433MHz Radio Links in Australia

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manuka

Senior Member
bpowell: Any chance of a few pix to better illustrate this?

Although indeed often a "cut & try" technology, simple vertical antenna at UHF indeed can be little more than a piece of wire cut ¼ wavelength long. One wavelength at 315 MHz is 3x10^8/315x10^6 so such a ¼ wave whip will be ~230-240mm long (~9 inches). The only real benefit of a tight helix is that the design will be more compact- hardly an issue in your case. If you have swampy signal spots perhaps try elevating the antenna (so LOS obstacles are cleared) or using a low angle radiator type (such as a Slim JIM) -or even a directional Yagi. Slower data rates will usually improve performance too- even try 300 bps. Stan (ZL2APS)
 
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bpowell

Senior Member
manuka said:
bpowell: Any chance of a few pix to better illustrate this?
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Sure, here you go.

Sorry for the quality, couldn't find a good angle to photo the receiver at. I put the home-made 1/2 wave length wrap antenna next to the project board for reference...currently I have the 1/4 wave length loop antenna installed, and working great. But I'd like a more compact design that works as well.

The transmitter has a duplicate of the wrapped antenna installed.
 

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westaust55

Moderator
bpowell said:
Hi Teddy,

I've set up a 315 mhz link that is pretty effective at about 30 meters and through two buildings...however, I'm finding a couple of "dead spots" in my house where I can't receive data, so I'm looking to remedy that. I'll be watching this thread.

The two antenna I use now are just 1/2 wave antenna (17.8 inches) made from hook-up wire, and then wrapped around a screwdriver...once tightly wrapped, I put silicone sealant around the whole job, let it dry, and then slid the antenna off and soldered one end to the TX and one to the RX.

This has worked to a point, but I have a feeling this is dumb luck. Just yesterday, I started experimenting with a better receiver antenna, and I got better performance with a 1/4 wave (8.9 inches) piece of hook-up wire formed into a loop and taped...so one end is soldered to the RX, the other end just loops around and is taped in place (no electrical connection)...that's cleared up one of the dead-spots in my house, but not the other.

I'm trying to learn how to optimize but it's complex stuff. I also think I might need to go out to the pump house (when it warms up) and swap out the helical wrapped antenna for a straight whip. I wonder if my wrapped antenna is actually not as efficient as a straight piece of wire (at the TX side).

I welcome any feedback!

FWIW, my current setup (with two wrapped antenna) has been shooting lets see....12 bytes every 15 seconds at 1200 baud with no problems!
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@Brendan,

consider adding a groundplane to your quarter wavelength monopole/whip type antenna.

While the usual theory is that a circular metal disk with a radius of a quarter wavelength is best, some research and reading of technical articles and books on monopole (whip) antennae suggest that a small metal plate still has a benefit.

Seems at 433 MHz, a metal plate of around 50 mm x 50 mm will provide a signal that is only 3 dB less than the quarter wave length sized disk.

If you look at post 23(?) my pics have a metal plate of approx 50 x 80mm dimensions.

You could even consider a half wavelength whip as see how that performs.
 
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bpowell

Senior Member
westaust55 said:
@Brendan,

consider adding a groundplane to your quarter wavelength monopole/whip type antenna.

While the usual theory is that a circular metal disk with a radius of a quarter wavelength is best, some research and reading of technical articles and books on monopole (whip) antennae suggest that a small metal plate still has a benefit.

Seems at 433 MHz, a metal plate of around 50 mm x 50 mm will provide a signal that is only 3 dB less than the quarter wave length sized disk.

If you look at post 23(?) my pics have a metal plate of approx 50 x 80mm dimensions.

You could even consider a half wavelength whip as see how that performs.
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I'll try to scare up some metal to try a groud plane...that might improve things.

I tried a half wavelength whip (no ground plane, just the hook-up wire) and even a full wavelength whip, the 1/4 loop still works better...it's bizarre, but it's a great learning expierence too!

Now, where to scare up some metal...maybe an Altoids tin lid? Hmmm...

Thanks!
 
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westaust55

Moderator
even an old cake or biscuit tin can be pressed into service for a test (but don't scare them :eek: )

If any distance from the Tx or Rx module you should be using some coaxial cable from the Tx/Rx moduel to the antenna otherwise even the wire length to the whip forms part of the antenna. Outer braid connects to the 0V and thegroundplane
 
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manuka

Senior Member

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westaust55

Moderator
Hi manuka,

So I presume that this is this the famous "contanga" antenna which you have mentioned in the past
 
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moxhamj

New Member
Amazing. Antenna design truly is a black art.

I once thought I understood physics. My school told me I was quite good at it. So did my University. I thought photons were tiny things that interfere with each other in double slit experiments and are both particles and waves. But a 433Mhz photon is a strange beast indeed. It is not tiny - you can hold one in your hand and 1/4 of it is 17cm or so. What are these things? And what amazing things can the radio hams persuade these things to do? With a coathanger, of all things?
 
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manuka

Senior Member
Not quite - the so called "Cotanga" (as coat hanger wire derived),is a normal 4 element Yagi beam => http://www.picaxe.orconhosting.net.nz/yagi433.jpg.

Check measurements & you'll see the Moxon rectangle is essential just a 2 element Yagi with the ends folded around.If you think this is bizarre consider the Japanese designed Hentenna (Japanese for weird).

Antenna design is indeed something of a black art. As a teen I well recall running a random length (~200 meter) fencing wire to a farm pine tree,and finding it rocked the socks off a carefully assembled resonant length 80 metre ham band dipole. Stan. (ZL2APS)
 

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Dippy

Moderator
I'm sure you are right, but antenna design is maths and physics really.
A very nice man once thought it was a black art until he was given the physics and maths behind the antenna design.

