CB/Ham antenna lengths

[Kevin B.]

Does the inherent length of an antenna affect reception at the wavelengths we use? Assuming everything else (ground plane, SWR tuning, etc) is equal, will there be a difference in reception between a four foot-ish antenna mounted low and a two foot-ish antenna mounted two feet higher? IOW, which matters more, the length of the antenna or the height of the tip?

 

[UFAB]

For UHF and VHF radios it is very important on length.

CBs are a different animal.

 

[sixstringsteve]

I believe what matters more is how much of your antenna is sticking up above the vehicle, where it can get the best 360* reception.

 

[frieed]

All of them, regardless of their physical length, are tuned to be resonant at the center of the CB band (talking CB antennas here) with some tweaking required to get best SWR. Generally, the longer the antenna, the wider the bandwidth. Specifically, if both are tuned with SWR = 1 on channel 19, the longer antenna will typically have better SWR on 1 and 40 than the shorter one.

 

[TurboMinivan]

For UHF and VHF radios it is very important on length.

CBs are a different animal.

I'd say this generalization is fairly accurate, if for no other reason than the meager amount of power CB radios use. CB is such a short-range offering that I don't think it's worth fretting over which height/style/brand antenna might offer a few hundred more yards of reception than another. For most street vehicles, where you mount the antenna on your vehicle will likely have a greater influence on its effectiveness than which brand/model you use. At least that's something that sticks in my mind after getting my HAM license.

Speaking of, I still haven't bough any sort of HAM radio.

 

[Kevin B.]

Specifically, if both are tuned with SWR = 1 on channel 19, the longer antenna will typically have better SWR on 1 and 40 than the shorter one.

How much better, would you say? Enough to matter for the purpose of trail comm? Eric is right that I'm not going to be trying to holler across the state with my CB. I'm just thinking about a permanent antenna to replace my magmount, and wondering if a four foot mounted on the rear bumper is going to function better than a 2 foot mounted discretely on the fender where the radio antenna used to be. Tweakers gonna tweak, and I don't want to advertise that there's a CB in the truck if I can help it .

 

[frieed]

From what I've seen, keeping the swr under 2 will do all you need for trail runs and for these short distances, placement is not too critical but getting a good ground is.
For longer distance, antenna placement gets more important since affects directionality. Most of these antennas are designed to be used with a ground plane. The ground plane reflects the radio waves and makes the antenna appear as a dipole (two element antenna) the best way to picture this is stand a pencil on a mirror. Pencil is antenna which now looks like two from the reflection in the mirror. Ideal placement for non-directional coverage is in the center of a large, flat, metal plate (center of the top on your runner(the metal part) would be the best location on your vehicle). Any variation in the ground plane around the antenna will affect performance in a directional way. Back to the mirror analogy; move the pencil to one corner. If you can see the reflection in the mirror(ground plane), performance between you and the pencil(antenna) will be better than if you see no reflection.
Ground planes that are not horizontal are bad, they act like reflectors as well. A non-horizontal ground plane (an obstruction) will cause shadows in the antenna's pattern because it reflects signal away from it's original path.

 

[TurboMinivan]

How much better, would you say? Enough to matter for the purpose of trail comm?

Based on my own experience, I would say typical trail comm will be satisfactory with virtually any setup, provided your cabling and grounding are done properly. My TJ is the case in point: its SWR measures out horribly for some unknown reason (~2.25 on ch 1, ~2.75 on ch 19, and well beyond 3.0 on ch 40), yet I have no trouble talking with the group on trail runs (even in big groups at Jeep Safari). As you've seen, my 3' fiberglass antenna is mounted over the left taillight with a TeraFlex bracket.

FYI

 

[gijohn40]

there are different types of antennas/// on is center loaded and one is top loaded... if you can get the top loaded and get it above your vehicle it will out do one that is center loaded and not above the vehicle... the reason being is like Frieed said the vehicle now becomes a reflector instead of the ground plane. even the fiberglass top of a jeep will effect the pattern of the radio wave. so get a top loaded one and make sure it is above the vehicle for best performance.

Dempsey your problem is probably what I just stated... I would be glad to help you with getting this down to the correct swr so that you don't burn up your radio....

 

[clfrnacwby]

I think sheer amount of antennas is what's going to give you the best reception. Just ask this guy

 

[frieed]

Curb feelers for when he flops

 

[MikeGyver]

CB antenna length matters very much. Adjusting the length is how you set the SWR. Antennas must be certain fractions of the wavelength to work properly.

 

[jackjoh]

The longer the antenna the better as long as it is tuned to the correct frequency. For trail use just about any length placed anywhere will work.

 

[Kevin B.]

Great info, thanks everybody.