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Author Topic: 6 Metre Quad  (Read 889 times)
EI5GUB
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Posts: 52




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« on: April 26, 2013, 03:21:26 PM »

Hi im planning on building a 4 element 6 mtr quad,can I use a quarter wave of 75 ohm coax into 50 ohm to match it,I will use an analyser to find correct length of the quarter wave 75 ohm piece
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WB6BYU
Member

Posts: 13171




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« Reply #1 on: April 26, 2013, 04:02:13 PM »

Depending on the quad dimensions, it may have an impedance closer to 75 or 50 ohms
rather than 112 ohms.  You can't generalize - you have to know what the value is
for the specific set of dimensions you are going to use.

I have had excellent results building quads using the W4RNL designs:  enter the
desired frequency and wire diameter into a spreadsheet and get dimensions for
2, 3 and 4 element versions.  I just feed the 3- and 4-element ones directly with
50 ohm coax.
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G4AON
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Posts: 526




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« Reply #2 on: April 29, 2013, 02:35:40 AM »

A 4 element single band quad should generally be around 50 Ohms feed, so you can feed one directly with 50 Ohm coax (via a choke balun).

There are both 4 element and 2 element 6 metre quad designs on my web site (www.astromag.co.uk/quad)

73 Dave
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WB6BYU
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Posts: 13171




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« Reply #3 on: April 29, 2013, 12:52:55 PM »

Here is an online calculator that implements the W4RNL formulas:

http://www2.mmae.ucf.edu/~ssd/ham/quadcalc.html

Unfortunately there appears to be a bug in the formula for the position of the
first director in the 4-element model:  this calculator puts it about 50% further
away from the driven element that W4RNL's original formulas.

It also gives feedpoint impedance and gain for the various models - you can
see how both vary (along with the exact dimensions) for different wire sizes.

Personally I've found that his 3-element versions are at least as good as many
of the 4-element designs I've found on the web (though I wouldn't include
G4AON's designs in that category.)
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WB6BYU
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Posts: 13171




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« Reply #4 on: May 10, 2013, 05:02:31 PM »

Quote from: WB6BYU

Unfortunately there appears to be a bug in the formula for the position of the
first director in the 4-element model:  this calculator puts it about 50% further
away from the driven element that W4RNL's original formulas.


In fact, the value given by the calculator for the distance between the driven element
and the first director is actually the reflector to first director spacing.  I just
verified this against the original spreadsheet.

With that change the values from the calculator should give good results:  I've been
able to simply cut the elements to length and get the expected results with no tuning.
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