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Old 13-Mar-2013, 6:16 AM   #1
Psdstu
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DB8e Question

I was wondering if anyone could explain how the new DB8e's UHF combiner works?

How does this antenna not have multipath problems with 2 UHF antennas?

Is there a difference between the DB8e UHF combiner and a reverse splitter?

Could someone use/order the DB8e UHF combiner to combine 2 different UHF antennas?
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Old 13-Mar-2013, 9:14 PM   #2
teleview
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I also have ask for answers to those same questions about the DB8e.

And not because I do not know any thing about the subject.

It is Ok to put forth ideas , ask questions and provide answers.

And here are more questions.

How is the situation of Antenna Cross Talk resolved/solved when the UHF reception panels are aimed in different directions.

How is the situation of Reception Strength Power Drain from one antenna panel to the other antenna panel resolved/solved when the antenna panels are aimed different directions??

Last edited by teleview; 16-Mar-2013 at 12:17 AM.
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Old 14-Mar-2013, 11:03 PM   #3
GroundUrMast
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I've been hoping that ADTech would weigh-in on this topic... I don't have a DB8E in hand at this point, just a DB4E and 91XG. So, combining my first hand knowledge of those designs with the published photos of the DB8E, here goes...

The 91XG and DB4E both employ electrically balanced 'bow-tie' elements. To be a bit more technically accurate, the 'bow-tie' element is a form of a folded dipole http://en.wikipedia.org/wiki/Folded_...#Folded_dipole, which inherently has a terminal impedance of roughly 300Ω at it's resonant frequency. The 91XG uses one directly connected and the the DB4E has four which are interconnected via balanced lines (the parallel heavy gauge wires running vertically on either side of the antennas center line). The upper and lower sections of the DB4E being the core components of two DB2E antennas. The wire diameter and spacing of the balanced lines is chosen to provide a characteristic impedance appropriate for use as a quarter wave matching line. http://en.wikipedia.org/wiki/Quarter...ce_transformer The net result is that both the Antennas Direct designs I've seen (and presumably most of their other original designs) appear to present a nominal 300Ω balanced antenna output to the input of the matching network.

The matching networks on both antennas I have in hand are micro strip lines built from what appears to be glass/resin PCB material. http://en.wikipedia.org/wiki/Microstrip The function of strip-line network is to transform the 300Ω balanced input to a 75Ω unbalanced output. There is very little ohmic loss in the small amount of copper of the strip-line, as a result this type of matching network is known for low insertion loss. (The matching network can be thought of as analogous to a car's transmission... allowing efficient coupling between an engine turning at high speed and the tires turning at low speed. To take the analogy further, a transmission designed with gears will be more efficient than one using pulleys and belts, given that gears slip less and have less friction than belts and pulleys. The micro strip being like the gear design and wire wound transformers being like belt and pulleys.)

So as I look at the photos of the DB8E, I see two DB4E's ganged, interconnected by two equal length coax cables and a 'black-box'. I'm left with two 'guesstimates' of what they have done...

1) The two cables are custom impedance (intended to act similarly to quarter-wave-transformers) and the 'black-box' is a simple set of connectors that parallel the two with one output.

2) The two cables are common 75Ω coax and the 'black-box' is a low loss micro-strip network. (Based on what I've seen from the engineering at AD, I would bet this is the more likely of the two possibilities.)

To the question of multipath, I don't expect any antenna design to overcome multipath by any means other than 'directionality'. In other words, the more directional the antenna is, the less off bore-site signals will be received... And if the interfering multipath(s) are arriving at the antenna from off bore-site direction(s) then the more directional design will help combat the multipath problem.

If you configure the DB8E to be a less directional array, you give up some of your multipath rejection. I don't see that as a reason to be critical of the design... That's physics.

A reversed splitter is a not an impedance matching devise, it's a power dividing network. As such, it will present more insertion loss than the micro-strip-line matching networks used by AD. When working with power transmission, impedance matching translates to efficient (low-loss) coupling of power. A matching network can have several ports, (the phasing lines running from each 'bow-tie' element on the typical panel antenna design for example). As power travels along the line, it will be adsorbed little by ports with mismatched impedance and absorbed efficiently by a port with a matching impedance. As a result, a correctly designed network will reduce the amount of power retransmitted by ganged elements.

http://www.antennasdirect.com/cmss_f...s/DB8E-TDS.pdf
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Old 15-Mar-2013, 4:42 AM   #4
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The information does not solve the problem of multipath (out of phase signals) when the antenna panels are aimed in different directions. And does not minimize the problem to the point it is not a problem.

The information does not solve the problem of signal strength bleed off of signal from one antenna panel to the other antenna panel when the antenna panels are aimed in different directions. And does not in any way minimize the problem of signal strength bleed off to the point that it is not a problem.

A modified directional coupler is one way to somewhat solve the problems , however there is a big signal strength loss (dB) when using directional couplers.

Last edited by teleview; 15-Mar-2013 at 8:27 AM.
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Old 15-Mar-2013, 8:51 AM   #5
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Circuit A.

Micro strip line printed circuits are a inductance and capacitance circuit.

____________________________________________________________

Circuit B.

A wire wound around metal core or a open air coil is inductance.

The the standard capacitor is capacitance.

