SteveRobey
Member
Hi guys,
A similar question came up yesterday on the Facebook group and I took an educated run at an answer, but I thought I would make a similar thread here in case somebody should happen to be looking for similar information but prefers to stay away from Facebook when in search of facts, it's usually a sound plan.
So the original question was "How should I mount my receiver antenna for best reception?" to which a few people quickly answered "Pointing straight down out of the bottom of the aircraft. While that will work, to fit the requirement of "For best reception" it's incorrect. This is because of radiation patterns and the same can be used for both your receiver antennas and FPV antennas as they both have essentially the same radiation pattern.
The radiation pattern of these antennas is basically a doughnut centered axially on the radiation portion of the antenna which you can see in this graphical representation that I swiped from google..
So imagine your antenna is running vertically along the X axis in this image, the wireframe shape is the radiation pattern. This is not representative of how far out the radiation pattern goes, just it's general shape. So you can see that we have the most power at the outer edge of the doughnut up to about 20 degrees of horizontal and the lowest amount of radiation or nulls are in the center top and bottom off the ends of the antenna. So pointing your antenna directly up or directly down in an aircraft, you're pointing the null at yourself. Depending on how high up the aircraft is, you could potentially have reception problems. Keep in mind that the antenna on your transmitter has the same radiation pattern but both the ground and the person holding the transmitter actually reflect a portion of that power back up and out. Mounting a receiver antenna sticking directly out of the top of an RC car, boat or other ground vehicle works because the antenna on the vehicle is always going to be more or less broadside the antenna on the transmitter, both with their nulls pointing more or less up and down, so the null will never be pointing directly at you.
With this, it would be generally recommended to mount your receiver antenna parallel to the ground. Mounting the antenna so that it is oriented to the front and back or side to side should not make much of a difference because even if the null is pointing in the direction of the transmitter, the aircraft is also (Hopefully) several feet off of the ground and thus the null would be several degrees above pointing directly at the transmitter. Another thing to consider when mounting your antenna is metal objects that could potentially couple to or interact with the antenna. If the antenna is touching other conductive surfaces, the RF radiation will want to travel along those parts as well, this can cause not only very irregular radiation patterns, but will also cause impedance mismatch and greatly reduce the power that actually leaves the antenna. The rest of that power can end up reflecting back down the antenna and into the radio itself. It sounds bad because it is. This is why you will always see people recommending that you mount your receiver antennas as far away as possible from your FPV antenna. Because they can interact and interfere with each other.
Don't let this scare you away from trying FPV on very small craft though, if they have the lift capacity to carry the extra hardware, there is likely enough room to mount your antennas a safe distance away from each other. The rule of thumb here is that antennas should be mounted a minimum of one quarter wavelength apart at the lowest frequency. Lucky for us, this equipment is fairly high frequency hence the small size of the antennas and thus one wavelength is relatively short. If you're using an older style receiver that uses a crystal, you might not have the same advantage as they are much lower frequency around 11 meters or so. So lets look at the frequencies that are common in 2017 and what their wavelengths are.
Your transmitter and receiver is likely operating at 2.4gHz. One wavelength at this frequency is .125 meters or 4.92 inches (round up to 5 inches). So a quarter of a wavelength would 1.25 inches.
If you're adding FPV gear operating at 5.8gHz, thats a full wavelength of .051 meters or almost exactly 2 inches. So a quarter wavelength there is one half inch.
Now obviously the longer wavelength is the lower frequency of the receiver at an inch and a quarter, so if the two antennas are at least that far apart you should be okay. If you want to be more certain you won't have any issues, you could always space them a full wavelength apart in which case you're still only looking at five inches. Typically I have seen FPV antennas mounted to the end of a small diameter, relatively short piece of co-axial cable and a lot of times something is added to that to make the cable more rigid, making some people think this means that it somehow needs to be "Higher up" and will mount it sticking out of the top of their aircraft. You really don't need to do this at all. You can mount the antenna either flat to your wing or fuselage or even on the under side, but it may be better to mount it inside the fuselage laying on it's side when you consider it will have the same shape radiation pattern as your receiver antenna. If there is room inside the body of the craft to do this. If not, simply mount it in such a way that it will introduce minimal drag to your aircraft and won't get snagged or scrape when landing.
