I need a quick lesson in RF - receiver antenna input grounded ?

Thread Starter

daba1955

Joined Apr 27, 2019
85
Ok my learned friends, I have a question.

On my latest project I am using these 433MHz Learning Code Receivers, and on final assembly of the project I was only getting about 12" of usable range. Not good, I hear you say.

Thinking that there was a problem with the antenna mounting, I put my meter on buzzer and discovered that the ANT connection was connected to the GND terminal of the receiver, ohms reading later, less than 1 ohm.
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This has surprised me, and need to know if this is normal ?

I'm using a short stubby aerial, M6F mounting onto a length of M6 stud which screws through a nylon carrier onto an exposed land on the PCB, this connects to the ANT terminal on the RX480-E. Obviously the outer casing of the project is also GNDed, so how does thsi work ?
 

DickCappels

Joined Aug 21, 2008
6,459
It is possible that the antenna returns to ground through an inductor in the impedance matching network or filter on the output of the transmitter, if it has such a network. 1 Ohm sounds about right.

Maybe you can trace out the antenna connection and see to what kind of component it goes.
 

Thread Starter

daba1955

Joined Apr 27, 2019
85
Thanks Dick, it's actually measured at 0.3 ohms.

1 Ohm sounds about right.

Maybe you can trace out the antenna connection and see to what kind of component it goes.
Need to get my USB microscope working again to do that. Windows explorer isn't adding it, so the software can't see it. It comes up in Device Manager just fine. A problem for another forum, methinks.
 

bertus

Joined Apr 5, 2008
20,567
Hello,

The drawing you gave is to small to read.
When there is a tuned circuit on the antenna, you can read a short with a DMM.

Bertus
 

SLK001

Joined Nov 29, 2011
1,548
Perhaps if we had a decent image, we could help you. It is not common to have an antenna shorted to ground at that frequency. In fact, the antenna should read as an open. My guess is that there is a problem with the assembly.
 

Thread Starter

daba1955

Joined Apr 27, 2019
85
Perhaps if we had a decent image, we could help you. It is not common to have an antenna shorted to ground at that frequency. In fact, the antenna should read as an open. My guess is that there is a problem with the assembly.
I have no way of measuring the impedance of the ANT input to GND at 433MHz. All I can do is DMM it.

And your guess is incorrect, a RX480-E receiver module measures 0.3 ohms from ANT to GND before assembly straight out of the packaging.

I will try again with the assembly image though....

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