Custom USB tyre pressure monitoring interface - Page 57

WuNgUn · 02-07-2009, 05:58 PM

Piece of wire, yes...but I'm thinking the diameter/length of the 'ring' and the thickness of the wire is a maybe a multiple or divisor of the wave length?

mrbean_phillip · 02-07-2009, 10:17 PM

Erm, no.

The length of the "antenna" is what matters, in this case.

mrbean_phillip · 02-08-2009, 06:38 AM

Hello Guys,
Ok, I have used an old WiFi booster antenna, and desoldered the original antenna-core, and used a 17.27cm (approx 6.80") coax core to make a new 433.92Mhz antenna.

I have used an original sma connector with a piece of thin coax, threaded that to the Alok-casing, and soldered the center, and screen to the original wire-antenna center, and pcb earth.

Now I have a decent rotateable, adjustable, and very neat-looking antenna for my TPMS.

I took the UTE for a drive, and it receives very well in the cabin.

I will take a few pics tomorrow eve (of the antenna) and host them on my photobucket account.

WuNgUn · 02-08-2009, 08:53 AM

I think I have a couple of old router Wi-Fi antenna's laying around...
Would these be suitable?

f1racr · 02-08-2009, 01:23 PM

That's exactly what I did............ works a treat.....

Unmodified 2.4ghz Wifi antenna's don't seem any better than the original internal antenna, and in most cases as you'd expect, worse.

You'll need to remove the original core from the 2.4Ghz antenna.

I'll look forward to seeing your pictures to see if there's anything I can do to make mine any more optimal as I was just winging it really more than knowing what I was doing.

WuNgUn · 02-08-2009, 01:38 PM

So if I can pull the 'guts' out of the antenna, it's better? We're talking about the little rubber, 4.5" antennas here, right?

f1racr · 02-08-2009, 01:45 PM

Well it's going to have to be around 6.5 inches as discussed above.

WuNgUn · 02-08-2009, 01:51 PM

Okay, so a standard Wi_Fi antenna is too short...
I'll have to rip apart the attic...lol
I'm sure I have a 5dB gain antenna up there somewhere (which is a little longer)

Blues · 02-08-2009, 03:31 PM

Quote: Originally Posted by Blues

If you can use another brand than this Antennafoactor / Linx Technologies, then look here at Farnell Australia and RS Components Australia:
http://au.farnell.com/jsp/search/bro...na+whip+433MHz
http://australia.rs-online.com/web/s...antenna+433mhz
Both Farnell and RS Components has locations almost worldwide including Australia.

Farnell offers 433MHz 1/4-wave whip antennas from RF Solutions: http://www.rfsolutions.co.uk/acatalo...p_Antenna.html
You can also by these antennas online diretly from RF Solutions loacted in the UK.

RS Componnets offer 433MHz 1/4-wave whip antennas from Centurion/Laird Technologies (Centurion Wireless Technologies is now Laird Technologies): http://www.lairdtech.com/pages/produ...o-Overview.asp
Old Centurion website: http://www.centurion.com

Another Australian suppiler of 433 MHz antennas in RF Modules Australia: http://www.rfmodules.com.au/misc/ant.htm

Blues · 02-08-2009, 04:20 PM

Quote: Originally Posted by Blues

Why 6.5 inch? Shouldn't it be 6.8 inch (17.2cm) for a 433.92 MHz 1/4-wavelenght antenna?
http://www.onlineconversion.com/freq...wavelength.htm

I think I know why you use 6.5 inch rather than 6.8 inch.
6.8 inch is of course theoretically the optimal length, but in pratice the optimal length of antennas is slightly shorter than the theoretical optimal length, "to account for the difference in the velocity of wave propagation in wire as opposed to the same wave in free space", to quote Wikipedia. I had forgotten about that for a moment.

http://en.wikipedia.org/wiki/Dipole_...haracteristics

Quote:

A half-wave dipole is cut to length according to the formula

[ft], where l is the length in feet and f is the center frequency in MHz [1]. This is because the impedance of the dipole is resistive pure at about this length. The metric formula is

[m], where l is the length in meters. The length of the dipole antenna is about 95% of half a wavelength at the speed of light in free space.

