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Antenna Conversion?

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SpugEddy

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I know I mentioned this in passing once before,
but I want to see if I can get some good suggestions
on the topic.

I have 20 year old Starduster M400 antenna that has been
taken down and replaced. The antenna has seen better days
and was mainly taken down because it was bent. Like I said,
it's been up for 20+ years. Now, being spring and I'm doing some
cleaning and thinning out of junk, I'm wondering if I can:

Take the old M400 and cut it down to work on 465 mhz.
I looked into the measurements and a 5/8 wave GMRS
would be somewhere around 15". I could just take the radiator
element and cut it down and the same goes for the ground plane
radials. I know this seems like a waste of an M400 antenna but it's
pretty much done. Its split from freezing, it's been hit by lightning,
it's bent, and so on. Some of the radials have already been used
to make a real nice Dipole for 10 -11 meter band and it works great.
The main question I have is the smaller radials near the top of the
antenna. If I'm correct, I won't need them for the GMRS band. Correct?
Like to see some nice options from our antenna guys.
 
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You can't make a 5/8 wave, a 5/8 wave doesn't have a 50 ohm impedance. You could make a 3/4 wave. And no, the top "radials" aren't needed
 

SpugEddy

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I just want to add that I'm really not a dunce,
but for some reason I have some kind of mental
block when it comes to 4NEC2. I just can't seem to
zoom in enough to be able to draw my antenna to
simulate and test it there.
 

SpugEddy

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Progress???
Can these numbers possibly be right?
I put this into 4NEC2 and here are some screenshots
of my results. The first shot is the Optimize the 2nd shot
is the Evolve result

Bottom right "Variables"
Len=length of main element
rp= radial length
h=height from ground
frq=frequency

15db at less than 1/2 wave seems like something is not right
 

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SpugEddy

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After spending about 3 days now of looking
at every kind of weird shape bat and butterfly
pattern, I don't think I'm too much closer to
a solution. Anybody here good enough with 4NEC2
to know why I get different results every time I run
Optimize and Evolve? Also, does anybody know how
to setup up a variable to change the angle of the
ground plane radials? The pic above shows them at 45 degrees,
but I want to be able to set a variable to change them from
about 65 degrees to 30 and see what 4NEC2 comes up with
 

nd5y

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I have only used MMANA-GAL for antenna modeling. I have enough experience with it that I can tell you that making slight changes to a 1/4 wave ground plane antenna, including the angle of the radials, doesn't change the radiation patter very much. The main thing that does affect the pattern and gain is the height above ground. The transmission line length and whether it is grounded on the opposite end that can also affect the pattern. From the screen shots it doesn't look like your antenna has a transmission line.

It's easy to change the angle of the radials or any other line you draw in MMANA-GAL. I haven't used 4NEC2 so I don't know how it works or how different the results would be. Probably not much different because math is math.
 

SpugEddy

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That's sort of where I'm needing some help.
This antenna was not supposed to be 1/4 wave.
I was originally going for a 5/8 wave then I was
steered into a 3/4 wave for better impedance.
Somehow the program keeps lowering the elements
closer to 1/4 wave and that's not what I want.
 

jonwienke

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3/4-wave does not have good gain--the signal goes up, not out. 5/8 or 1/4 are better.
 

nd5y

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I can't help with the program changing the parameters.

For maximum low angle gain you want the vertical radiating element to be around .6 to .62 wavelength with 3 or more horizontal 1/4 wave radials. Then you need an impedance matching network at the base.

A 3/4 wave vertical radiating element will make the impedance lower than 5/8 wave but then your gain moves to high angle lobes which defeats the whole reason of not making a simple 1/4 wave ground plane to begin with.

If you want maximum gain in a relatively simple antenna with one feedline then you need to model something like an AEA Isopole, Hy-Gain V-2R or MFJ-1764.
 
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