While researching how best to install my mobile amateur radio in my car, I didn't understand if there was optimum Antenna Feedline Length for mobile installations?

Understanding that this is caused when common-mode currents make the feedline part of the antenna, I want to know if effectively grounding the antenna mount (coax shield) to the vehicle body will help mitigate common mode currents with mobile antenna installations? (Effective meaning metal to metal under the trunk lip, and maybe an extra ground strap from trunk to car chassis.)

My feedline does run inside the vehicle along the body panels, so it is definitely not a good place to work as a radiator element and I certainly wish to shorten the feedline and obtain the best SWR reading possible. I am using a small 1/4 wave 2M VHF antenna that also doubles as a 5/8 wave UHF antenna (primarily for the shorter height for underground parking).

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    $\begingroup$ I recommend you read my answer to this question: ham.stackexchange.com/questions/754/… $\endgroup$
    – WPrecht
    Mar 24, 2014 at 18:18
  • $\begingroup$ @WPrecht-AB3RY Are you suggesting there is some capacitive coupling between trunk lip mounts as well? Interesting note, I tested my SWR at the same time as another local ham with a mag-mount and similar 1/4 wave antenna and we were both very close to 1.7 SWR (his mag-mount, mine trunk lip). $\endgroup$
    – BenSwayne
    Mar 24, 2014 at 19:52
  • $\begingroup$ @BenSwayne you can do just fine without a DC connection to ground. Probably more important is a good ground plane in all directions. Given the choice between a mag-mount in the middle of the roof or trunk, and a lip-mount with a good DC connection to ground, I'd go for the mag-mount. It will give you a more predictable pattern, and will behave more like an ideal vertical, which has an infinite ground plane under it. Either may work fine enough, though. If you don't want to drill holes in your car, or if you don't want a tall antenna on the top, you have limited options. $\endgroup$ Mar 24, 2014 at 19:57
  • $\begingroup$ Also, when you get in to questions of which of two less than ideal antenna setups are better (a situation occurring in virtually all mobile stations), the best option is probably to get or build a field strength meter and just do some experimentation. It's easy to get wrapped up in theory, but the particulars of each car are so different sometimes it's faster to measure than to predict. $\endgroup$ Mar 24, 2014 at 20:02
  • $\begingroup$ @Phil nailed it. What I was getting at was what you observed: it doesn't much matter. A mag mount is less work and less intrusive. $\endgroup$
    – WPrecht
    Mar 24, 2014 at 20:08

2 Answers 2


Does effective grounding of mobile (vehicle) antenna mounts address common-mode currents?

No. Well, not really. It might help, incidentally. Any 1/4 wave antenna is only half an antenna, and the other half is your car. The antenna designer made some assumptions about your car and how you'd use it to complete the antenna: if you violate those assumptions then you have a different antenna than the one that was designed, which may or may not be worse in any number of facets of performance, including common-mode current.

For example, the antenna designer probably assumed that the car is made of sheet metal. If you put a 1/4 wave vertical on a car with fiberglass body panels, you absolutely will have significant common-mode currents because the feedline is the other half of the antenna.

Consequently, the best advice is probably to install the antenna according to the manufacturer's instructions. Many mobile antennas are compromise designs, so by deviating from these instructions it may be possible to make an improvement, but it's also just as possible to make things worse. Without all the details it's hard to say which will happen.

My feedline does run inside the vehicle along the body panels, so it is definitely not a good place to work as a radiator element [...]

This is not necessarily the right conclusion. The feedline will be capacitively coupled quite well to the body panels, and the body panels can be decent radiators. RF currents want to radiate. If you don't absorb that energy in a loss somewhere, it will radiate, eventually. Indeed, it's more ideal if all the energy is radiated from your antenna as designed and there are no common-mode currents, but if that's not completely what happens I suspect the consequences are not as bad as you imagine.

The bigger problem, if there is one, is probably that some of the radiating structure comes close to electronics in your car resulting in noise when receiving. Remember reciprocity: if it's transmitting well, it also makes a good receiver. But, this is already a big problem in mobile installations: the RF noise from the electronics tends to find its way everywhere, thanks to the metal car. It's not like the antenna is on a tower in a rural area far away from noise sources. It's practically attached to a noise source.

With this in mind, there's an easy experiment you can do which will answer your question better than any musing over theory we can do here. Set up your radio so you can listen to or otherwise measure the noise received. Then, temporarily attach some proposed modification to the antenna system, be it a ground strap or whatever. Can you measure an improvement? If so, you've confirmed your modification reduces noise, perhaps by moving the radiating structures of the car farther away from noise sources. Repeat the experiment listing to some desired signal to be sure you are just reducing noise and not just killing the antenna performance (a dummy load is very quiet, also). If your tested modification reduces noise and increases signal, it is a good modification, and will have an equal improvement on your transmissions, by reciprocity.


