FM antenna technology

[artto]

I was looking at a Tivoli table radio for my wife's office at work and inquired their tech support about possible reception problems in her new building (typical steel frame and and aluminum siding office+warehouse with tall glass windows along the office side) about 25 miles from downtown Chicago. He said the radio uses the same antenna technology employed in cell phones and that reception should be absolutely no problem.

All this started me thinking. I have a dual array of large Winegard TV/FM antenna on the roof of my house that I still use for FM reception. My location seems to be in a null (weak) reception area for my usual favorite FM station, WXRT. Under certain conditions which seem to be a combination of weather, aircraft and interference from another station broadcasting on the same frequency near Rockford, IL, I sometimes get multi-path noise that even the Mac FM tuner can't seem to resolve adequately. This is typically an intermitent problem.

My question is are there any "new" outboard FM antenna that use the same cell phone technology that I can hook up to the FM tuner indoors that would outperform the dual array stack on my roof?

[jhoak]

I have been very pleased with the performance of the Magnum Dynalab ST-2 antenna in my application. I do have it mounted outdoors though.

From what "technical" notes I've read of late most if not all FM stations have moved to a "vertical" polarization for their broadcasts as opposed to "horizontal" polarization. This is the reason that your car radio picks up so much better than your at home FM tuner.In my case the ST-2 clearly outperforms the dedicated FM yagi antenna on my roof even though the yagi has a 10' height advantage.

There are a number of vertically polarized FM antennas available which brag of even better performance than the ST-2. One of the designs that I plan to experiment with soon is called "J-Pole".

[WMcD]

Interesting post.

Let me write with a little bit of ignorance.

I think the cell phone technology antenna notion is mostly bull feathers. There may be some tricks to get a working antenna in the small cell phone device. Yet physics still applies. The roof top yagi is probably doing as well as anything as far as having a given amount of metal in the EM field. It does not seem that you are suffering from lack of signal output from the antenna into the receiver.

I have lost an article from long ago about using diversity antenna set ups for FM. Maybe it out there someplace. Essentially you set up two antenna fed to a T connection. Then you adjust the placement of the two. They are to be perpendicular and equidistant to in your case, the Hancock Building. But you adjust the spacing so that they are 180 degrees spaced relative to Rockford. In this way the signal from Hancock adds and the signal from Rockford nulls.

BTW. A few decades ago the Chicago downtown transmitters had a power failure. My receiver about 3 miles north of Hancock suddenly picked up Rockford.

I've been considering HD FM to listen to WFMT at home, again a few miles north of the Chicago downtown. This because there is a lot of wicked multipath at my location. It looks like Sony had a nice receiver which is now out of production and going for premium prices,used. HD receivers are part of the higher end oriential AV receivers. Mac is very expensive, of course.

I notice that car receivers with HD FM are much cheaper and so, someday, I might try adapting one of those. for home use.

You can Google or call WXRT. It looks like they have three HD signals and programs. So I'm thinking that if you love WXRT and want to listen to all they have, you might consider an HD FM receiver hooked up to the roof top antenna rather than futzing with a new antenna. Of course there is also streaming WXRT.

Best,

WMcD

[russ69]

My location seems to be in a null (weak) reception area for my usual favorite FM station, WXRT.

Call the station and talk to the station engineer, he/she will be able to give you tips for best performance.

[JJkizak]

Cell phone antennas use "Fractal" technology, actually a bunch of squiggly formations to get a big antenna into a small space. If you put anything that "receives" into a grounded screen room it will not work, that is as long as the receiver is not hooked up to an external antenna through the shielding. Metal siding (which is grounded by the way with grounding rods) will impeade signals. So do trees, mountains, and buildings. Certain geographic areas not covered by the transmitting stations don't help at all.

JJK

[artto]

Thanks for the input guys! Plenty to think about.

Let me expand on this a little. The dual array antenna I'm using were set up on the roof probably around 1980 when I got my first big screen TV. One was intended to be used "upstairs" and the other "downstairs" for the then to be built "klipschorn room" (with TV intended to to be supported in there at some later date). Both of the antenna are VHF/UHF probably similar to current Winegard HD7694P Platinum HD Series Antenna, one mounted above the other on the same mast separated by about 5-6 feet vertically. After going cable I ganged the two antenna together and split the signal before going into the house, one going "upstairs" and the other the K-horn room. As Gil mentioned, there's plenty of gain. Both antenna are directional, originally aligned for maximum signal in the direction of Chicago (Rockford maximum rejection).

