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Antenna design for 434 MHz RF reciever module

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elsey.jack

New Member
Hi everyone,

I am working on a weather balloon as a research project over the summer. I need to get temperature measurements up to a height of 100 M.

I am using a 434 MHz RF transmitter and a 434 MHz RF receiver to transmit the data between the balloon and a hand-held display unit.

I am using a picaxe microcontroller in the balloon unit and another in the display unit. I am reading the temperature using a thermistor with the method discussed here.

The maximum range of the transmitter is 500 ft. The range is dependent on its supply voltage (2 to 12 V). My breadboard prototype worked perfectly when I had the transmitter powered at 5 V.

I have assembled the circuit on protoboard and am powering the transmitter with 11.2 volts from a three cell lipo battery. Now, however, the transmitter is starting to heat up the entire circuit through its connecting wires, falsely raising my temperature readings!

The data sheet (pdf) for the receiver unit recommends a 17 cm long wire for the antenna. If I improve the antenna can I get a longer range? Do you think that I could get 100 M range if the transmitter was powered with 5V?

EN0

Member
for the receiver unit recommends a 17 cm long wire for the antenna. If I improve the antenna can I get a longer range?

Yes, you sure can. Usually you want a short length of antenna wire which is why they suggest 17cm. If you had an ideal antenna you would make it according to the required wavelength. So you mentioned 434MHz, according to the following equation: Length in meters = 300/ f (in MHz) so 300/434 = 0.7 which is about 2.3 feet. So if you have the antenna that length it should work much better.

Do you think that I could get 100 M range if the transmitter was powered with 5V?

Yes, once more. Since the supply is 2-12V a 5V supply would certainly work better than 2V. You would have to experiment with the range and see how much farther you could go. The 12V supply would work the best.

Have you ever heard of Fractal Antennas? They are absolutely amazing, some of their primary advantages is being resonant at mutliple frequencies, a diminutive size, and more. In fact, they are based from fractal geometry and electromagnetic theory. My avatar is a computer generated fractal. I would advise you to research fractal antennas and google pictures of them. They are very pretty to look at! Hope this helps!

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birdman0_o

Active Member
Length in meters = 300/ f (in MHz) so 300/434 = 0.7 which is about 2.3 feet.

Out of curiosity where does this equation come from? Is this only for straight wire antennas?

EN0

Member
Out of curiosity where does this equation come from? Is this only for straight wire antennas?

Birdman,

Electromagnetic waves travel close to the speed of light, therefore that is 300,000 meters per second. Since we are dealing with the frequency in MHz we can cut this down to 300. If we were to have it just in the Hz range, it remain 300,000. I deal a lot with RF due to the fact I am an amateur radio operator. This equation is in the ARRL Antenna Handbook and so is this equation: λ(feet) = 983.5592 / f (In MHz) If you apply that equation to the frequency he is operated at it is the same answer about 2.3 feet. I believe it is for a dipole antenna, I'm not quite sure.

My only concern is whether or not he has a license. The 70cm band 420-450MHz is an amatuer radio band, and his frequency is within that band. he would have to transmitt his call sign in CW every ten minutes to identify himself. Note that if I wanted the length in centimeters I would multiply the number of feet times 30.48 and for 2.3 feet you get 70.104 cm.

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birdman0_o

Active Member
So we want the antenna to be one wavelength long? Why do they suggest ~ 1/4 then?

EN0

Member
So we want the antenna to be one wavelength long? Why do they suggest ~ 1/4 then?

A full wavelength would work better. Next is a 1/2λ (Distance in feet = 468/ f (in MHz) and 1/4λ (Distance in feet = 234/ f (in MHz).

Well-Known Member
The maximum range of the transmitter is 500 ft. The range is dependent on its supply voltage (2 to 12 V). My breadboard prototype worked perfectly when I had the transmitter powered at 5 V.

I have assembled the circuit on protoboard and am powering the transmitter with 11.2 volts from a three cell lipo battery. Now, however, the transmitter is starting to heat up the entire circuit through its connecting wires, falsely raising my temperature readings!

The data sheet (pdf) for the receiver unit recommends a 17 cm long wire for the antenna. If I improve the antenna can I get a longer range? Do you think that I could get 100 M range if the transmitter was powered with 5V?

Hi Jack
I'm not surprised that your transmitter is heating up. The data sheet for the transmitter states the amount of current drawn when you power from 5V and when you power from 12V. At 5V you are drawing about 100mW into the board, and you are transmitting around 10 dBm, which is 10mW. So the efficiency of the board in converting DC power into RF power is 10% and the amount of heat produced is 90mW. This isn't too much for that size of board. Now, when you operate at 12V, the data sheet says the unit draws 59mA at 12V which is 708 mW, but it is only generating 16 dBm output, so the conversion efficiency is 40mW/708mW = 5.6% and the amount of heat generated is 708-40=668 mW. This is a lot more heat for the board to dissipate, so yes it will warm up. The transmitter is just not very power efficient at 12V, even though it does function.

