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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  • gg555 - Friday, September 5, 2014 - link

    I'm confused though, because Motorola's website says, "front-facing and at bottom." Why "and"? If there's only one speaker, should it just say, "front-facing at bottom"? Reply
  • spikebike - Friday, September 5, 2014 - link

    Please motorola, er, lenovo. Please stop making disposable phones that turn into a useless brick when the battery dies. Such nice design and specifications could easily lead to a phone that would be useful, at least for secondary use for 3-4 years. If it's going to cost $500 it shouldn't ($20 a month for 2 years) I shouldn't have to throw it away to replace a $15 battery. Reply
  • semo - Friday, September 5, 2014 - link

    This must be planned obsolescence and I feel that it won't stop until smartphones are treated like fashion items. The S5 is almost the same phone, yet it has a microSD slot, removable battery and even has the option for Qi. If Samsung can do it, why can't Motorola? Reply
  • agent2099 - Sunday, September 7, 2014 - link

    The s5 is $300 more Reply
  • jhh - Monday, September 8, 2014 - link

    I used to think that replaceable batteries were a good idea, until I actually needed a new battery. This was the days of flip phones, but I doubt much has changed. The OEM battery was no longer available when a new one was needed. This left third-party batteries as the only exception, and their lifetime was much worse than the original. Since every cell phone seems to use its own form-factor, by the time a new battery is needed, the manufacturer has also discontinued the battery. I suspect that the third-party batteries ar also old at that point. Reply
  • steven75 - Monday, September 8, 2014 - link

    You and your subtle yet true facts! ;)

    Also the external USB battery packs and cases solved this issue long ago.
    Reply
  • Klug4Pres - Tuesday, September 9, 2014 - link

    "This was the days of flip phones, but I doubt much has changed."

    While your concern is valid, I think the situation with phones that have sold in high volume may be better than would be the case with a random flip phone. Some third-party battery suppliers seem to have a decent reputation.
    Reply
  • jjj - Friday, September 5, 2014 - link

    If they payed me i wouldn't use a phone without microSD. It just kills media creation(pics and vids) and consumption.They might as well get rid of the cam too.
    That being said good job on the slightly lower price compared to other phone makers from the old guard.
    It's funny how Moto X still feels like a midranger even if on paper if has high end specs. Just can't shake the feeling that it is a lightweight phone. Maybe the branding needs to be changed, might be hard to sell high end under it.
    The design of the front , not a fan, very dirty and looks like it's been through a war or something.It also makes it look bulky even if the dimensions are not bad for the screen size.
    Reply
  • nevertell - Friday, September 5, 2014 - link

    Why are they lagging behind HTC and Samsung in the SoC department ?
    I'd love to have this phone if it had a 64 bit SoC. Then again, I'm sort of a developer so I need a feature complete device with a feature complete SoC.
    Reply
  • jeffkibuule - Friday, September 5, 2014 - link

    Umm.. because none of the 64-bit Qualcomm SoCs currently available perform better than a Snapdragon 801? You want the Snapdragon 808 and 810, and those aren't due until next year as they were designed around 20nm, not 28nm. Reply

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