Sometimes I wonder if this setup is causing or contributing to some of the problems under certain conditions. The problems are worst on WXRT. WFMT affected somewhat while others like WLUP seem almost not affected at all. The multi-path problems usually occur on really bad weather days ~ stormy, high westerly winds, aircraft from O'Hare taking off into the winds. I live about 10 miles west of O'Hare. I've tried using only one antenna or the other but it doesn't make much difference. Way back I used to have an Apple II computer in the office which was located almost directly underneath where the antenna are on the roof. After the computer was on for a while the radio signal would start to deteriorate. After several hours it became virtually intolerable being mostly noise. Newer computers being shielded better don't contribute to the problem.

I guess one of my questions is, are the VHF/UHF directional antenna part of the problem? Same goes for the way I have them arranged? Maybe just a regular FM antenna should be mounted up there in place of one of the VHF/UHF? Or would one of the new fangled indoor antenna actually do a better job?

I should also mention that the listening room is partially below grade (42"). The aluminun siding has been replaced with vinyl w/integrated insulation so the house is no longer shielded like it used to be.

[JJkizak]

Airplanes are a huge factor in multipath and it can be short and severe.

JJK

[Arkytype]

Hello arrto, et al

Just a short checklist of things to consider:

1. Check your antenna-to-balun connections for corrosion.

2. Some VHF/UHF/FM combo antennas have a moveable shorting element to notch out FM reception. Make sure yours in in the FM-pass position.

3. Make sure your coax lengths are the same going to your combiner.

4. WXRT's frequency of 93.1 MHz translates to a wavelength of 10.56'. Your spacing is close to a half wavelength. You might set the boom to boom vertical spacing at 5' 3".

I've attached two links which have good info on vertical FM broadcasting and also a How To Guide for combining antennas.

I've always had good results using DX Antenna splitters and combiners. For short coax runs I use quad shielded RG-6 type. For longer runs, either use a quality preamp at the ntenna or low loss coax such as RG-8 type. All connections should be first taped with quality (e.g. 3M) electrical tape and then covered with a conformal tape for years of trouble free reception.

Lee

http://www.kyes.com/antenna/stackluge.html

http://iris.nyit.edu/~sblank/VPforFM.htm

[TheEvan]

Informative! As are the links. This is good timing as I will be installing the medium-range VHF/UHF/FM antenna I ordered in a few days.

[mustang guy]

The speed of light is 983,571,056 feet per second. 93.1 MHz is 93,100,000 waves per second.

Take the speed of light divided by the frequency, and you get the wavelength. 10.56' or 10' 6-3/4" long.

VERY VERY INTERESTING! Thank you for making me look at it this way. I actually understand it now.

The wavelength of my favorite station 106.1 is 9' 3-1/4".

Why do I find stuff like this so interesting?

[artto]

Geat stuff guys ~ Thanks!

I'm not sure I'll be able to do anything as some of the suggestions unless I have someone else do it for me. These antenna were put up 30 years ago, my eyes and balance are not what they used to be. Any youngsters out there that might like to help?

[]

[pi] [G] [8]

[LarryC]

Hi, Art -- I have a Magnum-Dynalab ST-2 and it works fine for me. Fine in the sense that stronger signals usually come in OK, while the Baltimore station that's 40 miles away and behind a nearby hill are touch and go. The dipole is bigger and higher up but hasn't worked any better, and working the rotor is a hassle.

You might consider using an antenna co., if not too costly, to mount an ST-2 and string or hook up the wire to see how well it works.

Obviously I don't have a surefire lowest-cost solution.

[WMcD]

Just to add to the thread.

One thing which may help is an antenna dedicated to the FM band. See attached. It has the benefit of on axis gain, but not too much. Importantly, though, it has good front to side and front to back ratios. If you are bothered by transmitters or reflections from other than downtown, this could be a worthwhile advantage.

It seems to me that you have an antenna meant to cover the VHF (2-13) and perhaps UHF (those higher number channels). We'd have to look but I believe all VHF channels are no longer used. The so called HD channels now active are in the UHF range, From what I understand, BTW..

The FM band and aviation communication are within the VHF TV channels but we can look up specifics. .

In any case. I believe the old TV/FM antenna's don't have the good front to back and front to side ratios which could benefit you. This is because they are they are a compromise to cover the now old TV channels.

I've also looked at FCC records on WXRT. It looks like it, and most other FM broadcasters, are set up with both horizontal and vertical polarization. This means the vertical whip on a car and a horizontal yagi both work well to receive it. Also slanted whips on cars.

To my understanding that (H and V) means there is a circular polarization in the broadcast signal which should be clockwise or counter-clockwise. Here I'm not absolutely certain.

However, it is pretty much well recognized that the action of reflecting a signal flips the polarization of a circular polarized transmission from CW to CCW and vice versa. A dipole or ordinary yagi senses mostly the vertical or mostly the vertical (you can change the orientation of the wire) and thus picks up only half the transmitted power (3 dB loss).

