Power Amplifier for LimeSDR mini - Page 1
Forum section: electronics / rf-microwave

author: mariolucas75
author_rank: Contributor
timestamp: October 14, 2021, 05:21:16 pm
content: Dear All,Could you please advise me on the following ....I am using LimeSDR Mini and i want to plug it into a power amplifier (PA) to transmit a stronger signal. The questions is which PA to use (relatively cheap one).As i know the selection of the PA depends on various factors such as what frequency i want to transmit, bandwidith, modulation... etc etc ...So my questions are ....1) Can you advise me what PA to use with LimeSDR mini if i want to transmit lets say GSM signal (let it be 900 Mhz or 1.8 Ghz)2) Can you also advise me the internet source where i can learn how to select AP and what additional components are needed depending on the task i want to do - amplify FM, AM, GSM signal (GMSK modulation) or if it is just a Morse code ... to learn how to select a PA for my LimeSDR mini. I know there is a LimeRFE designed for Lime products but it is very expensive 700$.Therefore i need cheap solutions and also to learn how to select these solutions....Thank you so much.

author: srce
author_rank: Regular Contributor
timestamp: October 14, 2021, 08:53:27 pm
content: First question is how much power do you need?

author: geggi1
author_rank: Frequent Contributor
timestamp: October 15, 2021, 04:46:02 am
content: Google "LimeSDR hamradio" and you will probably find plenty.Just be aware that some of these SDR units produces harmonics and spurious and you will have to filter to get this removed.By using a amplifier spurious and harmonics can be worse.

author: mariolucas75
author_rank: Contributor
timestamp: October 15, 2021, 08:59:41 am
content: Quote from: srce on October 14, 2021, 08:53:27 pmJust to cover ~ 20 meters....Is there anything cheap for this distance ?Also please could you advise me what / where to read to be able to know what type PA should i select depending on task (transmit distance, freq band, modulation types... etc etc ) i want to do ?

author: srce
author_rank: Regular Contributor
timestamp: October 15, 2021, 02:16:37 pm
content: I wouldn't have thought you'd need an external PA if the range is only 20m.

author: radiolistener
author_rank: Super Contributor
timestamp: October 15, 2021, 04:22:16 pm
content: Quote from: mariolucas75 on October 15, 2021, 08:59:41 amThere is no needs power amplifier to cover 20 meters distance.LimeSDR mini has 20 mW output, this is good enough power to cover 2000 meters.On the contrary, if you use power amplifier your receiver at 20 meters distance will be overloaded.

author: 9aplus
author_rank: Regular Contributor
timestamp: October 15, 2021, 04:22:58 pm
content: For 20 m range, just use proper resonant antenna...PSUser of 2 x Lime SDR USB, 1 x Lime NET micro

author: mariolucas75
author_rank: Contributor
timestamp: October 16, 2021, 08:48:34 am
content: I did FM transmission and it fades away just in another / next room .... I thinks it is an issue with low power transmission....I have standard alloy steel telescopic antenna.Another question - does limeSDR mini has an option to see what is it its power output at different let's say frequencies or modulations? Just to know how many dbm or mW is delivered into antenna and from antenna into air ?

author: radiolistener
author_rank: Super Contributor
timestamp: October 16, 2021, 09:07:51 am
content: Quote from: mariolucas75 on October 16, 2021, 08:48:34 amThat's the problem of your antenna and walls. Your walls working like shielding.And these Chinese "telescopic antenna" are very bad antennas, because they are not matched with 50 Ω connector. They are intended for receiver (for which matching is not so critical), but they are not intended to work with transmitter.For transmitter you're needs properly tuned antenna with matching between transmission line and antenna. Otherwise it won't accept the power from transmitter.Quote from: mariolucas75 on October 16, 2021, 08:48:34 amYou're needs to measure it. Here is power output for different frequency:http://www.dd1us.de/Downloads/LimeSDR%20mini%20TX%20output%20power%20in%20the%20VHF-UHF-SHF%20bands%20rev1.pdfSince you're using just a metal stick (with no tune and no matching) instead of antenna, I bet that very small part of transmitter output power is delivered to antenna, because your metal stick is not matched with 50 Ω transmission line and your metal stick doesn't have counterpoise. Without counterpoise it's impedance will be very high, so it has very high VSWR and power is just reflected back to the transmitter.You're needs proper antenna, properly tuned and matched.

author: mariolucas75
author_rank: Contributor
timestamp: October 16, 2021, 09:32:39 am
content: radiolistener could you pls please give me links to matching antennas or references what to consider / read to be able judge independently what antenna is good what is bad .....You also mentioned to measure output do you mean connect RF power meter to LimeSDR mini Tx port ? or just to measure in the vicinity (in the air) what power radio waves reach my RF power meter ?Thank you so much

author: radiolistener
author_rank: Super Contributor
timestamp: October 16, 2021, 10:04:59 am
content: Quote from: mariolucas75 on October 16, 2021, 09:32:39 amI don't use 900 MHz or 1.8 GHz bands, so I cannot suggest something. As a simple and dirty solution you can build your own simple classic antenna, such as half-wavelength dipole or something like that.If you're looking which antenna to buy, just avoid these which are marketed as "wideband", "all frequency" or something like that. Look for antenna which intended and tuned for the specific frequency of your interest.Quote from: mariolucas75 on October 16, 2021, 09:32:39 amYes, you can measure it's output power with RF power meter or with a spectrum analyzer. But they all have almost the same power output, so you can believe these values measured for LimeSDR mini by other peoples.

author: mariolucas75
author_rank: Contributor
timestamp: October 18, 2021, 07:43:29 pm
content: Can i plug LimeSDR mini Tx port to let say RTL-SDR Rx (It has only receive) port and thus will be able to measure power LimeSDR Mini produces at different frequencies (just as experiment) will not i damage either of them ? This is instead of buying costly RF power meters or spectrum analyzers ...

author: radiolistener
author_rank: Super Contributor
timestamp: October 19, 2021, 05:40:56 am
content: Quote from: mariolucas75 on October 18, 2021, 07:43:29 pmNo, RTLSDRv3 with disabled pre-amplifier (0 dB gain) has ADC overload input level about -13.87 dBm (measured at 40 MHz).You're needs to use at least 30 dB attenuator between LimeSDR mini and RTLSDRv3, otherwise RTLSDR ADC will be overloaded and signal will be distorted.