RF antenna 433MHZ

[danrogers]

Hi all, I'm using some of these for a project **broken link removed**

I have two sets, one came with no antenna on the RX module and the other came with a coiled up piece of wire. I have tested them and the one with no antenna has a poor range. I made one to pretty much the same size as the module that came with one and it worked.

I have removed the telescopic antenna from one of the TX modules and replaced it with another antenna, the same as I made for the RX and it seems to work OK with a fair range.

My question is really how do I get the best out of these modules - would a bigger antenna (i.e. a longer bit of coiled wire) give a better range or is there a science to using the correct antennas?

Thanks in advance!

 

[Tony Stewart]

There is indeed science to antenna gain, but a KEYFOB is intended to be omnidirectional and thus gain is near unity with some dead spots so orientation can make a difference.
The telescopic antenna is probably best and has a null when pointing towards the Rx, so dont do that. any other orthogonal direction is ok.

 

[JimB]

My question is really how do I get the best out of these modules

Those radio modules are a compromise on so many levels, as has been discussed here on ETO many times, that it is pointless to get all worried about what is the best antenna.
The cynical part of me says "If it works, be happy".

It would be interesting to set up a test one day to see what is good and bad with antennas for those things.
But at the moment it is well down on my list of things to try just for the fun of it.

JimB

 

[Tony Stewart]

If you know how directional AM radios are, then you will understand not to point end of antenna towards Rx.
Diversity is a tradeoff between directional beamwidth and gain and make sure the battery is good.

 

[audioguru]

Does it use a simple one-transistor super-regen as its receiver?

 

[MikeMl]

Read my posts to this forum thread.

 

[danrogers]

they are certainly pretty cheaply made. They seem have 1 chip that does the encoding / decoding. So it may be a case of having the antenna on the rx as big as I can fit in?

 

[Nigel Goodwin]

So it may be a case of having the antenna on the rx as big as I can fit in?

You don't want to be too long

What you're basically aiming for is a quarterwave whip, which is 16.5cm for 433MHz.

But it's likely to be something short of that, depending what can fit in - and coiling it up greatly changes things - but it's really not critical for short range devices like these.

 

[audioguru]

It does not have a crystal but instead it has a trimmer capacitor with red nail polish on it.

 

[MikeMl]

You don't want to be too long

What you're basically aiming for is a quarterwave whip, which is 16.5cm for 433MHz....

No. The pcb ground-plane is much too small to act as a proper counterpoise for a 1/4λ antenna. If all you do is stretch out the 1/4λ (originally coiled) wire so that it is straight, you are leaving many db of potential range on the table. A 1/4λ monopole antenna works only if it is operated against a conducting ground plane orthogonal to it. That ground plane must extend a minimum of 1/4λ in all directions away from the base of the monopole.

To increase the range, the antenna must be a half-wave dipole where the receiver/transmitter is connected at the center of the dipole, or a monopole operated against a ground-plane as described above. To create a dipole, the coiled wire becomes the driven half of the dipole (1/4λ), while you need to solder a second 1/4λ wire to the ground foil on the pcb and stretch it out in the opposite direction to create the 1/2λ length. (Think TV rabbit-ears). I calculate that the proper length for a 434MHz dipole is 28cm (1/4λ = 14cm).

Another hazard is cross-polarization. If the transmitter antenna is placed such that its plane is at 90 degrees to the plane of the receiver antenna (cross-polarized), you also lose 20db.

 

[danrogers]

Very interesting, cheers guys.

So a 1/4 dipole would need a 28cm antenna with the connection point in the middle? Is coiling OK in that case?

Good point about the cross polarization - I will bear that in mind

 

[MikeMl]

...So a 1/4 dipole would need a 28cm antenna with the connection point in the middle?

That is a 1/2 wavelength dipole.

Is coiling OK in that case?

No. Fore-shortening an antenna always reduces its performance. To get the full performance out of an antenna, its "aperture" (length) must be ~0.45*3e8/freq (meters).

 

[Nigel Goodwin]

No. The pcb ground-plane is much too small to act as a proper counterpoise for a 1/4λ antenna.

We're not talking about long range aerials here, while it's obviously not optimum without a ground plane a 1/4 wave aerial will far outperform whatever is there all ready

You could apply the same issue to hand held transceivers and walkie talkies (and cell phones for that matter), all of which have insufficient ground plane issues.

 

[MikeMl]

...
You could apply the same issue to hand held transceivers and walkie talkies (and cell phones for that matter), all of which have insufficient ground plane issues.

Yes, they are a compromise. All of my VHF and UHF walkie talkies ( I have several) make a ground connection to the metallic belt clip on the back of the transceiver. That way, there is several square cm of metal in contact with the palm of your hand such that your arm/body becomes the missing 1/4λ counterpoise. The 433MHz key-fob transmitter makers (and some cell-phone makers) could learn something from that trick.

Back to the 315/433MHz ISM modules: folks that buy these come here to ask how to extend the range. It is really simple; put proper antennas on them. Works for me. I am using those same (315MHz) EBay modules to open my garage doors and my motorized swing gate. The secret to getting more than 300m (1000ft) range in open air is the receive antenna. I am using a full-length 1/2λ dipole mounted up high on the outside of my metal aircraft hangar.

 

[Nigel Goodwin]

folks that buy these come here to ask how to extend the range. It is really simple; put proper antennas on them.

I would completely agree

But do you really need to open your garage door from over 1000ft away?

 

[MikeMl]

Actually, not the garage doors, but the drive-through swing-gate. The actuator I am using takes about 30sec to move the gate from closed to open, so if I send the command from about 1000ft away, the gate is open by the time I drive up to it...

 

[Nigel Goodwin]

Nice, but have you considered a lick of paint?

Looks a pretty big place though - and a bit dry?.

 

[MikeMl]

Nice, but have you considered a lick of paint?

Looks a pretty big place though - and a bit dry?.

The style here is to let pipe rail fences just turn rusty. In northern Az, it doesn't rain much; about 20in per year. I am located not too far from Sedona and Prescott.

 

[Nigel Goodwin]

I am located not too far from Sedona and Prescott.

You say that like I should have heard of them?

I'm off to google maps!

Edit:

Found it on google maps, bit brown round there isn't it?.

Zoomed out until I found somewhere I'd heard of, Phoenix - mind you, I've only heard of it - no idea where it might be

Zoomed out a bit further (to 20km per division), spotted more places I know now (including the Pacific) - so I understand better now roughly where you are.

 

[MikeMl]

You say that like I should have heard of them?

I'm off to google maps!

Try search key "Airport AZ27" in the Google Maps search bar. You may have to get to Arizona first...