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Re: [Discuss-gnuradio] power amplifiers on TX

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] power amplifiers on TX
Date: Thu, 31 Dec 2015 14:08:09 +0100
User-agent: K-9 Mail for Android

Output power of an SDR is both a function of gain and of course of the digital signal you're generating - for example, a digital signal with amplitude 0.5 should have, in theory, 25 times the power of the same scaled to have an amplitude of 0.1.

Best regards,

Am 30. Dezember 2015 22:43:04 MEZ, schrieb Daniel Pocock <address@hidden>:

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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