Directional FM

[davideduardo]

Here's an article from Radio World you might find interesting:

The Omni vs. Directional FM Antenna

That article is definitely worth a read. In particular, it mentions the tower mount as a way of achieving directionality to prevent reflection from terrain higher than the antenna "behind" the site.

When I built HCTM1's new site on a mountainside about 2500 feet above Quito in 1968, we used an adaptation of panel antennas with 4 elements, vertical only, spaced every 45° on a pole with mechanical beam tilt to cover the city directly below us. We adjusted the tilt with a winch and chain until we optimized it by observation, and then bolted it into permanent position. But the main objective was to not radiate behind us where the mountain continued on upwards.

We also used a full wave height reflector behind the array of radiators. It looked like a fence. All made with air conditioning copper and aluminum locally.

I've seen (or, maybe better said, "heard") quite a few FMs that did not take reflections into account and ended up with lots of signal coming back and causing what the listener called "a bad signal" in areas that were theoretically in the 70 dbu contour or above.

 

[Kent]

Why would the FCC set up full power FM allocations such that power reductions are required in certain directions to protect other full power FMs?

There are several reasons the FCC allows short-spaced allocations. So-called "contour protection" stations have been allowed, I believe, since at least the late 1960's. David mentions one reason the FCC allows that.

Here's another. What the FCC considers as short-spaced hasn't always been what it is today. In the case of many of those directional FM's on the East Coast, they weren't short-spaced when originally licensed but are today. Keeping those stations as they were when originally licensed isn't practical, if even technically possible. The FCC had to come up with a way to accommodate stations that were forced to move or make changes for any number of reasons. Requiring those stations to sign off or move further away from the cities they were licensed to serve wasn't an acceptable option to much of anybody. Allowing FM stations to directionalize was one way to ensure stations could make changes while not causing additional interference to stations that it would now be required to protect. The way the FCC wrote the rule, new FM stations could be licensed as contour protection stations as well.

I previously mentioned WKQC and WNOK being on 104.7 despite only being around 100 miles apart. That's a no-no today, but it wasn't when the two stations were licensed. WNOK signed on in 1959, WKQC in 1960. Both have made changes since signing on, and both are directional away from one another because those changes were made after the spacing tables were changed.

 

[Ai4i]

While the FCC lists the coverage as directional, the DCRM 10 antenna system is not designed to be directional. It is the mounting on the tower which makes the antenna system directional due to the reflections from the tower leg(s).

Question:
Would antennas that have vertical and horizontal element parts (rings with verticle dipoles at their ends) be more affected by the towers upon which they are side-mounted than ones that use diagonal elements that are perpendicular to and v'd toward each other? I would think...maybe

 

[Kelly A]

Question:
Would antennas that have vertical and horizontal element parts (rings with verticle dipoles at their ends) be more affected by the towers upon which they are side-mounted than ones that use diagonal elements that are perpendicular to and v'd toward each other? I would think...maybe

Not really. The tower just causes re-radiation.
The difference between 'ring-stub' (Dielectric, Shively, etc), 'arrowhead' (Jampro, SWR, etc), and 'rototiller' (ERI), involve the circular vs. elliptical polarity baked into the design. In mechanical terms; ring-stub dipoles don't have the vertical radiating elements equal in length to the horizontal portion of the element. It's not that there's anything wrong with the ring stub design from a performance perspective, but if you want closer to true circular polarity, the arrowhead or rototiller designs are superior.

 

[vchimpanzee]

I previously mentioned WKQC and WNOK being on 104.7 despite only being around 100 miles apart. That's a no-no today, but it wasn't when the two stations were licensed. WNOK signed on in 1959, WKQC in 1960. Both have made changes since signing on, and both are directional away from one another because those changes were made after the spacing tables were changed.

There is a similar situation with WXLK Roanoke VA and WKRR Asheboro (Greensboro) NC, both at 92.3.

And WPEG Concord (Charlotte) NC at 97.9 and WHZT Williamston (originally Seneca) SC (Greenville market) and WBRF Galax VA, both at 98.1.

 

[RF101]

A related question - What FM stations have some unusual antenna patterns not brought on by mountains? These would be stations with contours inherent to the directional pattern, e.g even something beyond "butterflies" and "dogbones" (end-to-end yagis) but with a serious (nulling) notch or two in an asymetric form? Any antenna manufacturers focus on that bizarre discipline of directional arrays? I would imagine off-angled panels or yagi antennae with a non-equal power divider would be part of the mix.

 

[PTBoardOp94]

The FCC will not license full-power FM or LPFM stations with extreme directional antennas. I'm not certain if the same rules apply to translators.

The FCC writes at 47 CFR 73.316:
(1) Applications for the use of directional antennas that propose a ratio of maximum to minimum radiation in the horizontal plane of more than 15 dB will not be accepted.

(2) Directional antennas used to protect short-spaced stations pursuant to § 73.213 or § 73.215 of the rules, that have a radiation pattern which varies more than 2 dB per 10 degrees of azimuth will not be authorized.

