Antenna Directions

[toter]

Have two antennas for long range drone. What is the best way to place them.
One-both horizontal for max gain or
Two- one horizontal and one vertical

Any thoughts

[OldSalt]

Advice on this varies widely.

Antennas all have a particular polarization as well as a 3D radiation patten. Polarization refers to the orientation of the electric field in a particular direction. Best reception (or transmission) of a signal happens when the transmitting (or receiving antenna) antennas are both aligned along the maximum gain of both and the polarization of both (E field alignment) are the same. The signal decreases when either the gain alignment or polarization alignment decreases. It is not well understood that both radiation patterns and polarization needs to be considered in RF links. Think of how polaroid sun glasses lighten or darken as the lenses are rotated. This is due to matched or crossed polarization. The same thing happens at 2.4 GHz but it is harder to demonstrate.

Assume a single dipole like antenna on the RC transmitter. The gain pattern is a torus (donut like) with pattern nulls in the direction of the dipole ends and the polarization is linear, aligned with the dipole. The strongest signal is in a plane perpendicular to the dipole provided the receiving antenna is polarization aligned parallel to the dipole.

If flying models did not change orientation antenna placement would be less of a problem and keeping antennas aligned would be easier. Since relative orientation does change other accommodations are often in place. Pairs antennas, one linear and one circular can be used. Polarization loss is less but there is a -3 dB loss associated with this configuration. Circularly polarized antennas are much more complicated to build and are physically larger. They are rarely used. They can have advantages but if not carefully aligned also suffer significant performance loss. They are also expensive. I see occasional use if circular antennas in drones with telemetry and cameras.

More common are diversity receivers where two antennas are provided. The receiver usually connects to the antenna with the stronger signal and ignores the other. Most common are two dipole or monopole like antennas. The idea here is to place the antennas 90 degrees apart with the idea that as the aircraft maneuvers and looses signal from one antenna the other will gain strength. Recognize that one can have bad luck and find a configuration where both antennas are "cross polarized" with third. Most of the time this does not happen. The general guidance is try to place one of the receiver antennas vertical and one horizontal. Occasionally the model configuration makes this difficult. A second configuration is for both antennas to be horizontal but again 90 degrees apart in the horizontal plane. This works well until the transmitter is held with it's antenna vertical. In that case neither horizontal antenna receives the signal well. Try to keep the transmitter antenna close to vertical and thus relatively well aligned with the vertical antenna on the receiver. If one is forced to use two horizontally configured receiver antennas the best situation is to also keep the transmitter antenna in the horizontal plane as well. Unfortunately now there are also now two patten nulls in the horizon from the transmitter and more from the receiving antennas. Another configuration to consider is to place the two receiver antennas 45 degrees from vertical and still 90 degrees from each other. I would choose this as my second choice rather that the two horizontal receiving antennas often suggested for consideration, again assuming a vertically aligned transmitting antenna.

More complicated communication systems sometimes combine signals from multiple antennas but it is not common in RC systems. Pricing, size, complexity and weight make this prohibitive.

I note that the original question did not describe the transmitting antenna nor the details of the receiving antennas. The answer is probably one vertical and one horizontal is preferred if the transmitter is a dipole and vertically aligned. Most of the time the vertical antennas would work best. When maneuvering the situation is not so clear. When the vehicle is directly overhead neither receive antenna would work well.

[jhsa]

According to my experience, if you fly gliders, normally overhead, both antennas horizontal on the plane and at 90 degrees to each other.. Radio antenna also horizontal.

For normal (acro) flying, in my opinion, one antenna horizontal along the fuselage, and the other vertical.
Radio antenna pointing down at an 45 degrees angle. Why this? Because in this case it is very rare to fly the model overhead or even behind us. With the radio antenna in this position, you always have the side of the antenna aligned with any position your aircraft might be, even when low and to one side on landing. And as the weakest points of the RF transmission are the tip and back of the antenna, the weakest link is towards the ground in front of you, and your own head (back of the radio antenna), which are the places your aircraft is very unlikely to be
There is one disadvantage with having the antenna in this configuration though. It is very easy to break it
So, be careful when you put the radio down.

João

[OldSalt]

If you really want to get into the subject of polarization here is a reference:

https://www.amazon.com/Polarization-Ele ... 1630811076

Polarization loss tables showing coupling between antenna types are posted in various locations on the internet. I have not looked lately but you might find one on Microwaves101.com or one of the many antenna related sites around. PhD candidates and Ham operators often post on antenna subjects.

