Author Topic: 433Mhz Antenna tuning  (Read 14431 times)

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Offline trevwhite

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433Mhz Antenna tuning
« on: June 10, 2012, 08:47:46 am »
Hi all.

I have a pcb I made that uses a 433Mhz transmitter. For the aerial I used a pcb track of approx. 165mm in length. I have read that you need to tune these sometimes to get the best distance. I do not know how to measure how well the antenna is working and how to tune them up. Can anyone offer advice, links, information on how to go about this?

Thanks

Trev
 

Offline G7PSK

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Re: 433Mhz Antenna tuning
« Reply #1 on: June 10, 2012, 10:36:12 am »
Seems a little short to me for a quarter wave 300/433 =0.69284 Meter wave length a quarter of that is 0.1732.
So you may need to make it longer for better results, I have found that cutting to calculated lengths for aerials normally gives very good results straight off for a fixed frequency.
 

Offline trevwhite

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Re: 433Mhz Antenna tuning
« Reply #2 on: June 10, 2012, 11:22:14 am »
Hi.

I read that for a pcb antenna the length needs to be reduced by 10 - 20%.

What I really would like to do is set up some test to measure the change in transmission power w.r.t. change in antenna length. I cant be walking 100m to test this every time I trim the length. I was hoping there is some test equipment that would give me repeatable measurements.

I do not know a lot about RF so am out of my depth at present.

Thanks

Trev
 

Offline G7PSK

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Re: 433Mhz Antenna tuning
« Reply #3 on: June 10, 2012, 11:45:39 am »
You need to introduce a meter to read the forward and reflected power in the system. This link explains it very well and also gives a circuit for making the measurements. In fact it gives you just about every thing you need to know.

 http://ludens.cl/Electron/swr/swr.html
 

Offline kripton2035

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Re: 433Mhz Antenna tuning
« Reply #4 on: June 10, 2012, 11:53:05 am »
this is a nice device to have when playing with 433 mhz devices ... (or other frequencies...)
cheap and efficient ...
see other products of same manufacturer for other frequencies : 868, 915 or 2400MHz or combo

http://www.seeedstudio.com/depot/rf-explorer-433m-p-784.html?cPath=174

Offline Fraser

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Re: 433Mhz Antenna tuning
« Reply #5 on: June 10, 2012, 11:58:04 am »

The expression "Tune for maximum smoke" comes to mind  :)

Its an old Rf techie term but you can achieve maximum RF output by using a simple diode detector circuit, a pick-up antenna and a multimeter or analogue meter. You basically need to build a field strength meter that is just a variation on a crystal radio design.

If you have a PCB antenna and/or unknown feed impedance to the antenna, SWR meters are a little difficult to impliment.

Take a look here:

http://users.otenet.gr/~meteo/project_rf-field-strength-meter.html

http://www.zen22142.zen.co.uk/Circuits/rf/sfsm.htm

Just do a google search on "simple field strength meter"


Make the transmit aerial too long and then cut a little off at a time whilst watching the meter reading. You should see a slight increase in meter reading as resonance is approached (at a test distance of 30 cm or so). When the reading peaks and then starts to drop with the next small cut off of the antenna, you have passed the peak tuning point of the antenna circuit but it will be close enough for your needs.

Aurora
 

Offline Fraser

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Re: 433Mhz Antenna tuning
« Reply #6 on: June 10, 2012, 12:03:10 pm »
Kripton2035,

Thanks for the link. I saw this unit at a much higher price in the UK (GBP149) and wondered where it came from. I think I will be adding one to my inventory  ;D

Aurora
 

Offline kripton2035

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Re: 433Mhz Antenna tuning
« Reply #7 on: June 10, 2012, 12:14:23 pm »
Aurora,

there are seeedstudio distributors in UK ; may be their price is competitive - think about VAT you will add to a china import ...
http://www.seeedstudio.com/depot/page.html?id=2

also this unit is from $99 to $199 from one frequency to all frequency.
I have one for some months and it is a nice device ...
I bought mine in germany ...
regards,

Offline Fraser

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Re: 433Mhz Antenna tuning
« Reply #8 on: June 10, 2012, 12:26:33 pm »
Hiya,

Many thanks,

I just ordered the 'full Monty' version ISM Combo edition from the UK supplier (Cool Components) ;D

Price GBP139 inc VAT and delivered .... not bad considering what it can do and the build standard. Sadly it's out of stock at the moment but will be sent as soon as new stock arrives.

Thanks again for the information... I'm a very happy bunny !

For others...read all about it here:  http://micro.arocholl.com/


UPDATE:

I did a quick search for the RF Explorer on ebay UK and as luck would have it someone was selling an 'as-new' 2.4GHz unit for GBP65  :) I bought it and then ordered the ISM wideband PCB upgrade from Seeed Studio for $55 (GBP37) delivered  :)  Happy Days. I will have the RF Explorer this week and the upgrade board for ISM frequencies below 1GHz in a week or so.

Order with Cool Components has been cancelled as there is no indication of when new stock will arrive.