 

[bigic]

Translators can use any pattern they want - the only limits are physical and economic.

 

[RF101]

Re: Directional FMs. Yes, the two biggies for directional antenna are (1) maximum -15dB null and (2) no more than -/+ 2dB change per 10dB radial. Yes, the FCC has made some rare domestic exceptions (LA Basin) due to mountains. Of course, in the international border regions the bilateral agreements take charge, e.g, max -20dB for US-Canada and -26dB US-Mexico. Translators/boosters are much more flexible for permitting bizarre patterns.

But what about strange full-powered FM patterns? Are there any memorable ones out there with multiple, relatively deep nulls?

 

[Kelly A]

But what about strange full-powered FM patterns? Are there any memorable ones out there with multiple, relatively deep nulls?

The Commission has a list of what's called 'standard patterns'. When you apply for a CP that includes a directional pattern, you need to specify the manufacturer and model number of the antenna, plus details on the structure the antenna will be attached. For typical directional antennas and mounting schemes, the brand, model, and standard pattern number will be listed in the CP and eventually the license. For antennas with special patterns and mounting schemes, the manufacturer must provide a pattern study showing that they've either computer-modeled a particular antenna and mount, or in some cases, the antenna was field tested mounted to a piece of equivalent tower section, to prove the pattern.

 

[RF101]

A station using a directional FM antenna will file a License To Cover (L2C) document typically whuch includes the manufacturer's "test range proof" of the antenna along . US rules state the range proof must meet 85% of the root mean square (RMS) value of the filed design envelope.

Question: Does Industry Canada or the CRTC post a similar document for Canadian stations with directional antennas? I have found a few Canadian border stations under- and over-performing their filed patterns but I don't have access to the field test data for confirmation.

Any insight would be welcomed..

 

[CentralFL]

I happened to be on a cruise last week. Out on the Berry Islands in the Bahamas, the only Florida FM with a usable signal was non-directional WRMF. The AMs pretty much all boomed in due to the saltwater path. Even at the top of the dial 1470, WWNN.

 

[DaveBayArea]

A related question - What FM stations have some unusual antenna patterns not brought on by mountains? These would be stations with contours inherent to the directional pattern, e.g even something beyond "butterflies" and "dogbones" (end-to-end yagis) but with a serious (nulling) notch or two in an asymetric form? Any antenna manufacturers focus on that bizarre discipline of directional arrays? I would imagine off-angled panels or yagi antennae with a non-equal power divider would be part of the mix.

Look up 101.3 in San Francisco and 100.5 in Sacramento. Both grandfathered at high power with somewhat bizarre patterns.

KZZO has an ERP of 115,000 watts (non-grandfathered power would be 50,000) toward Sacramento and 89,000 watts toward Stockton. But there's a null toward Elk Grove, where there's been a significant growth in population since they were licensed.

KIOI is also grandfathered at a very high power (I think 125,000 watts) with two powerful lobes aimed at San Francisco and another one toward San Jose. They're on San Bruno Mountain, where non-grandfathered power is very low - like 6KW.

In both cases I think these stations have to retain their odd directional patterns in order to maintain their grandfathered status, even if the desired market might have grown into the nulls of the pattern.

Dave B.

 

[kfbkfb]

KIOI-FM1 (101.3) & KIOI-FM2 (101.3) on channel FM boosters/translators:

https://en.wikipedia.org/wiki/KIOI

This is the first I've heard of on channel FM boosters/translators, maybe multiple FM (all on the same channel) translators could be used for translating 1 AM, freeing up unused (but usable for FM translators) frequencies in a radio market for translating other AMs.

edit: just looked at the radiation patterns for the 2 translators, they overlap, is special synchronization of the FM carriers required for this on channel tech to work?


Kirk Bayne

 

[davideduardo]

KIOI-FM1 (101.3) & KIOI-FM2 (101.3) on channel FM boosters/translators:

https://en.wikipedia.org/wiki/KIOI

This is the first I've heard of on channel FM boosters/translators, maybe multiple FM (all on the same channel) translators could be used for translating 1 AM, freeing up unused (but usable for FM translators) frequencies in a radio market for translating other AMs.

edit: just looked at the radiation patterns for the 2 translators, they overlap, is special synchronization of the FM carriers required for this on channel tech to work?


Kirk Bayne

In a lot of cases, those on-channel boosters do not overlap because of terrain. A Langley Rice will show that better.

In Los Angeles, a number of Mt Wilson FMs have on-channel boosters to cover the Santa Clarita area that is shadowed. A calculated coverage map shows total overlap, but most of the area is actually shadowed, and the booster fills that in.

 

[Schroedingers Cat]

Where all those 1.000 fields appear in an FCC CP DA application, the measured fields in the LIC application are often much less than 1.000. And with the actual pattern, the actual measured maximum to minimum ratio might also be greater than 15 dB in some cases. Also, International treaties allow 20 dB to be used in border situations. With nondirectional antennas, the actual ERP in some directions may exceed the licensed ERP, other directions, it's lower, due to tower interactions. That's why you see so many short spaced applications using the maximum allowable nondirectional power rather than a DA.

