[USRP-users] Amplifiers for USRP Transmission

Marcus D. Leech mleech at ripnet.com
Mon Jul 6 16:59:03 EDT 2015

On 07/06/2015 03:41 PM, Marcus D. Leech via USRP-users wrote:
> On 07/06/2015 03:32 PM, Evan Chavis via USRP-users wrote:
>> Hello USRP users,
>> I've been working with a USRP1 to transmit an amplitude modulated 
>> signal to a very basic improvised receiver, basically just a wire of 
>> length corresponding to the frequency I'm transmitting at attached to 
>> a microphone such that the microphone is picking up the signals on 
>> the wire as it would audible noises.  I'm using an SBX daughterboard 
>> (which can produce 100 mW) and a log periodic antenna (with 5-6 dbi 
>> gain).  I can get a decent transmission at a range of up to about 25 
>> cm, although it is best at a distance of just a few cm.  I've got the 
>> gain set with GnuRadio as high as possible without causing 
>> distortion, and I'm looking to improve the range of possible 
>> transmission.
>> My questions are, what do I need to consider when looking for an 
>> amplifier to use with a USRP, are there any special problems a 
>> beginner would be likely to miss when trying to use those 2 things 
>> together?  Also, if an amplifier's gain is listed at 20 dB, would 
>> that be 20 dB on top of the 31.5 the daughterboard is capable of 
>> outputting, or would the amplifier be useless in that it is capable 
>> of providing less power than the daughterboard itself?  And third, 
>> how is a device such as this one: 
>> http://www.minicircuits.com/pdfs/ZX60-V63+.pdf powered?  The 
>> data-sheet gives the numbers 5 V and 69 mA but I don't see an obvious 
>> way to power it aside from just soldering wires onto the marked bumps 
>> that say +5 and ground, is that the actual intended way to do it?
>> Many thanks for any input and have a great day
>> _______________________________________________
>> USRP-users mailing list
>> USRP-users at lists.ettus.com
>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
> You don't need an amplifier.  You need a better understand of how RF 
> works, and how RF receivers work.   The SBX covers 400Mhz to 4.4GHz.
> Given that you don't appear to know how RF works, I'd be very 
> reluctant to recommend that you augment your experiments with an RF
>   power amplifier on the RX side.
> How about studying how RF receivers work, how EM propagation works, 
> etc, etc?    The ARRL handbook is a good place to start on basic
>   RF topics. Adding an RF power amplifier to your setup will be both 
> illegal, and frustrating, and totally not the right way to go about these
>   experiments.
So, I'm going to augment my (admittedly, harsh) comments above with some 
comments that are more tutorial in nature.

Evan.  Consider that EM waves propagate on an inverse-square law 
basis--how might that  affect the efficiency of your receiver?

Consider that  an antenna + microphone is perhaps a wildly inefficient  
AM receiver, how  might that inefficiency  aspect affect your results?

Are there ways you can think of to improve your reception results, 
without using a power amplifier on the transmit side?

There are two problems with adding an RF power amplifier in your situation:

   (A) Legal.  Transmitting significant power into an RF band where you 
aren't licensed can lead to unpleasant visits from Federal Authorities.

   (B) Safety.  Significant RF power needs to be treated with utmost 
respect, and if you're a rank beginner (and it appears from the 
description of your
        setup that your are, and I apologize if that impression is 
incorrect) you really shouldn't be playing around with high RF power 
levels without
        knowing what you're doing, *particularly* as you move up in 

Evan, I'm guessing that you're a CS student at UMich, and probably not 
in a combined CS/EE program, so many of these concepts you may never
   have considered before, which is why I recommend something like the 
ARRL Handbook--the first several chapters go into  RF theory, receivers,
   transmitters, antennas, transmission lines, etc, etc.  RF design is 
an actual discipline, and it *REALLY* helps to understand it, even if 
this is
   *software* defined radio, the real, physical-world  aspects play a 
*huge* role in ultimate success.

Good luck, and don't take my previous harshness too seriously, I was 
primarily concerned for your safety, both in the physical-harm sense, and
   visits from unpleasant Federal Agents....

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