1/4 wave, 1/2 wave, 5/8 wave, full wave?

[k9rzz]

Ages ago, I worked my friend across town on 80 meters using a soda can and a 6 inch clip lead. Efficient? No, but it was a fun experiment. Other stories, but this isn't the place.

John K9RZZ

 

[chammy]

i work for fire dept. and am upgrading my mag mount 1/4 wave to something more permanent. i have a larsen/pulse nmo vhf 5/8 coil and a larsen w 490 whip. i came with a cutting chart but was hesitant to do so yet because we operate on a 154 mhz freq and also use occasionally 153, 155, 156 freqs. if i cut for 154 mhz will these other freqs become usless? thanks for help

 

[Hamop54]

As a general rule the bigger it is and the higher up it is the better it will work.
With ground plane antennas having radials is an advantage.
In your case the best performance would be from a 5/8 with ground radials, assuming you're talking about a ground plane antenna.
The 5/8 has a bell shaped pattern which conenctates performance on the horizon, so the higher up, the better.
Hope this helps.
Dan WA2CLP

 

[majoco]

Anything can be made to radiate, it's how well is the problem. Any length of wire can be made to have a low SWR at a particular frequency but the bit of wire itself may not be resonant at that frequency. An antenna length that produces an impedance at the feedpoint that matches the output impedance of the transmitter may be said to be resonant and be a good radiator but it still needs a good ground/counterpoise to work against.
For a very simple view, if you think of a quarter wave vertical rod, then the tip of the rod will have a very high voltage and no current, it's open circuit after all so it is a high impedance. The bottom end at the ground is all current and low voltage and so it is a low impedance. Somewhere in between is the perfect match for your feedline usually 50 or 75 ohms. At the high impedance tip you can attach a half-wavelength of wire and it will match as a half wave has a high impedance at the ends and low in the middle - a dipole is a half wavelength long. Think about it!

 

[profiledescent]

i work for fire dept. and am upgrading my mag mount 1/4 wave to something more permanent. i have a larsen/pulse nmo vhf 5/8 coil and a larsen w 490 whip. i came with a cutting chart but was hesitant to do so yet because we operate on a 154 mhz freq and also use occasionally 153, 155, 156 freqs. if i cut for 154 mhz will these other freqs become usless? thanks for help

You'll be fine. That close to 154, you'll be less than 2:1 VSWR.

 

[LtDoc]

Matching impedances has nothing to do with resonance. When speaking about resonance with an antenna it means that there are no 'un-neutralized' reactances present in that antenna. The input impedance of that non-reactive antenna can be almost anything and very seldom close to 50 ohms. A properly impedance matched antenna can be said to be 'tuned', but that has nothing to do with resonance.
The 'biggy' with different lengths of antennas is the resulting shape of the radiation pattern, where it put's a signal. The 'other' thingy with antenna length is a matter of convenience. Making one shorter to be able to have 'room' for it. That typically applies to mobile antennas but it also certainly applies to 'fixed' antennas too.
Antennas are not as simple as they may seem...
- 'Doc