The Metal Frame & External Antenna with Dynamic Tuning

While I’ve discussed material choices before in the context of mobile devices, the new Moto X requires a new depth of understanding in order to really appreciate the amount of work Motorola has done to enable the industrial and material design desired. In order to avoid issues with the metal frame detuning internal antennas and maintain radio performance, Motorola has developed their own custom antenna tuner that is supposed to be even better than the QFE15xx antenna tuner that Qualcomm has made as a part of their RF360 package.

Of course, at this point the iPhone 4’s “death grip” issue has been discussed to death. In short, due to a lack of antenna diversity, it was possible to easily detune the phone’s antenna and significantly decrease signal reception by putting a finger on the gap between two parts of the metal frame. Understanding how an antenna tuner can help to resolve this situation requires an understanding of impedance and how it relates to antennas.

The first and inevitable question is what impedance is. To briefly summarize this topic, impedance is essentially resistance in an AC circuit. Impedance in an AC (alternating current, or what comes out of most power outlets) is determined by resistors, capacitors, and inductors present in the circuit. In antennas, what’s really happening is that electromagnetic waves in the air are causing the antenna to resonate, and as a result the waves are converted in electrical signals. While this is easy enough to understand, the crucial portion of this is where the antenna connects to the rest of the system. Antennas inherently have an impedance determined by natural resonant frequencies, the height above the ground, and the conductors used to construct the antenna. For the most part though, this is relatively easy to tune for at the factory such that the impedance mismatch is small.

The major problem is that the real world is not just the inside of a factory. As mentioned before, the hand detunes the antenna due to its capacitive effects. This means that the impedance changes. For those still following along with the physics, the reason why an impedance mismatch causes reception to worsen is because the electrical signal is still in the form of a wave in the AC circuit, parts of the wave will reflect just like how some light is reflected when crossing from one medium to another, which is why water can appear to be a mirror from one side but a window from another.

Now that we’ve gone over the physics, let’s get back to the Moto X. Motorola has developed their own custom antenna tuner. While Qualcomm has their own antenna tuner, the major differentiator is that this antenna tuner actually detects capacitance changes at the antenna and adjusts impedance accordingly. In practice, the antenna is retuned incredibly quickly, with next to no hesitation. Motorola demonstrated this by showing two Moto Xs that were identical except one had the antenna tuner disabled. The Moto X without this antenna tuner rapidly dropped from ~23 dBm output power to ~7 dBm output power. The unit with the antenna tuner managed to achieve around ~15 dBm output power after detuning. Remember, decibels are a logarithmic scale so this represents around a 6.3x increase in power output.

In addition, in discussions with Motorola’s engineering team they claimed that the new Moto X improves upon the receive sensitivity of the first Moto X. This is no small feat as the original Moto X was known to have some of the best radio reception amongst its peers. Once again, this makes sense as even though polycarbonate is RF transparent there will always be some level of reflection, just like how there is reflection with impedance mismatches, and I would once again refer to our article on materials in mobile devices to get a better understanding of this subject.

Introduction & The New Moto X Moto Voice, Display, Actions, Attentive Display, Sound, Camera, and Final Words
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  • piroroadkill - Friday, September 5, 2014 - link

    Huge, pathetic battery, no microSD, bleh.
  • JBVertexx - Friday, September 5, 2014 - link

    I am cringing reading this article. There are pronouns missing antecedents, run-on sentences, and incohesive paragraphs throughout. Really, I couldn't even finish it the writing is so bad.
  • melgross - Friday, September 5, 2014 - link

    Seems as though you have a serious cognitive problem you need to get looked at. Even with some errors the article is well written and easy to understand.
  • JBVertexx - Friday, September 5, 2014 - link

    Serously? Who has the cognitive problem?
  • smorebuds - Friday, September 5, 2014 - link

    So where do you go for your grammatically perfect tech reviews?

    And like pitchers in the NL, it's nice when they are decent hitters but the pitching is far more important. In this case I'd put far more emphasis in Josh's ability to analyze smartphones than his grammar/writing skills (which are more than sufficient anyway... I mean, have you ever tried reading a DailyTech article?).
  • JBVertexx - Friday, September 5, 2014 - link

    Usually, Anandtech has the best writing of any of the tech review sites, which is why I read it every day. I'm just calling this one like it is.
  • RunAroundInCircles - Friday, September 5, 2014 - link

    You could always try the constructive criticism approach. You know, instead of bashing his writing with your first words. Try by giving credit where credit is due first, then suggesting improvements, and explaining how improvements can be made. I think you'll find that you will generally receive more agreeable responses.
  • JBVertexx - Friday, September 5, 2014 - link

    There was no credit due with this - it clearly was not edited - should not have been published like it is on this site.
  • Chaser - Friday, September 5, 2014 - link

    If you have any friends at all that sincerely liked you, thank them.
  • JBVertexx - Friday, September 5, 2014 - link

    Lots of friends buddy. This was legit criticism, so if you can't handle the heat,....

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