Re: Yagi design criteria.
There are several criteria besides boom length in a Yagi design. The advent
of computer modeling is, of course, a very useful tool. Not using equal
spacing of elements was pointed out by Dr. Lawson in his "PV-4" design.
The wide-spaced 4 element (i.e. 4el 20 on a 40-45' boom) seems to be a topic
of constant discussion. Sure, one can achieve about the same forward gain as
using 5 or 6 elements on the same length boom. The difference is that the
gain and F/B for the Yagi with more elements can be maintained across a
wider operating bandwidth. This means that the average gain across the band
for the antenna with more than 4 elements is higher, as is the F/B. The peak
values will most likely be the same, or in some cases higher than the 4
element. The feedpoint can also be kept at a higher value.
Element placement is obviously important for windload and tower torque. As
most have noticed using Yagi optimizers, if left to run on its own (not
freezing any locations), the optimizer will propagate the majority
of the elements to the rear of the boom (not so good for our rotators).
Carefully adding additionl elements allows for different positioning and
better mechanical design, while maintaining the desired electrical results.
Whenever possible, I tend to "overpopulate" a given boom length for these
reasons. The "extra" elements also provide the vehicle to peak the gain and
F/B right in the band and not have them rising or falling across the band.
Regarding the question of gain on a 60' boom, a practical 60' boom 20 mtr
design with either 6 or 7 elements will be in the range of 8.8-8.9dBd, which
is -0.6dB from the maximum theoretically possible for that boom length.
Anyone can crank this one up on their machine.
A Yagi with 10 or 11 dBd forward gain is a very large antenna for 20 mtrs.
We have made a 5.8GHz Yagi antenna measured (in an independent chamber) at
11.4dBi, which is about 9.3dBd. The model shows 9.5dBd. The antenna is 8
elements and has a boom length of 1.24 wavelengths. It is a broadband
design, which is why the forward gain is less (about .3 to .4dB) from what a
narrower band design can achieve. The gain and F/B are centered in the
middle of the target frequency range. The antenna has a 1.5:1 VSWR bandwidth
of 11.6%, which would cover about 1.64MHz if centered on 20 mtrs. The
equivalent boom length for 20 is 87'. This measurement implies one might
achieve 9.8dBd for an 8 element 87' boom to cover 20 mtrs. By the way,
adding a 9th element gets .1dB more at the peak and cuts the gain variance
across the band in half, as well as centering the gain in the middle of the
band. Now.......if someone doesn't agree with the model, that is something
else!
Maybe this measured gain will confirm your suspicion about high claimed gain
figures, at least for something less than 87'! My experience shows there is
no free lunch.
Adding elements "brings a lot to the party", but will not allow things to
exceed the old laws of physics; just smooth them out over a broader
frequency range!
Have a good day and 73,
Tom, N6BT
Force 12 Antennas and Systems
(Home Page http://www.QTH.com/force12 )
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