I have continued reading, trying to learn....Please, feel free to correct where I am wrong.
My understanding is....A 1/4 wave antenna REQUIRES grounding, so you would HAVE to do a thru roof NMO mount. A 5/8 antenna NEEDS to be grounded with a NMO thru roof mount (but is not required). A 5/8 wave antenna is the best for ultimate gain. If you are mounting on the bumper or a magnetic mount, you use a 1/2 wave antenna.
My question is....When looking at a dual band antenna at http://www.rfparts.com/diamond/Product_Catalog/mobile.html
I don't understand what antennas are 1/4 wave and 5/8 wave and 1/2 wave. Is it the column named "Element Phasing/Wavelength"? If so, what does 2 5/8 mean? Is the SG7500 both a 5/8 AND 1/2 wave antenna?!
I admit, I'm really tired of being confused....Thanks for all help guys...
There is a lot of confusing information, some of it misleading and/or downright false. It is easy
to get confused, especially when everyone has their own product or method to sell you.
Longer omnidirectional antennas have more gain, but only if they are carefully constructed so
all the parts radiate signal in phase with each other. If you just used a straight whip that is
over 1/2 wavelength there will be portions radiating out of phase, which will tend to reduce
performance (unless you are working airplanes or satellites.) So higher gain antennas are built
by combining multiple shorter elements: in the case you cited, internally the antenna consists
of two portions each 5/8 wave long, arranged so they radiate in phase. In some cases it can
be much more confusing - you can have a combination of 1/4, 3/8, 1/2 and 5/8 wave elements
in a common fiberglass shell to make an antenna.
But however it is made internally, the overall length sets a limit on the maximum omnidirectional
gain that it is capable of having. Antennas of approximately the same length will have about the
same gain, regardless of what the marketing department might try to claim. (They can have LESS
gain than the theoretical limit, but not more.)
The truth is there isn't that much difference among mobile antennas. I have often driven down
the road with a 1/4 wave and a 5/8 wave both on top of the car and switched between them
while listening to signals: generally the longer antenna has a slight edge, but not a lot. While
the 5/8 wave antenna has a theoretical gain of 3dB over the quarter wave whip, that presumes
it is over an infinite flat ground plane, and the roof of a car (even a van) is nowhere near large
enough for the 5/8 wave antenna to see the expected gain. In practice you might see 1dB of
improvement, and the signal strength will vary more than that due to terrain as you drive down
the road. Switching between antennas might cause an average improvement from 30% copy to
60% copy in a fringe area. In one case the higher gain antenna allowed me to maintain contact
about 200' further down the road than the lower gain one - both worked the same when I was
in the clear, and neither worked when I was behind a hill.
For an SUV that you might take off road, a 1/4 wave antenna may be the best choice. That's
what all the Forest Service trucks used, and many of the Search and Rescue vehicles I've
ridden in. You want a flexible whip that won't snap off when you hit an overhanging branch.
I'd strongly suggest looking for a 2m quarter wave designed as a dual band for 440 as well.
That will work about as well as anything else, and have a better chance of survival.
Both the 1/4 and 5/8 wave whips are fed at a point of maximum current - that means they
need a good ground connection to work well. "Half wave" designs are fed at a point of
minimum current (maximum voltage), so they aren't as dependent on the ground connection,
but they still need some place for current to flow: generally this is back down the outside
of the coax. But they also need some sort of impedance matching network at the base to
work with 50 ohm coax. If you are stuck without a ground plane (a fiberglass roof, for
example) they are a reasonable solution.
On VHF/UHF, a mag mount can provide sufficient grounding via capacitance between the
metal bottom of the mount and the metal under the paint, as long as the base of the antenna
is large enough. You can also use a mount on a bracket connected to the roof as you have
proposed. Mag mounts have problems with rust underneath, scratching the paint, getting
the coax in through the door seals, etc., but they can still work reasonably well. You might
want to start with one and see how it works (and find some of the inconveniences.) As
others have mentioned, try to choose one with an NMO mount so you can swap antennas
and compare the performance. The NMO is much better than the SO-238 type mechanically.
At some point you may choose to drill the hole and install a proper mount, and you can use
the same antenna elements on either one.
I keep a stash of mag mounts and antennas for SAR, ELT, etc. that I can stick on the roof
of a vehicle that isn't otherwise equipped. They do the job for temporary installations.
I also have scratches and rust spots on my roof from using them.
Probably the best advice, however, is not to trust numbers for antenna gain that are
provided by manufacturers. They likely were pulled from the depths of the Marketing
department rather than having any basis in Engineering. In the real world, when
antennas are properly installed there is only a small difference among them in terms
of RF performance. Pay more attention to the mechanical features: I avoid anything with
too many joints, or that isn't flexible enough to survive a branch strike. Simple is
And if you really are concerned about coverage when you are out in the hinterlands,
carry some sort of portable mast and antenna that you can set up when needed.
Antenna height above ground makes far more difference in performance than the choice
of antenna, and a simple antenna 15 or 20 feet in the air will outperform any of the
standard mobile antennas on the roof of the vehicle. (Especially if the vehicle happens
to be upside down in a ditch at the time!)