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I'm studying for my foundation license (in the UK) and I have these diagrams:

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Do the radials in a quarter or 5/8 wavelength antennas connect to the braid/shield of the coaxial cable? If so, how does that compare to the ground plane in a magnetic antenna, that is, the roof of a car, that is not electrically connected to the shield of the cable?

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  • $\begingroup$ I cannot recall, Pablo if mesh is a UK term but normally the outer conductor of a coax cable is called the shield (in an electrical sense) or the braid (in a physical sense). $\endgroup$
    – Glenn W9IQ
    Commented Oct 4, 2017 at 21:18

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Do the radials in a quarter or 5/8 wavelength antennas connect to the mesh of the coaxial cable?

Yes.

Every antenna, at the feed point, has two conductors entering it and connected to something. When an antenna is fed by a transmission line, that line also has two conductors (shield and center conductor, in the case of coaxial cable) which are connected to the corresponding points of the antenna.

In the case of a common vertical antenna, the shield is always connected to the horizontal conductive structure underneath the vertical element, regardless of the form that structure takes. (The reverse would not work as well, because there are usually incidental indirect connections between the coax shield and ground (whether earth or chassis) at the transmitter's end of the feed line — this does not totally “short out” the antenna system — it forms an unintended loop antenna instead — but it certainly means the antenna is not functioning as designed. Lots of things will work as antennas by accident, including mis-installed antennas.)

If so, how does that compare to the ground plane in a magnetic antenna, that is, the roof of a car, that is not electrically connected to the mesh of the cable?

The magnetic antenna mount has a wide metal disk in its base, close against the roof. This forms a capacitor, and as you should know, capacitors can pass RF. Thus, the roof is electrically connected to the shield of the feed line, just not at DC.

The best capacitive coupling would be obtained by using a mount with a large surface area. The other factor is the thickness of the dielectric (i.e. your car's paint), but if you're willing to scrape paint off then you might as well install a permanent hole mount instead.

The other differences are that the radials are short and sparse whereas a good roof is contiguous and thick, but they are serving the same function anyway. see this question and answer for more information on how the size of a ground plane or radials affects an antenna.

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    $\begingroup$ "Every antenna contains two conductive elements" What about loop antennas? $\endgroup$
    – Phil Frost - W8II
    Commented Oct 5, 2017 at 5:10
  • $\begingroup$ @PhilFrost-W8II I've been using that explanation for a while now, and you're the first person to catch that. Thanks; fixed to talk more about the feed point than the antenna. $\endgroup$
    – Kevin Reid AG6YO ♦
    Commented Oct 5, 2017 at 14:36
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The 1/4 radials are directly connected to the shield of the coax. That is what that diagram is illustrating.

A magnetic mount antenna is also connected, by capacitive coupling to the roof of a car through the paint to the steel. A direct connection to the metal would be superior (less losses), but the magnet has the convenience of being quickly removable.

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  • $\begingroup$ Would then the magnetic antenna be badly affected by me placing something to protect the paint, such as a piece of cloth? $\endgroup$
    – Pablo Fernandez
    Commented Oct 4, 2017 at 18:44
  • $\begingroup$ Yes. It would decrease the capacitance and increase the losses. There is likely already a piece of plastic on the bottom to protect the paint; but sliding it can scratch the paint. Instead, tilt it off. $\endgroup$
    – Mike Waters
    Commented Oct 4, 2017 at 18:58
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    $\begingroup$ @PabloLU5ARC (Please try not to ask followup questions in comments; edit your question if it is a clarification, or ask a new one.) It will reduce the coupling, moreso the thicker it is. Thin plastic would be a better choice than cloth, if you must. $\endgroup$
    – Kevin Reid AG6YO ♦
    Commented Oct 4, 2017 at 18:58

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