If you are using radio hardware that operates at a different frequency than the two I have addressed above, you can use an online wavelength calculator to determine what your longest wavelength will be and arrange your antennas accordingly. Here is a link to one such calculator, but there are plenty of others available that may give you lengths in more preferable units of measure. This one gives lengths in meters as it is intended to be used for calculations at much lower frequencies such as those typically used for long distance communications in the amateur radio bands.
http://www.rfcafe.com/references/calculators/wavelength-frequency-calculator.htm
So what can you do if you find that you are getting interference in your FPV setup or you have jittering servos? Probably the best thing you can do is add ferrite chokes to the leads of any affected equipment. Basically what tends to happen is that you end up with leads or wires that are at or close to a wavelength or multiple of a wavelength of that of your radio equipment which essentially means that it is working like an antenna. It's receiving radio interference because it is the correct length to do so. You can get snap-on ferrite chokes in a variety of sizes so it should be easy to find ones with an appropriate diameter center hole to fit around your servo leads and whatnot. Adding a choke to both ends of long leads will help to impede any stray RF signals that will be traveling along the outside of the wires. I have read some articles where people have talked about constructing tiny Faraday cages around everything and wrapping all leads in tin or copper foil to combat this and they usually end up frustrated because it doesn't work well and takes a lot of time. Chokes can be added in a matter of a few minutes.
Do research before you buy your equipment and mind the return policy. Check and see what other buyers have to say about a product before you decide to purchase it. The guy selling it is always trying to sell it and might not always mention "By the way, this thing emits RF noise like a Mexican radio station" but people who have purchased it and have had problems with it probably will. If you find you are still getting interference from your equipment after adding ferrites you might just have an FPV camera with a really dirty signal or noisy DC power supply. In these cases, moving things further apart in your aircraft or adding foil probably isn't going to help because everything is being powered by the same source and all of that noise can travel freely along the ground wires to everything.
Research before you buy, test it out, add ferrite chokes and if you're still having problems, hopefully you purchased from a seller that honors returns. When dealing with RF, it is always a good idea to educate yourself on best practices. You can also reference the ARRL handbook or similar resources for information on these subjects or ask an amateur radio operator. Most "Elmers" as they are called have been operating for decades and have learned all about RF and EM interference and what can be done to combat or eliminate it.
A similar question came up yesterday on the Facebook group and I took an educated run at an answer, but I thought I would make a similar thread here in case somebody should happen to be looking for similar information but prefers to stay away from Facebook when in search of facts, it's usually a sound plan.
So the original question was "How should I mount my receiver antenna for best reception?" to which a few people quickly answered "Pointing straight down out of the bottom of the aircraft. While that will work, to fit the requirement of "For best reception" it's incorrect. This is because of radiation patterns and the same can be used for both your receiver antennas and FPV antennas as they both have essentially the same radiation pattern.
The radiation pattern of these antennas is basically a doughnut centered axially on the radiation portion of the antenna which you can see in this graphical representation that I swiped from google..
So imagine your antenna is running vertically along the X axis in this image, the wireframe shape is the radiation pattern. This is not representative of how far out the radiation pattern goes, just it's general shape. So you can see that we have the most power at the outer edge of the doughnut up to about 20 degrees of horizontal and the lowest amount of radiation or nulls are in the center top and bottom off the ends of the antenna. So pointing your antenna directly up or directly down in an aircraft, you're pointing the null at yourself. Depending on how high up the aircraft is, you could potentially have reception problems. Keep in mind that the antenna on your transmitter has the same radiation pattern but both the ground and the person holding the transmitter actually reflect a portion of that power back up and out. Mounting a receiver antenna sticking directly out of the top of an RC car, boat or other ground vehicle works because the antenna on the vehicle is always going to be more or less broadside the antenna on the transmitter, both with their nulls pointing more or less up and down, so the null will never be pointing directly at you.
With this, it would be generally recommended to mount your receiver antenna parallel to the ground. Mounting the antenna so that it is oriented to the front and back or side to side should not make much of a difference because even if the null is pointing in the direction of the transmitter, the aircraft is also (Hopefully) several feet off of the ground and thus the null would be several degrees above pointing directly at the transmitter. Another thing to consider when mounting your antenna is metal objects that could potentially couple to or interact with the antenna. If the antenna is touching other conductive surfaces, the RF radiation will want to travel along those parts as well, this can cause not only very irregular radiation patterns, but will also cause impedance mismatch and greatly reduce the power that actually leaves the antenna. The rest of that power can end up reflecting back down the antenna and into the radio itself. It sounds bad because it is. This is why you will always see people recommending that you mount your receiver antennas as far away as possible from your FPV antenna. Because they can interact and interfere with each other.
