Jump to content

Recommended Posts

Hi folks

I'm new to antenna design and my knowledge to polarization are poor.

Can any of you explain to me the differense between vertical, horizontal and any other polarization i don't know about ??

And what is the best solution for my video downlink ??

Regards

Tommy ( Norway )

Share this post


Link to post
Share on other sites

Hi Tommy,

This is a great question, and a great thread to start on the new forum with it's nifty search feature.

** Side note to Mr. RC-CAM, I think this topic definitely deserves to be in a FAQ

First note that horizontal and vertical polarization are descriptions of a 'linearly' polarized antenna. Another polarization is 'circular' polarization, and can be described as 'left-hand circular polarized' (LCP) or 'right-hand circular polarized' (RCP)

Antenna's of a given polarization are most effective at receiving RF signals from antennas of similar polarization. So a vertically polarized antenna will receive the maximum possible signal from another vertically polarized antenna. Conversely, the vertically polarized antenna will receive a much reduced signal from a horizontally polarized antenna.

The kicker for our application is that the polarization of the antenna can change simply by changing the orientation of the antenna. So a vertically polarized antenna can quickly become a horizontally polarized antenna when our aircraft is banking hard.

One way I use to solve this is to match a circular polarized antenna on the ground to the linearly polarized antenna on the aircraft. In this manner, the changing orientation of the antenna on the aircraft is recieved more uniformly by the antenna on the ground. This method does not come without cost however, since I loose some sensitivity when I use a linear polarized antenna with a circular polarized antenna, however I accept this shortcoming in exchange for more uniform coverage.

Another way to solve it is like cyber-flyer. He used a 'turnstile' antenna on the airplane which simultaneously generates a horizontal and vertical polarized signal, and then receives it with a high gain linear polarized antenna on the ground. This is the setup he used to acheive some spectacular altitude. More recently I understand that he is using a CP antenna on the ground, mated with the horizontal/vertical polarized antenna in the air.

I hope this helps you get started. A lot of our success comes from trying different antenna combinations until we find one we like!

Bill

Share this post


Link to post
Share on other sites

Electromagnetic waves wich also includes light, can be thought of as if you swing a rope wich is attached to a tree in the other end. This is circular polarization.

If the rope passes a vertical slot on its way to the tree, it will only move up and down at the other side of the slot. This is vertical polarization. If the slot is turned 90° to a horizontal position, then the rope will swing sideways (horizontal polarization).

This is how polaroid sunglasses work, they have microscopic bars that polarizes the light. If you have two polaroid sunglasses, then you can look through both of them and see how the light actually is cancelled out when aligning them at the proper angle. Direct sunlight is not polarized in any way. When it gets reflected by a surface, such as water for example, at least some of it gets lineary polarized. This is how polaroid glasses removes some reflexes and makes it easier to spot a fish.

When a dipole-antenna is rotated around the horizontal-plane, it will only send truly vertically or horizontally polarized signals when oriented vertically and horizontally respectively.

As long as two lineary polarized antennas are parallell to eachother, the polarization is the same, no matter what angle they have relative to earth.

A wild guess from me, why not always circular polarization is used in radio transmission, is that traditional antennas are lineary polarized by design. Circulary polarized antennas such as Helix antennas, are not always the best choice for a number of reasons.

Another wild guess: TV-sattelites uses different polarization on different channels so that the channels can be closer to each and thus save frequency space.

Best Regards,

Bosse

Share this post


Link to post
Share on other sites

Bill, I had already purchased the 8db patch antenna, but your information

on your site about the circular polarization going well with the linear dipoles

that come stock made up my mind on which recieve antenna I wanted to try

next to eliminate dropouts. That combination works brilliantly!!! :D

Yesterday, I flew a SIG LT-40 using a 1 watt transmitter and the stock antenna transmitting to the patch antenna mounted on a camera tripod, no dropouts, great signal to 1000 feet agl!

