Connecting a Long Wire to an SDR?

[Tube Era Tuner]

I want to run some experiments to determine the effectiveness of various configurations of single wire antennas for my ancient Hallicrafters S-38C. I recently bought an RTL-SDR V3 with the optional small antenna kit. I get great FM reception, which proves that the dongle and antenna are working properly, but I have yet to have any luck with shortwave reception at night using it with my MLA-30+. I was thinking that taking advantage of the waterfall display would be a great way to find stations that my S-38 can pick up. The thought also occurred that the RTL would be a really handy tool for testing how different lengths and routing affect shortwave reception, giving an immediate and graphic representation of how that particular antenna is bringing in signals. I'm not particularly interested in going down the dipole antenna rabbit hole or messing with coaxial lead-ins, baluns, etc. So if I want to stick with single wire antennas in various forms, how can I connect them to the input on the RTL?

 

[groundwire]

a 4:1 or 9:1 unun will do the job nicely.

 

[Tube Era Tuner]

a 4:1 unun will do the job nicely.

What would that look like, how would I wire it, and how would I go about getting one? I have lots of experience with general electronics fabrication (I built a lot of wiring harnesses for the National Ignition Facility and earned an IPC soldering certification for space applications while at NASA) but I don't know jack doodly about actual electronics.
Thanks!

 

[groundwire]

simple, its the same as a balun. coax from receiver to the unun, then connect longwire to one terminal and the counterpoise wire to the other terminal. since you are using it for rx only, buy a cheap one. ldg $30. the unun will keep a fairly good balance between the unbalanced coax and wire. the rxcvr will thank you with good rx. a 9:1 unun would work fine as well.

https://www.dxengineering.com/parts/ldg-ru-4-1

https://www.dxengineering.com/parts/ldg-ru-9-1

 

[Tube Era Tuner]

simple, its the same as a balun. coax from receiver to the unun, then connect longwire to one terminal and the counterpoise wire to the other terminal. since you are using it for rx only, buy a cheap one. ldg $30. the unun will keep a fairly good balance between the unbalanced coax and wire. the rxcvr will thank you with good rx.

https://www.dxengineering.com/parts/ldg-ru-4-1

Okay, great!
What's a counterpoise wire?
So I need to find a 4:1 balun with a female SMA connector (or the appropriate adapter) to connect to the SDR dongle, and whatever length and AWG size wire for the counterpoise, and that's it?
My current single wire antenna is a 32 foot length of 12AWG wire strung in the attic in a roughly north-south direction. Am contemplating erecting a 25 foot flagpole just outside of an upstairs window and stringing 50 or 60 feet of wire from it to an elevated stanchion in the back yard. With a longer antenna would I need to adjust the length of the counterpoise wire?
I really do appreciate your time and patience.

 

[groundwire]

Okay, great!
What's a counterpoise wire?
So I need to find a 4:1 balun with a female SMA connector (or the appropriate adapter) to connect to the SDR dongle, and whatever length and AWG size wire for the counterpoise, and that's it?
My current single wire antenna is a 32 foot length of 12AWG wire strung in the attic in a roughly north-south direction. Am contemplating erecting a 25 foot flagpole just outside of an upstairs window and stringing 50 or 60 feet of wire from it to an elevated stanchion in the back yard. With a longer antenna would I need to adjust the length of the counterpoise wire?
I really do appreciate your time and patience.

a counterpoise is the wire that the driven element works off of. in other words its the other half of the antenna. counterpoise length is the same as the driven element, or a 1/4 wavelength if used with a vertical antenna configuration. in your case being rx only you could probably not use one at all or ground that terminal to earth without to much performance degradation. also you need the unun (unbalanced to unbalanced), not a balun, (balanced to unbalanced). your connecting unbalanced coax to an unbalanced antenna. for the coax connection just get a cheap sma to so239 adapter

 

[Riverman]

I have had great success simply using a single long wire antenna for SWL with no unun or balun as it was for receive only.
Doesn’t the S-38C have an antenna screw on the rear panel to attach the wire to?

