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I am trying to conduct an RF project using XY-MK-5V / XY-FS modules:

link here

My problem is that even though most blogs and Google searches for these modules use a quarter-wave length antenna (roughly 17.2cm), my transmission is worse than when I use a longer antenna. When the antenna is longer than 30 cm (close to 1/2 wave length) I actually get better reception at longer ranges. (7 meters vs 14 meters)

So my question is, how bad is it to use longer antennas? Is there a reason for the 1/4 wave length antennas being recommended?

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  • \$\begingroup\$ How much is "longer"? Have you checked out what the radio modules need re antenna length. They appear to be saying 32 cm for the receiver and 25 cm for the transmitter. With this in mind and also the anomaly in quoting their respetive frequencies (plus it's an ebay item), can you really be sure you are operating at 434MHz? \$\endgroup\$ – Andy aka Oct 5 '14 at 14:46
  • \$\begingroup\$ The modules may be tuned for a shorter antenna. Try cutting the length down to 10cm or less. Does the range reduce, or increase? \$\endgroup\$ – Bruce Abbott Oct 5 '14 at 19:06
  • \$\begingroup\$ I have tried 34 cm long, then at 22cm, and then at 17.2cm. It seemed to be worse the shorter I got. I'll try under 10cm and let you know. \$\endgroup\$ – parakmiakos Oct 6 '14 at 6:23
  • \$\begingroup\$ BTW, I've found the same empirical info (about the "perfect" antenna for XY's being longer than lambda/4) in misc. places, way too often to be a simple error or coincidence (mind me, even the datasheet provides a value different than lambda/4!) so it's possible that there's some "magic" going on here... I suspect that the kind of antenna (are you using insulated wire?) and its shape (straight vs spiral) is the main factor here, as it can influence the "perfect length". \$\endgroup\$ – vaxquis Dec 20 '14 at 21:54
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    \$\begingroup\$ Like many, I started out using the 1/4 lambda antenna and actually had pretty bad results. Now I am using this one: arduinodiy.wordpress.com/2015/07/25/coil-loaded-433-mhz-antenna and have a significantly better reach. I dont know the science behind it, but the increase in distance covered was remarkable \$\endgroup\$ – user96213 Jan 5 '16 at 21:58
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Actually, it's not that strange. A quarter wave monopole antenna (l = λ/4) is dependent upon a reflective plane normal to the antenna to act as a dipole. (Like a VHF antenna on a car.) Without this plane, the quarter wave antenna will not work properly.

The solution, as you've found, is to use a kind of half wave dipole antenna with a length equal to half the wavelength of the signal (l = λ/2).

There is no harm in using a longer antenna. The reason for recommending a quarter wave antenna is probably that it has a higher antenna gain than the dipole, as well as the fact that it simply takes up less space... It is also simpler than a "real" dipole.

The wikipedia articles on dipoles and monopoles are quite informative.

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  • \$\begingroup\$ "There is no harm in using a longer antenna." Really? I thought the signal strength nulled out at certain lengths? \$\endgroup\$ – endolith Jul 28 '17 at 19:17
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Actually, many 433MHz circuit boards have a coil with a few windings between the circuitry and the solder pad marked ANT. The XD-RF-5V that I'm using has a three winding coil with a 5mm diameter. 5mm x 3 x PI accounts for almost 5cm, so the external part of the antenna should be around 12cm to come to a total length of quarter lambda.

I always find antenna's to be black magic, but for me 12cm seemed to work!

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    \$\begingroup\$ But the coil isn't part of the antenna. It is a coil. \$\endgroup\$ – Johannes Jan 5 '16 at 23:02
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When calculating the length of antenna elements, remember to use the propagation velocity that is less than "c," the velocity of EM radiation in free space. For a velocity factor, 95% is a fair guess... the exact number of a simple wire escapes me at the moment. Moreover, the velocity of EM radiation in a coaxial cable is much slower, and it is listed for each type of coaxial cable. 66% is a fair guess. This has dramatic importance if one is trying to tune a length of feeder cable... not relevant here, but worth cognizance, just the same.