But, this also means something when transmitting and receiving antennas are both of a circular polarized design. If they are matched (say both CW) you get back that 3 dB loss. But if one is CW and the other is CCW, you get a very great loss. Let me say 20 dB in practice. So we have to be careful.

This is important. One item is that a circularly polarized receive antenna must match the "handed ness" (CW or CCW) of the transmit antenna.

The second is that a signal bounced off a surface (multipath) has it's polarization flipped. Therefore, if the transmit and receive antennae are matched in rotation, the receive antenna is very insensitive to signals from the same transmitter which are bounced from an airplane or building. This is not a matter of the front to back or front to side ratios.

It may be important here to step back. Elecrto-magnetic wave(s) (radio) have this polarization and that is because it has both a magnetic field and and an electronic field which have their respective phases (90 degrees ahead or behind). Sound waves in air don't have these two fields. Nonetheless, in a gross sense, in the case of multipath, the radio EM fields can add and subtract in phase, which makes multipath so destructive.just like refections and multiple sources in a room. So we can in the radio, use the polarization issue.

I have not found any home receive FM antenna which has circular polarization. There are some ham radio sources, though.

http://sv1bsx.50webs.com/antenna-pol/polarization.html

I don't know that I'm any better going up on your roof than you are, for the same reasons. Smile.

WMcD

CL-FMRX[1].pdf

[Arkytype]

Gil, et al,

When I opened the Scala info I thought to myself (with apologies to George Lucas), "Now, that's a name I've not heard in a long time. A long time."

Back in the '80s when I was a real radio injuneer, Scala antennas used for Studio-to-Transmitter-Links (STL) were noted for their high quality and low cost. They had a 48" high antenna that was UPS shippable! For short hopsp, they were great. We used six and ten-foot parabolic dishes for our hops which were tens of miles long.

The CL-FMRX looks rugged and like most all commercial antennae, its characteristic output impedance can be ordered with 50 or 75 ohms which negates the need for a 300 ohm to 75 ohm balun. (don't ya hate those things?) Wonder what would happen if you combined a vertical and a horizontal model?

VHF TV channels are still around. The FCC (seemingly run mostly by lawyers, these days) prodded by spectrum-grabbing wireless users are determined to "reclaim" the spectrum that over the air TV broadcasts utilize. I predict that in ten years TV stations will have so much ".....reasonable but defined interference" (to quote one FCC official) that they will throw in the towel. All full power TV stations just spent the last decade transitioning from analog to digital transmission at a cost of at least 1 million dollars per licensee. Now the wireless moochers want to utilize the spectrum between stations (the so-called white space) and promise that their gear won't cause interference. BS.

As far as circular polarization goes, Gil, you are correct regarding power allocation: half goes to the horizontal component and the other half goes to the vertical component. That is probably a FCC-mandated rule but don't quote me. Probably 99% of FM antennae are vertically "polarized". The old home stereo consoles AM/FM/Phono of the past used a 300 ohm twinlead dipole stapled horizontally to the back of the cabinet. The decision to require CP transmission made about as much sense as the 1961 decision by the FCC to approve the Zenith/GE system as the standard to use for stereo FM broadcasts.

FM and VHF TV antennae with more than one transmitting bay or element will utilize vertically stacked elements spaced either a half or full wavelength apart so as to take advantage of mutual coupling. Each element is fed either with the same power and phase as its neighbor or with a fixed phase shift in order to create a desired radiation pattern. In general, the more bays an FM antenna has, the higher the "gain" of the array and the higher the effective power that is transmitted. Of course, an antenna, like a transformer, cannot boost power.; it increases (or in some cases decreases) what is known as Effective Isotropically Radiated Power or EIRP.

In the case of KKYK-FM where I was the chief injuneer, we had an FM transmitter that delivered 30 kW to the transmission line flange located just above the output tube. As the 10-bay antenna was located about 900' on a 1000' tower, a low-loss 3 1/2" O.D. copper waveguide with center coaxial element (very expensive in any economy!) was used in order to meet our 100 kW EIRP licensed power.

Due to the antenna height and the fact it was located on a mountain west of Little Rock, the antenna manufacturer designed in a 5 degree downward (keep that word in mind) beam tilt in order to not direct (wasted) power above the radio horizon. When the antenna was installed (before my tenure there), the tower crew, not realizing there was a preferred orientation, mounted it upside down! The antenna was designed to achieve the downward beam tilt electrically, not physically, so the mounting brackets were the same top to bottom. DOH!

After a month of complaints from outlying listeners who suddenly found themselves in a "deep fringe" instead of a "city grade" signal strength profile, the station called in a consultant who flew a circular pattern around the tower in a Cessna 150. His RF field strength meter readings told him that the signal strength at 25 miles was stronger at 5,000 feet than at ground level! Re-hanging the antenna in the preferred orientation solved the problem!

