -0.40834 0.23570 7.62000 0.00200
GW 5 4 -0.40834 0.23570 7.62000 -0.40834 0.13470 7.62000 0.00250
GW 6 4 -0.40834 0.13470 7.62000 -0.40834 0.01270 7.62000 0.00300
GW 7 4 -0.40834 0.01270 7.62000 -0.40834 -0.01270 7.62000 0.00367
GW 8 4 -0.40834 -0.01270 7.62000 -0.40834 -0.13470 7.62000 0.00300
GW 9 4 -0.40834 -0.13470 7.62000 -0.40834 -0.23570 7.62000 0.00250
GW 10 4 -0.40834 -0.23570 7.62000 -0.40834 -0.33470 7.62000 0.00200
GW 11 4 -0.40834 -0.33470 7.62000 -0.40834 -0.43170 7.62000 0.00150
GW 12 4 -0.40834 -0.43170 7.62000 -0.40834 -0.49880 7.62000 0.00100
GW 13 2 -0.40834 -0.49880 7.62000 -0.40834 -0.50280 7.62000 0.00300
GW 21 2 0.00000 0.47804 7.62000 0.00000 0.47404 7.62000 0.00300
GW 22 4 0.00000 0.47404 7.62000 0.00000 0.43170 7.62000 0.00100
GW 23 4 0.00000 0.43170 7.62000 0.00000 0.33470 7.62000 0.00150
GW 24 4 0.00000 0.33470 7.62000 0.00000 0.23570 7.62000 0.00200
GW 25 4 0.00000 0.23570 7.62000 0.00000 0.13470 7.62000 0.00250
GW 26 4 0.00000 0.13470 7.62000 0.00000 0.01270 7.62000 0.00300
GW 27 5 0.00000 0.01270 7.62000 0.00000 -0.01270 7.62000 0.00367
GW 28 4 0.00000 -0.01270 7.62000 0.00000 -0.13470 7.62000 0.00300
GW 29 4 0.00000 -0.13470 7.62000 0.00000 -0.23570 7.62000 0.00250
GW 30 4 0.00000 -0.23570 7.62000 0.00000 -0.33470 7.62000 0.00200
GW 31 4 0.00000 -0.33470 7.62000 0.00000 -0.43170 7.62000 0.00150
GW 32 4 0.00000 -0.43170 7.62000 0.00000 -0.47404 7.62000 0.00100
GW 33 2 0.00000 -0.47404 7.62000 0.00000 -0.47804 7.62000 0.00300
GE 0
FR 0 1 0 0 146
EX 0 27 3 0 10
RP 0 90 1 0 90 1 0
EN
The resulting antenna pattern is shown below.
I was a bit underwhelmed by the size of the rear lobe. I was encouraged by another ham to consider making a three-element Yagi, as I would be much more pleased with the rear lobe. I returned to VE3SQB's design tool. I was a bit more careful in measuring the diameters of the whips, as they are actually metric diameters.
A new NEC file was generated for the three-element Yagi. All other parameters were kept the same.
CM NEC Input File for three-element, 2-meter collapsible Yagi
CM PT control card suppresses printing of element currents
CM TL control card specs transmission line in terms of Z,length,and shunt Y
CM Whip antenna specifications (sections fully extended):
CM Segment 1 is 0.122 meters long and 0.00300 meters in radius
CM Segment 2 is 0.101 meters long and 0.00250 meters in radius
CM Segment 3 is 0.099 meters long and 0.00200 meters in radius
CM Segment 4 is 0.097 meters long and 0.00150 meters in radius
CM Segment 5 is 0.102 meters long and 0.00100 meters in radius
CM Segment 6 is 0.004 meters long and 0.00300 meters in radius
CM Reflector center hex stand-off specifications:
CM 0.02540 meters long and approximately 0.00367 meters in radius
CM Driven element hex stand-off specifications:
CM 0.01111 meters long and approximately 0.00367 meters in radius
CM The reflector should be 1.01080 meters long
CM The driven element should be 1.00030 meters long
CM The director should be 0.91113 meters long
CM The reflector should be 0.56281 behind the driven element
CM The director should be 0.30746 meters in front of the driven element
CM The Yagi is suspended 7.62 meters (25 feet) in the air
CE
GW 1 2 -0.56281 0.50540 7.62000 -0.56281 0.50140 7.62000 0.00300
GW 2 4 -0.56281 0.50140 7.62000 -0.56281 0.43170 7.62000 0.00100
GW 3 4 -0.56281 0.43170 7.62000 -0.56281 0.33470 7.62000 0.00150
GW 4 4 -0.56281 0.33470 7.62000 -0.56281 0.23570 7.62000 0.00200
GW 5 4 -0.56281 0.23570 7.62000 -0.56281 0.13470 7.62000 0.00250
GW 6 4 -0.56281 0.13470 7.62000 -0.56281 0.01270 7.62000 0.00300
GW 7 4 -0.56281 0.01270 7.62000 -0.56281 -0.01270 7.62000 0.00367
GW 8 4 -0.56281 -0.01270 7.62000 -0.56281 -0.13470 7.62000 0.00300
GW 9 4 -0.56281 -0.13470 7.62000 -0.56281 -0.23570 7.62000 0.00250
GW 10 4 -0.56281 -0.23570 7.62000 -0.56281 -0.33470 7.62000 0.00200
GW 11 4 -0.56281 -0.33470 7.62000 -0.56281 -0.43170 7.62000 0.00150
GW 12 4 -0.56281 -0.43170 7.62000 -0.56281 -0.50140 7.62000 0.00100
GW 13 2 -0.56281 -0.50140 7.62000 -0.56281 -0.50540 7.62000 0.00300
GW 21 2 0.00000 0.50015 7.62000 0.00000 0.49615 7.62000 0.00300
GW 22 4 0.00000 0.49615 7.62000 0.00000 0.43170 7.62000 0.00100
GW 23 4 0.00000 0.43170 7.62000 0.00000 0.33470 7.62000 0.00150
GW 24 4 0.00000 0.33470 7.62000 0.00000 0.23570 7.62000 0.00200