____________________________________________________________

Circuits A and B preform the same functions.
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Old 15-Mar-2013, 9:00 AM   #6
GroundUrMast
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Quote:
Originally Posted by teleview View Post
... does not solve the problem of multipath when the antenna panels are aimed in different directions. ...

... does not solve the problem of signal power bleed off of signal from one antenna panel to the other antenna panel when the antenna panels are aimed in different directions...

A modified directional coupler is one way to somewhat solve the problems , however there is a big signal power loss (dB) when using directional couplers.
Quote:
And not because I do not know any thing about the subject.
It sounds like you are arguing against claims that have not been made. I don't see any claims that would suggest multipath has been eliminated, dealt with, overcome or solved... I see no claims of zero loss or perpetual motion. I haven't even seen a claim that the DB8E has perfected cold fusion.

The manufacturer's documentation is the most complete I've seen from any antenna builder... and it's clear, that if you opt to aim the panels in different directions, directivity and net gain will be reduced. It follows that one should not expect the split aim configuration to be impervious to multipath. Having said that, I fail to see any reason to be critical of the design or the detailed technical specifications provided. As with any other antenna from a reputable manufacturer, this is not a 'one size fit's all' solution and the antenna is not being promoted as such. I see a very well built high gain UHF only antenna, that has a well documented feature set that can provide an outstanding reception solution when properly spec'd and installed.

In theory, ganging two identical antennas would offer a gain increase of 3 dB. There are inefficiencies in the real world, so achieving 2 dB or more gain when ganging is doing well in my book. Given the credible gain specifications of the DB2E (12.0 dBi) and the DB4E (14.5 dBi) the engineers at Antennas Direct have done very well... achieving a net gain of 2.5 dB by ganging two DB2E arays. The results for the DB8E are even more impressive, at 17.4 dBi Max Gain, the combining networks incorporated into the design of the DB8E are doing extremely well... I see no evidence of "...signal power bleed off..." or "... big signal power loss...".

In the end, should one find they are beset with multipath, they have the option of cabling each DB4E section separately, and using an A/B switch or an axillary HD capable STB tuner. http://forum.tvfool.com/showthread.php?t=2882

Quote:
Circuits A and B preform the same functions.
Yes the function is the same, impedance matching and interfacing balanced to unbalanced... but the micro-strip-line has slightly less loss in it's designed frequency range.

Last edited by GroundUrMast; 15-Mar-2013 at 9:22 AM. Reason: defending the micro-strip
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Old 15-Mar-2013, 3:13 PM   #7
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Sorry, but I didn't see the post earlier in it's unexpected location.

The DB8e is pretty much as just described. Customers have been asking us for years to include the flexibility of independent aiming of the panels so we tossed the feature in there. When the antenna gets up in the air and the panels are aimed independently, either it will work or it won't. For those customers for whom it works, everyone's happy. If it doesn't then it will be necessary to fall back to the conventional practices of rotors, A-B switches, and the like. There's nothing magical or otherwise about the combiner - it's a very good two port splitter that is about as efficient as we could produce. If it's been treated with pixie dust or otherwise has "special" characteristics that eliminate signal phasing issues, I didn't get told about that. The coaxial cable's length has been optimized for this application.

As the DB8e is a UHF design, its application is going to be somewhat limited by the presence of VHF channels in many markets. It isn't for everyone (no antenna is) but, for those who need its peak boresite gain for "straight-ahead" locations, it's the best thing this side of a parabolic for UHF performance.
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Last edited by ADTech; 15-Mar-2013 at 3:39 PM.
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Old 16-Mar-2013, 12:09 AM   #8
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It is Ok to put forth information and ides.

As for micro strip line.

Micro strip lines are more susceptible to moisture that is always present in the air accumulating on the micro strip line circuit and this will detune the micro strip line circuit.

Incasing the micro strip line circuit in RF grade shellac or RF grade wax will reduce detuning.
And the micro strip line can be put in a sealed box.
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Old 16-Mar-2013, 10:59 AM   #9
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Quote:
Micro strip lines are more susceptible to moisture that is always present in the air accumulating on the micro strip line circuit and this will detune the micro strip line circuit.
Frankly, I disagree. Susceptibility to condensation is primarily a function of an objects surface temperature versus the dew point of the surrounding air. Water will condense just as readily on the windings and core of a traditional balun/matching transformer given the same environmental conditions. The dielectric properties of a transformer will also be affected negatively when subjected to moisture. In fact, I would expect a damp wire wound transformer would hold moisture longer as there is less air flow between the wire and core where moisture would cling than on a flat exposed surface. Ultimately the design of the DB4E balun provides plenty of shelter from rain, ventilation and drainage of condensation. The millions of functional strip-lines and wire wound transformers currently in service (some decades old) provides a strong argument that both are capable of reliable operation despite moisture in the air.

To give credit to the traditional balun/matching transformer, it generally offers greater bandwidth than a strip line or coax balun. So a wire wound transformer would usually be the better choice for interfacing an all channel antenna to the down-lead.

Enough nit picking... I hope ADTech's response has answered the OP's original questions. Unless @Psdstu invites further discussion it seems best to consider this thread closed.
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Old 17-Mar-2013, 5:17 AM   #10
Psdstu
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Thanks for all the replies, and responses.
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