My Yagi Design Dilemma

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My Yagi Design Dilemma

Postby VK4MDX » Mon Dec 14, 2009 7:38 am

Being a standard call, I wanted to design a yagi I could use on 52 MHz. I downloaded Quick Yagi 4 (QY4) and fed the numbers required into the program.

I added a folded dipole with 100mm spacing to simplify matching and ran QY4. I ran the optimised for gain option.

The resulting antenna showed a good plot on QY4.

Not knowing anything about QY4 credentials, I decided to model the suggested antenna in NEC. Being a Mac man, I run CocoaNEC. The rsulting plot was to say the least not impressive. I've checked my work for errors, removed the folded dipole in NEC, removed elements etc. Interestingly removing Director 3 which is the odd one out in terms of length progression, improves the antenna.

The plot on QY4 shows very small rearward lobes but the plot on CocoaNEC shows a huge rearward lobe. Do I trust QY4 or CNEC? Do I just build it and adjust using a FS meter?


Here is the QY4 Data

Operating Freq 52.1MHz
Reflector length 2.87849m
Fed element Length 2.71749m
Reflector spacing 1.02323m
Number Directors 4
Element Dia 13mm

Result gain 11.91
F/B 26.21
Imped. 53-2 - j0 Ohms

The plot looked excellent

Here is the CCNEC data

// My 6 element 6 meter for 52 MHz

model( "simple Yagi-Uda" )
{
real height, length, separation, reflector ;
element driven ;

height = 8;
length = 1.365875 ;
separation = 1.02323 ;

setFrequency( 52.1 ) ;



// driven element
driven = wire( 0, -length, height, 0, length, height, 0.013, 21 ) ;
voltageFeed( driven, 1, 0 ) ;

//driven folded dipole lower
wire(0, -length, height-0.1, 0, length, height-0.1, 0.013, 21 ) ;

//driven connection to bottom
wire(0, -length, height-0.1, 0, -length, height, 0.013, 21 ) ;

//driven connection to bottom
wire(0, length, height-0.1, 0, length, height, 0.013, 21 ) ;

//reflector, placed behind (negative x axis) the driven element
wire( -separation, -1.439245, height, -separation, 1.439245, height, 0.013, 21 ) ;

//Director 1
wire( .844753, -1.34225, height, .844753, 1.34225, height, 0.013, 21 ) ;

//Director 2
wire( 1.92563, -1.28019, height, 1.92563, 1.28019, height, 0.013, 21 ) ;

//Director 3
wire( 3.23865, -1.314185, height, 3.23865, 1.314185 , height, 0.013, 21 ) ;

//Director 4
wire( 4.96497, -1.25992, height, 4.96497, 1.25992 , height, 0.013, 21 ) ;


//
freespace() ;}

Here is the CCNEC Plot
Image

Here is CCNEC Plot with D3 removed
Image
VK4MDX
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Re: My Yagi Design Dilemma

Postby ZL1RS » Mon Dec 14, 2009 8:14 am

Hello David,

I ran your dimensions through 4nec2 and came up with a very similar plot to your CCNEC.

Perhaps you could try scaling a YU7EF or DK7ZB design from 50.1 to 52.1 and check the result in CCNEC.
(Google will find their web sites).

Happy to check your next version in 4nec2!

73, Bob
Bob, ZL1RS in the Bay of Islands at RF64vs
http://www.qsl.net/zl1rs
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Re: My Yagi Design Dilemma

Postby VK4MDX » Fri Dec 18, 2009 4:41 pm

Bob


Thanks, I'll give them as go when I get home after Xmas

David
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