Variable gain amplifiers and band pass filters for TX

    My club has a LimeSDR mini 2.0 and we would like to use it to transmit on HAM signals but there are a lot of harmonics and the output power is tiny (0.1W). We therefore looked around for some amplifiers and some band pass filters.

    Band pass filters

    We came across this nifty multiband band pass filter bank from nooelec: https://www.amazon.ca/Nooelec-…-Barebones/dp/B0CBSYS1JX/
    and thought it would be perfect for us as we'd like to practice TX on UHF and VHF bands.
    Unfortunately, from a video by tech minds on youtube reviewing the filter, the reviewer mentions that one should not transmit through this device. This has me wondering: is there a difference between TX and RX rf filters? Furthermore, where could we find a similar filter bank for UHF and VHF made for TX?


    Amplifiers

    We came across this tiny wide range amplifier: https://www.amazon.ca/Zerone-A…9Z-Antenna/dp/B07V9H5QXD/

    And realized it might be good to get the output of the LimeSDR mini into the 1W range. Although from there, how can we amplify more (I'm not certain 1W would be enough to make contacts, especially with AMSAT satellites)? Other amplifiers we've come across seem to mention at least 4W input. Is this input a reference to electrical power or RF signal in? Are there variable gain amplifiers that work on TX, if we'd like to fine tune our output power to be as small as it needs to be?

    All club members are pretty new to this and we're still learning. We've built a double cross antenna by following a guide and managed to downlink some weather images from NOAA-18, 19 and 15 using an RTL-SDR. We then got our HAM radio licenses but the world of SDRs doesn't seem as straightforward as what we learned for the exam.

    Any help would be greatly appreciated!

    Hi VA2SZM


    It's very good to have new colleagues at a club.


    The output of SDRs are low power, and generally don't have many filters at the input or at the RF output. This makes it flexible in terms of use (works on multiple bands), and at an affordable price.


    We avoid using relays or solid switching devices, as they have a lot of losses, noise, and cost a lot of money.

    The idea of switching the input, output, bypass generates many problems. In other words, having multiband equipment at a few GHz is unfeasible for a low-cost assembler.


    Ham prefers to assemble separate amplifier pallets. One for each band. These amplifiers have amplification stages, as needed P1dB, IP3, and have band pass filters within the desired frequency. Due to power levels, filters are separated for each stage and desired band.


    An comercial HAM equipment (below 1 GHz) is most often built with an output power between 5 and 100 watts. For VHF and UHF satellites with 5 to 50 watts.

    With 1 watt and depending on your location, you will make local contacts.


    The input and output of an amplifier is a reference to the RF signal.


    The variable gain amplifiers that I know of are for use in laboratories (generally under 10 watts) and cost a few hundred dollars. We generally calculate the gain of our antennas (VHF and UHF satellites we use directional Yagi antennas with at least 10 dBs of gain), the cable losses, and use a larger amplifier than necessary. Some commercial radios have adjustable power (0 to 100%) or programmed levels (5, 25, 100 watts).


    Sandro Ribeiro

    73 de PY1SAN

    GG77

    Rio de Janeiro - Brazil

    Hi,

    some additional ideas. I agree with Sandro fully. If you like to use LimeSDR or Pluto on the same band, with the same frequency, a good isolation of the receiver input is nessecary to avoid overdriving when amplified transmit power can reach the receiver. Probably a direct short cut at the boards input is required. With separate connectors for transmit and receive only an antenna relay is used. But it's isolation must be perfect. It is funny to play around with SDRs on VHF and UHF. But using it for QO-100 it is a complete different story.

    73
    Andreas

    Thank you Sandro, Andreas for your advice, it is much appreciated.


    As for sending you a message, Stefan, I've signed up to QRZ but my callsign cannot be registered, unfortunately. Please be patient, I'll message you as soon as I can.


    73

    -Simon