A 1/4 wave vertical whip is basically a 1/2 wave dipole with the counterpoise (where the coax shield would connect) being the car's body instead of a wire. They, but defination, would have no gain since it's the reference for the dBD gain factor (decibles over dipole). 1/4 wave at 440 is about 6" (the size of each leg of the 1/2 wave dipole). Not sure why you'd say "it would be a sloper antenna" since that would make it only about a foot long (not much slope needed).
For some gain, you may want to look at a J-Pole (
J POLE ANTENNA DESIGN CALCULATOR by K4ABT) - pretty easy to make and can be wire (flexible, portable) or copper pipe (sturdy).
A little semantics here. Not aimed at your comment specifically n5ims, but rather a statement to help newer users understand a slightly convoluted principal. I understand what you mean by a 1/4 wave having no gain. You mean it does not have more gain than a 1/2 wave dipole and are quite correct. But so a new user might understand, _everything_ has quantifiable gain.
A 1/4 wave vertical does have gain (as I said above so does everything, even if it is negative gain compared to something specifically), but it does not have gain over (more than) a half wave dipole. So on a scale of dBd (gain referenced to a dipole) a 1/4 wave vertical has about -3 dBd (that is negative 3 dBd, less than one). On a scale of dBi (gain referenced to an isotropic) it is about -0.85 dBi. Still less gain than an isotropic, but a better "number", less negative.
Gain in dB is not an "absolute", but always a "relative" (as is anything represented in just "dB", while dBi, dBd, dBm, dBV, etc are absolutes because you have given the references in the last letter or last letters). As such even a 3" paperclip on 80 meters has a gain value in dB, it is just a pretty large value negative unless the reference is also pretty inefficient. But understanding the difference between dBd and dBi is important, because some designs and some manufacturers state gain in one reference, and others will state it in another, and others yet might use another "standard", such as gain compared to a 1/4 wave vertical itself.
So, if you see a 1/4 wave vertical mobile antenna advertised as "0 dB gain" this might be right, if they are using another 1/4 wave vertical as their reference. If they are calling it "0 dBi gain" they are stating an impossibility, as would be "0 dBd gain" By the same token a 1/2 wave bottom fed could be said to have "0 dBd", or "+2.15 dBi", or even "+3 dB" if the later is referenced to a 1/4 vertical. Also, some makers actually measure the gain, normally against a half wave dipole, and then state the gain as dBd. Other makers might just model or calculate the value, and calculated are often done vs an isotropic model. Sometimes the vendor does not bother to include the "d" or "i" to tell you the reference.
In other words, "gain" for a given antenna is sometimes an iffy thing. One must keep in mind the specific reference used. One must also have an idea of if it is calculated or measured (measured is almost always less than calculated, but is often more honest). And last, keep in mind that makers like "big" numbers on antenna advertisements and packaging, so don't take any of them at face value without some thought. I have seen more than one antenna package with inflated, or very hard or impossible to explain theoretically, gain claims.
And, we have not even talked about where that antenna might be making that said gain, at what take-off angle. A whole new variable to consider. One that can sometimes make a lower gain antenna more desirable for a specific application.
And to the OP, if you are bringing up the repeater with your handheld on the stock antenna now, from the location you want to install this antenna, it is very likely you will do better with a 1/2 wave vertical dipole, assuming good feedline and a short feed run.
T!
