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Does smaller = better?

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#1
Okay, this may be a dumb question, but: Space constraints aside is there any practical reason to use fractional wavelength antennas? Why not use a full wavelength or taller? The specific application I am looking at is a 110w mobile in the 150-160 mhz range, the best condition/easiest to mount antenna on hand is an old Motorola low band(?) unit (102" whip+4" spring) that I would rather not cut any more than necessary. It would be mounted to the side of a steel box on a relatively slow vehicle so wind deflection shouldn't be much of an issue. Thoughts?
 
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#3
A 102" whip will not resonate at the VHF frequencies you mentioned. All else being equal, bigger is better, but resonance is part of the "all else being equal", as is the radiation pattern.

Location is important. An antenna mounted on the side of a metal box will always have higher SWR and poorer performance than an antenna mounted on the top of the box. Always mount the antenna on top of the vehicle whenever possible. 1/4, 1/2, and 5/8 -wave antennas have the best resonance and radiation patterns. If you want to go larger, you need to look into antenna arrays, which are generally directional.
 
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#4
Okay, this may be a dumb question, but: Space constraints aside is there any practical reason to use fractional wavelength antennas? Why not use a full wavelength or taller? The specific application I am looking at is a 110w mobile in the 150-160 mhz range, the best condition/easiest to mount antenna on hand is an old Motorola low band(?) unit (102" whip+4" spring) that I would rather not cut any more than necessary. It would be mounted to the side of a steel box on a relatively slow vehicle so wind deflection shouldn't be much of an issue. Thoughts?
So, gotta ask....

110 watt VHF mobile. You want a BIG antenna. Who are you trying to talk to?

As stated the 102" whip isn't resonant at 150-160MHz, so that isn't going to work well. SWR will be high, might even damage your radio.

Longer antennas tend to have more gain. More gain can be a good or a bad thing, depending on what you are looking for.
Antennas achieve gain by focusing more energy towards a specific point or points. On a mobile antenna, a higher gain antenna will focus more RF energy towards the horizon. This CAN be useful, if you are out on the flatlands. However, it can also be a drawback in the mountains. Lower gain antennas radiate more in all directions. This can be helpful in some situations, like a mobile in the mountains, or a VHF antenna on top of a sail boat mast.

Also, to make the antenna provide a proper 50Ω impedance match, which the radio --really-- wants to see, you need to match the impedance of the antenna to the rest of the system.
With a 1/4 wave length antenna (about 18 inches tall on VHF) their characteristic impedance is 50Ω, so no matching is required.
1/2 wave, 5/8 wave, full wave, collinear, etc. will all require some sort of matching.

In the end, the coverage you are trying to achieve AND the ERP limitations on your FCC license will all dictate what sort of antenna you need. Bigger isn't always better or legal.
 
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#5
I'll toss in my 2c on things. A 1/4 wavelength whip over a ground plane like a vehicle will match fairly well to 50 ohm coax and will radiate mostly sideways toward the horizon. Other popular lengths like 1/2, 5/8 and .64 wavelengths over a ground plane will squish a little more signal at the horizon and provide a little gain but these do not match well to 50 ohm and require additional stuff to match them.

Other antenna lengths like 3/4 or 1 full wavelength have radiation pattern problems that will fire your signal up at high angles or down to the ground with very little radiation at the horizon where you want it. It happens that a 3/4 wavelength whip matches well to 50 ohms but its cloverleaf pattern keeps it from being used, except maybe a full 1/4 wavelength VHF whip used on a dual band handheld where its a 3/4 wavelength on UHF and provides a good match. You can move the handheld around to optimize the pattern where you can't do that on a vehicle.

There are big VHF/UHF antennas where separate dipoles or vertical elements are fed and spaced properly to work in concert and provide more signal at the horizon (gain) by taking signal away from other directions that are not needed like straight up, as someone else mentioned.
prcguy
 
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#6
Thanks, that's the information I was looking for. I'm installing this for a friend and was going to contribute the antenna, but not if it meant hacking 7 feet off of it.
 
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#7
Antennas are naturally resonant at odd quarter wave lengths. So an antenna is naturally resonant at 1/4, 3/4, 5/4, 7/4 and presents a 50 ohm load.


There are some other mobile antenna designs but for many circumstances on VHF there isn't a ton to be gained by the average user by using a taller antenna IMO.
 
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#8
While antennas 'match' (SWR wise) at odd quarter wavelengths (1/4, 3/4 ect), the longer they are the more of the signal goes toward the end. In the case of a 1/4 wave the signal is at right angles to the element, that is toward the horizon for a vertical element. A 3/4 wave sends a significant amount of the signal basically straight up with more and more going in that direction as length increases. For most uses this energy is unusable and therefore is 'wasted'.

Amateur radio has used this as a trick (a 2 meter 1/4 wave works on 70 cm and can be used for LEO satellite work if the bird is nearly overhead. Even for this there are better ways.

73
 
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