Where have all the mainsprings gone? (3)

＝Semi-Automation of 2 elements cross-yagi =

This problem can be solved by turning the antenna upside down.
The length of the antennas used for amateur satellite communications are about 1m or 35cm. Unlike TV antennas that have a dozen or so elements lined up, with a little ingenuity it could even be rotated using a kitchen timer.
That said, the upside-down antenna is attached to the end of an inverted J-shaped pole, like a street light on a highway, and is then turned around. So the smaller and lighter the object to be turned the better.



Here, we will shorten the larger (145 MHz) element of this antenna by inserting a loading coil.

With HF band antennas, a loading coil is a common way to shorten the length of the antenna, but when the wavelength is as short as 145 MHz, you don’t see many examples of this.  Simply it’s not necessary.  We can do some calculations to get an idea of ​​how much coil we need to insert, but we’ll try this out as we go.

　　

(1) First, using the antenna analysis software MMANA, I shorten the element length (which is usually about 1m) by half, setting it to a total length of 50cm.

(2) Next, I decide a position of inserting a loading coil on the element, and calculate what constant (inductance) is required.

(3) Then I adjust the numbers we get to make it easier to make and wind, and enter them into the solenoid coil calculation formula to determine the number of turns.

(4) We then make a coil wound exactly like that, connect it to an aluminum pipe to make an element, and measure the tuning frequency with an antenna analyzer.

(5) We then remake the coil based on the deviation to make it the desired frequency, and the element is complete.


The coil uses copper wire, so it is a little heavier, but the aluminum pipe is half the length, so it is lighter, and most importantly, the “J” shape at the front can be smaller.
I haven’t assembled it yet, but I’m sure the bottleneck for this antenna will be the “chicken.”
I’ll keep that in mind as I design and build it.


(To be continued)

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