Antennas - One, two or Four?

[dataman19]

I have a seemingly newbie question..
Maybe because I am new here?
Broadcast Engineer in 60's & 70's (Fun years for AM/FM), then again after Government Retirement in mid 90's...
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Never did an FM Antenna Install (Gonna rectify that in the next couple of months though...)
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My question is based upon seemingly mindless gibber from a sales Tech Rep to one of my Clients (who is building a "new" tower and wants "new" antenna Array).

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Antennas in question (2 different radials)
1st::: AKY3
http://www.broadcastwarehouse.com/label-/aky3/aky5-fm-yagi-antenna/59/product
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2nd ::: AKG/1M 2KW
http://www.broadcastwarehouse.com/label-/akg/1n-high-power-dipole-antenna/50/product
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Enter the Power Divider:
http://www.broadcastwarehouse.com/label-/5kw--power-divider/731/product
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Now in "my" world a power divider is a "divider" say 100W in to 2 ea 50W outs (-3db)..
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The customer wants to put two or four AKY3s North and South (in pairs because the will have 7.5 dB gain (??))
And two AKG/1M 2KW's East and West (2 dB gain each direction???)
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This to be run by a 60way power divider...
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DAH...
am I missing something here???... The power divider will like decrease the signal by (-9db???) and the so called gain of having two antennas (the AKY3's - 2 each direction) isn't valid if you use a power divider... Or is it?
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The idea of having two pairs of radiators situated north and south (in pairs) would put a large contour north and south (especially since they are Yagi's)... but then adding a single radial east and another west is kinda like
putting a VW Bug between you diesel pickuo and a trailer to get more towing power (a waste).. I al;so do not see how the contour will benefit.
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Told the customer that, even suggested he hire an engineer with extensive RF antenna experience (I'm just a lowly Broadcast Engineer - what do I know. And I also admit that I have never spec'd or installed a "new" antenna system for an FM Transmitter from scratch - just fixed them...)..
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So what is all the hoopala... hype or fact... I inherited this gig, and I kinda gotta dance this dance - but I don't want a lot of stuff sitting behind the transmitter bay when all is said and done. Crap - where was this customer in the 70's when I sold my Station in Muscle Shoals???
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Can somebody who has actually done all this explain to me how a power divider will allow you to hook up two antennas radiating the same direction and preserve the +5db Gain that you are supposed to get by using pairs?
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I asked for Patterns for the antennas, and more specific specs for the power dividers... Still don't see how this isn't a big waste of cash...
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Still haven't factored in the RF line either... and I know that the transmitter isn't going to clean this up either..
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What has happened in the Broadcasting Industry? has everyone gone nuts???
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Like I said... please enlighten me on this...
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Dave
dataman19

 

[Cynyster]

Out of all that I think I missed the actual question.

What are you trying to accomplish? :-\

 

[rfry]

Is this project to be installed in the US?

If so, and it is to be licensed by the FCC, then the antenna cannot be designed to produce a radiation pattern that is directional in the horizontal plane, unless the FCC requires it for that installation.

In that case a specific, directional pattern will be defined for that application (usually by a consultant). Once that pattern data is available you will need to find an antenna manufacturer that can quote a complete antenna assembly designed to produce that pattern when installed on the mounting structure that the station will use. The antenna manufacturer will define the power gain, weight and windload of the antenna system when the antenna is quoted. So the questions about antenna types, power dividers etc really don't need answers.

If this station is located in the US but has no plans to apply for an FCC station license, then it is a pirate operation that probably you'll want to avoid.

 

[Lazy J]

Yeah, I have never seem an FM antenna pattern like what you are describing. You will definitely want to see what the license (or construction permit) calls for. I'll bet it doesn't call for a yagi in one direction.

As for the gain from pairing them up... The easiest way to imagine it would be like this. One bay will give you a spherical signal like a basketball. A second bay will flatten the signal out a little, more like an oval. As you add more bays, it flattens the signal out more and more. This will take the power that would normally go up and down, and send it out to the sides, increasing the gain from the antenna and increasing the coverage area.

