he's talking about one antenna, mounting to the vehicle vs. mounting using a magnetic mount........the key issue here is ground losses and in most instances running a wire from the magnet to vehicle ground is no answer. the vehicle is not a ground plane for the lower frequencies, but rather a capacitor to ground, especially when monopole antennas are operating at frequencies low enough that the vehicle body doesn't provide anywhere near 1/4 wavelength of body metal in most or all directions from the antenna mounting location to the boundaries of the vehicle proper. the vehicle body becomes one plate of the capacitor and earth ground the other plate with whatever in between acting as the dielectric. ground loss can be minimized by proper grounding (bonding) all bolted-on parts including doors, hoods, trunks, tail and exhaust pipes, bumpers, etc. mounting the antenna as high as possible on the vehicle also helps as this reduces the coupling to ground (as in loading coils on short shafts, we want the vehicle coupled to ground, not the antenna). avoid using bumper mounts, magnet mounts, trunk lip mounts, and similar devices.
n just about every case, an HF mobile antenna will have an input impedance of less than 50 ohms, and typically between 12 and 35 ohms. obviously if we wish to obtain a low VSWR, we need to match our 50 coax feed to the antenna. there are several ways to do this, but the easiest is to use a 4:1 unun.
unun stands for unbalanced to unbalanced, as opposed to balun, which is balanced to unbalanced. on one side we have our 50 ohm input, and on the other a 12.5 ohm output. by tapping 1/4, 1/2, and 3/4 of the way along the top winding, 1.5:1, 2:1, and 3:1 ratios can be obtained. Or, 33 ohms, 25 ohms, and 16 ohms. In all but the extreme cases, one of these taps will closely match just about any mobile antenna.
there are other issues to consider in magnetic mounted antenna scenarios including the fact that if it flies off the vehicle and causes damage to any person or property this liability is not covered under most auto insurance policies. in addition, if you're still going to insist on using magnetic mounts then know this. the amount of pull exerted by the magnets upon the surface they are mounted to is directly proportional to the thickness of the metal they are attached to, so if you don't want them flying off of the vehicle you might want to reinforce the roof area where they will be attached with some additional plate so that those suckers stick as well as they do when you see them on the displays in most retailers where they are sold.
when the ground losses from the magnet mount and the lower coil loaded type antennas are added together efficiency can suffer as much as 15 - 30% and that just might be a conservative figure. whether on a magnet mount or attached to the vehicle we have already seen first hand here in this forum the results of using extended shafts under these types of antennas to increase usable range and the benefits obtained are indisputable. when the current loop created by the inductor or coil is elevated high enough so that coupling between the antenna and the vehicle is minimized the antennas simply perform much better. the use of top loaded antennas virtually eliminates a large portion of the ground loss associated with antenna coupling to the vehicle as does using the extended shafts with the open air coil loaded designs on the shortened shafts.
bond the vehicle components mentiioned above properly to tighten the coupling between the vehicle body and earth ground and decouple the antenna from the vehicle body. that will reduce ground loss and increase antenna radiation efficiency.
balance these factors for best compromise between performance and convenience.
