Tech Talk   From the Breakfast Table

November, 2006

 

Antenna Gain, Fact or Fiction?

For the past two months we have been discussing gain as it applies to power and voltage.  This month we will take a look at another type of gain; as it applies to antenna design.  Remember, in any discussion of gain (or loss if negative) we need to specify "over what?"

In September the discussion centered around gain (or loss) over a specific power level.  If we started with 10 Watts of power and increased it 10dB, we ended up with 100 Watts.

In October, we talked about S-Units and then our starting point was specifically identified as S-9, or 50 µV and the gain or loss was a value compared to that.

This month the standard will be the dipole and there just happens to be two standards this time.  First we have the "Isotropic" dipole, which is a theoretical model only. It exists only in space and radiates equally in all directions since it is unaffected by nearby objects or nuisances such as feedlines or masts. Even though it is theoretical it is a useful design tool and real world antennas that are compared to it have gain figures written as dBi.

The second dipole is in fact a real world antenna complete with feedline and support structure but it has been optimized by the builder so that it radiates according to accepted standards.  When an antenna is compared to this standard, the gain is written as dBd.  If a manufacturer claims his antenna is measured as having a gain of 6dBd, then he is comparing it to his dipole.  Now most Amateurs don't have the ability to perform gain measurements because the accepted method requires a huge amount of real estate. At a manufacturer's test range, his dipole is mounted taking care to optimize the height, relation to nearby objects (there won't be any) and correct feedline trim and support structure. In other words, this antenna, and any other antenna to be tested will be adjusted to perform as well as it possibly can.  Once the antenna is mounted, a carefully measured RF signal is applied to the antenna and field strength measurements are taken downrange while the antenna is rotated.

Once the standard is established, the dipole is removed and the antenna to be tested is installed in it's place. Field strength measurements are then taken from the same azimuth stops and the values compared.

Amateurs who have more than one antenna for a specific band often conduct similar tests with the receiving station evaluating the "field strength" by reading his S-meter.  By flipping a switch and swapping antennas, the Amateur can get a quick and dirty comparison between the two antennas, relying on the receiving station to give honest readings.  There are a number of things wrong with this technique however.  First, all antennas are at least somewhat directive, even verticals.  The likelihood of both antennas being "pointed" in the same direction is slight. The comparison is meaningless if either or both antennas is unable to be rotated  Often, running a test like this will yield a superior reading for antenna A with one receiving station, and the next test run with a receiving station in a different location will give antenna B a superior reading.

Secondly, quite often our antennas are not optimized. I put up a three element Yagi several years ago and spent a lot of time tuning it up, getting it "just right".  It still seems to work very well but is it still optimized?  Probably not, it's seen a lot of weather since then.  If your antenna is located in a mild climate, chances are you can neglect it for quite a while before you notice signals aren't quite what they ought to be.  If you live in an area exposed to high winds or corrosive air such as is found along the coast, then you probably ought to do a rehab every couple of years.  One of the most common causes of "poor performance" is a degraded feedline.  Most of us use coax cable because it's relatively cheap and very easy to work with.  But it wears out and water often finds a way in. If the connectors are not properly installed and sealed they will admit water when it rains.  From the day you put it up it begins to degrade and if you leave it in the weather long enough it will become a dandy long skinny dummy load.  So, if you compare a great antenna with a lossy feedline to a poor antenna with a great feedline, what have you determined?  You can wear two watches and if they don't agree you still don't know what time it is.

Another point to consider in running a test on HF is signal conditions.  It is not unusual for signals to fade rapidly from moment to moment. Your receiving station might be reading one of your antennas during a peak and a moment later read the second antenna during a dip.  The two reading may vary but have nothing whatsoever to do with your equipment.

So, what is antenna gain and how can I get some?

 This is a very complex subject and entire books have been written on the subject. ARRL Bookstore    The biggest problem in describing gain, is the fact that it is 3 dimensional and anything you read or see on a computer screen for that matter is two dimensional.  Gain is described on two planes:  The E plane (E for electrical) and the H plane (H from Henry for electromagnetic).  Trying to visualize both planes at the same time can tax your imagination.  Lately I've seen an analogy become popular where a small light bulb is used to help visualize the E plane.  Even though it isn't particularly accurate, it can be very helpful in understanding how we get something for nothing. (we don't, really)

Here's how it works.  Dismantle a flashlight so that it can be lit without the reflector.  In a dark room you will notice that the illumination from the bare bulb will be quite faint; probably not much more than a candle. We will call this the reference point, which corresponds to a unity gain antenna such as a dipole.

Now reassemble the flashlight and once again turn it on in a dark room.  Notice now that the reflector is causing the light to be much more intense in one direction but much weaker in other directions.  This is equivalent to "forward gain" as found in directional antennas such as the Yagi.  The total amount of light emitted remains the same, the focus has changed.

So in antennas we can use "focus" as a substitute term for gain. When we use an antenna that has gain, expressed in dBd, we are saying that the focus of this antenna will be more intense in certain directions as compared to a dipole in the same location.  What if that focus is not pointed at the station we want to communicate with? Most likely, our signal will be weaker (dimmer) than if we used the dipole, but if we point our gain antenna at the desired station our signal will become much stronger (brighter).

When we use a vertical antenna in a mobile installation, the gain takes on a slightly different look.  Looking down on the antenna from an imaginary point in the sky, we see that the signal radiates more or less evenly in every direction which is what we want.  In reality, there are many things that affect this circular pattern however and the most significant is where the antenna is located on the car.  Put it on the trunk and the pattern will become kidney shaped, favoring certain directions over others. Remember, though that gain is 3 dimensional and when we look down on the car from the sky we are only seeing two dimensions.  Lets get back on the ground and take another look at the same car:

The following figure shows a mobile installation with 3 different radiation patterns.  Sorry, not available in stores.

It will help to visualize each pattern as a "donut" with a very small hole, that has been sliced in two so you can see it's profile.