 

[TomT]

Let's back up even further.

1. U.S. or overseas project?

2. If U.S.--what is this? An FM translator? As noted, full power broadcast, with a few very specific exceptions, can't use this type of antenna array. The exceptions are usually educational band stations with very specific design requirements which would be specified in the construction permit.

3. If it is an overseas project--site requirements or limitations? (e.g.--is commercial power available or is this generator, requirements of licensing entity, where is population in relation to site location.)

4. Also, what will be the supporting structure?--how fat is the tower?

Very generally, there are more disadvantages than advantages to using a high-gain directional array over a lower gain non-directional array. Another factor: circular polarization will work far better in most instances than v-Pol, or mixed V-Pol/H-pol.

I used to favor V-pol for low power translators, rationalizing that this at least gets the car radios (most table radios are deafer than a post anymore). However, more and more cars are going to the raked back antennas--look at the Ford Focus, or low-end VW models for example (they are sold in most parts of Europe and the Americas).

 

[dataman19]

Hi gang..
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Sorry about the over all confusion..
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Flew the Antenna Installer Supervisor in today... Jerry is a wealth of info... This is not the antennas we are going to get - he spec'd the antennas two years ago (and has them with the crew) doesn't matter what the "owner" has bought - we are going to get a "legal" antenna install.
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Yes this is a US Install - it is on a Reservation and all the weird things were happening because the tribal elders were making all kinds of crazy requests (although out of respect I have to point out - They were just asking and not directing. See Bart and Lisa - sometimes it does hurt to ask..). This construction permit has been delayed and extended due to Tribal Issues (hey I'm new to this project, I don't know why the Tribal Council has so much input - they just do. It's "Their Reservation, and being a sovereign nation within a nation brings a lot to the table)... The tribal council is aware of FCC guidelines (but that's what they see them as - "guidelines"). They can not control a Radio Broadcasting company broadcasting onto the reservation from out side the reservation - but they darn well can control what goes on inside the reservation.
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Now we are back to normal and I can relax. In a couple of months the antennas will be in and I can go back up and do the antenna field patterns.
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The owner will be happy, the Tribe should be happy (hopefully) and I will be happy...
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Thanks for the inputs guys.. I appreciate it.
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Jerry is sending me some simulations that he is going to do on the crazy antenna configuration, and the ones he did before the circus came to town. This will be interesting. He's sending it to me on a MPeg Vid CD/DVD (swears I should be able to watch it on my computer with a Combo CD/DVD drive, or watch it on my TV with the CD/DVD Player..).
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This should clear up any confusion I have on the matter, as well as enlighten me as to what the real world does for a living...
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Thursday I'm gonna fly him back to South Georgia where he and the crew are wrapping up their projects there. Then he and the crew are taking a few weeks off before getting ready to head to Injin Territory (a little pseudo cowboy humor).....
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Note: You can get a construction permit for a Station on Tribal Land - but that doesn't mean the Tribe will let you build it.....This kinda goes back to the broadcast law blog entry's about buying construction permits for stations that cannot be built...
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One again... thanks gang
Dave

 

[ChiefEngineer]

You note something here that screams of a problem.

If you plan to do field patterns for FM you are in way over your head. Listen to others on the board. It looks like you might have stepped in it.

Patterns are not measured in the field by a station engineer. The pattern is regularly and almost always measured by the manufacturer or presented to the Commission before a CP is built. It is never measured in the field by the station for any reason. The process is : submit a possible pattern from a manufacturer, present it to the FCC, get a grant, build it as described.

The pattern is theoretical and actual from the manufacturer. You can submit your own pattern and with some help build it. The FCC has to have the exact antenna on file to approve it. They will never allow anyone to build something and do field measurements. By the description something is terribly wrong.

 

[CaptBob92]

Chief Engineer is correct. The only field readings taken on an fm could be readings from the ground to verify a null is situated in the right direction and then that reading is only approximate and doesn't hold much water. I'm of the understanding that if you wanted to do a field pattern tune up you would need to take field readings from the air at the altitude of the antenna using a copter or some sort of aircraft. It's not likely that is your plan.