433 Mhz antennas for 3Dr radios

Please guys i need some help. I got a pair of 433 MHz telemetry modules from RCTimer, but the antennas just don't perform. And i want to buy some new ones. I am thinking in buying one of this dipole for the Ground Starion and one of this to use in the Quad. Is this the best configuration? Please help me soon.

Replies

• From my first Amateur Radio Manual:  The Radio Amateur's License Manual, 75th Edition, March 26, 1976

The General Class License, Page 33, Question 56:

Question:

What is Standing Wave Ratio(SWR)? How can the SWR on a transmission line be determined from the incident and reflected voltages?

In an unmatched transmission line, there are "Variations", also called Standing Waves, of current and voltage along the line. SWR is the ratio maximum current to minimum current(or maximum voltage to minimum voltage) along the line. It can be determined by measuring those values or by the use of a bridge circuit(reflectometer) which measures incident and reflected voltage or power separately. The SWR can be calculated from the formula:

SWR= Vf + Vr/Vf - Vr

Where Vf is the incident or forward voltage and Vr is the reflected voltage.

I thought you said there are no publications with this reference in it? For many years it was called Variable Standing Wave Ratio. Voltage Standing Wave Ratio is inaccurate to a degree because current's can be measured as well, however you do see it printed more often as Voltage Standing Wave Ratio because that is the common method of calculating SWR.

As for the rest of the info, there is much too much for me to go over in explaining BALUN's, Pi Networks, Transmatches, etc, where they are used and their purpose. There should be some items left open for those in this forum to explore on their own, in a Library or online.

Yes, I still have the reference manual I took the information from.

• @Vega,

as you so clearly indicated -

SWR= Vf + Vr/Vf - Vr  - Where V is VOLTAGE   and NOT 'Variable'..  The term VSWR has NEVER in any Text Book anywhere, ever been referred to as Variable Standing wave Ratio. The 'variations' are in the voltage, or current, or power, whichever you choose to use as the measurement term. If you read carefully, and understood what Standing Waves are, you would realize that the SWR measurement is a RATIO of terms and REMAINS CONSTANT for constant conditions. The SWR does NOT vary with measurement - it is not a variable ratio. The measured SWR will only vary if the transmitter impedance versus antenna ( the load) impedance are changed.

As for the rest being to much for you to explain, perhaps you might care to enlighten us as to your background on these subjects? Nothing should, for a suitably knowledgeable Elmer, be 'too much' to explain...

The Nampilot...

• Yes, VSWR can be stated as Voltage Standing Wave Ratio both terminologies refer to the same thing. Variable is just an older term, like Cycles is now referred to as Hertz, both mean the same thing.

Coax cable at frequencies above 400mhz should have as minimal as line loss as possible and the velocity of the cable should match the frequency it is being used at. I prefer solid conductor coax, but twisted is easier to work with. Connectors should also be soldered properly to ensure a good connection. I have made hardwired connections when connectors were not effective.

A very easy way to see what you are getting as an output is to use a field strength meter to characterize the output of your system, as well to determine if you are getting a good omnidirectional signal. This will change with a beam antenna where the lobes of energy will be concentrated in one direction with small lobes from the sides and back of the antenna.

Yagi antennas can be used effectively to 2.4ghz, but parabolics can be used at this point. A good way to determine the length of a half wave dipole for any frequency to 2.4ghz, is 468/Frequency in Mhz(or Cycles). This formula can be used for higher frequencies, but it becomes less effective as the dipole becomes very small. You can use a full wave at this point as the drive element in your parabolic.

A Balanced to Unbalanced System Coil(BALUN) is effective, but trimming capacitors can be used as well a transmatch(Tunning Capacitors & Coils). The old method was to use a Pi Network of a trim capacitor and tuning coil to tune the transmitter to the antenna. The coax itself feeding the antenna can be coiled to be an effective BALUN at these frequencies.

A VSWR of 2:1 will work, but you are losing energy from the reflected voltages. It makes no sense to have an antenna with gain, running in to an unmatched system and losing energy. I have worked on several projects were a 6db gain omnidirectional antenna basically was unity gain because of a high VSWR.

