It just goes to show, if you follow instructions and build an antenna in a way that has been proven, you can benefit from the thousands of dollars worth of software I use!
Jan, ON6ZG is the most recent ham to benefit from both the accuracy of my software and of his own fair hands! After being impressed with his 50MHz LFA, Jan decided to follow my instructions carefully and build 2 x 15el LFA Yagis. Below are the two antennas which do look impressive. However, more impressive than the antennas themselves is the antenna analyser plot which is identical in every respect to the software model.
The 2 x 15el 144MHz LFA Yagis at ON6ZG with a 6m version between them
Impressive - the Analyser plot showing an SWR plots between 144 and 145.2MHz almost exactly as the prediction below
The NEC4 SWR prediction showing the small lift or 'double dip' exactly as the finished antennas
Take a look at the analyser plot above at it's lowest, flattest point just above where it says 'Center = 145MHz' Here you will see two point where the blue line merges with the green line where it is a perfect 50Ohm (1.02:1) and a small section between them where it raises to 1.03:1 and turns blue once more. Looking at the plot prediction, you can see the very same with the dips between 144.300MHz to 144.500MHz and 144.900MHz to 145.200MHz
This is a result of using a configuration which requires no correction. Sadly many hams are mislead by designers that give incorrect guidance in how to build their antennas and it is for this reason, they will never see such a mirror-image of these plots. One typical scenario is when through-boom-and-element insulators are used. around 6mm correction is needed for an antenna using these insulators to get near model and 144MHz!
As an example, if an antenna is built with such insulators, perhaps a wider, flatter SWR plot will be seen (than model) but without a dip as low as the model shows. This is because the antenna elements (with the through-boom-and-element insulators) has electrically shortened the elements. The dipole is over-sized to give a reasonable SWR in the part of the band you need it (lower in the band) but the reality is, the antennas best performance is a few hundred KHz above this point.
The building could further be mislead because he has not actually added anything to the dipole length. This is because he has done so with the 'tails' between the coax and the feed point. A correctly built antenna will require the dipole ends being trimmed in order to remove the over-sizing.
If your antenna is 1.2:1 across a wider bandwidth than the designer suggested without the minimum he predicted either, now you now why! Want an antenna that will work as it should rather than OK? Well, you have come to the right place!