GW 25 4 0.00000 0.23570 7.62000 0.00000 0.13470 7.62000 0.00250
GW 26 4 0.00000 0.13470 7.62000 0.00000 0.01270 7.62000 0.00300
GW 27 5 0.00000 0.01270 7.62000 0.00000 -0.01270 7.62000 0.00367
GW 28 4 0.00000 -0.01270 7.62000 0.00000 -0.13470 7.62000 0.00300
GW 29 4 0.00000 -0.13470 7.62000 0.00000 -0.23570 7.62000 0.00250
GW 30 4 0.00000 -0.23570 7.62000 0.00000 -0.33470 7.62000 0.00200
GW 31 4 0.00000 -0.33470 7.62000 0.00000 -0.43170 7.62000 0.00150
GW 32 4 0.00000 -0.43170 7.62000 0.00000 -0.49615 7.62000 0.00100
GW 33 2 0.00000 -0.49615 7.62000 0.00000 -0.50015 7.62000 0.00300
GW 41 2 0.30746 0.45557 7.62000 0.30746 0.45157 7.62000 0.00300
GW 42 4 0.30746 0.45157 7.62000 0.30746 0.43170 7.62000 0.00100
GW 43 4 0.30746 0.43170 7.62000 0.30746 0.33470 7.62000 0.00150
GW 44 4 0.30746 0.33470 7.62000 0.30746 0.23570 7.62000 0.00200
GW 45 4 0.30746 0.23570 7.62000 0.30746 0.13470 7.62000 0.00250
GW 46 4 0.30746 0.13470 7.62000 0.30746 0.01270 7.62000 0.00300
GW 47 4 0.30746 0.01270 7.62000 0.30746 -0.01270 7.62000 0.00367
GW 48 4 0.30746 -0.01270 7.62000 0.30746 -0.13470 7.62000 0.00300
GW 49 4 0.30746 -0.13470 7.62000 0.30746 -0.23570 7.62000 0.00250
GW 50 4 0.30746 -0.23570 7.62000 0.30746 -0.33470 7.62000 0.00200
GW 51 4 0.30746 -0.33470 7.62000 0.30746 -0.43170 7.62000 0.00150
GW 52 4 0.30746 -0.43170 7.62000 0.30746 -0.45157 7.62000 0.00100
GW 53 2 0.30746 -0.45157 7.62000 0.30746 -0.45557 7.62000 0.00300
GE 0
FR 0 1 0 0 146
EX 0 27 3 0 10
RP 0 90 1 0 90 1 0
EN
The rear lobe is significantly reduced in size, but the front lobe is still significantly broad beamed. For my first Yagi, the predicted antenna pattern is satisfactory.
Construction
The boom of the Yagi consists of a one meter length of 3/4", Schedule 40 PVC pipe. A BNC bulkhead connector is attached to one of the shortened spigot caps.
The reflector and director elements are attached to slots cut into the boom. These slots are used to adjust the spacing of the elements. The two photos show how flat washers and lock washers are used to attach the whips to 1" hex stand-off placed in the slots. The center point in each slot provides the appropriate spacing for an operating frequency of 146 MHz.
The whips for the driven element are attached to 7/16" hex stand-offs. An access hole drilled into the boom allows attachment of the coax to the stand-offs. A hatch made from a short section of PVC spigot cap is to cover the access hole.
A PVC compression irrigation tee is used for the antenna support. Loosening the compression couplings allows the boom to be rotated, enabling both horizontal and vertical antenna polarization. The 3/4" FIPT tee is fitted with a bushing to reduce to a 1/2" fitting. A 1/2" MIPT to 3/4" slip fitting is then connected to the bushing. The 3/4" slip fitting has a Acme thread for a painter pole epoxied in place. The picture below shows the completed Yagi with the whips retracted for transport and storage.
Testing
Tuning of the Yagi turned out to be very simple. The whips were extended to the provided the elements lengths specified by VE3SQB Yagi design tool for operation at 146 MHz. The antenna was tested using an MFJ-269 antenna analyzer.
Testing was first performed with the antenna oriented for horizontal polarization.
Here are photos of the MFJ-269 showing low SWR readings across the 2 meter band when the Yagi was tested with horizontal polarization.
Testing was then performed with the antenna oriented for horizontal polarization. There was some concern that there might be some interaction between the antenna and the aluminium mast and the feedline crossing close to the reflecting element.
Here are photos of the MFJ-269 showing low SWR readings across the 2 meter band when the Yagi was tested with vertical polarization. They appear to show that my concerns were unwarranted.
I'm looking forward to doing some actual on-the-air testing soon.
Below is a PDF containing detailed instructions for constructing the collapsible, three-element, Yagi.
Belated Update:
On January 25, 2014, I decided to see how far I could reach with a 5 Watt hand-held (Wouxun KG-UVD1P) and the Yagi. After trying a number of repeaters without success, I was able to conduct a short QSO through the W5WB/W5AUY Repeater (SW Dallas ARC). The straight-line distance is just over 29 miles. Not bad for power and antenna!
