Subj: Re: F/B, Gain, & stuff
Date: 97-03-19 12:02:38 EST
Wanted to offer a couple of comments relating to F/B and gain:
Having a higher F/B does not mean the antenna has more forward gain. What it
indicates is only that the antenna is tuned for a high F/B. It means that the
energy has been pushed somewhere else and most likely it has broadened out
front lobe of the antenna and the front-to-side will likely be less, too. In
most cases, the antenna will have less forward gain when the antenna is tuned
for a higher F/B.
Unfortunately, the notion that having a high F/B meaning the antenna has more
forward gain has led to an over stated importance on F/B, similar to having a
!:! VSWR means the antenna is "working." This is one reason I try to talk to
clubs and at conventions and also one reason ARRAY OF LIGHT came into being.
Redistributing energy is, of course, how we acquire gain; however, with
increasing F/B, most often the forward lobe becomes broader in terms of
half-power beamwidth. Also, to achieve high F/B, Yagis with few elements
than 5) on moderate boom lengths (<.5 wavelength) generally need to be tuned
less than optimum forward gain. More elements allow for different tuning
settings than with a simple 3 or 4 element Yagi. Adding elements to some
will allow for maintaining gain, F/B and operating VSWR across the entire
Boom length and number of elements still is the primary control over forward
There are directional antennas that have high gain and little, if any F/B.
these antennas have is an narrow lobe, translating to an very high
ratio. Sometimes, this type of pattern is desired, because the high side
rejection is beneficial (i.e. the interferers from the rear are not as
as those from both sides).
Catch you later...........73, Tom, N6BT
Force 12 Antennas and Systems
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