ERI .78 spacing

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fm-engineer
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ERI .78 spacing

Post by fm-engineer » Mon Apr 03, 2017 3:59 pm

Anyone here ever custom space a multi-bay ERI rototiller at .78 instead of using half-wave? Seems half-wave spacing is becoming popular again but I'm still not sold on it. I've found just as many engineers that have regretted half-wave spacing compared to others that think its the best thing they ever installed. This particular installation is in hilly terrain with the antenna about 650 feet above the main service area. I've read about some fractional spacing of Jampro's at .78 with some very good results.

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Deep Thought
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Re: ERI .78 spacing

Post by Deep Thought » Mon Apr 03, 2017 4:31 pm

Without knowing the whys of people's opinions it is almost impossible to even comment here. What are the applications? Why did they decide to use a 1/2-wave spaced antenna (or a full wave spaced, or a 0.78 wave spaced)?

Like everything else inter-bay spacing is a design parameter which is influenced by the design goals and project limits.

I've designed antennas using all sorts of odd spacing, especially when used for translators which are squeezing into a second or third adjacent and have to limit downward radiation to some level at some distance from the tower. It's also handy in high EMR power density environments. What is your goal here?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Re: ERI .78 spacing

Post by fm-engineer » Mon Apr 03, 2017 5:35 pm

Class A FM with a 30 year old Shively, 4 bay, full wave spaced. It is a very narrow bandwidth antenna. Two goals on this proposal. Lessen near field multipath conditions in hilly terrain by focusing energy on the main lobe and eliminate some of the downward radiation of the side lobes. Of course, the concern is reduced signal strength at ground level in the main service and population area which averages 400 to 600 feet below the antenna. Second goal is to increase bandwidth to improve IM and synchronous noise, and improve any phase delay problems caused by a long coax run (about 680'). Looking at ERI 4 bay rototiller which will easily improve bandwidth, but at full wave spacing, also has side lobe characteristics similar to the Shively. Some studies of fractional spacing has indicated .78 delivers a reduction of side lobe energy similar to half-wave but with only a moderate loss in gain. There is also an natural increase in bandwidth, but that really isn't an issue with ERI's. ERI will build at .78 on a custom order but they favor using half-wave over .78 or other fractional spacing options.

R. Fry
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Re: ERI .78 spacing

Post by R. Fry » Mon Apr 03, 2017 5:44 pm

ERI will be glad to quote anyone an array of Rototiller® elements having 0.5λ, 0.78λ, and other (practical) vertical spacings, and to send them the calculated, free-space elevation patterns for those arrays.

Then the broadcaster and/or consultant can select whichever configuration is thought best to meet the needs of that particular installation.

Just to note that for typical installations and other things equal, most of the difference in the field strengths near the earth for those various element spacings occurs within the first ~ ten miles of the antenna site.

Suggest that for best performance no matter what the vertical spacing of the elements, the azimuth pattern of the array should be measured by the antenna OEM when mounted on an accurate physical model of the supporting structure that will be used for it at the transmit site -- to guide the buyer toward the physical installation details that might best serve his (legal) goals.

R. Fry, CPBE
http://rfry.org

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Re: ERI .78 spacing

Post by R. Fry » Tue Apr 04, 2017 5:54 am

Followup -

Here is a link to a paper showing how changing the element spacing and power distribution across an FM transmit array affects the fields near the transmit site, for the conditions given there:

Zero Sidelobe FM Antennas.pdf
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TPT
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Re: ERI .78 spacing

Post by TPT » Tue Apr 04, 2017 7:34 am

Have two stations in our area with 1/2 wave spaced antennas--two class A's mounted below the antenna for a Class B. Space considerations on the relatively short tower & RF levels at ground level were the main considerations in using the half wave spaced antennas.

Not particularly impressed by the results. My inclination would be to look at other options, including new transmitter. We're in hill country--Ohio River valley. One of our stations was a Class A with a 3 bay ERI and was put on the air with an old Harris FM 2.5H. Changing to a solid state Nautel immediately improved multipath --probably because of the reduction in AM noise.

What is the ERP on this installation?

fm-engineer
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Re: ERI .78 spacing

Post by fm-engineer » Tue Apr 04, 2017 11:01 am

TPT wrote:
Tue Apr 04, 2017 7:34 am
Have two stations in our area with 1/2 wave spaced antennas--two class A's mounted below the antenna for a Class B. Space considerations on the relatively short tower & RF levels at ground level were the main considerations in using the half wave spaced antennas.

Not particularly impressed by the results. My inclination would be to look at other options, including new transmitter. We're in hill country--Ohio River valley. One of our stations was a Class A with a 3 bay ERI and was put on the air with an old Harris FM 2.5H. Changing to a solid state Nautel immediately improved multipath --probably because of the reduction in AM noise.

What is the ERP on this installation?
6 kW, also hilly terrain as you describe

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Re: ERI .78 spacing

Post by fm-engineer » Tue Apr 04, 2017 11:01 am

R. Fry wrote:
Tue Apr 04, 2017 5:54 am
Followup -

Here is a link to a paper showing how changing the element spacing and power distribution across an FM transmit array affects the fields near the transmit site, for the conditions given there:


Zero Sidelobe FM Antennas.pdf
Appreciate the information R. Fry

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Re: ERI .78 spacing

Post by TPT » Tue Apr 04, 2017 2:26 pm

May be a useful exercise to first run the numbers comparing TPO vs. antenna gain to get your ERP.

Let's assume you have a 300' run of 1 5/8th air to the antenna now--6 kw ERP on the Shively takes about 3300 watts TPO
If you went with an off-the-shelf 1/2 wave spaced 4 bay ERI, you would need about 5300 watts TPO for the same ERP, because of the lower gain of the antenna. To get back close to where you are now, you would need a 6 bay 1/2 wave spaced ERI, then the TPO is about 3600 watts.

Let's go back and REDUCE bays--which also seems to help multi-path in certain situations. Same coax length, 3 bay ERI 1 wave length spacing, tpo would be 4500 watts.

Of course, this is a hypothetical, but you get the idea. And the dollars come into play. As a guide, we had to buy a new ERI 2 bay back in '13, was about $7,500.

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Re: ERI .78 spacing

Post by fm-engineer » Tue Apr 04, 2017 3:37 pm

Thanks for the info TPT

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