The OP inquired about using a "longer wire" and I want to introduce caution to that point. Johannes added, with excellence, that the OP really started out with a quarter-wave dipole that uses a phantom second half (the earth, as a mirror) to make a more proper antenna... the half-wave dipole. Proper orientation of the quarter-wave element...the original wire... would be NORMAL and STRAIGHT... i.e, so as to find that mirror (the earth) that it depends on. I do not know how high above the earth this configuration must be; perhaps Johannes can answer that.

More importantly, the half-wave dipole is FORGIVING to neophytes because of its simple "doughnut" (omnidirectional) radiation pattern, at right angles and all around, to the wire(s). In other words, it communicates with other antennas that share a mutual horizontal relationship. There is no gain in the direction of the wire itself... (vertically).

A principal of "reciprocity" says that transmitting and receiving antennas share the same rule book! Well, that is easily taken, in low power situations like this.

If you start using longer dipole antennas, you are instinctively searching for higher "gain." It's not a simple thing! You SHOULD adhere to the rule of using overall lengths that are odd multiples of half wavelength (reduced by velocity factor). If your dipole is symmetric, that's good for novices. Here's the rub: Longer antennas have higher gain... but also have increasingly complex dispersion/reception patterns; iow "lobes." (1 for a simple 1/2 wavelength dipole, 3 for a 3/2 wave dipoole... including both elements of the dipole in this description of length), etc. You must grok these lobes or you're going to be doing some serious scratching of your behind, wondering what is going on. Again, what's good for the transmitter is good for the receiving antenna, too.

Then there are reflections and screens. Keep away from metal objects. Look up (people never look up, ha ha) at any common rooftop television antenna and you'll see one active dipole (horizontally polarized, by the way, typically) and many horizontally-polarized reflective elements... VHF antennas have DIPOLE reflectors in DIFFERENT lengths. When a pigeon sat on THE LONGEST ONE and damaged it, you might recall the loss of "Channel 2" ... if you're old enough to remember that people used to depend on the air waves, rather than cable, for their television viewing.

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The science behind the antenna radiation is this: The transmitted signal is an alternating current, a sine wave. At 0, 1/2 or 180 degrees, and back to 0 degrees of the wave, the current is at 0 or minimum. At the quarters or 1/4 (90 degrees) and 3/4 (270 degrees) wavelength the current is the highest and the radiation is greatest. 1 quarter wavelength is the shortest antenna length that will radiate signal. 3/4 and 1 1/4 and 1 3/4 and so on are also radiation points. Especially on receive the more length the better. The tuning of an antenna is the result of trying to get the most current out of the transmitter while transmitting. The most current the most radiation and the greater distance the signal will travel. At that best tuning the receive will also work best. When the tuning is off some of the output signal is reflected back causing resistance and less radiation causing loss of distance. This matching is called the swr of the antenna and is represented by a ratio. The lower the ratio the better. A tuning circuit can be used to balance the swr of most any length of wire.

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There are already some answers relating to differences between \$ \frac{\lambda}{4} \$ monopole and \$ \frac{\lambda}{2} \$ dipoles and their associated gain patterns, so I'll augment the answers with something about impedance matching and characteristic impedance.

It's possible to show that maximum power transfer occurs in a DC circuit by analysing simple equivalent circuits and setting the derivative, \$ \frac{dP}{dR} = 0 \$ The same applies to complex conjugate matched reactances (AC circuits with inductors and capacitors). Your transmitter has an effective output impedance and by virtue of the antenna's geometry, location relative to other structures and the materials involved, also has an associated complex impedance. In order for maximum power transfer to occur impedances must be complex conjugate matched.

The coil on the board may be part of this matching network. The reflection co-efficient tells you how much power is being reflected back into your transmitter and possibly being lost. Depending on how mis-matched you are, you might dwarf the gain properties of the antenna.

To answer your question, changing the length of antenna is likely to cause mis-match and lost power. The actual value of length might be influenced by the network already on the board.

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