Antenna aperture is a concept that may need some explanation. It is a way to describe how effective an antenna is at absorbing RF energy from the signal passing by. It is expressed such as . . "An aperture of 1 square meter." This means that the antenna will absorb an amount of RF energy equivalent to all the energy coming through a "window" of 1 square meter area. Note that it does not refer to the physical size of the antenna (as viewed by eye from the "front.").
A good example of this is the long Yagi beam antenna. Viewing it from the front, it looks to the human eye to be no bigger than a single dipole. Yet, its aperture is very much bigger than a dipole. The long Yagi is what is called a Slow Wave Structure. The director elements interact with the moving wave front to slow down the speed of radio signals whose frequency is close to the design frequency for the antenna. This slowing effect causes the wave front close to the directors to lag behind the energy farther off-axis with the antenna. This causes the wave front to become curved, like the surface of a bowl, with the open face of the bowl facing towards the driven element. This bending of the RF wave front acts to bring energy that initially was not directly in line with the antenna to a focus at the driven element. It does so because energy flow is always perpendicular to the wave front. The curvature of the wave front has been bent (by the directors) such that the perpendicular to the wave front points towards the driven element. Thus energy flows to the driven element from positions considerably off-axis. Hence, a large aperture.
From the above discussion we see that a long Yagi is able to sample energy from a large surface area of the incoming wave front.
Now consider a multipath situation. There are signals coming from several directions. When the beam is swung around the compass, as it comes to the bearing of one of the multipath signals, the energy from that path is focused on the director. The aperture of the antenna as it faces the signal, is very much larger than the antenna aperture for signals coming in off-axis. This on-axis energy can likely be stronger than the off-axis energy, even though the signal it is "looking" at is weaker, therefore we get a peak in signal strength as seen by the receiver.
But, except in special cases, a long Yagi is not practical for mobile radio direction finding.
Short Yagis can provide useful gain for T-hunting, and produce a bearing. However, they cannot sort out weaker multipath signals and give clean bearings on the several paths that may be present. They cannot bring a weak path up enough in response to overcome the energy of stronger paths which are hitting the driven element from off-axis (without benefit of the bending effect of the Yagi). Its (forward-looking) aperture is not big enough to absorb enough energy coming from the front to be able to overpower the energy picked up from the stronger off-axis signal (coming from the side).
So what's a T-hunter to do? There is another way to obtain a large aperture for T-hunting while using a small aperture antenna (such as SuperDF or Doppler). Its called a synthetic aperture. It is "created" by moving the antenna through space while averaging the data collected at each point along the path of travel. This creates a synthetic aperture equal in size to the distance traveled while collecting the data. This method is not able to sort out the various paths to give a bearing of each, but it does produce a much improved bearing of the strongest path.
George Russ Andrews
President
George R. Andrews (Russ, K6BMG)
BMG Engineering, Inc.
9935 Garibaldi Avenue
Temple City, CA
91780, USA
Voice 1(626)285-6963
Fax 1(626)285-1684 (24 hour automatic)
America OnLine: Grandrews
Web: http://users.aol.com/bmgenginc
(7 Feb 1996)
Send E-mail to grandrews@aol.com.
(A message window will open.)
Return to This article,
Contents.
Go to Next Page of this
article.
Go to Prior Page of this
article.
Return to TOP PAGE.