 

[groundwire]

I have had great success simply using a single long wire antenna for SWL with no unun or balun as it was for receive only.
Doesn’t the S-38C have an antenna screw on the rear panel to attach the wire to?

thats true too. if you just wanna get it done and cheap just get an sma to so239 adapter and shove the end of the wire into the connector. your choice

 

[Tom Line]

Does that RTL-SDR require the hardware control panel (the one with the gain in it) mode box changed to go below 28 or so Mhz ?? I use SDRsharp (SDR#) and prefer the older version of the software.

 

[Tube Era Tuner]

I have had great success simply using a single long wire antenna for SWL with no unun or balun as it was for receive only.
Doesn’t the S-38C have an antenna screw on the rear panel to attach the wire to?

Yes. It can accommodate either a single wire or a pair. I've currently got it hooked to an MLA-30+ using one of these gizmos:

But I'd like to see if I can get better reception with an outdoor long wire antenna, or by trying different configurations of long wire antenna in the attic, and I'd like to use the RTL-SDR to give me a visible verification of antenna function. I can quantify both signal strength and the number of available signal sources using the SDR, rather than depending on "this seems to work better" subjective judgement when switching from one antenna to another.

 

[Tube Era Tuner]

Does that RTL-SDR require the hardware control panel (the one with the gain in it) mode box changed to go below 28 or so Mhz ?? I use SDRsharp (SDR#) and prefer the older version of the software.
View attachment 62784

Good question. I'm using SDR# as well, but have only just started, and am a bit overwhelmed by the options available. Might be helpful if I understood what quadrature sampling is...

 

[Tube Era Tuner]

a counterpoise is the wire that the driven element works off of. in other words its the other half of the antenna. counterpoise length is the same as the driven element, or a 1/4 wavelength if used with a vertical antenna configuration. in your case being rx only you could probably not use one at all or ground that terminal to earth without to much performance degradation. also you need the unun (unbalanced to unbalanced), not a balun, (balanced to unbalanced). your connecting unbalanced coax to an unbalanced antenna. for the coax connection just get a cheap sma to so239 adapter

Okay, this is very helpful! Again, I really appreciate your patient and well-explained replies.

 

[Tom Line]

I ran a long wire up to my attic and just stuck it in the rtl. Works fine. Have fun learning, but there's nothing special about picking up shortwave stations running very big watts.
If one switches between quad to direct, the stations above and a below 28mhz may appear in the wrong place. Remember to switch it back to quad when going over 28 mhz or so or you'll start picking up HF traffic up in the FM broadcast area and have a WTF moment. If you actually understand how direct and quad works, you're never going to be fun at parties.
I understand there's a lot of enthusiastic individuals repeating what they've heard about antennas and matching baluns for shortwave and enjoying purchasing products, but for shortwave listening, in my opinion, it's not necessary or even better than a simple, no cost wire.

 

[nasanction]

I want to run some experiments to determine the effectiveness of various configurations of single wire antennas for my ancient Hallicrafters S-38C. I recently bought an RTL-SDR V3 with the optional small antenna kit. I get great FM reception, which proves that the dongle and antenna are working properly, but I have yet to have any luck with shortwave reception at night using it with my MLA-30+. I was thinking that taking advantage of the waterfall display would be a great way to find stations that my S-38 can pick up. The thought also occurred that the RTL would be a really handy tool for testing how different lengths and routing affect shortwave reception, giving an immediate and graphic representation of how that particular antenna is bringing in signals. I'm not particularly interested in going down the dipole antenna rabbit hole or messing with coaxial lead-ins, baluns, etc. So if I want to stick with single wire antennas in various forms, how can I connect them to the input on the RTL?

Check out DXCommander on YouTube He is an antenna experimentalist who can explain about long wire listening. I can tell you it isn't a random length, it is still measured in wavelengths. but he uses 3 antenna's, a transmit antenna, and 2 listening antenna's, one of which is a very long wire.
and also this video: