That makes sense, at that point in the circuit the load resistance might be a few ohms and so only require a few volts to generate 4W - At the output though, the load will be 50 ohms, and at that point the peak to peak voltage must be 40V (given 4W output power). I'm after any sort of formula or even 'spoken' words that says "it will be 40V because...."




James

 

If you are a fan of the film "Blast From the Past" you might recognize the finish to this line....it will be 40V because...it must.

In RF work, we do our network design using power and impedance and let the volts and amps work themselves out. There is no doubt that at the antenna terminal, the peak to peak voltage must be 40V to give 4 watts at 50 ohms. As I mentioned in a previous post, the components used between the drain of the transistor and the antenna output are all reactive, so they don't use up power, but they do shift voltages and currents (voltage goes up, current goes down, power stays the same). If we pay attention to the impedance going from 1 ohm at the input of the matching network to 50 ohms at the output of the matching network, then the voltages and currents will also transform from 5.6 volts and 5.6 amps peak to peak at the input, to 40 v and .28 amps peak to peak at the antenna terminal (if i've done my arithmetic correctly that is).

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RadioRon

 

RadioRon, I thought power level was supposed to be tested using a EM field meter at a specific distance from the transmitter? If I'm not mistaken transmitters/receivers have to be tested with their antennas as a unit to receive validation. I've read many warnings that even though the antenna's on many devices are removable changing the antenna can void the FCC certification. Mind you that's a whole lot of they gotta catch ya doing it =)

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Good point, but this only applies, in the USA, to some "part 15 devices". These are unlicensed transmitters operating at low power and are a special case. My comments about measuring conducted power applied in general to licensed transmitters and some unlicensed devices and I regret not making this distinction in my last email. It would be interesting to know how many devices of each category are out there. I would guess that there are many millions of unlicensed part 15 transmitting devices out there. I would also guess that there are several hundred million licensed devices out there. Cell phones are licensed devices of a special sort and there are many more cellphones than there are all the other types of licensed transmitters combined.

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RadioRon

 

Hrrm, here's a question then. If cell phones are licensed on their power output through a resistor load could I hook a micro yagi or biquad to my cell phone and still be legal?

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Man, you had to ask a question that takes some serious time to answer! First off, understand that there is two aspects to this. One is the equipment authorization, which applies to the individual cellphone and certifies that the phone meets certain technical requirements. Then there is the license, which is different but the licensee cannot use anything but authorized equipment. The license is granted to the cellphone company on whose system your phone operates, so they have a responsibility to FCC to maintain their system within technical limits, and this includes "their" phones.


Here is an example. Nokia FCC ID EO25ZVPC105A.
The grant of equipment authorization states that the device is regulated by Part 22 of the FCC rules, and has an output power maximum of 0.6 watts (conducted). So, then when you look up part 22 it says that mobile transmitters must not exceed an ERP of 7watts. (part 22 covers cellular channels below 1 GHz, while part 24 covers PCS which is 1850Mhz and thereabouts). This implies that you would not be illegal if you put an antenna with gain of up to10.6 dB on this device. But wait.

PCS devices under Part 24 must not exceed 2 watts EIRP peak power (ref CFR47.P24, 24.232). If you were dealing with a CDMA mobile phone in this band, then the conducted authorization would again be 0.6 watts, but now you could only add a 5.2 dB gain antenna. Slightly lower for a GSM phone. But wait.

Now, having said all that, remember that it is the cell company (Verizon or whoever) that holds the license for your phone, and I am going to guess that you need their authorization to change anything in one of their phones, in order to be strictly "legal". I would also guess that they might say no if you asked their permission because when you increase tx power, you mess up their cellular coverage structure, depending on where you are and where you point your antenna.

Now, there is also the issue of the fact that the phone is also certified as a part 15 device when it is not transmitting. This certification probably also does not allow you to change the antenna.

Then there is the other rules that apply to personal communication devices, those that deal with radiation hazards. These dictate how much power you can feed into your antenna, at a maximum, depending on the distance between the antenna and any people.

So, I'm afraid it is a bit more complicated than we all would like it to be. From the cellphone system operator's point of view, adding an external antenna might mess them up, so they probably wouldn't like it unless their engineers gave the go-ahead.

Whew, that was a lot of typing. Can we change the subject now?

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RadioRon

 

I followed everything you said with perfect clarity, and it only makes me wants to cause the FCC to cease to exist that much more. They at the LEAST need a full reboot. I think we're already more than a few generations beyond it's ability to manage the RF spectrum, and in many of the 'of major interest' bands that the FCC manages they act as nothing more than a pimp taking their cut while the industry itself acts as it's own regulator commission because they need every single microscopic bit of data they can squeeze out of what spectrum they're allowed. 99.99% of their mandates and laws are unenforceable in the first place so they almost completely exist by acquiescence.

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I knew you were going to say that! :-)

And of course it's true, the laws of conservation of energy also support that logic.

I'm just going to have to knuckle down and do the maths to get to the voltage part!


Many Thanks
James