Re: 137MHz "V" Antenna advice needed ...


David Slipper
 

It's NOT a Yagi - this picture (see HERE)  is similar to what I am building but for 137MHz (the one in the pic is for a higher frequency).

The two dipole elements are horizontal and 120 degrees apart.

Do the reflector elements need to be connected together??

Do they need grounding ?? or connecting to the feed in some way ??

Dave



On 21/12/2021 23:30, Kriss Kliegle KA1GJU wrote:

Slightly confused... any images of said 8 element horizontal V dipole?
With that said...
A beam (Yagi) with 8 elements will be highly directional, not something you want when chasing a satellite that is moving across the sky... unless you plan on using an azimuth/elevation rotator (more complexity).
I know one satellite operator in the club uses the 'egg beater' type antennas, they're omnidirectional  and no there are no moving parts to fail.

Typically in a beam antenna, the elements are +/- 1/2 wave long (two +/- 1/4 wave elements connected together to form one +/- 1/2 wavelength long element). Since the center of each parasitic element (essentially a dipole) is at 0 volts, the element is usually clamped to a metal boom without being insulated.
Lots of info on the web to get dimensions of each element and the spacing (google "VHF beam antenna dimensions" or similar)

i.e: https://www.rfwireless-world.com/calculators/3-element-Yagi-Antenna-Calculator.html
Yagi Antenna calculator Formula
Yagi beam antennas are great for narrowing the 'field of view' of RF, but once not aimed within the beam width, the RX'ed signal can drop off dramatically.
Antenna specs of commercially available antennas will state what the half-power beam width is. For example a -3dB drop or half power drop from the main lobe:

Antenna Beamwidth

The more elements added, the more forward gain (better front to back ratio)... at a cost of a more narrow beam width. Of course there are lots of variables that can be tweaked, height above ground, element spacing, element lengths, etc to change the specs of a beam.

Confused yet?

73 Kriss KA1GJU/AM

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