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So I've been looking at my different options for 6 meter mobile antennas.

My question mainly pertains to grounding. I've heard hams talk about needing a ground to the frame of the vehicle when using HF. I don't know if this is also important for 6 meters or not. It would be much easier for me to get a mag mount, but I'm unsure if the grounding will be good enough at 6 meters.

If I do go the mag mount way, there are two options I'm looking at: A standard 1/4 wave NMO or a DC grounded 1/4 wave NMO. The DC grounded one has a coil. I'm assuming this coil is going to ground because there is no need to load this antenna as it is already 1/4 wave long. This leads me to think that the DC ground is serving some other purpose. Could it be to help the ground performance on a mag mount? (This is what I'm hoping.)

If I do manage to ground the antenna properly, my impression is that 6 meters would be the apex of mobile communication. The size of the roof is very close to the right size for a ground plane on this band. In addition, the antenna is pretty much the largest without being loaded. Being in a hilly environment, I think this should be the best band.

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The bonding of automotive panels and structures is generally done to reduce RFI that can affect in-vehicle electronics. On 6 meters, this may not be at all necessary but you will not know until you try it.

A 1/4 wavelength vertical antenna will produce an SWR of ~1.5:1 in an efficient installation. Keep in mind that the relatively high RF resistance of the steel panels of the vehicle add to the inefficiency of the ground plane. Due to the short length of coax involved, a base matching coil to reduce the SWR is generally not warranted from a feedline loss perspective as the losses in the matching circuit will nearly equal any coax losses due to the slightly elevated SWR in the no coil version. However, your 6 meter transmitter may put out slightly less power when faced with a 1.5:1 SWR. In this case you may be able to justify the use of an antenna with a base matching network.

The height of a 1/4 wave antenna for 6 meters can place a fair bit of strain on an NMO mount. Regularly check your installation to make certain that everything is properly tightened and snug.

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  • $\begingroup$ Thanks for the response! It's still not enough for me to make a decision though. Your mention of 1/4 wl producing a 1.5 SWR doesn't make sense to me as I've had 1/4 wave ground plane antennas in the past produce 1.1 with no matching coil. What is the point of the DC ground/coil on these mobile antennas then? And is it worth it to purchase a more expensive one with matching? $\endgroup$ – Synaps3 Dec 24 '18 at 14:42
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    $\begingroup$ @Synaps3 A quarter wave, ground plane antenna has a feedpoint impedance of ~32 ohms (largely non-reactive). Any inefficiencies (e.g. a poor or lossy ground plane) will increase the feedpoint impedance bringing it closer to 50 ohms and thereby lowering the SWR. Other techniques such as angling the ground plane downward will also improve the SWR at the expense of the radiation pattern of the antenna. A base coil can change the otherwise 32 ohm impedance to something closer to 50 ohms, thereby lowering the SWR. $\endgroup$ – Glenn W9IQ Dec 24 '18 at 14:57
  • $\begingroup$ That makes sense. So the antenna I was working with before must have just had a less than optimal ground plane, giving me the 1.1. So the only thing that's not cleared up is if a mag mount can create a good enough ground plane for 6 meters. I know people use mag mounts for UHF and 2 meters, but 6 is getting closer to HF. $\endgroup$ – Synaps3 Dec 24 '18 at 15:13
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    $\begingroup$ @Synaps3 The mag mount will probably "work" on 6 meters. The mag mount obtains its ground plane connection through the capacitance of the mag mount to the vehicle body. Since there will be capacitive reactance with some degree of ESR (effective series resistance), this is a form of inefficiency that will also add to the impedance of the antenna. It is difficult to predict the exact effect since the construction and geometry of the mount and other factors such as paint chemistry and thickness come into play. $\endgroup$ – Glenn W9IQ Dec 24 '18 at 15:57

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