Im very pleased to find a combination that finally works after a year of

frustration. Thanks guys!!

Mike Robinett

KE4UVQ

Eyes Overhead RPV

Share this post


Link to post
Share on other sites

I suppose I really should send people to Bill's site.

http://www.blackwidowav.com/patchantenna1.html

I got this antenna from somewhere else, but it is the same antenna.

I tested it a few months ago with Dave Jones of AUAV in

Palmetto, FL while he was testing a UAV for the Univ. of Florida,

I pointed it at the plane using my separate setup and it didnt drop

out! I was impressed, so I use the same setup now with pleasing

results! :D

Edited by KE4UVQ

Share this post


Link to post
Share on other sites

KE4UVQ,

1 Watt ?

I am not trying to be smart here but 1 Watt is relatively a lot of energy, isn't it ?, I mean dropouts are not supposed to occour within 500 meters no matter what polarization mismatch you might have at any given time, at least not according to my experience ?

I am just curiuous and interested so take my reply right.

Best Regards,

Bosse

Share this post


Link to post
Share on other sites

Correction:

Sometiomes, people, including myself, are being picky about details and exact formulations:

"Energy" should have been "Power"

The words "Power", "Energy" and "Force" are commonly misused and despite of myself knowing that, I fall into the trap anyway.

Best Regards,

Bosse

Share this post


Link to post
Share on other sites

Dropouts are going to happen, I know. But I had

quite alot with the 100 mw transmitter because placement

and polarization of both antennas became crucial..and

frustrating. I witnessed the 1 watt transmitter in action

and saw what a difference it made and said "I gotta have that!"

Also, I want to go a little higher than "fun-fly" range and

really get some great views.

Your point is well taken and questions are how we learn.

I know I havent learned everything yet! ;)

Share this post


Link to post
Share on other sites

I think the point Bosse was making is that it is hard to say if your aerial system is working well as you are using a lot of power which will hide any failings.

I myself had poor range using 100mW TX and was thinking I needed to increase power but as soon as I tryed a low noise pre-amp it transformed the set up !

Since then I have increased power to 1/4 W and at 1 mile range have VERY few dropouts, its the r/c link that fails first.

Terry

UK

Share this post


Link to post
Share on other sites

As long as I fly my model at level flight and the view angle to the model does not exceed ~ 45°, I have perfect video signal within 500-1000 meters horizontally and substantially more at higher elevations. And that is with a nominal radiated power of 25 mW.

To achieve this, I have to point a GP patch to the model. If I use a Bi-Quad, range is at least doubled.

If I wanted to fly at extreme altitude (several thousands of meters), above my head, I would install a patch or Bi-Quad on the aircraft and replace the otherwise very well working ground plane antenna of Yb2normal's design.

Best Regards,

Bosse

Share this post


Link to post
Share on other sites

Bill,

I few more questions about your CP patch. Have you compared it's maximum range against the goof proof patch, (assuming the GP patch is recieving a properly polarized signal)? I would think that the CP patch would still have further range with it's higher db gain. also, what are the implications of using a right hand polarized versus a left hand model when pairing it up with a non CP groundplane of a single polarization? does it matter?

Share this post


Link to post
Share on other sites

I am by no means an antenna expert but I'll take a stab at answering your question.

First, the theoretical gain of the GP patch is 8dbi, so the gain should be about the same.

Having said that, a properly oriented GP patch *should* give you better range than the CP patch because there are losses involved with matching a CP patch with a linear polarized antenna. The main benefit of the CP patch is that your signal reception should be more consistent (albiet down in gain) regardless of the polarization of the linear antenna.

Note that the CP antenna will not magically pull signal out of the air when it is not there, like in the case when you are in a null point of your linear antenna.

Regarding LHCP versus RHCP, my understanding is that there would be no difference between the two when matched to a linear polarized antenna.

Regards,

Bill

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×