If you have an SDR radio is there no point in buying different band transceiver - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: September 26, 2017, 08:16:18 pm
content: I am getting into ham radio and was going to buy a fancy transceiver but was thinking since the SDR will pick up all bands you could just use the SDR to pick up signals then have a few different transmitters for each band and transmit on that. Could this be done with a multi antenna switch for each band and maybe something where you could use the same antenna on the SDR and transmitter? Just a thought don't know if this would work in real life.

author: awallin
author_rank: Frequent Contributor
timestamp: September 27, 2017, 05:13:30 am
content: maybe it's a bit like USB/PC-oscilloscopes vs. "real" oscilloscopes.Sometimes it's nice to have a dedicated screen on the device, real physical knobs and meters and LEDs etc.. no crashing operating systems or constant software updates..what you describe sounds doable. Some SDRs could have a TX/RX switch inside so they can use the same antenna for TX and RX. I'd imagine you have to be careful with a power-amplifier feeding back (reflections etc) power into the SDR input.did you look at what computer peripherals are available for SDR use, e.g. usb-connected CW-key or headset with push-to-talk or similar?

author: DimitriP
author_rank: Super Contributor
timestamp: September 27, 2017, 07:03:01 am
content: QuoteIt sounds a lot easier than it is in "real life".I'm not trying to talk you out of it, but I believe life will be a lot better if you embark on this project after you have a decent antenna up and connected to your "fancy" tranceiver !

author: bd139
author_rank: Super Contributor
timestamp: September 27, 2017, 08:21:21 am
content: SDR feels like cheating to me. Anyone can find a CW signal on a waterfall display. Seems to take the fun out of it.As for what you want to do, it doesn't sound like a terrible idea. You'd need a TX/RX switch, possibly transmitter controlled on keying. You can probably get away with one transmitter for all bands with a DDS VFO or something like that if you want to build something. Typical QRP style transmitters is a good bet. Millions of ARRL articles out there on them.

author: dmills
author_rank: Super Contributor
timestamp: September 27, 2017, 10:56:45 am
content: Good RX design spends a lot of trouble to remove as much out of band energy as possible before hitting the active devices, and this is at least as much true of SDR as it is of the classic superhet.I would at the least add a switched band pass filter ahead of the SDR to limit the RF power at the mixer or AD converter, it will likely make a worthwhile difference to what the apparent noise floor is in band by limiting reciprocal mixing and improving the linearity of the mixer or allowing more gain ahead of the ADC.73 Dan.

author: bd139
author_rank: Super Contributor
timestamp: September 27, 2017, 11:27:02 am
content: Also don't some of the SDRs have problems with LO leakage and imaging?

author: Beamin
author_rank: Super Contributor
timestamp: September 28, 2017, 12:51:36 am
content: I don't want the SDR to transmit just receive and get a HF and separate VHF transmitter. I like how you can get kits for 40 meters and just use it as a dedicated transmitter. I ordered one of those and built it the other day cheap and Its around 7 or 8 watts; I want to do QRP. I use the SDR to find signals then switch to the 40 meter unit but its receive isn't as good as the SDR. I was thinking some relay type device or arrangement using PIN diodes to block the TX into the SDR. I have several different antennas I have made from HF to VHF to UHF and a whole bunch in between. I want to find and build a TX for each band that's <10 watts and use one SDR.There is a TX/RX SDR but its like 800 and the TX is more for running experiments like messing with your neighbors door bell and breaking into your own car. Plus I would be pissed if I got the SWR wrong and killed the SDR.

author: cdev
author_rank: Super Contributor
timestamp: September 28, 2017, 01:02:36 am
content: Have you gotten your license yet?You need to get a license to transmit with ham radio equipment.If you dont already have one, you should start with an RTLSDR, maybe the rtlsdr blog version 3.. very cheap and you'll be able to listen to a lot.Even HF if you build a band pass filter for it.

author: ChuckDarwin
author_rank: Regular Contributor
timestamp: September 28, 2017, 01:48:24 am
content: I was going to note that using a series of transceivers UHF/VHF/HF, depending on your preferences, is probably going to create far less operational hassle, if your interests lead in that direction. Using a SDR in tandem to see spectrum, isolate and possibly decode signals is desirable; SDRs have lots of cool add-in filters and decoders which I am beginning to appreciate. Many newer transceivers have mini-spectrum analysers built-in. An elmer friend prefers hunting-the-dial and using "an ear--even have a spare one", so to each his own. Seeing your last comment, which clarifies your interests a bit more, you might look for a electronic coaxmatrixswitch (CYTEC CXL or JFW) for the rigs and antennae. Cutting off the SDR, and possibly terminating it into an attenuator/load, so you don't overload it or let the magic smoke out when transmitting elsewhere would be a strong consideration.