Offsetting the transmitter 45 degrees introduces 3 dB of loss between the vertical and horizontal antennas on the aircraft. This is as well as any pattern loss. A 90 degree angle difference results in around 30 dB in loss unless the misalignment is precise and then it is even more. Fortunately perfect misalignment is hard to achieve and hard to maintain in moving systems.

The all horizontal antenna solution has an issue if the aircraft is going up or down and the either transmit or receive gain patterns nulls are aligned. No signal is relayed and things get bad quickly if the aircraft is in a dive.

My intent in the post is to describe briefly how both pattern gain and polarization both play into how much signal is received. Both should be considered as mis-alignments in either can result in loss of connectivity. A 6 dB loss of signal halves the maximum range and loosing another 6 dB cuts the range to a quarter. I also wanted to suggest several alternate configurations as examples. There is no best configuration and all are compromised. Adjustments can be made for particular situations.

It is unfortunate that aircraft have to fly after the antennas are installed. Antennas are always too big and small ones do not work well.

[jhsa]

Offsetting the transmitter 45 degrees introduces 3 dB of loss between the vertical and horizontal antennas on the aircraft. This is as well as any pattern loss.

Yes, but you have more chances that the side of the TX antenna is pointed at the aircraft. Also, normally the aircraft flies at an angle in front of you. I mean, up and in front of you.
You always have a loss even with the TX antenna is in the most ideal position.. you just have to bank the aircraft to mess it up So, the antenna pointing down towards the ground at an angle might ensure a more stable link in all directions, except behind you.

The all horizontal antenna solution has an issue if the aircraft is going up or down and the either transmit or receive gain patterns nulls are aligned. No signal is relayed and things get bad quickly if the aircraft is in a dive.

That is why I said that this setup is only good for gliders flying overhead, not acro flying.

João

[OldSalt]

Each configuration has advantages and disadvantages. One can focus on the "good" features or "bad" features or try to consider both and thus the greater opportunity to have a discussion of how to consider a particular configuration. Generally increases in signal levels are modest but the nulls can be very deep. Safety needs lead me to try to eliminate as many of the regions where there are deep fades as practical and I advise others to do the same. Both polarization and radiation pattens need to be considered for greater understanding and performance. I see the type of aircraft flown as just special cases and do not want to confuse those discussions with the basic ideas of polarization and radiation patterns.

The loss I am referring is separate and distinct from what is normally considered in the Friis or one way radar equation. Yes, there is always loss but that is like saying entropy of a system increases over time. I am not sure how helpful those statements are to the understanding of polarization and radiation patterns. Link budget analysis can get very complicated and the results are not easy to evaluate. Parameters include signal to noise ratios, energy per bit, system sensitivity, link margin, background noise and interference levels and other issues. There are probably two dozen factors if not more that could be added to the discussion. I am discussing only two of a myriad of cogent factors. I could also go down a rabbit hole to discuss if the loss factors are really attenuation or just a result of the spherical signal spreading in space as it radiates further. There are numerous books on the subject. I did not go into details in a few sentences of my first post.

Actually installed antenna patterns are not the idealized patterns people often use in discussions like this. They are very changeable depending on objects and materials nearby. The antennas used in RC models usually favor "cheap" and "small" over performance. The transmitter antenna is the same and the radiation pattern changes as the user moves his/her body and hands around in the near field. Consider the moving propellers on a drone, they introduce a modulation into the received patterns and the modulation can occasionally be visible in the receiver. Control surfaces on sailplanes can also disturb/change polarization and radiation patterns. Carbon and metal in the structures can also be very influential. Shadowing is the most obvious effect but it does not stop there. In terms of detail we almost never know antenna parameters like gain and directivity, polarization characteristics in all directions, s parameters and the effect of nearby structures, isolation and interaction with nearby antennas, various bandwidth considerations and changes of characteristics with frequency, etc. Those factors are strong contributors and a discussion of antenna performance can also get very intense. I could make more book recommendations for the interested individual.

I also note that as the transmitter is in your hand and one can change the transmitter antenna orientation to make the situation more favorable as the aircraft changes orientation. I have suggested two parameters to keep in mind if one wants to move the transmitter to a more favorable position. Some RC systems now link RSSI back to the user and thus give feedback when adjustments might be appropriate and also tell when the link is performing well.

I have provided some guidance to decide how the RF link behaves in response to (both/either) polarization and radiation pattern. There is also a very detailed reference cited if someone wants to dive deeper into the polarization subject. There is not a configuration that I am willing to label as "best" in general. If one has something that works in a particular situation by all means use that and if one were to make additional changes I have hopefully provided some insight for how polarization and radiation patterns interact and change how much of the transmitted RF signals arrives at a receiver.