The recommendation of using a 17 cm long wire is based on over simplistic assumptions, and you need to be aware of the assumptions. I would guess that this number comes from a guy with a calculator who calculated
0.25x(3x10^8)/(434x10^6). This equation calculates the length of a quarter wavelength in free space at 434 MHz. It assumes that the wire is an ideal quarter wavelength monopole antenna. In order for your 17 cm wire to come close to being a good quarter wave antenna (which is a very good and simple type of antenna) it needs to be straight, and perpendicular to a large conductive surface which acts as a ground plane. Usually such an antenna is oriented so that the wire is going straight up (or straight down) and underneath the bottom of the antenna (or under the tx module) is a sheet of metal (copper, or whatever) that is at least 2 x 17 cm across, either round or square is ok. If you can't manage a complete sheet, then you would need to have at least two 17 cm wires, each one oriented away from the tx module at right angles to the antenna wire.

If you plan to put this transmitter on a balloon, then it might be a good idea to consider other antenna configurations. Its too bad they don't specify the output impedance of the module, but it is pretty common for this to be 50 ohms, so let's consider the most popular 50 ohm antenna of all, the half wave dipole. This may be a better antenna for you, because it simply requires that you have one 17 cm wire connected to the ANT pin draped away from the tx module (hanging down is ok) and another 17 cm wire, connected to the GND pin of the module, and draped exactly in the opposite direction. So the two wires line up with each other but go in opposite directions away from the module. This might be easy to attach to your balloon, because it is all in one line.

I think that if you try to improve the antenna by making the antenna arbitrarily long it won't work better than using a half wave dipole antenna. There is one thing you need to beware of. In ideal circumstances, these antennas, like the half wave dipole, radiate strongest in the direction that is broadside to the antenna, and weakest along the axis of the wires. This is true of the ANT wire of a quarter wave antenna too. So try and orient your antenna wire axis perpendicular to the direction you want to RF to go.

Although I havn't studied the Receiver board to see if it has an ANT pin, everything that I said also applies to the receiver. The best antenna for that one is probably also a half wave dipole, if there is a 50 ohm feed pin on the module.

One thing you need to know about antennas is that they must be "in the clear", or in other words you can't have metal or your body or any other conductor beside the antenna wires or else it won't work well. The wires should be dressed away from all the electronics. Its OK for the antenna wire to be next to nylon, or string, or a balloon, or anything that is nonconductive, just no metal, or arms, or other wires, or circuit boards.

PS: although 17 cm is close enough for now, you should be aware that if everything else is ideal, then the wire should actually be a bit shorter to account for the fact that the RF energy does not travel along the wire at quite the speed of light, it actually goes a bit slower, so the wire ideally should be a bit shorter. I usually recommend shortening by about 10%, or in other words, a better length might be about 15.3 cm, but for now this is being overly fussy and you shouldn't worry about it.

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MikeMl

Well-Known Member
You will increase the range by a factor of ten or more by doing the following:

Modify the transmitter so that it feeds a horizontal dipole which is center-fed, 1/2 wavelength (λ) long. A horizontal dipole has maximum radiation at right angles to the dipole, with deep nulls off the ends, where you might lose the signal as the balloon rotates. Even better, would be two crossed dipoles called a Turnstile, which has an omnidirectional pattern.

For the receiver, obtain or build a 3 to 6 element Yagi for 433Mhz. Such an antenna is directional, so you will have to "aim" it at the balloon. It will enable you to "direction find" where the balloon is going, and possibly even to find the crash site

Get yourself a copy of the ARRL Handbook. It does not have to be the latest edition. It has design info for all types of home-buildable antennas. While you are it, get a ham license, too...

elsey.jack

New Member
Thanks for the help everyone!

I figure I will start out with a ½ wavelength dipole antenna both in the transmitter and the receiver. I will then start looking at yagi antennas for the receiver if I can't get the range I need.

I'll pick up a copy of the ARRL handbook from the library today.

Nigel Goodwin

Super Moderator
Thanks for the help everyone!

I figure I will start out with a ½ wavelength dipole antenna both in the transmitter and the receiver. I will then start looking at yagi antennas for the receiver if I can't get the range I need.

Assuming this is a licence free module, isn't the licence free status dependent on using ONLY the specified antenna?.

EN0

Member
The amateur radio band (70cm) goes from 420-450MHz and his frequency of operation is within this band. He would definitely need an amateur radio license. If he doesn't, he can take his Technician and have full priviliges within this band. It is a thirty-five test question and very simple. You should receive your call sign within a week. Next comes the General Class License (35 test questions), and finally the Extra Class license (50 test questions). It used to be where CW (Morse Code) was required, but since 2007 the FCC only has the question now.

https://www.electro-tech-online.com/custompdfs/2009/08/Hambands_color.pdf

The ARRL Handbooks do have antenna information but the ARRL Antenna book is much more informative when it comes to antennas. You might consider picking up both.

Since you're doing a 1/2λ, then you use this equation: length in feet = 468/ f (In MHz). So your antenna length should be about one foot.