The ideal commercial broadcast transmitting antenna would have an omni-directional pattern. This would make the station manager happy as there would be listeners/viewers in a 360 degree circle around the tower. In reality, the tower structure and the antenna array will usually create a null or notch on the "backside" of the antenna. In the case of KKYK, the decision was made to aim the front of the antenna in the most populated direction and the null was aimed at a more rural population density.

The ideal receive antenna would have infinite front-to-back ratio, narrow acceptance angle (beam width) and never ice up! Because we live out in the boonies of NW Arkansas, we've got a 15-foot-long VHF/UHF antenna atop a 55' tower. Even tho the antenna is shooting into a mountainside a quarter mile away, it brings in a 5 by 5 digital TV signal from several stations up to 60 miles away. So much for the "You have to have a line-of-site path to receive digital TV" argument!

Lee

[Arkytype]

ARGHH!

Guess

my

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key

wasn't

working.

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[dtel]

I notice that car receivers with HD FM are much cheaper and so, someday, I might try adapting one of those. for home use.

I use one in the workshop. What got me thinking about a car stereo was the better reception even for just FM, get tired of changing cd's out there.

I remembered the Pioneer "Super Tuner" it was called, I had many years ago and started looking and found the new version. It has CD, HD radio, AM, FM and a plug in for I-pods or other contraptions and It also has pre-outs for under $150.

I connected a Crown D-75 to the pre-outs and power a pair of mwm bins with a diy split LaScala top with AA crossovers.

I does Fm well but being 60 miles from New Orleans I was only able to get one HD channel ( on occasion ). Now this is with just a auto replacement antenna mounted on a block of wood.

The HD station was the Delta, all Blues, and the HD really does sound better. I plan on building a bigger workshop and I will probably put a taller more permanent antenna.

One thing I was surprised at was the car radio seems to have more bass than anything I have tried out there, I don't know why, it sounds good. [Y]

Not long ago I unplugged the Crown and tried just the radio's amp, I was surprised how loud it went and this was outside. The reason i did this is because I can use the radios amp and not need AC power. I leave it connected to a car battery in the workshop with a trickle charger connected.

This all might sound crazy [] BUT I have forks like on a forklift than connects to the front end loader of my tractor, the speakers are sitting on a frame I made with wheels under it to move them easily. I scoop everything up with the forks and move it anywhere in the yard I want, 6 acres.

Portable party, they can even go in the truck or trailer and bring them anywhere with just the car battery and play all day without a charge.

Sorry it took so much to explain, didn't mean to drag it out.......but it really works good, and sounds really good outside. []

They went up a little since I bought it.

http://www.pioneerelectronics.com/PUSA/Car/CD-Receivers/DEH-P5200HD

[jcmusic]

Let me add my expirence with my small Yagi antenna. I had it on my roof for awhile it worked really well except, when trying to get my two favorite stations there was always multipath. Funny thing happened I put a new roof on after a hail storm and decided to try the antenna in the attic instead of back on the roof. Well what do you know!!! With the help of my wife who hears like a dog, the multipath is gone and the two stations now come in crystal clear!!! Go Figure!!!

Jay

[garyeanderson]

There are a number of vertically polarized FM antennas available which brag of even better performance than the ST-2. One of the designs that I plan to experiment with soon is called "J-Pole".

After reading this and dealing with my lame wire antena I looked up the "J-pole" on google and found a couple of good sites

http://www.hamuniverse.com/jpole.html

After a bit of reading and a think about it, I went an found some scrap copper pipe in the garage and made up a couple. They seem to pull some of the lower powered stations in as well as I can remember so I am pleased. Cost out of pocket was zero so it fit nicely into the winter budget. I have them indoors as I don't want to get into ground rods or any perminate solution.

[mustang guy]

Is there a reason we can't just take a car antenna from a junk yard and hook it to the receiver?

[artto]

Lots of great stuff here, thanks a million!

Now, to make matters just a little more complicated ~ there's something I just remembered. [8-|]

Between me and O'Hare are two large transmitting antenna, one for WBBM and one for WGN. Basically they are about 3 miles east of me, both at aprox. 45 degree angles, one SE and the other NE of me on a line from my house to O'Hare. The planes usually are somewhat south of me on takeoff, kind of depends on wind speed and direction.

But like I said, this typically occurs during lousy weather, like the blizzard last winter, or those cold high wind rainy fall storms that hang around for a while. Those two towers in combination with the air traffic pattern and cloud cover are probably the main culprit.

I think I might try a Godar FM1-A indoor antenna since the room is a little more than half way above grade and the aluminum siding is gone. That way I can move it's location and/or orientation when things get dicey and see if I can dial in the station better.

This IS an issue because I sometimes record live concert broadcasts of material that is not commercially available and the multipath noise and dropouts has ruined more than a few recordings.