I've pulled a couple of these apart.
There are multiple matching coils, loading coils and various element 
arrangements all the way up the antenna.
It's not a simple thing to build.
far easier to build a yaggie or quad for those bands.
I expect it would also be more expensive to build something like the 
x200/300 as well than it is to buy one.

73
Colin, V A6BKX
-----Original Message----- 
From: Neil k8it
Sent: Tuesday, April 28, 2015 2:24 PM
To: [log in to unmask]
Subject: Re: plans for high gain 2m 70cm antenna

I have been thinking of building a base station 2m and 70cm antenna. I like
the diamond x-200a or x-300a designs but would like to try building one
myself. the antennas are rated at 6 to 6.5 dB on 2m and 8 to 9.5 dB on 70cm.
But does anybody have dimensions of this type of antenna? I found several
J-pole designs but no 5/8 wave collinear designs with radials similar to the
Diamond x-200A or x-300A antennas.
Anybody have any thoughts?

Would a 8 foot center element (two 5/8 waves in series simply work? would it
match to 50 ohm coax? will it match on 70cm?

thanks
73 Neil k8it