There may be some weird geometries that work nicely, but a clever bloke could describe the performance mathematically given time and inclination.

Sadly, the arts of Maths and Physics are not so popular with the man on the street ... and once you remove the science then you have to insert magic or religion to explain the workings.
But I'll totally agree with you if you have other views.
 
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BeanieBots

Moderator
Dippy said:
There may be some weird geometries that work nicely, but a clever bloke could describe the performance mathematically given time and inclination..
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Most aerial designs are fairly simple in so far as they relate to a function of the Tx wavelength such as 1/2 wave dipole or 1/4 wave whip.
Could somebody please explain how &/or why a 5/8ths whip works so well:confused: For decades that one has bugged me:mad:
 
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John West

Senior Member
I prefer to use a full half-wave dipole most of the time, meaning wherever it will reasonably fit. No sense using a quarter-wave radiator (and wondering if the circuitry it's attached to has enough capacitance to work off of it,) and tossing away 3dB. Of course, wherever there's room and the RF link is between two fixed points, I'm all in favor of coat-hanger Yagis (Yagi-Yuda's.)

It's a lot easier to use more antenna elements than it is to squeeze more power from the battery.
 
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manuka

Senior Member
It's a lot easier to use more antenna elements than it is to squeeze more power from the battery.
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It depends on the setup and intended coverage of course. Although superior antenna indeed appeal they may be too directional & costly, pick up excessive noise or be bulky & inconvenient- almost all cell phones now use internal antenna for the latter reasons.

IMHO at 433 MHz it's often better to use more sensitive receivers and more energy efficient modulation techniques (GFSK espec.), plus smart narrow band filtering. At lower freqs classic SSB (Single Side Band with supressed carrier- long popular with hams & marineers) gives a 9dB system gain over a plain AM link. Stan (ZL2APS)
 
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william47316

New Member
i havnt delved into the HF wireless stuff much but i have made some modules which work on my 8 digit LED displays, it'll take 8 bytes and with a simple protocol and checksum it will send it over those simple bare bones wireless units using a picaxe 08m as its brain and it turns out to be reasonably reliable and can notify when the data is getting flakey

the transmitter can take 24 or 4800 baud and transmits at the 433Mhz at 4800 baud with 50% header the 8 byte data + a check value which is the average of the data, which works ok for major data flakes but sometimes the minor errors get through at the receiver

Code: 
setint 00000100, 00000100
symbol checksum = b9
let dirs = 00010011
let pins = 00000000
pause 250

setfreq m8

let pins = 00010010


TXloop:
setint 00000100, 00000100
if bit0 = 1 then
serin 3,n1200, b1,b2,b3,b4,b5,b6,b7,b8
end if

if bit0 = 0 then
serin 3,n2400, b1,b2,b3,b4,b5,b6,b7,b8
end if

'calculate a crude checksum
w5 = b1 + b2 +b3 +b4+b5+b6+b7+b8
checksum = w5 / 8 'set a byte variable to the average value of the data input for checksum

serout 0,n2400, (170,170,170,170,170,85,170,b1,b2,b3,b4,b5,b6,b7,b8,checksum) 'broadcast a quailifier to prep the receiver and burp the bytes out
pulsout 4,5000 'pulse out a confirmation LED to say its finished

goto txloop

interrupt:
if input2 = 0 then

if bit0 = 0 then
bit0 = 1
LOW 1
goto exitsub
end if

if bit0 = 1 then
bit0 = 0
high 1
goto exitsub
end if
end if
goto interrupt

exitsub:
'goto interrupt

return
the receiver end is a bit simpler as it waits until it receives at 3 bytes of the 50% duty data and the other 9 bytes of whatever be it actual data or noise from the ether then it gives it a bit of math to figure out if the data is ok and handles it, if its ok it sends the 8 bytes on its way if its a bad egg it ignores it and records it as a "bad packet" get 20 and it sends the message and a bell character to the display, originally i had it for every bad packet but it got a little annoying (beeping) so i made it reduce the responsiveness to errors a little
Code: 
setfreq m8
symbol checksum = b9
symbol loccheck = b12
symbol baddata = b13
high 4
mainloop:

serin 3,n2400, (170,85,170),b1,b2,b3,b4,b5,b6,b7,b8,checksum
w5 = b1 + b2 +b3 +b4+b5+b6+b7+b8
loccheck = w5 / 8 'set a byte variable to the average value of the data input for checksum
if loccheck = checksum then
serout 0,n2400, (b1,b2,b3,b4,b5,b6,b7,b8)
pulsout 4,5000
end if
if loccheck <> checksum then
inc baddata
if baddata >= 20 then
serout 0,n2400, ("BAD",7,"DATA")
baddata = 0
end if
end if
goto mainloop
 
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westaust55

Moderator
Not knocking anyone, but for completeness of information for those who read this thread in the future, the use of $55 or “U” or 85decimal (all three are one and the same value expressed in different ways) is the preferred and technically more correct value for the preamble as originally highlighted by womai here and used by myself in post 35 of this thread.

hippy has provided a very good explanation as to why $55 is recommended value for a preamble to RF comms at post 14 here.
 
hippy

hippy

Ex-Staff (retired)
To add to what westaust55 says - a lot of effort was put in by many people in getting SEROUT/SERIN wireless communications working and working well, establishing the basic principles and mechanisms for reliable solutions. As a result of that community effort we have a very good reference design for wireless comms which it is recommended to follow.

It's not so much dictating that it must be done 'this way' but that 'this way' solves many of the flaws and problems that doing it some other way often introduces - problems which may not be realised by those new to wireless.

When there's a perfect round wheel, lovingly created for everyone to use freely, it doesn't really make a lot of sense to craft your own square wheel and then find it doesn't perform as well as expected :)
 
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