Second Adjacent Options

[RF101]

I need to resolve a second adjacent issue with using "Living Way" and "Downward Radiation" approaches outside the 60 dBu.

Here's a clumsy situation for an LPFM applicant. I have second-adjacent stations on either side of a proposed frequency.
:
The low-side interferer I can handle with Living Way. (There is no residence inside the "interference zone".)

The high-side interferer becomes problematic. At the proposed transmitter site the station-to-be-protected has just a 59.5 dBu F(50,50) signal which is below the threshold. Yes, that station's protected contour starts actually some distance away. I can protect that outlet with distance separation by reducing the LPFM's power ( e.g. to 15 watts). Question: Could I apply Living Way or Downward Radiation Limits for a station outside the 60dBu? I want to get those full 100 watts ERP..

(I would use a directional antenna but the tower owner said "no".)

 

[Greg Strickland]

RF101- Perhaps a consulting engineer can work with you on financial terms you can afford. Do you think it is fair for participants here to guide you through the engineering, considering consulting engineers are trying to earn a living?

 

[Kelly A]

RF101- Perhaps a consulting engineer can work with you on financial terms you can afford. Do you think it is fair for participants here to guide you through the engineering, considering consulting engineers are trying to earn a living?

Scott has a boat payment due. I'm sure he'd be please to assist.

 

[Michi]

Why would they not want to allow a directional antenna? A single Scala CL-FM is also side mounted and does not require any more tower space than a single bay Nicom BKG-77 or any other similar single bay antenna. Perhaps there's a misunderstanding by the tower owner on what this actually entails in this case.

 

[Cold Coffee]

Why would they not want to allow a directional antenna? A single Scala CL-FM is also side mounted and does not require any more tower space than a single bay Nicom BKG-77 or any other similar single bay antenna. Perhaps there's a misunderstanding by the tower owner on what this actually entails in this case.

Mr Bradshaw at the FCC told me second adjacent waivers may require stacked antenna bays to prevent blanketing on the ground.

 

[Michi]

I I can protect that outlet with distance separation by reducing the LPFM's power ( e.g. to 15 watts). Question: Could I apply Living Way or Downward Radiation Limits for a station outside the 60dBu? I want to get those full 100 watts ERP..

From the outset, if your HAAT allows for you to have 100 watts ERP maximum, you cannot propose 15 watts ERP. The minimum service contour distance on LPFM is 4.7 km, which equates to 50 watts at 30m HAAT. (§73.811(b)). In most (but not all) cases, the minimum ERP allowed is half of the power (in rounded ERP) of the maximum.

By rule of thumb, in most (but not all) cases where a directional antenna is not in play and there are two short-spaced stations, you only need to analyze the weaker of those stations (the one with the lower dBu) because the resulting interfering contour of the stronger station will always be smaller and if the antenna can protect the weaker station, it will protect the stronger station as well.

If you are just outside of the contour, you still have to protect the occupied structures that are inside of the service contour. Using the normal U/D method, let's say that a station arrives at your location with a 59 dBu contour, this means you would have to use the 99 dBu interfering contour to perform the U/D method. At 50 watts, this would mean that you would have to protect all of the occupied structures and four lane highways that are both inside the 60 dBu contour and within 557 meters of the tower site. Without a very tall height and/or a massive amount of bays, there's no way that you would be able to find an elevation pattern that will properly protect everything.

When you are outside the contour, the only way around it is to assure that the LPFM's 100 dBu interfering contour remains outside of the 60 dBu contour of the short-spaced station.

Now, if you do use that method to show a lack of contour overlap (100 vs. 60), you still have to do an analysis on the second (stronger) station. For example, let's say you want to use a Scala CL-FM to keep the 100 dBu out of the 60 dBu contour on one station, but that antenna cannot properly protect any occupied structures in the directional pattern based on the U/D determined contour of the other (stronger) second-adjacent station because of the antenna's elevation pattern, then you still can't show a lack of interference to satisfy the waiver.

 

[Michi]

Mr Bradshaw at the FCC told me second adjacent waivers may require stacked antenna bays to prevent blanketing on the ground.

Multi-bay antennas have different radiation patterns than a single bay antenna, so it is very possible that using multi-bay antennas could prevent penetration of occupied structures. It all depends on the elevation pattern specifications provided by the manufacturer. At REC, we have an internal system that has stored in it the elevation patterns for dozens of antenna and bay configurations. We are able to enter basic parameters (height above ground, interfering contour and height above ground to protect) and it will show which antennas can be used. Other folks who do what I do also have methods of determining this. The stronger the incoming station(s) is/are, the less complex the antenna system needs to be.

 

[RF101]

Thanks all for answering this one. This was a non-directional diplexing case so any beam (CLFM, etc.) was not possible. I will work up a two-bay halfwave solution and see if the numbers crunch out.

BTW, did everyone note the madness in some of these LPFM applications? Some groups propose using eight-bay halfwave arrays to overcome the second-adjacent issue. Do they realize how much weight,/windload will transpire? If the group rents from a major tower provider like American Towers they will pay a premium for vertical space.

 

[Michi]

Thanks all for answering this one. This was a non-directional diplexing case so any beam (CLFM, etc.) was not possible. I will work up a two-bay halfwave solution and see if the numbers crunch out.

BTW, did everyone note the madness in some of these LPFM applications? Some groups propose using eight-bay halfwave arrays to overcome the second-adjacent issue. Do they realize how much weight,/windload will transpire? If the group rents from a major tower provider like American Towers they will pay a premium for vertical space.

People will do this just to get the application in the door. If they are granted, they will try to (1) move it somewhere else or (2) build with a smaller antenna and hope that the second-adjacent station does not file a complaint. In 2013 and on later modifications, applications with second-adjacent waivers rarely included conditions on what specific antennas must be used.

 

[Kelly A]

People will do this just to get the application in the door. If they are granted, they will try to (1) move it somewhere else or (2) build with a smaller antenna and hope that the second-adjacent station does not file a complaint. In 2013 and on later modifications, applications with second-adjacent waivers rarely included conditions on what specific antennas must be used.

Or build it with an antenna purchased from Amazon or EBay, forgetting whatever they had submitted on the application.

 

[Greg Strickland]

RF101-

I wrote my earlier post assuming you are associated with a group seeking an LPFM license that is not interested in paying for engineering assistance. I apologize for assuming that, and for the tone of my post.

From a low antenna height above ground level, reducing signal on the ground close-in is challenging.
I hope it goes well for you.

Greg