Complex antennas need to be modelled correctly in order to maintain accuracy. One very comon antenna these days is the openselve dual band antenna and if it is not right, the second band will move from where it should be.

There are some photos and comments from PA3FXO. Need I say more?

The 9el 50/70MHz Duo (bottom) at PA3FXO

Hi Justin,
 
As the 144MHz 12-LFA lies in the garden waiting to be put up ..... the dualband Yagi 4m-6m (5 and 4 elements) has been put up today. Was easier to do and the former 4 el DK7ZB was taken out of the pole.
My photos are a bit large in Mb so I will send you 3 emails. This one with a photo of the antenna at the bootom of the pole .... above are 55 elements for 23cm, then 2x 16 for 70cm and at the top the 10 element ZB which will be replaced soon.
The second email is of the frequency sweep where 50Mhz and 70Mhz are cleary to be seen. 

And a shot of the coaxial balun with the radiating element connections wrapped in rubber type tape that makes it waterproof.
As soon as the 12-LFA for 144MHz is on the roof I'll mail you the results. And planning on a video for You-tube viewers.
I will also update my website with more pictures.
Thanks for all your info on the web, your email replies and best of all our chat at the Budel VHF day in PA-land. Now to make a qso on the bands between us, to top it off :-)
 
Best regards, Cees PA3FXO

The analyser plot shows the antenna perfectly centred on both 50MHz & 70MHz

Want to build this antenna? Here it is: http://www.g0ksc.co.uk/sc6-4-9d.html