I am involved in a project related to CubeSat communications. Currently, we have a VHF Cross-Yagi antenna with a 12.7 dBi gain. We would like to test it by sending RF signals to a local receiver for now. To my basic knowledge, for the transmitting front end, we need a power amplifier, a SDR, and software to encode, modulate, etc., the signal (we are using GNU Radio for this). Is this front end optimal for sending signals?
I have been looking for a suitable power amplifier that can be compact and placed near the antenna, but I can’t find any compact ones. Do you have any recommendations?
Hi, nice to hear you are experimenting with this (:
Depends on what you mean by “nearby”, meters, kilometers ?
For lab testing you do not need power amplifiers, if you put 0dBm in a small antenna it will be screaming strong in any receiver in the same room.
To calibrate any of my receivers that is already connected to antennas, I usually send a continuous calibrated tone to a rubber ducky at -50dB or so, it is clearly visible some tens of meters away.
If you are connecting TX to RX in some way, you want a lot of attenuation to keep the signals to a reasonable level.
If this is over the kilometer range, either use the SDR TX as is, or put a LNA capable of putting out 20-30dBm as PA. Transmit into a dummy load and monitor the spectrum as you increase the TX power, pushing it too hard will just mess the signal up.
Always use low pass filter after the PA if it’s not already built in.
You can probably find a lot of modules at the usual suspects, search for “Linear Power Amplifier SDR” or “vhf amplifier sdr” that has a bunch of gain and decent output power. These almost never have low pass filters built in and needs to be added.
Having everything down in the lab is way easier than putting a lot of equipment at the antenna. Use a mast mounted preamp for the RX and put the rest down in the lab. Compensate for losses when TX by increasing power (: losses at VHF is not that high in decent coaxes (no 5mm stuff).
Do use a sequencer to control preamp/PA to avoid frying the preamp.
For now, we only want to test the antenna for a short range (one of my colleagues will do it since he has a license). Later, we plan to test it over longer distances, possibly using a balloon or drone with an attached receiver device. The VHF antenna is mounted on an aluminum boom that also supports a UHF antenna used for receiving CubeSats or LEO RF signals. The antennas look like this: LEO Pack for Satellites. The antennas are mounted onto a mast that is mobile with caster wheels with the goal to make it transportable.
On the receiving end, we have an LNA with a built-in bandpass filter for 430 - 440 MHz and a HackRF One. I was initially confused when you mentioned that it is possible to use an LNA in the TX part as a PA. I now understand that an LPF or bandpass filter is needed to avoid amplifying unwanted components.
Do you have any sources or books where I can learn more about the transmitting part? I have seen some setups that use SWR meters, but I think our antenna doesn’t need one yet. The description states, “SWR Across Entire Band: <1.2”.
Some of this is covered in the study material for becoming a radio amateur. There’s also a lot of diy and kit building out there, some of them cover more theory as well as the practical circuits and what they do.
QRP-labs mostly does low power (as the name suggests) and some stuff that is in the VHF territory. His kits and instructions are good quality. the PA’s and LPF have som good info, but is mostly specific to HF, theory applies to VHF as well.
The W6PQL site that has the sequencer also has a ton of kits and info.
SWR is a term that ham’s often use, it is centered at 1:1 at a perfect 50ohm load and it tells how much of the transmitted power is reflected back into the amplifier (bad, heating and overvoltage, terminate or severly limit the tx power).
When talking about antennas and RF it becomes more important to speak about the nitty gritty details like the impedance both in the reactive and resistive. Like when using a VNA to measure something you get a lot more details than just SWR. It quickly gets into the smith charts and all kinds of good stuff (:
When building stuff you will need to measure, depending on what you already have access to you will soon have a shopping list of modules and instruments (:
I agree with Daniel SA2KNG…. A lot of the questions you have been asking are the sorts of things that Amateur Radio operators learn as part of getting their licences and have built on over the years through the practical application of that theory…
If you want to learn a lot of practical skills about building radio systems I suggest you have a look at the series of YouTube videos by VE2ZAZ/VA2IW where he goes about designing and building a 10GHz Transverter system from scratch. He goes through all the aspects of radio systems including some of the questions you are asking about levels, filters, losses, power, sequencing, testing etc. What he says about 10GHz applies at all frequencies.
Do your self a favour and invest the time in watching them. They will greatly add to your skills in resolving issues and make getting an Amateur Licence easier.
Thank you for your response. Unfortunately, I didn’t know that these were taught in radio amateur courses, as I am a student working for the university and was never informed about the radio amateur stuff, lol. They just told me to work on it, and since my specialization is aerospace engineering, I was lost at the beginning, but I’m figuring things out slowly. I will try to look for the online videos you provided. However, I am short on time due to exams, so I won’t be able to attend classes now. Regarding getting my license, I won’t pursue it since my colleague/boss has one, and he is, of course, the one who wants to transmit. However, the stuff that I having learning in the process is awesome!
I appreciate your advice and apologize for any inconvenience!
At least try to get hold of study material for radio amateurs, it covers a lot of the basics (:
While recommending channels is on the topic (which I have viewed a lot): W2AEW “Mostly videos on hobby electronics, test & measurement, ham radio, and other stuff.” TSP “science & electrical engineering video blog for industry professionals, students and hobbyists.” FesZ “channel about various electronic projects I spend my free time doing.” RobertFeranec “Electronics, PCB Layout, High Speed, Board Design and more”
Good luck in the project, and keep us posted on how it goes. Remember to give back to the community and be involved, even if not revealing any secrets (:
Thank you for the recommendations and sources! W2AEW was actually the first source I watched when I started a couple moths ago, lol! I’ll continue learning and will keep you updated!