author: radiogeek381
author_rank: Regular Contributor
timestamp: September 28, 2017, 02:08:05 am
content: First off, I'm a ham, originally licensed in 1979, so not yet an old timer. Most of my operating time in the last fifteen years has been VHF/UHF/microwave. I've owned lots of "normal" HF radios, and a few VHF and UHF radios. (Coolest was a Signal/One CX-7A, sorry I ever sold it.) I also spend a lot of time with a modern SDR.SDR is a whole lot of fun. But it is very hard, and perhaps impossible, to find an affordable starter's SDR based HF radio. VHF/UHF is probably a different story. (I have an Ettus N200/UBX on my desk here. It tunes from 10MHz to 6GHz. It is <not> a starter's radio.)Well designed HF radios carefully apportion gain in the RX chain between the antenna and your ears to ensure good performance in the presences of very very strong interfering signals. Imagine trying to hear a 100mW signal from 300 miles away while your radio is being blasted by a 50000 W transmitter just 30 miles away.* That's a 60dB difference in signal level BEFORE we account for the path loss. In the ideal free-space world, the actual difference would be something like 80dB. Yes there are tricks in SDRs to improve the dynamic range, but it costs money to get to that point. If you want a radio that can deal with the very very noisy HF environment, your best bet is to go analog.Yes there are SDR HF transceivers with good IMD performance. But they are very expensive. I can't imagine that spending more than $4000US for your first HF radio is a good idea.VHF radios are a different story. There are still intermod issues and dynamic range considerations, but they are much easier to treat (in my experience.) An SDR receiver for VHF/UHF could provide some interesting opportunities even in the beginner price range.But if you want to get your feet wet on HF, find an "analog" or at least not-so-SDR rig. I have an FT817 (HF,VHF/UHF) that is just fine. Yes there are better. (especially since the FT817 makes some compromises in that balance between front-end-gain and dynamic range) There are also radios that are much cheaper and a joy to operate. I've owned a few TenTec transceivers that were simple, fun, easy to work on, and performed well. No doubt other posters will have suggestions of their own. Find a local ham who can help guide you, and don't spend a lot of money on that first rig.I won't be surprised if someone posts that they've been operating on HF with an SDR built out of two TTL inverters and a length of coat-hanger wire. Good for them. But I have and use a moderately expensive SDR with software that I've been developing for a long time. I have an FT817 right next to it. The 817's performance is head-and-shoulders above the SDR on frequencies below 20 MHz or so. Above 50 MHz the SDR wins, hands down. In time, I'll add front-end filtering to the SDR. It will improve things a bit, but there are lots of intermod sources that will still be in the passband. (Note that most of the off-the-shelf inexpensive SDR's have very broad front-ends -- some have no filtering at all, others have filtering that isn't so good.)All that said, the RTLSDR is loads of fun. There are interesting signals to be found in the VHF spectrum. Yes, it can get crunched by strong signals. But for $20, it makes a good platform to learn a little about SDR and play with some neat tools.And if, after all this, you do go with an SDR for HF, find one that is designed for HF. Radios that tune from "DC to Daylight" mix products from "DC to Daylight" and create lots of signals that aren't really there. They really need front-end filtering to be adequate.*The illustration is based on an AM broadcast station (well out of band) vs. the desired signal in an HF ham band. Say an AM broadcaster at 1300 kHz and your desired signal at 3.9MHz. There are no "perfect" filters, and that AM signal is going to look pretty tasty to the first semiconductor junction it sees.

author: Beamin
author_rank: Super Contributor
timestamp: October 01, 2017, 07:08:00 pm
content: I recently bought an SDRPlay because in what I have read it out performs everything under 300$ and even some more expensive ones like the HackRF for 800. It also seemed the best for under 50MHz. I have a balun and I run different lengths of wire in a long wire ant. setup. It does have some issues with stations in HF appearing in multiple spots but seems to only do this with a few stations. I tried the dongles under 50 MHz but they don't really work. Dongles are good for specific digital stuff you know is there.I built some VHF and UHF antennas for it and I never really hear any HAMs on 70cm or 2m. These antenna are pretty directional and I haven't spent tons of time trying to hear them unless they are operating digital and I don't hear them. I have some software that will decode digital but it can be a pain because the software that comes with the SDR is much more sensitive and easier to use/see signals. Unfortunately you can't run both programs so if I find a signal I have to close the sdr program open another program and tune it to the frequency. I have only decoded morse and that one digital format on 14.somethingMHz I can't remember what its called. I want to use wisper but haven't really got it to work because it seems to want to use a sound card and I can't get it to understand that there is an SDR attached.Another cool program is the virtual radar that picks up plane transponders. That alone is worth the cost of an SDR. I still haven't been able to make it work with the cockpit view. That would really be cool.