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Re: [Discuss-gnuradio] power amplifiers on TX

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] power amplifiers on TX
Date: Wed, 30 Dec 2015 22:24:19 +0100
User-agent: K-9 Mail for Android

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 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.

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.
Best regards,

Am 30. Dezember 2015 21:32:46 MEZ, schrieb Daniel Pocock <address@hidden>:

Based on the USRP specs (up to 100mW output power), I started looking
around for amps that would get me up to a level comparable with other
amateur rigs. This is one of the first I found:


Specs mention a low-pass filter so harmonics shouldn't be amplified.

It takes 100mW in and puts out 120W on 2 meters. It needs up to 21A
when running, that probably means getting quite a serious DC power
supply or just running it off a car battery. It looks like it needs an
external heatsink.

Has anybody else tried solutions like this with USRP or other SDR

For simplex operation, if the application is not transmitting, will
output power completely drop to zero when using something like this?

Can anybody comment on how to wire the USRP and a power amp for TX and
RX through a single antenna?



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