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I am currently looking for a good solution to create a simple VHF dipole connected to a RF preamplifier / band pass filter. The intended use is 88-108 FM RX. I see basically two solutions for a rigid antenna design :

  • two classic screw terminal consumer telescopic antennas for each dipole leg.

  • two BNC terminated telescopic antennas for each leg.

I prefer the BNC solution because it is sturdier and easier to streamline and connect to a PCB with through hole sockets. However I wonder what to do with the BNC ground terminal. Keep it floating, connect it to the circuit common ground, or shunt it with the center pin and dipole leg ? FIY I will use a surface mount common mode choke rated for VHF, so the dipole will balanced. The preamp case is unshielded, only the RF preamplifier circuit path will be copper clad.

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  • $\begingroup$ Not connecting seems indeed the safe way to go. I was asking since the two traces connecting the two terminals come apart at a right angle for 4 cm inside the enclosure, so technically the line stops there and the antenna starts here. I suppose that the BNC jacket when not connected would capacitively couple with the antenna legs over the length of the connector. I assume that this would not change a lot given that the wavelength is more or less 3m. Nevertheless, I still wonder what is the best solution between leaving it not connected or shunt from a purely theoritical point of view. $\endgroup$
    – rodv92
    Nov 29 '18 at 13:45
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There is no problem with using the BNC version of the telescoping elements. Because the shield of the BNC is not connected to anything, there is no reason to connect it to your circuit in any fashion. So leave it floating and proceed with your project.

Have fun!

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