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Understood that most of the stations on this forum are AM, where polarization probably wouldn’t likely make much of a useful difference due to the shortness of the antenna. So matters like antenna polarization don’t come up much.
But on FM, it could be a practical measure. In some areas (like this one) it’s not uncommon for houses to have an old TV antenna on the roof. TV antennas may not be as ideal for FM reception as a tuned specialized receiving antenna, but being mounted updoors on rooftops and since they cover the frequencies like the audio for TV channel 6, they obviously cover at least close to the FM range. Years ago, I and friends used to use the TV antennas for tuning in distant stations on our stereos.
I have one “fringe” listener, maybe 60 yrds or a little further away (and with another house between us) , where we somewhat solved her not being able to recieve the signal clearly enough by hooking up the old TV antenna on her roof to her stereo, but also had to hook an old preamplifier to her receiver before we actually got it mostly solid for her. But there are some problems with that, mostly because a preamplifier also amplifies noise, not just signal, and so it fuzzes out when people are upand around in my household and walking around the house.
My antenna location is far from “optimal”, being indoors on the ground floor of a building with aluminum siding on the first and second floors. As such, people walking around in the house sometimes results in the signal fading out for a moment for some listeners.
A couple weeks ago, walking down the street I happened to look at her antenna, and realized (duh) that it was horizontally polarized while my dipole is vertical. So I made an experiment of rotating the dipole and sure enough, the fringe listener’s reception cleared right up. Even tried taking her antenna preamp out, and it sounded good. Except another listener closer in (the house right next door) lost the signal. They were vertical polarized (boombox telescoping antenna) polarized and now almost directly “on the end” of the antenna, so that’s no huge surprise. Moving their antenna to horizontal helped some, but not enough, so I put things back as they had been.
One of the projects I’ve been considering for this summer would be making new and less “jury rigged” antenna (there’s a pic of my current antenna in my blog here, if anyone is curious, it’s rathe cobbled together) and mounting it outdoors and high on the house to eliminate some of the problems mentioned and kill off some of the possible multipath complications. Now, thinking of TV antennas already on some buildings as a possibility for potential listeners within a block or so, I’d already been thinking of the possibility of two horizontally polarized dipoles mounted at right angles to each other on a small mast as a way of doing it without the “off the end” problems. Horizontal dipoles have a bit of a directional pattern, but it isn’t extreme, so I was thinking that a pair of them could provide a *reasonably* omnidirectional pattern.
Then in another thread, WDCX mentioned that broadcast TV and FM stations use circular polarization. Wondering that was and why, I went and looked it up and also checked out some of the homebrew single antennas of that type after finding out it has a fairly omnidirectional pattern and can be picked up by both vertically and horizontally polarized receiver antennas, I’m now thinking it might be the best overall option. More challenging to construct than a simple dipole, and it has (as WDCX mentions) negative gain when only one antenna is used. But considering for legal operation on part15 AM, almost any practical transmitter will have to be attenuated anyway, the -3db or so would actually be “good news”.
I’ve found an acquaintance that actually has a reasonably good car radio, and checked with the manufacturer’s (Alpine) customer service department who bounced me to tech support who told me that the receiver sensitivity specs (at least for their company) in the manual assumes a stock commercial antenna that has been professionally or factory installed. The receiver in this vehicle fits the description and is a simple vertical whip, not an “in the windshield” dipole type, so the specs in the receiver manual should be fairly close to what is listed. So I’m getting a bit more interested in seeing what can be done with gain antennas on the listener receivers.
What I am wondering is with the others here that do anything with part15, does everyone run vertically polarized dipoles or groundplanes or verticals? Is anyone experimenting with horizontal or circular polarized antennas for part15 FM, or has anyone worked with them on something with some similar aspects like ham 2M operation where ideas, problems and observations might shed some light on use of other polarizations for part15 FM?
Daniel
Total posts : 45366When I operated on the 6 meter band (around 50.2 MHz), horizontal polarization was standard, but many operators used vertical polarization. I personally never experienced any problems because of “cross-polarization.” As far as I was concerned, there was always enough polarization orthogonal to the orientation of the antenna to facilitate communication. Horizontal polarization gives slightly better performance at VHF frequencies in urban areas because there is less sensitivity to man-made noise.
The easiest way to get both vertical and horizontal polarization is to tilt the antenna. The radiation pattern is not omnidirectional, but this method is about as simple as it could be.
A tilted antenna was used when the first commercial MW AM station, KDKA, went on the air in 1920. I don’t think that this was by design. It was just a convenient way to hang the antenna wire. The vertical component was useful during daytime and the horizontal component aided skywave propagation at night. Economical radio receivers with excellent performance were not available to the general public until about ten years later. The original MW AM audience was made up of radio amateurs.
Circular polarization simultaneously provides an omnidirectional signal with both horizontal and vertical polarization. The vertical polarization is provided by a verical rod and the horizontal polarization by a loop parallel to the earth. One kind of antenna that provides circular polarization is a large diameter helix wound in the vertical direction. I think that circular polarization is too much trouble to produce on Part 15 FM.
It is not at all true that polarization does not matter in Part 15 AM. Groundwave is generated only by a vertically polarized signal, and groundwave propagation is very important in Part 15 AM. Marconi originally discovered groundwave propagation, which was later theoretically explained by scientists. The early years of radio were dominated by groundwave propagation from electrically short vertical antennas.