One other note, Effective Radiated Power(ERP) should also be calculated and checked physically to make sure the entire system functions properly.

• Hi ther eVega et-al--

No, there is no such term as Variable Standing Wave, etc - It is Voltage. Please consult any antenna handbook for the definition, even those from the early part of this century...

The simple definition is - The Ratio of the maximum Voltage along the transmission line to the Minimum Voltage along said line is defined as the Voltage Standing Wave Ratio. This is often 'shortened' and referred to as SWR, or simply, Standing Wave Ratio.

However, the ratio of the Max current to Min current in the transmission line yields the same ratio and so can also be used to determine SWR.  But VSWR is Vmax versus Vmin, no variables in there..

'and the velocity of the cable should match the frequency it is being used at'

Absolutely no reason for this. Velocity factor of the cable is only important if you are trying to determine an electrical equivalent length of cable, eg, for a 1/4 wavelength of coax, the length will be the physical 1/4 wavelenght multiplied by the velocity factor.

This formula can be used for higher frequencies, but it becomes less effective as the dipole becomes very small.

Actually, this formula is 'true' regardless of frequency. It does not loss effectiveness at all.. What is important are ALL the other factors that affect element lengths, for example, a dipole length will be affected by the length to element diameter ratio. What does become less effective are the simple 'rules of thumb' for these ratios when the frequencies go up into the microwave region. But the formula for wavelength remains intact.

You can use a full wave at this point as the drive element in your parabolic.

Well, I suppose you can, but it would make a very poor antenna. A full wave dipole does not have a radiation pattern that will illuminate a parabolic dish properly at all. It will be very inefficient, and you will not realize the gain potential of the dish antenna at all. It will also have funny lobes in odd places..

A Balanced to Unbalanced System Coil(BALUN) is effective, but trimming capacitors can be used as well a transmatch(Tunning Capacitors & Coils). The old method was to use a Pi Network of a trim capacitor and tuning coil to tune the transmitter to the antenna. The coax itself feeding the antenna can be coiled to be an effective BALUN at these frequencies.

I am afraid you are mixing two completely unrelated subjects here.   Transmatches and Pi-Networks are part of many methods used to MATCH the antenna impedance to the transmitter output impedance. This has nothing todo with BALUNS at all. Baluns are not 'matching devices' although they can be made to perform impedance transformations as well ( 4:1, 9:1, etc). A Balun is a device employed to interface an UNBALANCED system to a BALANCED system, such as a dipole antenna ( unbalanced) to the single ended output of your RCS transmitter.

Without this Balun, you have RF currents propogating down the outside of the coax cable from the dipole interface.

As a result, the coax radiates as well as the dipole, and the desired clean radiation pattern of the dipole is destroyed. Coiling the coax does work as a balun - NOT as a matching device.

A VSWR of 2:1 will work, but you are losing energy from the reflected voltages. It makes no sense to have an antenna with gain, running in to an unmatched system and losing energy. I have worked on several projects were a 6db gain omnidirectional antenna basically was unity gain because of a high VSWR.

A VSWR of 2:1 is a loss of only 10% of TX power, as I said. You will NOT notice this in any average RCS setup at all.You must consult some SWR tables to see what this means. SWR does NOT affect antenna gain. ( Antenna 'gain' is a misnomer in any case - an antenna does not really have gain, but rather directivity - there are no active elements - amplifiers - in an antenna to create gain). Since SWR 'measures' how much energy is reflected from the antenna back to the transmitter , it is therefore indicative of the energy not radiated by the antenna.

An SWR of 1.5:1 results in only 4% power reflected, 2:1 gives 11%, 4:1 gives 36% and 7:1 gives around 50%.

Regarding your '6dB gain antenna' -  your SWR had to be in excess of maybe 10:1. Not sure why several projects had antenna with such high SWR - not really antenna were they...

SWR is not the Boogie-Man - an SWR of 2:1 is quite acceptable and easily achieved without fancy test equipment, so most DIYers can do it.

One other note, Effective Radiated Power(ERP) should also be calculated and checked physically to make sure the entire system functions properly.

