Why a Yagi?
There are four 2 meter FM frequencies I want to talk on regularly. Three of these are club repeaters to my Northwest. The fourth is the simplex calling frequency.
K4ITL 145.21 -offset 144.61 TX
W4DW 146.64 -offset 146.04 TX
NATIONAL SIMPLEX 146.52 TX
AK4H 147.27 +offset 147.87 TX
I use simplex to talk to my friend Rod any time there is a big storm or power outage. Rod is also to my Northwest.
Yagi antennas are passive, meaning they do not have active components like transistors. They rely on simple construction with precise measurements in order to produce gain. The gain is the same for transmit and receive. It is also highly directional. This can be viewed as a trade-off for increased gain, but it can also work to your advantage if you only want to operate in the direction you are pointing and ignore other signals.
Since everyone and everything I normally want to reach is in the same direction, it makes sense to put up a Yagi antenna and aim it that way.
Selecting the center frequency
Three of my four desired frequencies are bunched together in the 2 meter band within 2 MHz. I’m going to try and tune my beam for 146.52MHZ. This should give me very good results on all three, and give me the most benefit on the SIMPLEX frequency, which is where I suspect I will want the most help to pull in weak signals and be heard. Hopefully the K4ITL repeater, a couple MHz lower, will still be workable.
Sizing the Yagi Elements
Sizing the Driven Element using the formula DE = 470/f
DE = 470/146.52
DE = 3.2078 feet
DE = 38.493 inches (I will measure it @ 38 and 1/2)
Sizing the Director using the formula D = 425/f
D = 425/146.52
D = 2.9006 feet
D = 34.808 inches (I will measure it @ 34 and 3/4)
Sizing the Reflector using the formula R = 510/f
R = 510/146.52
R = 3.4808 feet
R = 41.769 inches (I will measure it @ 41 and 3/4)
My planned measurement rounding will skew the design, but the greatest measurement difference of the three is only 0.058 inches which is roughly 1/16th inch. I’m hoping that tolerance is going to be OK. We shall see.
Calculating the distance between elements
The formula to calculate distance between elements is this:
Dist1 = 0.18^ (Distance from reflector to driven element)
Dist2 = 0.15^ (Distance from driven element to director)
To get the free space wavelength ^, divide 984 by frequency in MHz
D1 = 0.18(984/146.52) feet D1 = 0.18(6.7158) feet
D1 = 1.2088 feet
D1 = 14.506 inches (I will measure 14 and 1/2 inches)
D2 = 0.15(984/146.52) feet
D2 = 0.15(6.7158) feet
D2 = 1.0073 feet
D2 = 12.088 inches (I will measure 12 and 1/16th inches)