Antennas come in all shapes and sizes, and which one is best depends wholly on what you are doing with it. A very popular choice for sending video from drones is the cloverleaf antenna. It is circularly polarized which is an advantage when you have a moving vehicle. It also reduces multipath interference.
A cloverleaf contains three closed loops spaced at different angles. The antenna works well for transmitting but isn’t ideal for receiving. It is also difficult to tune after building it. However, for the right job, it is a good performer. [Vitalii Tereshchuk] shows how he made a cloverleaf antenna that fits a WiFi router.
The starting point for this antenna is a stock whip antenna. Some soldering iron work and a cardboard jig to get the angles right can convert that whip into a cloverleaf. You can see the whole process in the video below.
We’ve seen tiny antennas and even some tuned by liquid metal. The cloverleaf antenna is probably more practical for a weekend project.
“The antenna works well for transmitting but isn’t ideal for receiving.”
Pardon my ignorance, but I thought most antennas had reciprocity – the same capability to transmit as receive (the exceptions having non-linear characteristics). What about this antenna causes it to not have the same transmit response as receive response?
My guess is that for an FPV drone setup, you want an omnidirectional antenna on the drone to transmit and a directional antenna on the receiver. The drone will be pointing every which way, but your receiver can usually be pointed at the drone.
To answer your question- Yes, emission and reception are reciprocal. The “directional” antennas are referring to the emission (and reception) pattern, not the preferential direction of signal flow.
There are some interesting things that can be done with meta-materials to create “one-way” antennas, but I have not seen them on the market.
Adding a guess of my own (and yes, “reciprocity holds” was my first thought, too): Cloverleaf might not be the most efficient antenna; hence, it’s not that great if you try to get capture much field energy, so on RX, you might want to get something bigger with higher effective area/efficiency, but that might not matter that much in TX, where you just crank up your amplifier.
Though hobbyist rarely care that much about FCC/CE limits, so with a better antenna one could get even more transmit power. Also if this is used for wifi, it seems doubtful whether one-directional connection could work at all.
Transmit power remains the same regardless of radiation pattern (assuming we are talking about equally matched antennas), only EIRP changes and comes with directionality…
With the caveat that simulation doesn’t always faithfully model real world results, I’ve run an NEC2 model of the cloverleaf and gotten very good results across a fairly wide band. My guess is some receiver circuits don’t like the DC short.
Nice – this one I just might do
http://www.antenna-theory.com/antennas/cloverleaf.php
+1
Wouldn’t you want to have a circularly polarized antenna on the computer side as well? Otherwise, wouldn’t you experience lowered signal strength once your laptop antenna was out of phase? (I’m absolutely no antenna expert here, just curious.)
AFAIK that’s the situation with antennas in laptops/routers nowadays, you can get better reception by repositioning one of them in a certain way. Hope somebody clarifies this for us, I just read it somewhere.
Ok…so after a little reading on the Internet, it appears that the laptop antennas are commonly linearly polarized (LP) and is why you have to reposition them for optimal signal. Due to something called the signal reciprocity theorem (which was mentioned in an earlier comment) the transmit and receive antenna, if linearly polarized, should exist on the same plane for optimal signal strength. By having a circularly polarized antenna, which rotates its signal around a circular path, would mean that you would be out of phase 99% of the time in this LP to CP antenna configuration (like I said before, I really don’t know what I am talking about, so this might be completely wrong). References: http://www.astronwireless.com/topic-archives-antennas-polarization.asp & http://www.antenna-theory.com/basics/polarization.php
You would not be “out of phase”. All other things being equal, if you use a circular polarized transmit antenna and a linear polarized receive antenna, you’d lose 3dB signal as compared to both being linear polarization and of the same polarization.
However – with circular transmit and linear receive, you always lose the 3dB no matter how you rotate the receive antenna. But with say a vertical transmit and a horizontal receive, you can lose a LOT more than 3dB of signal.
I think you are conflating phase difference of a wave and polarization of an antenna.
Picture this: You look through a vertical polarizing light filter at a source with random polarization. What comes out is 3dB down (half the light) and all polarized the same as the filter. You DON’T get just the waves matching the filter’s orientation. Keep in mind that this means that photons with other angles are now vertically polarized, but only half of them.
So if I set up a polarizing filter vertically, I get out vertically polarized photons. Here is where it gets weird: Set up another polarizing filter at 45 degrees. Again, I lose photons, but what comes out of the second filter is at 45 degrees.
Now it gets even weirder – set up a horizontal filter after the 45 degree, and you get horizontally polarized photons. Remove the 45 degree filter, and it goes black.
It is not an all-or-nothing proposition. A photon (light or radio) at 45 degrees polarization has a 50/50 chance of being passed by a filter or detected by an antenna that is vertically polarized. And when it is, it becomes vertically polarized.
Thanks for the clarification. That is weird…
With modern MIMO devices, it’s possible to emulate a circularly polarized antenna with a vertical and horizontal pair of antennas. In fact, it’s even possible to emulate two circular polarized antennas of opposite polarity, each working with a different signal.
With a circularly polarized antenna on one end and a linearly polarized antenna on the other you get a 3dB loss from polarization mismatch, independent of which linear polarization. Between 2 linear polarized antennas you can get anywhere from no loss to practically total signal loss from polarization misalignment.
To answer the post above receive antennas can be just about anything to deliver a signal and even noise to a receiver. Transmitting antennas need to resonate to work, and improve reception. Ham radio gets you lots of antenna experience….. lots hit or miss.
Looks like a short/squat quadrafilar helix…
Was about to say the same thing….