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Author Topic: 2M Ground Plane Radial Lengths Measurments  (Read 2095 times)
OZEPETS
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Posts: 1




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« on: January 19, 2006, 12:38:24 AM »

I have a question regarding the measurment of the radial lengths on a simple 2M  ground plane I am building. I want to put a small loop at the end of each radial for safety reasons. Using provided measurements (from various sources), What is the correct method for measuring the radial lengths. Is it the overall electrical length (tip to tip, including the loop) or is it the physical length (tip to outside edge of loop)?

Does all that make sense? Hope so ;-)

73
Laurie
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AI4KK
Member

Posts: 107




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« Reply #1 on: January 19, 2006, 06:03:13 AM »

If the loop is closed, measure to the end of the loop, but including the distance around the half of the loop that you follow.
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KE4SKY
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Posts: 1045


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« Reply #2 on: January 20, 2006, 05:10:36 AM »

This web site has a simple plug and play utility to provide suitable dimensions for a quarter-wave ground plane vertical for any frequency and takes the guess work out of it.

http://www.csgnetwork.com/antennagpcalc.html

I've listed examples for some common frequencies to give you some ideas.

2m SSB    - 144 MHz - 19.5"
2m packet - 145 MHz - 19.4"
2m FM     - 146 MHz - 19.2"
70 cm     - 446 MHz - 6.3"
GMRS      - 467 MHz - 6.0"    

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WB6BYU
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Posts: 13486




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« Reply #3 on: January 20, 2006, 01:38:59 PM »

The bandwidth of such a ground plane antenna is wide
enough that it really doesn't matter how you measure it,
unless you use a very large loop.  And if you are going
to adjust the antenna for minimum SWR anyway, that process
will accomodate any reasonable radial length.

I'd just measure the straight line distance to the tip
of the radial and figure that is close enough.  (Technically
it isn't just a question about the length of wire that
makes up the circle, but how much capacitance it
contributes to the circuit...)

My favorite dimensions for a 2m groundplane are 18" for
the radiator and 24" for the radials.  These came from
a design by Woody W6BCX in QST some years back, designed
to minimize RF current on the coax.  Adjust the angle
of the radials for minimum SWR.  I like this because I
can cut two radiators from a 36" length of brazing rod.
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