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Author Topic: 630M Dipole????  (Read 659 times)

W9IQ

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Re: 630M Dipole????
« Reply #15 on: February 24, 2020, 02:09:15 PM »

... If my numbers are correct one can meet the 5 watt EIRP FCC limit using a modest vertical driven with less than the 500 watt FCC max RF power. ...

It seems likely that when evaluating the radiated fields of these antennas so as to meet an FCC EIRP limit using only a NEC far-field calculation that higher fields are being unreported, and may exceed the fields that would be generated by the FCC EIRP limit at distances fairly close to the radiator.

The FCC guidelines specify far fields are to be used: "... EIRP or ERP from the results of a power measurement performed under far-field conditions". So if an antenna is properly modeled, the far field results of the model should be sufficient.

- Glenn W9IQ
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God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KH6AQ

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Re: 630M Dipole????
« Reply #16 on: February 24, 2020, 02:19:49 PM »

Matching network losses are figured into my numbers and measured base impedance measurements can be used to modify the NEC model.

Out in Hawaii only low elevation angles matter (as in less than 10 degrees). One thing that may throw off power calculations for my QTH (by a couple of dB) are ground constants. In our simulations we've used 0.005S/13 ground but I believe mine may be closer to 0.001/5. This might be estimated from how the E-field falls off with distance when measured near the surface. For 0.005/13 it's close to 6 dB and for 0.001/5 it's closer to 8 dB for a doubling of distance. Toward the mainland there's 25 wavelengths of lava between me and the ocean.
« Last Edit: February 24, 2020, 02:26:57 PM by KH6AQ »
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RFRY

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Re: 630M Dipole????
« Reply #17 on: February 24, 2020, 02:25:36 PM »

... If my numbers are correct one can meet the 5 watt EIRP FCC limit using a modest vertical driven with less than the 500 watt FCC max RF power. ...

It seems likely that when evaluating the radiated fields of these antennas so as to meet an FCC EIRP limit using only a NEC far-field calculation that higher fields are being unreported, and may exceed the fields that would be generated by the FCC EIRP limit at distances fairly close to the radiator.

The FCC guidelines specify far fields are to be used: "...

However definition for the distance to the far field found in antenna textbooks is not the same as the far-field definition applied by MoM software such as NEC.

The NEC far-field is calculated for an ~infinite distance, over a flat ground plane if one is defined.

The textbook far field applies to all distances beyond 2L²/lambda, where L is the length or aperture of the radiator.

Which one is favored by the FCC?
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KH6AQ

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Re: 630M Dipole????
« Reply #18 on: February 24, 2020, 02:45:25 PM »

In the paper 630m Antennas, by NO3M, the author comments on the limitations of field strength modeling. He advocates using perfect ground to be on the safe side. For the vertical I modeled over 0.005S/13 ground this makes a 2 dB difference.

https://www.kkn.net/dayton2018/no3m-dayton-2018.pdf
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RFRY

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Re: 630M Dipole????
« Reply #19 on: February 24, 2020, 02:50:23 PM »

In the paper 630m Antennas, by NO3M, the author comments on the limitations of field strength modeling. He advocates using perfect ground to be on the safe side. For the vertical I modeled over 0.005S/13 ground this makes a 2 dB difference.

At what horizontal path distance from the radiator?
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KH6AQ

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Re: 630M Dipole????
« Reply #20 on: February 24, 2020, 03:04:59 PM »

What NEC uses; essentially infinite. Perhaps I should have said EIRP rather than field strength.
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W9IQ

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Re: 630M Dipole????
« Reply #21 on: February 24, 2020, 03:15:26 PM »

I understand the comments about the modeling. But to be pragmatic, rather than academic, consider how the FCC would actually enforce EIRP. They won't be flying a drone around to find the peak radiation. They likely would accept a well engineered model as a reasonable approach.

- Glenn W9IQ
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God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

RFRY

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Re: 630M Dipole????
« Reply #22 on: February 24, 2020, 03:36:56 PM »

... consider how the FCC would actually enforce EIRP. They won't be flying a drone around to find the peak radiation. ...

Likely no drones, but the FCC is known to measure field intensities 2-3 meters AGL and close to the radiator (but in their far field as defined by 2L^2/lambda), where fields are attenuated very nearly by their inverse distance from the radiator.

It is easy from that measurement to learn what ERP/EIRP was needed to produce that field. which if excessive may support a citation.
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W9IQ

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Re: 630M Dipole????
« Reply #23 on: February 24, 2020, 03:49:59 PM »

That may work for ground wave measurements but extrapolating to far field elevation would require a reference model in itself. At best they could guesstimate gross violations.

- Glenn W9IQ
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God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KH6AQ

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Re: 630M Dipole????
« Reply #24 on: February 24, 2020, 05:40:20 PM »

The paper A Pragmatic Approach to 630m Antennas, by NO3M shows three methods to determine EIRP. I'll apply two of them to my vertical model.

(1)
Peirp = PTX x Refficiency * 3 (Gain of 4.77dBi)
5.0W = 166W x 1.0% x 3

(2)
Peirp = (E * D)2/30
5.0W = (12.2mV x 1000m)2/30

Running the simulation with real ground (0.005S/13) at 173 watts produces 12.2 mV/m at 1000m. This is measured 2m above ground. So, in this case the two methods result in just about the same RF power to produce 5.0W EIRP. Running this again with my suspected 0.001S/5 ground the RF power needed is 205 watts. Again, pretty close and I don't happen to have an RF field strength meter let alone one with such small uncertainty.

https://www.kkn.net/dayton2018/no3m-dayton-2018.pdf
« Last Edit: February 24, 2020, 06:00:28 PM by KH6AQ »
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KH6AQ

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Re: 630M Dipole????
« Reply #25 on: February 24, 2020, 06:07:47 PM »

Let's run the same exercise using the second formula for the dipole. In this case the dipole produces maximum field strength off the ends. To minimize the effect of the dipole length the distance is set to 3000 meters.

(2)
Peirp = (E * D)2/30
5.0W = (4.1mV x 3000m)2/30

173 watts produces 4.1 mV/m at 3000 meters, meeting the 5.0W EIRP spec. Broadside, the field strength is only 0.25 mV/m at 3000 meters.
« Last Edit: February 24, 2020, 06:25:18 PM by KH6AQ »
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RFRY

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Re: 630M Dipole????
« Reply #26 on: February 25, 2020, 04:40:57 AM »

Below is a spreadsheet calculation showing the ERP & EIRP for 200 watts across the feedpoint terminals of the antenna system described there.

Assumptions were made for (hopefully) typical values for the ESR of the loading coil and the resistance of the system connection to r-f ground.  I may have set the ground R rather low, but it could be possible with a decent set of radials buried in Earth of high conductivity.

The big problems here are that few users accurately will know what those values really are.

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W9IQ

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Re: 630M Dipole????
« Reply #27 on: February 25, 2020, 05:30:27 AM »

Peirp = PTX x Refficiency * 3 (Gain of 4.77dBi)

The use of correct terminology in this context is important:

     Peirp = PTX x Refficiency * 3 (Gain Directivity of 4.77 dBi)

This is because antenna gain is the product of efficiency time directivity:

     Gain = Efficiency * Directivity

As originally written, the efficiency would be twice factored.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.
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