Antenna Impedance of RF Modules

[amirahmadian]

Hi dear friends.
I've purchased a RF ASK transmitter module and an ASK receiver module in order to build a remote control system. They are WXC810 and PCR1A chinese modules that work at 315MHz frequency.
The problem is that the datasheet of module doesn't say anything about the impedance of the antenna that I shoud use (Or maybe I haven't found a complete datasheet). I have this problem with both transmitter and receiver.
Now is there a way for me to calculate the suitable impedance for the antenna (for transmitter and receiver)? Or is there a general standard for that?
After all, is it really necessary to know the right antenna impedance? Or maybe my system will work fine even if I choose an arbitrary antenna?

 

[vedagarbhamsandeep]

you will have to use 50ohms or 75ohms impedance though 50ohms is standard for most applications. What connector is present in you module ? .. this will tell you the impedance of the device / module.

 

[chuckey]

iIf you use an arbitrary length of wire you will lose aerial gain so the range will not be as great. However, whatever aerial you decide to use it can be matched the the 50 ohm input/output impedance so you compromise on the aerial gain. Most UHF "short"aerials are 250mm of wire loosely coiled, say 20 turns on a 3mm diameter.
Frank

 

[amirahmadian]

you will have to use 50ohms or 75ohms impedance though 50ohms is standard for most applications.

Is that a standard for both "transmitter and receiver" antenna impedance? Is it true for all ASK modules?

What connector is present in you module ?

The modules have an ANT pin that I connect the antenna directly to it. I don't see any special connectors. Or maybe I haven't got what you mean exactly by "connector".

 

[vedagarbhamsandeep]

Is that a standard for both "transmitter and receiver" antenna impedance? Is it true for all ASK modules?

The modules have an ANT pin that I connect the antenna directly to it. I don't see any special connectors. Or maybe I haven't got what you mean exactly by "connector".

How do you intend to connect the antenna to your ANT pin ?

Most antennas are designed for 50ohms impedance except for few parabola antennas with LNB (Low-Noise Block Converter) wherein it receives signal from satellites use 75ohms and also I suppose cable used is RG6 (75 ohms cable)

 

[amirahmadian]

How do you intend to connect the antenna to your ANT pin ?

Most antennas are designed for 50ohms impedance except for few parabola antennas

Please note that I'd like to make a simple antenna myself , I haven't purchased an antenna. I suppose I can make one with some wire,coil,etc. I is that a difficult job? Anyway, should I make an antenna with 50 ohms impedance?
Sorry because my poor knowledge of antenna theory!

 

[vedagarbhamsandeep]

yes, you can make it yourself. The simplest and cheapest antenna would be a lambda / 2 monopole using a co-axial cable.

 

[drkirkby]

Although your chip has only one output pin for an antenna, you can probably assume they mean you to connect the other conductor for the antenna to ground.

 

[amirahmadian]

Although your chip has only one output pin for an antenna, you can probably assume they mean you to connect the other conductor for the antenna to ground.

Thanks. But some simple antennas don't need the second conductor. For instance, monopole antenna. Am I right or not?

 

[drkirkby]

Thanks. But some simple antennas don't need the second conductor. For instance, monopole antenna. Am I right or not?

You are half-right. A monopole needs a groundplane - a piece of of metal, typically a half-wave or more across (ideally infinite in extent), mounted at right angles to the monopole. The signal should be connected between the monopole and the ground. In that instance the input impedance is about 37 + j 22 Ohms, though if you shorten the monopole, you can bring it to resonance.

In practice, in portable devices, one just has to make do with what one has. In this case, you are best to use the groundplane on the printed circuit board. You have no choice in this instance. But this sort of works, because however the RF is generated, it must be somehow connected to a positive and negative supply voltage.

Dave