[TowerTalk] F/B and Gain

[K7LXC@aol.com]

Subj:	Re: F/B, Gain, & stuff
Date:	97-03-19 12:02:38 EST
From:	force12@interserv.com
To:	K7LXC@aol.com

Hi, Steve.

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
the 
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
(less 
than 5) on moderate boom lengths (<.5 wavelength) generally need to be tuned
for 
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
designs 
will allow for maintaining gain, F/B and operating VSWR across the entire
range. 
Boom length and number of elements still is the primary control over forward 
gain. 

There are directional antennas that have high gain and little, if any F/B.
What 
these antennas have is an narrow lobe, translating to an very high
front-to-side 
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
imposing 
as those from both sides).

--------------
Catch you later...........73, Tom, N6BT
			Force 12 Antennas and Systems



--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Sponsored by:             Akorn Access, Inc. & N4VJ / K4AAA