Quite impossible for the average person - To check ERP against theory would require the use of an EMI chamber and test reference antennae - and would serve little purpose for most users. In any event, the system ERP cannot be 'calculated' - it can be modeled to a greater or lesser extent, but the result will only be as good as your model. The antenna ERP can be calculated, but the actual radiation pattern of the resulting system is what is important to the user, so antenna ERP is useful only as a guideline.

In general, whatever the radiation pattern of your antenna, it will always be effected by its environement, especially so in the small aircraft RC folk fit them to - there will always be additional reflectors and directors in place - the aircraft wiring, motors, batteries, etc, so ERP will not give the user much idea of how well his system will perform. If a full omnidirectional radiation pattern scan of the aircraft could be done, that would be fantastic - you would know exactly what the pattern is, how to improve it, what causes the broken pattern, etc. But that does require some fancy test equipment and a test range devoid of interfering RF signals - and lots of money....

Obtaining optimum range from your RF system is all about optimising each installed aspect - Antenna type and directivity, radiation pattern, polarisation ( very important! - cross polarization can give very high signal losses - 30db or more if you are unlucky..), antenna location, cables and connectors. There is no single antenna solution that is best for all applications.

You will notice from my various posts -  I have 20km plus range on 868MHz @ 300milliwats on my datalink, and on the 2.4GHz video link @ 500milliwatts - this does not happen by accident - check out the posts on SurVoyeur, Guppy, Hornbill, etc.

The antenna used are a vertical dipole on ground and a Ring Radiator on the A/C @ 868MHz, and a Skew Planar Wheel on the A/C and a Short Backfire Turnstile tracking antenna on ground @ 2.4GHz

I did try to cover a lot of this in my blogs...

The Nampilot..

• There have been many discussions along these lines with some useful info provided - problem with these forums and the way the process works, the discussions are lost in the mist one they fall down in the queue and there is no easy way to find them or refer to them again - Maybe some of the info in these blogs here might help some of you a little.

The info is not limited to 433MHz, so ignore if not applicable...

@Vega - VSWR is Voltage Standing Wave.....not 'Variable' Standing-

VSWR is blamed for far to many poor performing antenna. An SWR of 2:1 will result in an effective TX POWER loss of around 10% - that will not reduce your range from a possible 'expected' 2km to a few hundred meters.  There are other factors - some transmitters, esp high power ( 10's to 100's of watts) ones have protection on the TX output , reducing the TX power as the SWR rises. It would be most surprising to find any of the typical RC world transmitters with this function.

The most important is to ensure the antenna is resonant at the desired frequency, that its polarisation and gain is appropriate for the application, that if it requires a BALUN one is present, and that the SWR is 'acceptable'.  Use good quality COAX, esp if the length is more than a few hundred mm, good quality connectors - poor connectors can easily have 1-1.5dB loss at 2.4GHz for example - and 3dB loss is half your power gone.

Anyway, take a look at the blogs, maybe there is some info you find applicable.

The Nampilot..

http://diydrones.com/profiles/blogs/more-433-mhz-antenna-the-1-2wav...

http://diydrones.com/profiles/blogs/dipole-style-antenna-for-433mhz

http://diydrones.com/profiles/blogs/antenna-feedpoint-matching-balu...

http://diydrones.com/profiles/blogs/some-more-antennae-and-when-is-...

• More than likely the dipole antennas you have are not matched to the transmitter. The VSWR(Variable Standing Wave Ratio) is high. This cause a lot of issues, such as poor distance, static and high interference from local RF sources(computers and other transmissions), as well it can eventually destroy the final out put transistors of the transmitter.

A well designed antenna system is essential for good performance. For your base antenna you should use a Yagi antenna tuned toi the frequency you want to use. You will get great distance from just a few Milliwatts, as well the dipole should be tuned to the frequency you want to use.

J-Poles are great in this situation. Easy to build and work fantastic at these freqencies. Ham Radio operators have used J-Poles since the 30's.

• There is no need for expensive antenas...I got this and since then I forgot problems with reception.....

• Would these work with a 915mhz transmitter?