# Designing a simple monopole antenna that operates at 433MHz

#### KaiL

Joined Aug 30, 2014
69
This is the circuit model that I have designed using the ANSYS HFSS software
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Now I would like to implement it but I am not really sure how to do it.
2 things that come to my mind is I need a straight metal rod and a ground plate.

The length of the metal rod should be around 173.2mm since I want it to operate it at 433MHz but how big or small should the ground plate be?

Using the software HFSS , when the length of the conductor is 173.2mm and a 340x340mm ground. The Result is not optimal.

After I have changed the length of the conductor , the reflection coefficient has improved from -9 to -15 db. The following is the result

So now I want to implement it in real life . I am not really sure how. Could anyone shred some lights for me?

#### BR-549

Joined Sep 22, 2013
4,938
You do the same thing with a real antenna. Start with a conductor a little longer than resonance. Watch SWR as you trim antenna into resonance.

#### SLK001

Joined Nov 29, 2011
1,548
Start with the type of connector that you want to use. Let's say BNC. Select a connector like this:

Now select the ground plane that you want to use. Let's say you are using a double sided PCB blank. Let's make the ground plane 12" in diameter (why did you use a square ground plane in HFSS?) Drill a hole for the screw part of the connector in the center of the PCB material large enough so that the BNC fits in the hole. Attach the BNC to your ground plane and tighten securely. Now go down to the local hobby shop and buy a copper tube with an inner diameter that will fit over the pin on the BNC connector (K&S Engineering has a variety of such tubing). If you can't find copper, then brass will also work.

Cut the acquired tube long and solder it to the BNC pin. Attach your signal to your antenna and trim the tubing to best VSWR for your desired frequency.

Done!

P.S.: A square ground plane will also work fine.

Joined Nov 5, 2010
215
What are you connecting this to? Some commercial transmitters are inductively pre-loaded which would need to be taken into account in your theoretical antenna design. But the trim-and-check SWR technique is the best.

When you're testing/trimming it, make sure you have the exact feedwire (I assume some kind of coax connecting the transmitter to the antenna) you plan to use. This will affect the impedance matching and reflections and may change the resonance of the antenna.

#### SLK001

Joined Nov 29, 2011
1,548
What are you connecting this to? Some commercial transmitters are inductively pre-loaded which would need to be taken into account in your theoretical antenna design.
No inductive loading necessary at the OP's frequency of interest. Inductive loading is for lowering the resonant freq of an antenna (¼λ - ½λ), and mostly used when the λ is in the tens of feet range or more (< 30 MHz).

Joined Nov 5, 2010
215
No inductive loading necessary at the OP's frequency of interest. Inductive loading is for lowering the resonant freq of an antenna (¼λ - ½λ), and mostly used when the λ is in the tens of feet range or more (< 30 MHz).
I agree completely. It wouldn't be necessary at this frequency. However, in his initial experiments, the antenna resonated at a lower frequency than its ¼λ length should have.

And it's possible, though not necessary, that the manufacturer of the module intended it to be used in a space too limited for a full 17.3cm antenna, and did something on the PCB to allow for a shorter resonant antenna (albeit at a loss of signal).

And in my perusal of 433MHz antennas, I ran across loading for decreased space:

In addition, I've designed several consumer products that used 433MHz in too-small a space for 17.3cm monopoles. We had to use various techniques to get them to resonate at a shorter antenna length.

#### KaiL

Joined Aug 30, 2014
69
Hi guys

Thanks for the guide. I can't really find any PCB board (In my area) so I use Aluminium plate for the ground (20 by 20 inches) with a brass rod of 165mm.

Currently , it looks something like this. This picture below is taken online and my design looks like this picture except that the plate is aluminum

I would to check now if I need a balun for my monopole antenna ? I have some friends that say need it while some said don't need

#### SLK001

Joined Nov 29, 2011
1,548
No balun is necessary. However, the attachment of the connector to the plate could use some improvement. Use a countersink and countersink the four holes, then use flat head screws from the top and put the nuts on the bottom. In your application, it probably won't matter much, but each nut on the plane shown above will degrade the signal.

#### Lestraveled

Joined May 19, 2014
1,946
The simple mono pole antenna you describe has been used for a long time. The flat ground plane antenna has a minor draw back in that the antennas impedance is about 35 ohms. Most RF cables and systems are 50 ohms. Also a "solid" ground plane is not needed. Radial wires are as effective. Look at the below drawing from the ARRL. It is simpler, cheaper and it has an impedance of 50 ohms. Use your dimensions. the radials should be angled down about 45 degrees.

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#### KaiL

Joined Aug 30, 2014
69
No balun is necessary. However, the attachment of the connector to the plate could use some improvement. Use a countersink and countersink the four holes, then use flat head screws from the top and put the nuts on the bottom. In your application, it probably won't matter much, but each nut on the plane shown above will degrade the signal.
Can I have some explanation why balun is not needed ?

I only know that balun is a type of transformer that convert balance to unbalance sign or vice versa.

#### SLK001

Joined Nov 29, 2011
1,548
Can I have some explanation why balun is not needed ?
What was the impedance of your excitation?

Look at your second plot. What VSWR do you get with an S11= -15dB?

Also, you are not using a balanced system anywhere, so why do you think that you need a balun? You probably mean "matching transformer".