2
$\begingroup$

I have a SDR device and have a question about using a monopole antenna with them, but the question could be applied to any device which uses an antenna.

SDR comes prepacked with a monopole antenna which essentially is a single wire connecting to the middle of the Coax output (picture attached).

Here are my questions:

  • What happens to the ground wire in this case (the outer part of the Coax output)? is the shield of the coax cable acting as the "ground"? if so then does it not matter that the shield is not 180 degrees opposite of the antenna like a dipole is?

  • Is there a better way to "ground" it if I don't have access to the ground itself.

enter image description here

$\endgroup$

3 Answers 3

2
$\begingroup$

What happens to the ground wire in this case (the outer part of the Coax output)? is the shield of the coax cable acting as the "ground"?

Yes, and so is all other connected metal, possibly including the USB cable and the attached computer. There might or might not be a path to earth there, but I find it better to think about it not as “ground”, but as “the other half of the antenna” — because it all affects the radiation pattern. The point of adding a “ground plane” is not to make the antenna “grounded” but to provide that “other half” in a consistent fashion so the antenna will have predictable behavior.

However, for hobby receiving purposes, there is no reason to worry about using the antenna “correctly”. Just play with it and find what receives your signal better.

if so then does it not matter that the shield is not 180 degrees opposite of the antenna like a dipole is?

It matters but it doesn't make the antenna ineffective. An L-shaped antenna (the cable running sideways out of the antenna base) is still a usable antenna. Picture an antenna with radials at the base.

If you decrease the angle between elements all the way to zero, the antenna would theoretically not receive anything because it is now more like a transmission line (intended not to radiate!) than an antenna. But outside of that perfectly null scenario, there will be reception.

Is there a better way to "ground" it if I don't have access to the ground itself.

The earth itself is actually nearly irrelevant to setting up your antenna unless you are putting it right up against the earth. It's too many wavelengths away to matter, otherwise.

As already mentioned, any large sheet of metal, like a baking pan or a metal cabinet or desk, will work well for this purpose. It should have at least as much radius (length from the antenna base to the edge of the metal) all around the antenna as the length of the antenna rod itself.

$\endgroup$
3
$\begingroup$

Usually the shield is connected to the magnet at the base of the antenna and is capacitively coupled to the metal to which the antenna is attached. Typically a steel pizza plate acting as a ground plane.

jm

$\endgroup$
3
  • $\begingroup$ I'm guessing a ground connection is not as important as the main antenna? how can a tiny little plate act as ground? $\endgroup$
    – Dan
    Commented Jun 30, 2023 at 12:22
  • 1
    $\begingroup$ @Dan at VHF and above, it can have good capacitive coupling to the roof of a vehicle. (By itself, it's a terrible ground.) $\endgroup$ Commented Jun 30, 2023 at 12:57
  • 1
    $\begingroup$ Adding to mike's comment, frequency matters. A pizza tray is perfectly adequate at VHF, but might as well be non existant at HF. At VHF and up, you can even use a steel bowl to approximate downturned radials. At HF and below, a dummy load is a good approximation for the shipped antenna configuration. $\endgroup$
    – webmarc
    Commented Jun 30, 2023 at 18:41
1
$\begingroup$

Dan, All simple antennas have two parts, think of the two arms of a dipole, or a rabbit ear tv antenna. The whip or monopole antenna has two parts also, the wire in the air, and the earth below it. The magnetic base antenna connects to this earth in two ways, one by connecting through the magnet to a metal surface, which acts like one plate of a capacitor, with the earth being the other final plate. Mounted on a car, the roof, through the sheet metal, to the chassis, makes a very lossy plate to capacitive couple to the ground under the car. Through this path the signal goes through the 180 phase shift. This antenna is connected to coax, the monopole to the center, and the magnet to the shield. There may be a coil and capacitor to help match the antenna impedance to the coax impedance. Think of the coax as three wires. The center wire, the inside of the coax, and the outside of the coax. Because radio waves have a high frequency the electrons moving in a conductor get pushed to the outside surface of a wire. This is called skin effect. Both sides of the coax shield are connected to ground at DC, but because of the skin effect they act as two wires at RF. Radio waves traveling to your RTL split at the coax, some going on the outside of the coax shield surface and being wasted, and others traveling down the inside of the shield surface between it and the center conductor. You can force this energy off the outside of the coax by putting a coaxial choke or two on the outside of the coax, snap on chokes are pretty convenient and work well for VHF/UHF, but if you are receiving HF, then several wraps around a type 31 material toroid will work very well as a coaxial choke. This type of choke is also called a balun or common mode choke.

$\endgroup$
2
  • $\begingroup$ Welcome to ham.SE, Jimbo. Respectfully, this would be a little easier to read if it were broken up into paragraphs. :-) $\endgroup$ Commented Jul 5, 2023 at 20:34
  • 1
    $\begingroup$ thank you Mike , jim $\endgroup$
    – Jimbo47
    Commented Jul 8, 2023 at 3:39

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .