Basic Antenna Length Question

[oberlino]

I'm new to CB, and I wonder if anyone can help me by clearing up a persistent question. I read over and over that 108" is the length of a 1/4 wave antenna, and that a half-wave dipole would be 18 feet long in total. These numbers are consistent with each other, but the 468/f formula I see everywhere suggests that something like 103.2" would be more appropriate for the middle of the CB band. If I've done the calculation correctly, 108" is 1/4 wave at 26 MHz, which is not in the CB band.

So... what am I missing?

Practically speaking, why is 108" the best length for a SS whip, i.e. why add a 6" spring to the 102"? For a vertical dipole, are the elements likely to be closer to 108" or 103"?

Thanks!

 

[DaveJacobsen]

A dipole is not the same as a whip. You would not use a dipole on a vehicle.

The length is related to the actual wavelength of the signal.

All antennas will perform their best at one frequency, for the rest the antenna will be either too short or too long.

For CB, a 108 inch & tuned antenna is going to be a superior antenna (when you're not moving, as the whip will bend when you are moving)

A 108 whip also needs a fairly stout mount, you cannot use a mag mount with it.

A spring helps absorb shock when the whip hits something (like a garage roof)

 

[jhooten]

108 is a quarter wave in free space space. It does not account for the change in the velocity of propagation of the radio wave in a conductor. The 468/f formula is an attempt to allow for those variables. It give you a starting point. It is not an absolute. A 108“ whip is much to long for mid cb band for most real world conditions.

 

[ko6jw_2]

Formulas will get you in the ballpark. For receive only they are fine. For transmitting you need a good SWR meter or an antenna analyzer to accurately tune any antenna.

Vertical dipoles are very difficult at CB frequencies. They are balanced loads and need some form of balun to connect to coax. Yes, you can connect directly without a balun, but it may affect the directional pattern. The other drawback of a vertical dipole is that the feed line should run at right angles to the dipole for a quarter wavelength.

By the way the 468 constant is supposed to take into account end effects that make wire antennas a little shorter than the free space wavelength.

Don't obsess over lengths that you calculate. Get on the air and tune the antenna.

 

[mmckenna]

Good explanation.

A 108“ whip is much to long for mid cb band for most real world conditions.

Since antennas will need to be tuned for specific uses/installation, it's MUCH easier to start with an antenna that's too long and make it shorter. Trying to take an antenna that is too short and make it longer is a much more difficult task….

 

[nosig55]

I thought whips were 102" back in the day.

 

[alcahuete]

I thought whips were 102" back in the day.

They still are. 102" and 6" spring. 108"

 

[jhooten]

Good explanation.



Since antennas will need to be tuned for specific uses/installation, it's MUCH easier to start with an antenna that's too long and make it shorter. Trying to take an antenna that is too short and make it longer is a much more difficult task….

Add an appropriate amount of capacitance at the base if it is too long. If too short inductance.

The problem with cutting the whip is it is not easy these days to find a whip that has the removable ferrule. To trim a whip with a welded on ferrule the tip has to be cut which means the corona ball goes away leaving a sharp point that can poke people or things. Also cutting the thin flexible end instead of the fat stiff end creates other problems (more shock transmitted to the base when the whip hits something for example).

 

[FiveFilter]

I always cut from the "top" end when adjusting my mobile antennas. Although it does remove the corona ball, I've never experienced a sense of performance loss.

There are arguments to be made both for and against the efficacy of corona balls. I choose the "against" side because, well, I always cut from the top. The main reason I cut from the top is that the mag mounts I use hold the antenna to the base via the depth the antenna goes into and thus is supported by the base, and in addition there is a small horizontal set screw designed to further secure the antenna. I always figured I'd rather the antenna to be as deep as possible into the base as it seems more secure that way when traveling at Interstate speeds, rather than the antenna not all the way into the base and thus depending more on the tiny set screw.

Besides all that, I have several antennas that came from the factory without corona balls. So, I can't be missing much by cutting it off of those that do.