Aurora
« Last Edit: June 10, 2012, 01:41:59 pm by Aurora »
 

Offline trevwhite

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Re: 433Mhz Antenna tuning
« Reply #9 on: June 10, 2012, 04:31:06 pm »
Well I have a feeling that the RF Explorer will do what I need. Brilliant, thanks. Anyone know how accurate this unit is?

Thanks all.

Trev
 

Offline trevwhite

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Re: 433Mhz Antenna tuning
« Reply #10 on: June 10, 2012, 07:53:14 pm »
Reading through the below link on the RF Explorer it states that the resolution is 0.5dbm? I do not quite understand what this means? Can anyone explain how well the unit can measure the signal strength?

http://micro.arocholl.com/index.php?option=com_content&view=article&id=71:accuracy-precision-and-resolution&catid=42:faq&Itemid=64

Thanks

Trev
 

Offline Fraser

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Re: 433Mhz Antenna tuning
« Reply #11 on: June 10, 2012, 08:18:13 pm »
Here is a dBm to other unit conversion chart for you to look at:

http://217.34.103.131/pages/pdfs/dg03-110.pdf


I attach a dBm to mW chart for your information.

To fully understand dBm I think it best to Google it as there are some excellent explanations of this Logarithmic unit of measure on the Net.

http://www.maxim-ic.com/app-notes/index.mvp/id/808

http://www.radio-electronics.com/info/formulae/decibels/dBm_dBW_table.php

http://www.moonblinkwifi.com/dbm_to_watt_conversion.cfm

http://www.rapidtables.com/convert/power/dBm_to_mW.htm

http://ifmaxp1.ifm.uni-hamburg.de/DBM.shtml

Aurora

« Last Edit: June 10, 2012, 08:27:40 pm by Aurora »
 

Offline kripton2035

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Re: 433Mhz Antenna tuning
« Reply #12 on: June 10, 2012, 08:50:47 pm »
@Aurora:
too bad for you, another expansion board is coming for the rf explorer ...
pretty sure you'll buy it !

http://micro.arocholl.com/index.php?option=com_content&view=article&id=84:an-update-on-rf-explorer-wsub3g&catid=3:newsflash&Itemid=50

Offline trevwhite

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Re: 433Mhz Antenna tuning
« Reply #13 on: June 10, 2012, 08:59:03 pm »
Thanks for the links. What I meant to say is that I thought the resolution was the smallest step of measurement. So a resolution of 0.5dbm seems a lot. I think I am not understanding the parameter. I understand dbm itself. I was just wondering how far out the measurements might be. 
 

Offline Fraser

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Re: 433Mhz Antenna tuning
« Reply #14 on: June 10, 2012, 09:01:31 pm »
Hiya,

I saw that news on the new upgrade board, but to be honest this is just a quick use 'gadget' for me. I do have some 'real' spectrum analysers that cover 5kHz to 3.6GHz for serious work. The ISM bands are most likely to be where I will need a portable unit, and at $140+VAT (GBP112)  the new board isn't exactly cheap, and we will have to see how it performs over such a large range.

Nope, for me the ISM Combo spec for just over GBP100 'all-in' will do nicely  :)

Aurora
« Last Edit: June 10, 2012, 09:03:31 pm by Aurora »
 

Offline Fraser

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Re: 433Mhz Antenna tuning
« Reply #15 on: June 10, 2012, 09:15:19 pm »
Trevwhite.

In the tables you will see that a 0.5dBm change in amplitude equates to 0.12mW.
Aurora
 

Offline trevwhite

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Re: 433Mhz Antenna tuning
« Reply #16 on: June 10, 2012, 09:21:42 pm »
Yes, doesnt that seem quite a lot for a measurement? Anything below 0.12mW might actually be zero?

 

Offline Rufus

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Re: 433Mhz Antenna tuning
« Reply #17 on: June 10, 2012, 09:51:04 pm »
Yes, doesnt that seem quite a lot for a measurement? Anything below 0.12mW might actually be zero?

The scale is logarithmic. 0.5db resolution means there are 20 divisions between each * 10 change in power.

The difference between 0dbm and 0.5dbm is 0.12mW, the difference between -90dbm and -90.5dbm is 0.12pW.
 

Offline trevwhite

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Re: 433Mhz Antenna tuning
« Reply #18 on: June 10, 2012, 09:54:08 pm »
That is great. Thanks Rufus.

Trev
 

Offline Fraser

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Re: 433Mhz Antenna tuning
« Reply #19 on: June 10, 2012, 09:57:23 pm »
Trevwhite,

In answer to your question...

Nope that's not how dBm works as we are talking about a Log ratio measurement. i.e.

A change in signal power from -10dBm to -10.5dBm is 0.0109 mW
A change in signal power from -17dBm to -17.5dBm is 0.0022 mW

Now the scary figures

A change in signal power from -50dBm to -50.5dBm is 0.0000010875mW  :o

Remember the unit goes all the way down to -115dBm so it is very sensitive as well.

This even had me confused for a minute so well done for raising it. I'm ill at the moment so the old brain struggles a little with the maths  ;)

Aurora
« Last Edit: June 10, 2012, 10:00:51 pm by Aurora »
 


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