Anyone who has taken enough Physics and Engineering in their studies knows that radio waves are coherent, so no matter what you do, a booster will create interference fringes somewhere no matter what you do to make it less noticeable. It's not the same as putting an incoherent light bulb in a dark corner. If you look at boosters in line of sight situations in shadow areas, the actual field is closer to inverse field than FCC Propagation predictions. A few watts in a small shadow area often creates less problems than anywhere near the maximum 20% ERP.

Here's an example of an actual DA measured pattern illustrating that you have shallower nulls and arcs of roughly 2 dB below maximum, roughly 64% of the maximum ERP, even in the major lobe where the CP APP shows 1.000, and a much deeper null. WLAV-FM 96.9 Grand Rapids, MI lost its tower site, and due to the multiple rule changes over the years, had to go to a DA with a deep null, to protect stations in Zion, IL and Chicago. The old Section 73.213 allowed full 50 kW/500 feet nondirectional facilities at the old site. The measured LIC DA vs. the CP APP are way down at the next to the bottom link.

Draft Copy « Licensing and Management System « FCC

Licensing and Management System

enterpriseefiling.fcc.gov

 

[Signal Geek]

KIIS in Los Angeles is listed by radiolocator.com as directional, but I fail to see how.

Several other FM's besides KIIS are also a bit directional from Mt. Wilson. It may be an effort to push more signal into the L.A. basin and Orange County, and reduce signal to the northeast. Very little benefit to push RF in that direction, due to mountains and low population.

 

[Kent]

Several other FM's besides KIIS are also a bit directional from Mt. Wilson. It may be an effort to push more signal into the L.A. basin and Orange County, and reduce signal to the northeast. Very little benefit to push RF in that direction, due to mountains and low population.

Not sure what the logic was in making KIIS-FM directional, but it definitely wasn’t done to push more signal into LA and Orange County.

As I mentioned previously in this thread, FM directional antennas only subtract coverage. AM directional works somewhat like a balloon; FM does not. I understand it may be technically possible to push FM more in one direction, but current FCC rules don’t permit that.

 

[Schroedingers Cat]

Up until the early 1960s, commercial 92-108 MHz stations were licensed with contour protection, but many were just a few kilowatts from an AM antenna tower, so often just 100-200 feet above average terrain, and AM towers are usually at low elevation, not high elevation like FM and TV. Those could be squeezed in closely together. That's where all the short spaced stations originated. Also, all stations including Class Bs, were only protected to the 1 mV/m contour, now usually called the 60 dBu F(50,50) contour, not 54 dBu F(50,50) like Class Bs. The FCC created short spacing by constantly changing the rules, not the stations. Short spacings are mutual, one station didn't just squeeze in short spaced, the FCC rules allowed it.

 

[Michi]

The FCC will not license full-power FM or LPFM stations with extreme directional antennas. I'm not certain if the same rules apply to translators.

The FCC writes at 47 CFR 73.316:
(1) Applications for the use of directional antennas that propose a ratio of maximum to minimum radiation in the horizontal plane of more than 15 dB will not be accepted.

(2) Directional antennas used to protect short-spaced stations pursuant to § 73.213 or § 73.215 of the rules, that have a radiation pattern which varies more than 2 dB per 10 degrees of azimuth will not be authorized.

These rules do not apply to LPFM stations. Since LPFM does not have a rule like §73.215, which REC has proposed in the past and are required to meet the §73.807 minimum distance separations on the books at the time that the Local Community Radio Act was enacted (REC proposed at the time that LPFM stations be permitted to use the old "LP-10" distances as the minimum distances as the "lower" distances of a §73.215-like arrangement in order to meet the statutory requirement), LPFM stations must meet the "LP-100" distances in all directions towards all stations including translators (even though no statute exists that requires distance separation to translators), even if the station is operating a directional antenna. On the LPFM side, the only real reasons to operate a directional antenna include:
(1) To achieve a second-adjacent channel waiver;
(2) For public safety agencies to limit radiation in certain directions (no LPFM is doing this right now);
(3) To comply with international agreements; and
(4) For reserved band LPFM stations (88.1~91.9) to protect Channel 6 TV and LPTV stations.

LPFM stations are not required to submit a proof of performance when licensing a directional antenna if the reason for the DA was either (1), (2) or (3) above, but are required to provide a proof of performance for (4) and for any other reason that a station would want to run a DA. Other than (1) and (4) above, LPFM stations must meet the §73.807 distance separations to other facilities. (1) is just an extension of the existing rules that permits LPFM stations to waive a second adjacent channel and (4) allows LPFM stations to short space a TV station, especially LPTV stations, which as REC pleaded and the FCC agreed, the minimum distance separation requirements in §73.825 significantly overprotect a large majority of LPTV stations as the rule was originally written to assume that all LPTV stations are 3kW @ 600m HAAT.