There are several issues to be considered here. First, does the antenna type naturally want to sit over a ground plane (for example, in order to radiate effectively or at all)? An end-fed half-wave antenna for example would not need (or maybe even "want" a ground plane, and would generate only a very small ground-plane current if mismatched slightly. Some mobile antennas from Japan are advertised as being "no ground plane" types because they are designed in this way. Other antennas, such as a quarter wave monopole or a 5/8 wave monopole (with base loading coil) DO want to sit on a ground plane in order to generate a good (or just the expected) radiation pattern.

The second point is that to which you refer, i.e. the issue of a poorly-matched antenna generating current on the outside of the coaxial cable shield. If this is the situation, and especially if the "natural" feed-point impedance is low, then make every possible effort to sort out the mis-match. This will have the most beneficial effect. A tool such as the mini-VNA can help with diagnosis and measurement. Remember to resonate the antenna first, and then to adjust the impedance matching. If the match cannot be improvedthen it is worthwhile attempting to "kill" the current on the coaxial cable outer shield. Grounding is one way, but it is very hard to do. You need an RF ground with very low inductance. The mis-match current must "want" to flow down the ground connection much more than it wants to flow on the outside of the feeder cable. Especially for VHF / UHF, use short, wide straps, NOT wire, and even if you can make a good connection to a panel, some car panels are actually not well bonded to each other as far as RF currents go. If you need the ground plane for reasons of radiation efficiency, then the plane must be about half a wave-length in diameter to begin to be effective (or at HF as big as you can get!). At VHF and especially at UHF, a single aluminium roof rack bar can act as a ground "plane" and has the advantage of being drillable without affecting the paintwork of the vehicle! When you have made your best attempt at grounding, consider adding a choke to the cable close to the feed-point. This could be a small coil of coax held with cable ties, or wrapped around a ferrite rod, or a few turns of coax through a ferrite or iron-dust toroid, or one of those handy "snap-on" ferrites that come in two halves and close around the cable to make a tube. This will reduce the braid outer current, make the transceiver happier (less RF flying about near the radio) and also reduce unwanted couplings and variabilities in matching performance.

Finally, if grounding is impractical, consider installing a quarter-wave resonant wire (or wires, or even a quarter-wave helical antenna) extending from the mounting point ground and orthogonal to the whip. This acts as a resonant "trap" which will soak up quite a bit of the mismatch current by trying to simulate a ground "plane" (albeit poorly!). If this approach is not practical, and you cannot drill into the centre of the car roof, then a capacitive RF ground might be the answer - basically if you can make a good RF ground connection at the antenna base and attach that to a conductive plate or foil that is then held very close to the car body panel, the effective impedance will drop (although it will remain reactive). This is a good way to "try" the effect of grounding in fact - try using aluminium foil smoothed out and taped over the roof for example. I should warn that this technique is only really effective at VHF and UHF, not so good at HF.

  • $\begingroup$ When you say "mis-match" I assume you mean impedance matching between the antenna and the feedline, but this is unrelated to balance, or the lack thereof, resulting in common-mode currents. An antenna can be matched perfectly but still have common-mode currents, or it can be very poorly matched but have no common-mode current. The two are not the same. $\endgroup$ Dec 8, 2014 at 14:05
  • $\begingroup$ If an antenna is perfectly matched to the line, it presents the same impedance as an infinite length of extra line. The energy carries on flowing without any change or reflection. If the feed-line is coaxial cable, then the energy carries on flowing between the screen and the centre conductor. The line stays unbalanced (which is how a coax cable operates). Any current flowing on the outside of the coax implies a lack of proper match - after all the current has to be conserved. Proper match on an unbalanced feeder cannot result in external braid currents. $\endgroup$
    – G4ZLZ
    Dec 8, 2014 at 20:55
  • $\begingroup$ That's simply not true: a good match does not imply no common-mode current. I put this challenge to you: build a dipole and feed it with coax but no balun. Match it however you want, by trimming the length or adding a matching network at the feedpoint. Then measure the common-mode current and tell me what you get. Alternately, try significantly changing the feedline length or orientation and tell me if you still get a good match. $\endgroup$ Dec 9, 2014 at 0:32
  • $\begingroup$ See also How is impedance matching related to balance and common-mode currents?, which I might answer when I have more time. $\endgroup$ Dec 9, 2014 at 0:33

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