Total posts : 45366Rattan,
You can make an inexpensive CP antenna using crossed dipoles. One should be vertical, and one horizontal. You need to introduce a 90-degree phase shift in the feed line to one of them (the amplitude should be equal if you want true CP).
This arrangement works fine, although the pattern will not be omnidirectional. Off the ends of the horizonal dipole, the polarization will be primarily vertical. But you might be able to orient the antenna so that doesn’t cause any problems for your listeners.
Also, the direction of rotation will be reversed off the “front” of the antenna, as compared to the “back” of the antenna. This will not matter to people using linear antennas (dipoles or whip antennas).
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Total posts : 45366Regarding:
“It is not at all true that polarization does not matter in Part 15 AM. Groundwave is generated only by a vertically polarized signal… ”
… I should have spoken more clearly. I meant to say that polarizing the electrically short antenna used on part15 AM horizontally rather than the usual vertical didn’t seem to me like it would provide useful/desirable effects so far as the typical part15 AM station would be concerned.
So far as it being too much bother on part15 FM, well, a resonant antenna for the FM BCB isn’t a lot of copper (or whatever it’s being made of). The designs I’ve been looking at are probably a little finicky to tune, but since apparently circular polarization is sometimes used by licensed low-mid power FM stations, I’m thinking it might be worth tinkering with. Anyway, a couple of the homebrew designs I’ve been looking at are:
http://www.wa7x.com/cycloid_info.html#notes
..which I figure is a 2M design but could be adapted and..
http://members.tripod.com/AMN92/cp_ant.htm
..either of which would take some work and tinkering, but don’t seem overly difficult. Rather than copper pipe, I’d be more likely to try 4 guage copper wire to start with and there’d be some things I’d have to learn and figure out, but I’m not looking at having it all done tomorrow or something, and building a few test models wouldn’t cost much from the looks of them.
Daniel
Total posts : 45366Check out the twisted dipole on the left of this page on PCS’s ad. That will provide simple circular polarization on FM! And they are stackable!
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
Total posts : 45366Yeah, I’d noticed that, Lee. Assuming you mean the “PCS-0210 Single Circular Dipole for low power FM transmitters”? Not a bad price for a commercial antenna, either. Sure does look interesting.
I’ve only really started looking into the circular polarization antenna concepts, it’s new stuff to me, and sounds interesting since some of the commercial FM stations apparently feel the circular polarization is worth their bother.
My thought on it is that if one can get a signal that’s better for the possibility of people nearby but out of “boombox” range to pick up with their old TV antennas if they really want to listen, then it’s worth the bother of exploring. Those antennas are usually horizontal polarized and turning the standard dipole I use sideways made a noticeable difference with the one “test subject” I was able to try it with. From what I’ve read the circular polarization has a “cost” of being about 3 db lower to a vertical or horizontal polarized antenna.. But attentuation is something we have to do on part15 FM anyway. So if it’s a little smaller signal, that’s good if there’s any other advantage that can be gained.. Like adding a low-pass filter attenuates the signal, but attenuates any possible “junk” going outside the band up into the aircraft band even more, so it’s more useful than just doing it with a few more ohms of resistance when we can.
For any practical transmitter likely to be used for part15 FM, we need to attenuate it a lot anyway, so it only makes sense to do some of the attenuation in ways that clean up the signal more or, in this case, possibly offer more antenna options to the *listener*. Also, from what I understand, the circular polarization is less prone to “dead spots” from multipath (or whatever it is) within the close ranges we’re talking about. I’m still working at figuring out the specifics of how that works, though.
A high gain antenna on the transmitter is pretty much counterproductive for FM part15 anyway, since the maximum allowed field strength would logically be measured where the gain pattern is highest. So it’d actually reduce the potential listener area if you’re legal. But high gain antennas on the listener/receiver end are another story, and one of the best potentials I see for getting to more listeners with legal part15 FM. And if some of them already just happen to have such an antenna sitting on their roof collecting corrosion, well.. That’s an intriguing prospect.
Daniel
Total posts : 45366Yes,you are right,circpol does reduce multipath somewhat and covers both vert and horiz receiver antennas. It also fills in the nulls better. The specs say 0db vertical and -3db horizontal gain,so no worse than a 1/4 wave really. I just may have to try one out here. One could literally bend a folded dipole and make a circpole quick and dirty. AND they are broadbanded! It is one way of getting better coverage without adding more uv/m….never use a gain antenna!
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
Total posts : 45366It *looks* like basically a simple folded dipole that has been shaped to do the circular polarization. Not sure how that would affect the impedance or what sort of balun they may be using, but if it’s maybe still 300 ohm at the antenna and 75 or so to the coax, that would be easy enough to put together for the low power used for part15..
Not that the price isn’t reasonable (since it certainly is), but I enjoy making things. *shrugs*
Daniel
Total posts : 45366Email me at wdcx@earthlink.net
I will send you details. Less than $20 bucks for all materials. You Build. Unity Gain.
NO EXTRA CHARGE FOR MY SAGE LIKE ADVICE.
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
Total posts : 45366This is a simple approach giving some radiation that is horizontally and vertically polarized — but not in all directions (click here ).
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