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Author Topic: Verticle YAGI upside down?  (Read 1394 times)
KK4RHF
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« on: March 25, 2013, 02:24:57 PM »

I am hunting a new three or four element YAGI and have been perusing the various sites. I got to one vendor and there was a YAGI shown with the Gamma Matching right next to the rotor mast.
By all appearances the antenna was upside down. The radiating element was positioned right behind the mast.  To me it seemed that they were trying to talk through the wall rather than over it. I have never owned or assembled one so I'm not sure if I'm upside down or the antenna was.

What say you?
WQNP788
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G4IJE
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« Reply #1 on: March 25, 2013, 02:42:03 PM »

Not sure there is such a thing as upside-down for a vertical yagi. The whole element is "driven" - not just the half with the gamma match. Vertical yagis are best mounted on a non-metallic mast or rear mounted behind the reflector, but a metallic mast through the middle of the antenna may not be total disaster, as the mast will not normally be resonant.
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KK4RHF
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« Reply #2 on: March 25, 2013, 06:06:26 PM »

I thought for sure that the radiator was separated from the other half like a dipole and the half that had the gamma match went to the radiator.
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WB2WIK
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« Reply #3 on: March 25, 2013, 06:12:42 PM »

I thought for sure that the radiator was separated from the other half like a dipole and the half that had the gamma match went to the radiator.

That's not true at all.

A gamma-matched dipole is used to allow grounding of the center of the driven element, which leads to a reactive off-center feed; the gamma match tunes out that reactance and makes  it work the same as a center-fed dipole.

But in all cases, both "halves" of the driven element are equally "hot" and contribute equally to radiation.  It wouldn't matter if a gamma match were "up" or "down."

However, placing a vertically polarized beam in the plane of a conductive mast does detract from performance; nobody in the commercial world would do that.  Vertical beams are always either end-mounted with the supporting mast behind them; or used with a non-conductive support; or stacked side-by-side on opposite sides of a conducting support.
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KK4RHF
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« Reply #4 on: March 25, 2013, 06:25:36 PM »

Thanks for clearing that up for me. Now I know when I get my other YAGI it's going to the flatside. I have a two meter radio and a beam for it and it is end mounted and I am going to place it below the other antenna.
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WA3SKN
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« Reply #5 on: March 27, 2013, 06:03:39 AM »

With most yagi antennas you will find the elements connected to the metal boom and the boom connected to the metal mast and tower... this makes sense for lightning purposes.  The tower is normally grounded and keeps the potential problem outside!
This is one of the best reasons for using the gamma match.
Now you can design a yagi with an insulated boom and elements, but why?
73s.

-Mike.
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K0JEG
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« Reply #6 on: March 27, 2013, 06:25:01 AM »

How the antenna is mounted might have something to do with how water is handled. I've seen a few matches that if they were mounted "on top" would pool water and destroy themselves.
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KE3WD
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« Reply #7 on: March 27, 2013, 06:38:58 AM »

With most yagi antennas you will find the elements connected to the metal boom and the boom connected to the metal mast and tower... this makes sense for lightning purposes.  The tower is normally grounded and keeps the potential problem outside!
This is one of the best reasons for using the gamma match.
Now you can design a yagi with an insulated boom and elements, but why?
73s.

-Mike.

Not just lightning protection, which may or may not work depending upon the type and duration of strike, but DC grounded antenna elements are also useful for bleeding off static charges that can make undesirable NOISE on the received signal. 

The yagi would work pretty much the same with Gamma Match located on either side of the boom. 

It is imperative that you don't attempt to analyze the antenna element situation as a simp;le DC path, often it is not the case.  Since your RF is basically AC at some high frequency, the Gamma Match actually is acting as a transformer (autoformer) within a specified frequency band.  The whole thing is at DC ground potential, but at the specified frequency of operation, it appears as a nominal load to the RF device attached to it. 


73
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N4JTE
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« Reply #8 on: March 28, 2013, 05:34:51 PM »

Wow, what band or freqency are you talking about, better yet what antennas are you on about??
Bob
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KK4RHF
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« Reply #9 on: March 28, 2013, 05:52:39 PM »

I was under the mistaken impression that they were insulated from each other and only one half radiated and the other was counterpoise. Each half was a 1/4 wave and that's why thought the one was upside down. I know the reflector and the director are one piece but it never occurred to me that the gamma match would electrically separate the radiator from the other half.
Thanks for getting me straightening me out!

I was looking at a ten or eleven meter yagi.
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