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elsey.jack

New Member

Even if my transmitter only has a range of 100 M or so? Hmm. I'll have to do some research into this. Thanks for the heads-up.

EN0

Member

Even if my transmitter only has a range of 100 M or so? Hmm. I'll have to do some research into this. Thanks for the heads-up.

Range doesn't matter in this band, you'll need a license. If you were in the FM broadcast bands, and were under a certain amount of power, you would be fine. There are three different types of licenses, the Technician, General, and Extra. The first two are 35 questions and the third is 50. All you need is to take the Technician to have access to the 70cm band. According to regulations, you'll need to transmit your call sign (to be received after you pass your license-mine is KD0FAA) every ten minutes. Then you can relay the balloon information. You can't do it for business purposes, just pecuniary intrest from the operator.

elsey.jack

New Member
Where would I go to take these questions?

Is there a class that I would have to take?

EN0

Member
Where would I go to take these questions?

Is there a class that I would have to take?

You should study first. I would advise you to pick up the following book:

Amazon.com: ARRL Ham Radio License Manual: All You Need to Become an Amateur Radio Operator (Arrl Ham Radio License Manual) (Arrl Ham Radio License Manual) (9780872599635): H. Ward Silver, American Radio Relay League: Books

You might be able to get it at your local library. Or, you could practice at this website:

To find an exam session near you go to the following website and enter the information required:

ARRLWeb: Exam Session Search

Hope that helps! You'll enjoy ham radio, it is a very fun hobby!

JimB

Super Moderator
The modules are intended for licence free operation under FCC part 15 rules. Have a look at this document, page 20 of the pdf describes what is allowed at this frequency:
https://www.electro-tech-online.com/custompdfs/2009/08/oet63rev.pdf

434Mhz is only authorised by the FCC for "Intermittent Control Signals" which is not your application.
However 315Mhz modules are commonly available in the USA for any data transmission. The datasheet on the Sparkfun website shows two variants of the module, 434 and 315Mhz.
434Mhz is for use in Europe, 315Mhz is for use in the USA.

I dont think you have to worry about getting your amateur radio licence for this.

JimB

EN0

Member
The modules are intended for licence free operation under FCC part 15 rules. Have a look at this document, page 20 of the pdf describes what is allowed at this frequency:
https://www.electro-tech-online.com/custompdfs/2009/08/oet63rev-1.pdf

434Mhz is only authorised by the FCC for "Intermittent Control Signals" which is not your application.
However 315Mhz modules are commonly available in the USA for any data transmission. The datasheet on the Sparkfun website shows two variants of the module, 434 and 315Mhz.
434Mhz is for use in Europe, 315Mhz is for use in the USA.

I dont think you have to worry about getting your amateur radio licence for this.

JimB

But I don't think he's in Europe. He's in the USA. Therefore, you're saying he'll have to modify his frequency to 315MHz.

JimB

Super Moderator
But I don't think he's in Europe. He's in the USA. Therefore, you're saying he'll have to modify his frequency to 315MHz.
He shows his location as Detroit which I believe is in the USA some where, which is why I quoted the FCC regulations. Contrary to popular belief by some Americans, the FCC does not have jurisdiction beyond the borders of the USA.
So, yes, if he wants to be kosher in his operation, he should use a 315Mhz module.

JimB

unclejed613

Well-Known Member
try this simple antenna made from coax cable (and you can use RG-174 cable which is the same diameter as 16Ga wire). make the dimension for each of the two elements (the exposed center wire and the folded-back braid) equal to 19cm. you can skip the part about copper tubing, and just fold back 19cm of the ground braid over the cable insulation. keep the braid intact so it acts like a metal sleeve over the outside of the cable. this will give you a good half-wave dipole (each element, the folded back braid and the center wire are each 1/4 wave) that doesn't need to be supported because it's the end of the cable. at the other end of the cable, the center wire connects to the transmitter antenna terminal, and the shield braid connects to the transmitter ground (either a metal shield or a ground trace on the pc board will do)

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Shax

Member
try this simple antenna made from coax cable (and you can use RG-174 cable which is the same diameter as 16Ga wire). make the dimension for each of the two elements (the exposed center wire and the folded-back braid) equal to 19cm. you can skip the part about copper tubing, and just fold back 19cm of the ground braid over the cable insulation. keep the braid intact so it acts like a metal sleeve over the outside of the cable. this will give you a good half-wave dipole (each element, the folded back braid and the center wire are each 1/4 wave) that doesn't need to be supported because it's the end of the cable. at the other end of the cable, the center wire connects to the transmitter antenna terminal, and the shield braid connects to the transmitter ground (either a metal shield or a ground trace on the pc board will do)

Unclejed613 beat me to this answer!
I use this same type of antenna in my rockets for finding the buggers when they land.. I believe they are commonly called bazooka dipoles.. They do work well.. I've tracked and found rockets to well over 16,000 feet and at distances of over 3 miles on the ground and only 10mw RF...

SHAX.

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