Yagi Antenna
I have a pretty sweet set up for my HF radio. I have an Older ICOM - 7100 base station with a Yaesu FT-450 I won in a drawing at Sea-Pac a few years ago. I use the yeasu as a back-up and sometimes as a mobile in my hunting rig. I run this into an inverted V wire antenna and once I have pulled a station out of the mud, I use a couple Mosely 5 element yagi antennas which all sit on top of a telescoping tower. I hate climbing towers, and I am not getting any younger, so the telescoping aspect is wonderful, especially when I let out the pins and set the whole thing on the ground stand.
On my rounds, I noticed that the wind had knocked out one of the elements on my yagi, so I started to size up the project and do some repairs. Fortunately, it was only one of the parasitic elements that got broken, so it is just a matter of putting up another piece of aluminum tubing. On an HF yagi, that piece of tubing can be quite large.
This got me to thinking.
There is nothing magical about a Yagi antenna. They work on a couple principals of physics, which involve induced current and the ability of radio waves to interact with capacative and inductive surfaces. The principals are highly technical, but basically simple to understand and using these, you can make a 2m or 75cm band yagi that has astonishing gain.
A yagi antenna has one driven element and two or more parasitic elements that are reflectors (those element(s) behind the driven element) and the directors(those element(s) in front of the driven element). Ideally the driven element is a multiple or harmonic length of the frequency band you wish to transmit. Most are 1/4 or 1/2 wave length. with 2m, a full wave antenna is only 6 feet in width, so it is very possible to create a full wave yagi, that is almost portable.
Placement of the antenna elements is along a center pole, horizontal, and perpendicular to the antenna mast. I make mine out of PVC if I am putting together a 2m antenna. The reflector is made inductive, and the induced currents are out of phase with the driven element which reflects the power away from the parasitic element back at the driven element. This causes the RF antenna to radiate more power in the opposite direction to this form of parasitic element. Adding capacitive elements, tightens the beam and increases gain.
An element can be made inductive by tuning it below resonance. This can be done by physically adding some inductance in the form of a coil (expensive and a real pain), or more commonly by making it longer than the resonant length. Generally it is made about 5% longer than the driven element as this saves cost and keeps the element mechanically as one piece which makes it cheaper and stronger. I have experimented with multiple reflectors and it increases the gain, but not so much so that it makes it worth it. One of the interesting things it does, is lower the take-off angle of the signal. This comes in handy when making a DX call.
But Crazy Uncle, what about the directors? You remember we made the reflector 5% longer to reflect the wave? Well, directors are shrunk by 5% in relation to the driven element to make it capacitive. That way the induced currents are in phase and get directed away from the driven element and the antenna. That's why they call it a director. Doing this several times with smaller and smaller directors increases the gain of the antenna and tightens the beam of the RF signal.
Yagi.jpg
I have found that spacing out the elements no further than a 1/4 wave is optimal. Connect your driven element, which is electrically isolated to your radio via co-ax.
Have fun and let me know if this helps. I am going to try to draw a diagram and copy it into this write-up.
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