The magic numbers above are derived from a one Hz wavelength which is the distance that light radio travels in one second. For English that is 186,282 miles times 5280 feet per mile. To convert to metric multiply the previous total by 12 inches per foot and then, by definition, multiply that by 2.54 cm per inch. Divide this number by 100 to convert this length to meters. Then divide the result by one million to account for MHz rather than hertz. This will give a number which must be divided by two for a dipole antenna. To correct for resistance and impedance multiply the dipole wavelength by about 95% to account for the difference in the velocity of wave propagation in wire as opposed to the same wave in free space. If the wire velocity is known, that value should be used to get the magic numbers of 468 feet or 142.65 metric. All that is left is to divide by the desired frequency as measured in MHz to obtain the length of the antenna element.

For a quater-wave antenna I guess the optimal length must be half of this.

468 / 2 / 433.92 * 12 = 6.47 inch
or
142.65 / 2 / 433.92 = 0.164 m

I also found an online calculator to calculate the pratical rahter than the theoretically length for quater-wave antennas and it gives the same result (16.4 cm or 6.47 inch): http://www.csgnetwork.com/antennagpcalc.html
This website use the formula l = 71.3232 / f ;where l is the length in meters

mrbean_phillip · 02-08-2009, 04:28 PM

@ Wungun: I used a WiFi booster antenna, but it was still to short by a couple of cm's - all I did was to make a u-bend in the conductor, and making sure it doesn't touch itself anywhere - whew, that sounds ambiguous.....

I just used heatshrink-tubing to prevent it short-circuiting

So, make sure your conductor is around 17cm long, and use shielded coax for the short run from the pcb to the antenna connection, if you don't mount the antenna directly to the pcb - which I didn't - I used a sma connector.

edit: Currently sititng in my house, closed doors, and approximately 10m away from my UTE, and the TPMS refreshes data from the tyre-sensors - have it connected to my laptop

So, +1 for the home-made antenna jobbie.....to be fair, I never tested Alok's antenna at this distance, and I am sure it will yield approximately the same results.....

mrbean_phillip · 02-08-2009, 05:26 PM

@ Blues: Yep, correct, that is the "shortening-factor" I referred to earlier, without going into the theory - I was to lazy

But, taking my calcualtions of 172.7mm, and multiplying by 95% gives you exactly the 164mm your formula does.

I used 172mm, it won't play a big role in our application, it will only mean the 164mm length antenna will be tuned slightly better - i.e, maximizing reception-distance.

But, good info though, not all for nought!

Lok · 02-09-2009, 10:36 AM

Let me know when you guys have found something that can be sourced. I will place an SMT connector on the board so that we can use a rigid antenna on the next run, I think it will look much more professional and it should yield better results. MrBean, I like the fact that you can use your laptop from your home and still get reception, this is especially useful for the next application I have for the product, the Pits

. Believe it or not, a lot of car enthusiasts who spend time on the track on the weekends will love this device especially once I get the logging working correctly. I am currently testing this out on my buddy's NSX and so far so good. Keep up the good work guys!

Robby BMW · 02-09-2009, 12:25 PM

Hi guys, I read with interest the last part of this topic.
Using an external antenna can be useful, especially in those cases where the car body acts as a shield, but also an excessive gain may cause some problems.
During the last test I made with the receiver produced by Lok (which seems to be more sensitive than the original 3rd eye), I noticed that if the distance between the sensors and the receiver is less then 1.5 - 2 meters, the receiver goes into saturation and it can't properly receive the data.
So, be careful with the external antennas, my advice is to use them only if you really have problems with the internal antenna.

Lok · 02-09-2009, 01:06 PM

Hmm, that explains why sometimes my rear tires have quicker refresh times then my front tires as my reciever might be sitting too close to the front tires. I guess we will have to do some testing to find out what is the optimal range and placement for the reciever. From what I can put together it looks like it should be at least 1.5-2 meters away from the sensors and will work as far out as 10 meters.

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