HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: MDK2 on July 10, 2017, 1812 UTC
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Hello all. After a year, more or less, of using my homebrewed loops, I've decide that I need to improve them. I have rather cheap RG-58 cables which I think are susceptible to picking up RFI, and ferrite chokes haven't really helped there. So I need something that's better shielded. My question to HFU at large is, what kind? I'm leaning toward RG-6 quad shielded.
I've also decided to alter the larger loop for lower coverage. I was advised that additional loops was a good way to do this, but my question there is, Do I use the same 15' length of copper line and turn that twice, resulting in two loops about 7 1/2' in circumference, or get a 30' length and turn that twice, so that the loops have the same 15' circumference that it currently has? (And if the answer is two loops of 7 1/2' circumference, will I also need to change the coupler's circumference, currently 3', down to 1.5' to maintain the 1:5 ratio of coupler to loop?)
If anyone needs to familiarize themselves with my loops, I posted about the bigger one last year.
https://www.hfunderground.com/board/index.php/topic,29416.0.html
Thanks in advance for your help.
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Do you know for a fact that the loops themselves are not picking up the RFI? If that is the case, then changing coax will not help.
Are the loops inside?
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I think it picks up some, but the variable capacitor acts as a kind of preselector, and there isn't much of anything, signal or RFI, if you tune away. The reason I think changing the coax will help a bit is this. Sometimes I set up my loop outside, and run a length of LMR coax to connect it to my RSP2. The difference is pretty stark, even though the same bit of RG-58 is still part of it.
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The capacitor is resonating the loop, which is why you only pick up a signal at that one (resonant) frequency. I don't think that suggests that the loop is not picking up RFI. If it is inside your house, with the usual zoo of RFI emitting power supplies and so on, I'm almost certain it is. The insides of modern homes are extremely hostile to DX signals. Not to say the coax might not also be picking up some RFI as well.
TV coax is dirt cheap, so it is an easy enough experiment to try. You may also want to consider some sort of balun or transformer where the coax connects to your pickup loop?
If you can run your RSP2 just via USB power, try that with a laptop, while you switch off your main circuit breaker, and see what happens to the RFI. You might want to let other people in the house know, first ;D
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If I had a laptop capable of running RSP2-compatible software, a lot of my troubles would be solved. Unfortunately all I have in that department is a chromebook (the one OS for which there doesn't seem to be any SDR software for SDRplay), and there simply isn't anything in the budget to get a decent one for running the SDR. We have major expenses that we're saving up for, so the hobby can only get by on pocket change for the time being.
Anyway, I'm sure the loop is picking up some RFI - how could it not? I will consider those suggestions, but one thing at a time. It just seems too likely that this cheap coax is picking up a lot of it, too.
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OK, how about this - disconnect the loop from the coax, but leave the coax connected to the radio. Maybe connect a 50 ohm resistor across the coax where the loop was, but try not to have any long leads. Do you still get RFI?
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First, I'll have to get a 50 Ohm resistor. ;D
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Or you could just try it open ended, and then with a simple wire aka aligator clips to short the end. 0 ohms is close enough to 50 ohms for broadband rx porpoises.
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I think open ended would be a fair first test. Not sure about shorting.
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Thanks for the suggestions. I'll give it a try next week sometime (too much happening this week to spare any time for this).
There are still a couple of open Radio Shacks near me. I wonder if they have a 50 Ω resistor?