Don't let this scare you away from trying FPV on very small craft though, if they have the lift capacity to carry the extra hardware, there is likely enough room to mount your antennas a safe distance away from each other. The rule of thumb here is that antennas should be mounted a minimum of one quarter wavelength apart at the lowest frequency. Lucky for us, this equipment is fairly high frequency hence the small size of the antennas and thus one wavelength is relatively short. If you're using an older style receiver that uses a crystal, you might not have the same advantage as they are much lower frequency around 11 meters or so. So lets look at the frequencies that are common in 2017 and what their wavelengths are.
Your transmitter and receiver is likely operating at 2.4gHz. One wavelength at this frequency is .125 meters or 4.92 inches (round up to 5 inches). So a quarter of a wavelength would 1.25 inches.
If you're adding FPV gear operating at 5.8gHz, thats a full wavelength of .051 meters or almost exactly 2 inches. So a quarter wavelength there is one half inch.
Now obviously the longer wavelength is the lower frequency of the receiver at an inch and a quarter, so if the two antennas are at least that far apart you should be okay. If you want to be more certain you won't have any issues, you could always space them a full wavelength apart in which case you're still only looking at five inches. Typically I have seen FPV antennas mounted to the end of a small diameter, relatively short piece of co-axial cable and a lot of times something is added to that to make the cable more rigid, making some people think this means that it somehow needs to be "Higher up" and will mount it sticking out of the top of their aircraft. You really don't need to do this at all. You can mount the antenna either flat to your wing or fuselage or even on the under side, but it may be better to mount it inside the fuselage laying on it's side when you consider it will have the same shape radiation pattern as your receiver antenna. If there is room inside the body of the craft to do this. If not, simply mount it in such a way that it will introduce minimal drag to your aircraft and won't get snagged or scrape when landing.
If you are using radio hardware that operates at a different frequency than the two I have addressed above, you can use an online wavelength calculator to determine what your longest wavelength will be and arrange your antennas accordingly. Here is a link to one such calculator, but there are plenty of others available that may give you lengths in more preferable units of measure. This one gives lengths in meters as it is intended to be used for calculations at much lower frequencies such as those typically used for long distance communications in the amateur radio bands.
http://www.rfcafe.com/references/calculators/wavelength-frequency-calculator.htm
So what can you do if you find that you are getting interference in your FPV setup or you have jittering servos? Probably the best thing you can do is add ferrite chokes to the leads of any affected equipment. Basically what tends to happen is that you end up with leads or wires that are at or close to a wavelength or multiple of a wavelength of that of your radio equipment which essentially means that it is working like an antenna. It's receiving radio interference because it is the correct length to do so. You can get snap-on ferrite chokes in a variety of sizes so it should be easy to find ones with an appropriate diameter center hole to fit around your servo leads and whatnot. Adding a choke to both ends of long leads will help to impede any stray RF signals that will be traveling along the outside of the wires. I have read some articles where people have talked about constructing tiny Faraday cages around everything and wrapping all leads in tin or copper foil to combat this and they usually end up frustrated because it doesn't work well and takes a lot of time. Chokes can be added in a matter of a few minutes.
Do research before you buy your equipment and mind the return policy. Check and see what other buyers have to say about a product before you decide to purchase it. The guy selling it is always trying to sell it and might not always mention "By the way, this thing emits RF noise like a Mexican radio station" but people who have purchased it and have had problems with it probably will. If you find you are still getting interference from your equipment after adding ferrites you might just have an FPV camera with a really dirty signal or noisy DC power supply. In these cases, moving things further apart in your aircraft or adding foil probably isn't going to help because everything is being powered by the same source and all of that noise can travel freely along the ground wires to everything.
Research before you buy, test it out, add ferrite chokes and if you're still having problems, hopefully you purchased from a seller that honors returns. When dealing with RF, it is always a good idea to educate yourself on best practices. You can also reference the ARRL handbook or similar resources for information on these subjects or ask an amateur radio operator. Most "Elmers" as they are called have been operating for decades and have learned all about RF and EM interference and what can be done to combat or eliminate it.
