A directional antenna, such as a yagi or quad, is great for T-hunting, but there will come a time when you will wonder just what its beamwidth is, and how precise your bearings can be. Likewise, if you are planning to use a directional antenna when you hide the transmitter, you will want to know how narrow the beamwidth is, and whether there are any weird lobes that might give the game away.
Don't take the manufacturers' word for the antenna's beamwidth; check it yourself. What follows is a procedure to measure an antenna's receiving pattern. I guarantee that you will find the time taken to do this well spent, and the results informative.
In fact, why not plan an "antenna-calibrating Saturday" with other T-hunters in your group? As long as you are going to the trouble of setting up to check your antenna, you might as well let the gang help out and check theirs, too. Encourage others to bring models by different manufacturers, so that you all can compare the various designs. You could compare quads-vs-yagis.
What you will need:
The antenna mount is the tricky part. Basically, you want to either place the antenna high above the nearby people and environment, OR you want to use your standard vehicle mount, if you do mount the antenna on your vehicle during hunts. If you mount the antenna on the side of the vehicle, such as through the window, then you will definitely want to duplicate the measurements from BOTH sides. You will find this very instructive.
Also, you will want to be able to measure antenna rotation during the test. This will require a compass rosette, marked off in increments no greater than five degrees, and a pointer mounted on the antenna mast. Orient the rosette so that 0-degrees is in the direction of the transmitter.
If you are mounting the antenna on a vehicle, point the vehicle in the direction of the transmitter. It is best to remove any other antennas from the vehicle, as they can re-radiate signals and throw off your measurements (you might consider re-running the measurements with them on, later, for comparison). Set the polarization the same as the antenna used at the transmitter.
Set the transmitter up in a clear area, and as far away as possible. You can use a small 1/4-wave antenna, but a directional antenna aimed at the test site might be better, to minimize the effect of possible reflections. Again, make sure that the polarization is the same as the antenna to be tested.
Turn on the transmitter, and adjust the transmitter output and your attenuator until the S-meter stays on scale when the antenna is oriented to receive the strongest signal. Adjust the attenuator so that the needle rests exactly over any mark on the meter. You must have at least 40db of attenuation switched in with the attenuator. This is a critical part of the setup, to ensure that you have plenty of signal to work with, but not so much that you cannot keep the S-meter on scale.
Record the attenuator setting.
Rotate the antenna clockwise in 5-degree increments, stopping at each position to adjust your attenuator so that the needle once again rests exactly over the same mark on the meter. Record the attenuator setting at each position. Continue until you have completed the 360 degrees.
During this process you will see sharp dips in the readings that may not occur at the 5-degree increments. Take measurements at these positions, too, and record them. These "nulls" will usually be quite narrow. Usually, peak readings will be fairly broad, but if you can pinpoint one at some point other than at a 5-degree position, record that, too.
Review your readings. If any readings look out of place, go back now and re-check them.
By the time you have completed measuring an antenna, you will probably be quite surprised at the results. Didn't know that beast had so many nulls and "lobes", did you? You will want to transfer your readings to a Polar plot, so that the pattern of your antenna is more readily seen. (Of course, you could have plotted them directly on polar graph paper, to begin with, but numbers are more precise).
Remember, the goal of this exercise is not to measure the overall gain of an antenna, but rather its radiating/receiving pattern. The measurements you've taken will tell you the front-to-back and front-to-side ratios, as well as the locations of any lobes or asymmetries.
Do you know how manufacturers specify "beamwidth"? Usually, it is described as the angle between the "half-power" points. In terms of the measurements you just took, it is the angle between the two points left and right of 0-degrees rotation, where the signal drops off 3db from maximum.
I was quite surprised to discover that my 4-element Cushcraft 147-4 yagi antenna, in vertical orientation, has a 66-degree beamwidth(!). Surprisingly, this antenna does take good bearings. The Front-to-back ratio is only 14db, though, which explains why I get good signals "off the back", too.
There are four lobes, at 0, 105, 180(!), and 256 degrees. There are four pronounced nulls, at 86, 135, 225, and 284 degrees. With this many lobes and nulls, I have to be careful to swing the antenna full circle when taking bearings, in order to be sure that I'm identifying the strongest signal direction.