HFU HF Underground
Technical Topics => SDR - Software Defined Radio => Topic started by: syfr on May 27, 2020, 1621 UTC
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Hi All,
I have an SDR or two I am going to deploy for HF reception. I hope to link to them via WiFi to avoid a DC connection to my house. Power will be a DC feed (yes, a dc connection) but via a run of "fusible link" thin wire , before connecting to a choke and onward to a DC supply. The whole arrangement is going to be assumed to be somewhat sacrificial as I live in NC, and lightning is a way of life. The idea is that my house is not sacrificial.
I have a tower and a yagi etc, but that stuff is all disconnected during lightning season except when I'm using it. I do have about an acre of woods back there with a ton of trees that I can hang things from, though I really don't want to erect something super complicated at this point .
Here's the question. What recommendations do you have on a broadbandish wire antenna ? Let's assume it's not tunable (so broadband is a poor term!) for HF ? I've never messed with a loop and I do have one of Chris's multitap impedance transformers available for this , so those are on the radar screen. If that's a good solution, what design/dimensions would you recommend? I guess a G5RV is kind of a loop/folded dipole/squashed loop.
I've got a lot of wire if needed.
Your thoughts appreciated. Chris, what works well for you?
I don't want to put my Kiwi out there, but an MSI SDR might be expendable (I want to stick a RTLSDR dongle out there first just do do a proof of concept on the wireless link, etc). I know their limitations!
Thanks!
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If you are concerned about lightning damage, I wonder if a LoG (Loop on Ground) might be a good choice? While there's always the risk of damage due to induced currents even if there's no direct strike, it might minimize the chance?
Also while I am not personally a big fan of active antennas, this might be another area where they have some protection advantage, as there's a lot less wire in the air. And maybe the amp takes the hit instead of the downstream radio? I'm thinking something along the lines of a crossed parallel loop with LZ1AQ style amp (see my posts elsewhere here on the HFU for construction details).
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Hi Chris
I'm not all that far from you (to the south). How do you protect your loops etc from lightning damage ?
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Hi Chris
I'm not all that far from you (to the south). How do you protect your loops etc from lightning damage ?
I disconnect antennas during storms. Also all of my coax lines run underground for some distance in conduit, and the shields are grounded at both ends of the conduit run.
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Understood Chris, thanks!
Part of this is an experiment to see what a PiZero and an RTL dongle (I know, I know) can survive . I might just erect a full wave loop for 22M , do my best to choke the heck out of a long DC feed line (the last 10 feet of which might be pretty small wire as a fusible link) and see what happens through a summer. The worst I'm out is about $50 including a small DC supply.
What I really want is a constant feed , wirelessly from out back. I'm pretty sure the Pi can work the across the yard to my router DX with the digital feed. If I could get a year out of such an arrangement, I'd call it a success . I think the MSI dongle is pretty affordable, and I could allocate one of those a year or so, if this works, but I"m not likely to put a spendy solution out there. For WSPR and QRSS, I think with a dongle of some sort, and a bandpass filter, I can put up a dedicated WSPR/QRSS monitor ... but we'll see.
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What is the real impedance of the antenna input of the SDRs?
I've seen that for a (pricey) Red Pitaya, the impedance is high, and a guy recommended taking the HF on 50 ohms terminating resistance.
My idea would be to accept the native high impedance as is, with just an untuned loop as receiving antenna. That loop could be turned to give the least signal from the nearest MW station (or any annoying RFI), and the length of wire/coax for the loop chosen to give the least signal on the 88-108 band. So just the simplest wide band HF receiving antenna.
But I would not like the idea of a long DC line to feed the device, even with a choke. An alternative way of power supply is to be found. All the set should be left floating : no DC line, no AC line, no HF line! Well, if a supply line is unavoidable, maybe an AC line with screened isolating transformers at each end could be the solution.