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I have a transmitter setup from my router like shown below. The backplane is made from cardboard and has an RPSMA male wire held up in the middle. It is attached to my router.

Goal: design a monopole that can attach to the middle of this cardboard backplane and transmit a signal better than a simple 3.1 cm paperclip.

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I am trying to design a monopole antenna that can attach to the RPSMA male pin in the cardboard through a female pin in this connector. It is the same on both sides.

enter image description here

The RSSI without any antenna using this setup is about -80 to -85.

I have a reference monopole antenna that is just copper wire of length 3.1 cm (the router is transmitting at 2.4 GHz so that is 1/4th wavelength).

However, unlike what I have seen online, the RSSI value only improves about 5 to 10 when this is plugged in. Either way, I expected using some sort of ground radials to improve this.

Using the shell of a motor and a soldering iron, I created this:

enter image description here

The main copper through the middle has an insulting layer of heat shrink tubing to stop it from conducting with the motor casing. It connects to my female SMA on the inside of the casing through the other side of the middle copper wire.

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The 4 ground plane radials were soldered on, and they are also 3.1 cm wire in order to match the 1/4th wavelength.

Supposedly this is supposed to reflect the signal, however, this does no better than when the radials weren't there and it was just a plane copper wire. I assume I don't know something about the way ground planes are supposed to work. I was told any conductive radials would do the job but maybe they actually have to be grounded through the coaxial somehow?

This is my entire setup, and I'm not sure where specifically the problem is. My bet is on how I set up the radials, but that wouldn't explain why the improvement is only 5-10 in RSSI from no antenna to antenna.

Cross section of SMA:

enter image description here

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  • \$\begingroup\$ What is this motor housing, how does it connect to the SMA? The connector cross section seems unrelated, unless you're saying that you're using hardline, the two kinds of SMA connector shown, and you've sectioned it somehow? \$\endgroup\$
    – Tim Williams
    Jul 30, 2022 at 23:04
  • \$\begingroup\$ And, under what conditions is your reference signal level? With standard antenna? What orientation and distance to (and obstacles/reflectors near) the client radio? And what activity i.e. passive connection, active download? (Radios will often increase power to increase SNR and bandwidth on demand only!) And what type of coax is used? \$\endgroup\$
    – Tim Williams
    Jul 30, 2022 at 23:07
  • \$\begingroup\$ The wire through the middle of the motor housing may be causing a significant signal loss. The SMA connector and associated coax are probably design with a 50 ohm impedance. Do you have any idea what the impedance is of the wire-through-the-motor-housing? \$\endgroup\$
    – SteveSh
    Jul 30, 2022 at 23:46
  • \$\begingroup\$ Look at the cross section of that SMA connector you posted? Do think the dielectric and conductor-to-shell dimensions were just arrived at arbitrarily? \$\endgroup\$
    – SteveSh
    Jul 30, 2022 at 23:48
  • \$\begingroup\$ And why are you going through all this trouble to make a monopole antenna? Why not just make a vertical half wavelength dipole? \$\endgroup\$
    – SteveSh
    Jul 31, 2022 at 1:41

1 Answer 1

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I was told any conductive radials would do the job but maybe they actually have to be grounded through the coaxial somehow?

Yes, they need to connect the the coax outer shield/screen which, in turn, grounds them back at the router (due to the way coaxial cable works).

Why don't you just get rid of the ambiguous effects that the motor casing might produce and solder the radials directly to the coaxial outer shield.

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  • \$\begingroup\$ so by outer shield you mean the gold part of the sma connector? \$\endgroup\$
    – Saveer Jain
    Jul 30, 2022 at 18:29
  • \$\begingroup\$ No I means the outer shield (one insulation layer down) in the coax wire. It may work fine if the "gold part of the sma connector" (the outer surfaces) are connected to the radials. \$\endgroup\$
    – Andy aka
    Jul 30, 2022 at 18:43
  • \$\begingroup\$ I added a new image of the cross section of an SMA connector attached to the coax. It looks like the copper shielding of the coax doesn't make its way through the SMA connector due to the dielectric. Am I false? If I'm right, how do I ground it. \$\endgroup\$
    – Saveer Jain
    Jul 30, 2022 at 20:27
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    \$\begingroup\$ The copper shielding MUST make a connection or you will have a poor antenna. It's vital. \$\endgroup\$
    – Andy aka
    Jul 30, 2022 at 22:19
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    \$\begingroup\$ @SaveerJain You are misunderstanding that cross section. The coax shield is directly connected to the brass part of the sma connector, as it must be for the connector to form an electric circuit. However in this case I wonder why you left the sma connector on ? I would cut it off and use the central wire as the antenna and solder the radials directly to the shield. \$\endgroup\$
    – user1850479
    Jul 30, 2022 at 22:43

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