Members kevinnem Posted March 13, 2007 Members Share Posted March 13, 2007 so you have a antenna that will trasmit a FM signal. to be recieved by another antenna. I have run accross a number of different polarizations ie vertical , horizontial, eliptical, and circular. Can some one explain the differance to me? I know GPS is cicularly polarised, however I don't knwo of any others. what would a "stick in the air" aka car antenna be? what about the little FM loop antennas that come with stereos? IF I want to broadcast to cars (with a antennea sticking up) and radio with the same, what kinda of trasmition would I want to use? Kev. Link to comment Share on other sites More sharing options...
Members Scodiddly Posted March 13, 2007 Members Share Posted March 13, 2007 You want to do the same polarization - so for an antenna sticking straight up on the receiver, you want the antenna sticking straight up on the transmitter. Of course if you've got a diversity receiver (two antennas), you'd want to cheat the two antennas slightly towards horizontal but in different planes, in case the receiver is tilted a bit. In general wireless audio stuff is vertically polarized. Helical antennae are interesting because they're more able to pick up both vertical and horizontal signals without needing special orientation. Link to comment Share on other sites More sharing options...
Members Prog Posted March 13, 2007 Members Share Posted March 13, 2007 AM/FM is vertically polarized. Television is horizontal (in the US). Link to comment Share on other sites More sharing options...
Members preacherman672 Posted March 13, 2007 Members Share Posted March 13, 2007 We control the HORIZONTAL and the VERTICAL............. Link to comment Share on other sites More sharing options...
Members karlw Posted March 21, 2007 Members Share Posted March 21, 2007 Hi Kevin, I can't answer everything you are asking, but maybe this will help. Scodiddly is headed in the right direction... First off, a car antenna is known as a "whip" type and is really a ground-plane antenna with the car acting as the ground. The short "whip" antennas on typical wireless systems act in a similar manner. This design depends on a "ground plane" to be adjacent to the antenna for it to work. Thus, you can't just extend your antenna closer to the transmitter by running it out there with a BNC cable, unless you set up a ground plane as well. This can be something as simple as mounting the antenna on a blank rack panel, for instance. Whip antennas are vertically polarized but the interesting thing is that they have what I would call a donut-shaped pickup or transmission pattern, and the donut itself is horizontal. In other words, as scodiddly pointed out, you want to have your transmitter and receiver antennas parallel to each other for maximum sensitivity. A slightly more efficient design based on a similar principal is the dipole antenna. This is an antenna the contains its own ground, so it can be extended simply by running it out on a cable. It is also oriented vertically and has a horizontal donut-shaped pattern. These are good receiver antennas when you need to cover a large area but are fairly close to the transmitters since they don't have any extra gain. Like whip antennas, they need to be tuned by being a specific length to be most efficient at a specific frequency. LPDA (log periodic dipole - "shark fin" or "paddle" type) are a wideband antenna with some forward gain. And these have more of an elliptical pattern in that they have +4-6 dB of gain in the forward direction and are more dead to the sides and back. These are useful where you need more reach. They are also vertically polarized and thus you need to orient them properly for good operation - they should be upright, with the narrow end pointed towards the transmitter/s. So if you want to broadcast to cars, you could use just about any type of antenna. Most FM stations use a tower-type antenna oriented vertically (parallel to the car antennas) and they are half-wave or full wave in length to be tuned to a specific frequency for maximum efficiency. You could use an LPDA if you wanted greater efficiency in a specific direction. However, it would be fairly large because of the typical FM frequencies (88MHz - 108MHz). I would suggest reading up in some hobbyist forums on the subject before deciding what to do. The shortwave guys probably have the most collected amateur knowledge about clever antennas. -Karl Link to comment Share on other sites More sharing options...
Members Scodiddly Posted March 21, 2007 Members Share Posted March 21, 2007 I would suggest reading up in some hobbyist forums on the subject before deciding what to do. The shortwave guys probably have the most collected amateur knowledge about clever antennas. I found an old ARRL book about antennas at a thrift store. Circa 1950 and still damn near state of the art - they figured a lot of RF out during WWII. Tough reading, but the chapter on transmission lines was well worth the effort. Link to comment Share on other sites More sharing options...
Members Ear Abuser Posted March 21, 2007 Members Share Posted March 21, 2007 I found an old ARRL book about antennas at a thrift store. Circa 1950 and still damn near state of the art - they figured a lot of RF out during WWII. Tough reading, but the chapter on transmission lines was well worth the effort I have a very well worn ARRL Handbook I bought in 1970 that is still the first place I look for answers to RF questions.(plus it has a bunch of tube data in the back!) There are probably as many weird antenna designs as there are weird loudspeaker designs, and many of them use the same principles such as line arraysor horns. Google fractal antennas sometime for some interesting reading. Link to comment Share on other sites More sharing options...
Members kevinnem Posted March 22, 2007 Author Members Share Posted March 22, 2007 Thanks for the info, it helps alot. I understand teh basics of teh DI pole antenea, and the "log" style, and how they work, and tune. I was just a bit confused as to the relationship between its tramit direction, and its polaristation direction. in teh end, I will use a vertical antenna, to trasmit, .. this will give me even coverage in teh horizontial plane, and null spots above and below the antennea right? The gain would be 0, this is fine (Ithink). I will recieve with standard anteneas for FM radio. I think this will work, .. I sure hope it does, cause people will be paying me for it Link to comment Share on other sites More sharing options...
Members karlw Posted March 23, 2007 Members Share Posted March 23, 2007 Kevin, Yes, I think you have a grasp of it - a vertical antenna will radiate horizontally, with nulls above and below, and provide 0dB gain. -Karl Link to comment Share on other sites More sharing options...
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