Quote:
Originally Posted by awattam
What does it mean when someone says 1/4 wave or 5/8 wave when describing antennas? How will this affect me choosing or modifying an antenna?
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An antenna should be resonate at the frequency being transmitted on. The most simple type of antenna in a mobile environment is the 1/4 wave antenna. The radiating element is 1/4 wave length long. This design gives a proper 50 ohm impedance match to the transmitter. The design has a radiation pattern that is very even around the antenna and between the horizon and overhead.
A 5/8's wave length antenna does not provide a 50 ohm impedance match on it's own, so they will have a coil at the bottom of the antenna. This design provides some additional gain, but flattens out the radiation pattern more. There are lobes directed more towards the horizon, and less upward.
On the VHF band, a 1/4 wave antenna will be about 18 inches long.
A 5/8's wave antenna will be about 46 inches long.
An easy way to think about the antenna gain is to think of a lightbulb. A single lightbulb on a flat plane will radiate light out more or less equally in all directions. This is similar to a 1/4 wave antenna.
Now, think of that lightbulb inside a light house. The Fresnel lenses focus that single light out more towards the horizon. A ship at sea would be able to see the focused beam of light better than the plain lightbulb.
Different variations of antennas would appear to have different patterns, with most vertical antennas using different designs to focus more or less of the power out towards the horizon.
So, you may be asking yourself "why doesn't everyone just use a higher gain antenna?" Well, there are various reasons.
Think of a sail boat with the antenna up at the very top of the mast. If that antenna was a high gain design, the signal would be focused out towards the horizon. Great, you'd say! Well, now add in the rolling of the sail boat out in rough seas. each time the sail boat rolls off in one direction or the other the antenna pattern is sending your signal down into the water or up into the sky. For this very reason, most sail boats have low gain antennas to help reduce this. The common design used is a 1/2 wave antenna that has the added benefit of not needing a ground plane underneath it to work. This is ideal when the antenna is of top of a thin mast.
So, using that example, you _might_ find that using a higher gain antenna out on the flat plains would work well for providing better coverage. Now, put that vehicle in the mountains where the repeaters are up high above on the peaks. If you had a high gain antenna most of your signal would be focused into the mountains. In this situation a 1/4 wave antenna would likely work better.
As the old saying goes, "you can't get something for nothing", and that applies here. Higher gain antennas isn't a source of free power. Higher gain antennas just focus what power is fed into them more towards the horizon, kind of like a light house. In some instances though, you don't want just one narrow band of radiation going out mainly towards the horizon. This is where a lower gain antenna might be more beneficial.
Now, your milage may vary, sometimes it takes some experimenting to find out what works best for you. Often people will go out and purchase the highest gain antenna they can find thinking they'll be able to "talk farther". Sometimes this works, sometimes it doesn't. Chances are, a mobile transmitter out on the plains will be able to reach a far off repeater with a quarter wave antenna, or a high gain antenna might work well in the mountains.
Other things to consider about antenna design has to do with how high profile or low profile you want the antenna to be. On my full size truck, a quarter wave antenna gives me the results I want here with mountains around me. I also benefit from a lower profile install and don't have to worry about hitting ever tree branch along the way.
Often something you can do is to watch for police cars or fire trucks in your area. See what kinds of antennas they are using. Chances are they are using a specific design for a good reason, years of use has led them to favor a specific design.
On a base antenna, you have all the same variable, but you can add in some additional tricks. Commercial base antennas have a feature that is referred to as "down tilt". Down tilt is when they use certain designs to bring the radiation pattern downwards from the horizon. A high gain antenna up on a mountain top might have poor coverage down near the base of the mountain due to more power being focused off towards the horizon. Down tilt can be used to lower the patter more towards the area around the mountain or tower.
When it all comes down to it, you can guess, run the numbers, spend hours agonizing over it, but for hobby use, sometimes it's just easier to experiment a bit and see what happens. That is what is fun about this sort of stuff, overall, antennas are cheap and experimenting around and learning how each design works is part of the fun.
This may sound like a dumb question...
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