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Lplate

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Hi just joined up Im a beginner and I need help with a question, please !

Why is a folded dipole sometimes used as the active element in a yagi antenna array ?

Thanks
 

kb2vxa

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Hi L and all,

Because a folded dipole has a somewhat broader frequency range and 300 ohm balanced feedpoint impedance. The former has questionable value in most applications while the latter makes impedance matching when stacking them a little easier. For example a two stack (antennas in paralell) gives a 150 ohm balanced impedance easily matched with a 2:1 balun and 75 ohm coax. A four stack gives 75 ohms so a 1:1 balun is used. Fed with a 50 ohm transmitter output the resulting 1.5:1 SWR is insignificant. Don't be confused, coil baluns are common on HF but on VHF and UHF coaxial baluns are used, just an odd arangement of a short length of cable from the balanced to the unbalanced side.

Edit;
I just thought of something you may have seen before. Right up the street at the power substation is this huge telephone pole with a four element folded dipole vertical colinear aray on top. HUH? OK, that's four vertical folded dipoles each mounted 1/2 wave above the other evenly spaced around the mast for 360 degree coverage. It gives about 5 or 6dB gain over a single dipole and the impedance is matched as described above.
 
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Lplate

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Thanks alot kb2vxa,

could you also help with this ?

Calculate suitable values for the element dimensions and spacings, assuming it is to operate at 100mhz. Also in what way would it have to ber mounted to obtain directional properties ?

As you can gather im on the yagi chapter :)
 

VintageJon

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A folded dipole has an impedance of 300 Ohms, but the directors and the reflector as capacitively-coupled elements reduce this impedance to get it near 50 Ohms, asuming enough elements.

-Jon
 

prcguy

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The folded dipoles on the VHF or UHF vertical array Warren mentions are each matched individually at 50 ohms and they use a phasing harness made of 75 ohm cable to get the whole thing back to 50 ohms. These particular dipoles are grounded in the center and only one side is fed with coax, so they behave a little different than a folded dipole in free space. This design will give the most gain you can get in a vertical antenna, 6dB over a dipole omni or 9dBD in almost a 180 deg pattern for a 20ft high VHF high band antenna. You can also use these as a mast for other antennas without affecting the performance of the dipole array below. I love vertical exposed dipole arrays!
prcguy
kb2vxa said:
Hi L and all,

Edit;
I just thought of something you may have seen before. Right up the street at the power substation is this huge telephone pole with a four element folded dipole vertical colinear aray on top. HUH? OK, that's four vertical folded dipoles each mounted 1/2 wave above the other evenly spaced around the mast for 360 degree coverage. It gives about 5 or 6dB gain over a single dipole and the impedance is matched as described above.
 

gcgrotz

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prcguy said:
....These particular dipoles are grounded in the center and only one side is fed with coax, so they behave a little different than a folded dipole in free space. ..... I love vertical exposed dipole arrays!
prcguy

And that is why they survive lightning better, especially if top mounted on a tower. I've seen a few fiberglass "empty pipes" after a lightning hit but never a melted dipole array.

Back to the original post, it sounds like you are wanting an FM antenna. It would be easier to just buy one. But if you want to build, pick up this month's QST magazine ( www.arrl.org )antenna issue, there is an article about building yagi antennas and it uses a folded dipole feed directly to 50 ohm coax. There are several other good articles too, including one on grounding.
 
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