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Re: [Discuss-gnuradio] power amplifiers on TX

From: James Humphries
Subject: Re: [Discuss-gnuradio] power amplifiers on TX
Date: Wed, 30 Dec 2015 18:14:07 -0500

Hi Daniel,

The output power of the USRP is controlled by the TX gain setting. Keep in mind that the output power of the USRP is not calibrated and will vary from device to device. Some may have higher output power than others.

Isolation between the transmit and receive sections of the USRP is a pretty big issue. I have issues with a very modest TX amp (~15dB) and am putting out just over +20dBm. An external switch was the only way to get the isolation needed. I guess some circulators also have pretty good isolation, but they aren't very cost effective for the isolation you will need.

Which USRP are you using? You'll need to properly shield the RF sections as well.

I'm on Marcus' side with that output power, that's a scary high output. I start to sweat at 10W... :) Good luck with your experiments.


On Wed, Dec 30, 2015 at 4:43 PM, Daniel Pocock <address@hidden> wrote:

On 30/12/15 22:24, Marcus Müller wrote:
> Hi Daniel,
> Cannot stress this enough:
> Don't try to do everything to the max right from the start. Sure, 100mW
> is a lot less than what can do in the licensed bands, but then again,
> not coming from an amateur background, 120W right out scare me. Please

Maybe some clarification is in order:

- need to check if the power amp's output is proportionate to the input

- does the USRP and GNU Radio provide a way to manage output power
(power into the amp), or is it always constant at 100 mW?

- it is legal - e.g. 400W is the limit in the UK and regs are similar in
other countries for fully licensed hams:

> make sure no more than -15dBm are fed into the USRP RX. So at an output
> power of 120 W ~= 51dBm, you need isolation of at least 66dB between
> your TX antenna and the RX port of your USRP for the TX frequency. Also,
> considering you're buying a device that can potentially cover 70MHz to
> 6GHz, spending lots of money on a powerful amplifier that can but
> operate on a few MHz really sounds like an unbalanced investment. Maybe
> reduce the output power (unless you want to do moon bounce, maybe), and
> get separate filters, just to keep the option of not operating in
> 144-147MHz; your whole operational range is much smaller than the amount
> of spectrum you can control with the USRP at once.

Agreed - many people would be satisfied with something up to 50W,
similar to a mobile rig, this was just the first thing Google found when
I included "100 mW" in my search query.

I have had SAREX contacts with 5W from a handheld device and J-pole, but
that was in Australia where the population is very thinly distributed
and I could have been the only person transmitting.  There were no
nearby mountains (unlike my current QTH) and no tall buildings.  In a
densely populated region like Europe, more power may appeal to some people.

> Of you're not going to use two highly directive antennas for RX and TX
> to achieve isolation:
> You will either need an RX/TX switch (that you should definitely control
> using the USRPs GPIO pins, which can be programmed to certain states
> when transmitting, receiving or doing full duplex) to isolate RX from TX
> when transmitting, or an extremely expensive circulator-based device.
> To be honest, you seem to be throwing money at your problem, that partly
> including having a bit of uncertainty what you want to do. I'd rather
> start small, buy the USRP, and maybe a preselective filter, experiment a
> bit with "short links", then invest in antennas, filters, switches and
> or amplifiers as you build up wishes and experience.
> My feeling is that you'd probably want to first RX only on different
> bands with different antennas, see what's possible with and without
> preselection, then decide on one or two bands, get more specific filters
> for these, switches, and then an amp.
> Don't start with something that you can use to defrost your antenna, and
> then try to figure out where you fried which parts of your rig.

Thanks for this feedback, I wasn't going to go out and buy this first, I
just wanted to start putting together a vision of what the big picture
looks like and what all the pieces will cost.

Buying an amp that can fry an egg is a poor substitute for having the
right antenna.  On the receive side, this fact is obviously even more




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