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I have these common eBay 433 MHz module pairs pictured below and am trying to hook up an external antenna.

On the TX there is a little hole marked ANT which works for that.

On the RX pictured (XD-RF-5V), there is a similar hole with no markings that increases range if I hook it up to an antenna.

So I was looking at making dipole antennas for them, but I got confused when trying to find the ground to connect up to that part of the dipole antenna.

You might be thinking easy trace the pin marked GND, which I did, and while doing that I traced the 'antenna' hole I'm using.

The hole is connected to that little copper coil and then onto the SMD components 'above' it.

But it confuses me that there is 0 ohms resistance (as if it's connected) to that GND ground pin...

So how can it work as an antenna if it is connected to ground?

And as a second question, wouldn't I have to take into consideration the length of that coil in my antenna length calculations? What is it anyway? Could I remove it?

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I can't disclose the full circuit because of an NDA but here is the antenna input section for a similar receiver. This one includes a capacitor in series with the antenna connection, which presumably yours doesn't if you're able to measure a DC resistance:

Receiver antenna input

So when you're measuring the DC resistance the inductor in the parallel LC circuit will appear as a short to ground, but of course that won't be the case at the RF frequencies of interest. So don't try to remove it because it'll compromise receiver performance and you shouldn't need to take it into account when calculating the antenna length.

The PCB and pin lengths may have a bit of an effect of the optimal antenna length but if you're keen enough probably the easiest way to tune it without any test gear is to start with the antenna a bit longer than theoretically required and trim it down until you get the maximum range. That's assuming you're using wire for the antenna that can easily be replaced if you end up making it too short before finding the best length.

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  • \$\begingroup\$ ok , well i understand now mostly, enough to know its all ok. As for tuning, i can just cun it off in bits but was wondering can i fold it on itself ? if the bit folded over is touching the main bit all its length does it behave as a shorter antenna ? \$\endgroup\$
    – Hayden Thring
    Nov 14, 2013 at 10:52
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    \$\begingroup\$ @HaydenThring, I guess if you fold it in really well so that the outer surfaces are in relly good contact that should be OK. Helical antennas might be worth a look into as well if you're after something small, 433MHz is a common frequency and they are available for a dollar or so on e-bay or you could try making your own. \$\endgroup\$
    – PeterJ
    Nov 14, 2013 at 11:09
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    \$\begingroup\$ thanks, i made a 1/4 wave whip, and then a 1/2 dipole and it gets more range so im happy to keep experimenting buildng my own as well as ordering some to compare \$\endgroup\$
    – Hayden Thring
    Nov 14, 2013 at 22:13

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