DB-224/DB-420 NEC Models

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MotoBill

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Has anyone that's into NEC modeling ever modeled the former Decibel Products folder dipole arrays such as the DB-224 or the DB-420 series?
I've been lead to beleive that the driving point impedance of each element ahead of the phasing/matching harness is approximately 100 ohms.
However, I an only get my EzNEC model down to approximately 119 ohms.

I suppose the 119 ohm value is realistic, but am looking for input from anyone that has experience in accurately modeling, or measuring these types of antennas, and if you'd care to share your model files.

Thanks!

MotoBill
 
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MotoBill

MotoBill

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Apparently no takers. However, I seem to have answered my own question about feed impedance. I discovered that I had incorrectly modeled the feed point (source) after making a closer inspection of a "Real" DB-224. It turns out the feed actually constitutes an "Off Center" feed configuration. Having made the corrections, the impedance now drops in at almost exactly at 100 Ohms + J0 at resonance.

I typically only use NEC to model tower cross sections with antennas of interest in an attempt to determine optimum placement about the structure for side mount installations. This has proven very helpful to solve or prevent coverage issues due to the complexity of pattern distortion. I generally only use simple center fed dipole elements in my models for this type of analysis but wanted to test the water by replicating folded dipole radiators on various bands to see if there notable differences when I kept coming up with the incorrect driving point impedances. Not that this should be a factor on the pattern, but I just like to account for everything possible.
One thing's for sure! Don't put much stock in all of those nice side mount pattern graphics you see in the various antenna catalogs. Every antenna near environment offers its own unique set of characteristics. Anything metallic in the near field of the antenna is a parasitic radiator!
 

CARC383

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Looking at the dipoles on a DB-224/420, to me they should model as a typical center fed half wave dipole. The ground side has both ends of the loop grounded making it just a fat element. The hot side is a grounded 1/4 wave, the same as used on many 1/4 wave ground planes that match direct to 50 ohms. When you take a 1/4 wave monopole and extend the far end back to the feedpoint area and ground it about the only thing it does besides putting the element at DC ground is it also gives it a bit more BW due to the extra surface area.

So I would model them as a dipole with roughly 50 ohms impedance in free space. Since the dipoles are mounted near and probably less than 1/4 wavelength from the mast, the impedance will be pulled down some and I suspect less than 50 ohms. Maybe 30-40 ohms is my guess. I have made replacement coax harnesses for these antennas using 75 ohm coax cut to critical lengths and they match up well telling me they are not that far from 50 ohms.


Apparently no takers. However, I seem to have answered my own question about feed impedance. I discovered that I had incorrectly modeled the feed point (source) after making a closer inspection of a "Real" DB-224. It turns out the feed actually constitutes an "Off Center" feed configuration. Having made the corrections, the impedance now drops in at almost exactly at 100 Ohms + J0 at resonance.

I typically only use NEC to model tower cross sections with antennas of interest in an attempt to determine optimum placement about the structure for side mount installations. This has proven very helpful to solve or prevent coverage issues due to the complexity of pattern distortion. I generally only use simple center fed dipole elements in my models for this type of analysis but wanted to test the water by replicating folded dipole radiators on various bands to see if there notable differences when I kept coming up with the incorrect driving point impedances. Not that this should be a factor on the pattern, but I just like to account for everything possible.
One thing's for sure! Don't put much stock in all of those nice side mount pattern graphics you see in the various antenna catalogs. Every antenna near environment offers its own unique set of characteristics. Anything metallic in the near field of the antenna is a parasitic radiator!
 
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