Receive-only Loop Antenna with Wideband Amplifier (Part 1)


Receiving loop antennas are a good solution for the ham who has an electrically noisy environment. Loop receiving antennas are superior because:

  • They can be positioned far from local noise sources. Today, one’s house is a major source of wideband noise.
  • Because they are more sensitive to the magnetic component of the RF than the electrical component, they are relatively insensitive to nearby power line discharge noise.
  • Being directional, they can be positioned to null out a specific nearby noise source.
  • Because they are not used for transmitting, they can be lightly built and near the ground.

So I set out to design and build one. This article describes my first version. (I was so impressed with it that I went on to design a much more elaborate receiving loop system. The first entry of that design work is here. That project and its several entries has the acronym of ESOLA. As of June 2022, it is still in development. Return for updates!)

Design considerations: Loop antennas are high impedance antennas, and the smaller they are the smaller the signal they produce, so they always need an impedance converting amplifier to amplify and match their tiny signal to a 50 ohm coaxial cable. The smaller the loop, the smaller the signal, and the more amplifier gain is required. Amplifiers add noise, and as one shrinks the antenna, the noise added by the amplifier begins to offset the noise eliminated by the loop configuration, until a point of diminishing returns is reached. So, the smaller the loop, the less noisy the amplifier must be, and the more gain. This wideband amplifier (WBA or LNA) has to be located near the loop, and far from the receiver. To get power, there are two strategies: one is to have power storage at the WBA (batteries) and the other is to feed power down the coaxial cable. Most of the commercial powered loop antennas receive their power down the cable, but the experienced ham eventually learns that coaxial cable conducting DC current generates a bit of RF noise itself. This means, to get the lowest possible noise, power should be stored at the WBA, with recharging done when the amplifier is offline via the cable, or by solar power. I chose the cable charging method for my design.

Today, many WBA’s are designed using Gallium-Arsenide transistors (GALI-74) and other technologies, but I wanted to start with something I was more familiar with, the classic J310 (or U310) depletion mode N-channel junction FET, combined with a fast bipolar transistor. Doing a dual amplifier and then combining the signals using a transformer was a good way to get added amplitude. I did some hunting and found I had some 2N5770 transistors, so I chose to use them. Operating voltage of 5V would be all that was needed, so 5 NimH batteries would supply enough voltage, though of course it would need to be regulated. In addition, I’d need the charging circuit to stop charging when the total battery voltage went too high. And a power-on LED, and a switch.

I did the WBA design using LTSpice. Here is the design I ended up with, after a few iterations (click to enlarge):

I cannot attach the LTSpice source file directly (this hosting service will not allow that file type) so I will append it to this post in Text format, and you can copy it off and save it as an ASC file if you want. Ask me questions if you need help. Below, we see the amplifier throughput characteristic simulated in LTSpice, showing it is fairly flat from 3 MHz to 30 MHz, and usable output gain from 500 kHz to 100 MHz.

The LTSpice design led to the final schematic, after a few alterations and tests: (click to enlarge)

I need to admit that the battery management circuitry is unnecessarily complicated. I designed it using discrete parts because that’s what I had on hand, but there are better approaches using linear ICs. Do me a favor and redesign it yourself, and send me an update! [Edit: my redesign of the battery management section is here.] I’ll skip a detailed circuit description — if you’re an experienced electronics hobbyist, there’s probably enough information in the schematic above for you to duplicate the design, but feel free to ask me questions. If I’m asked questions about this, I’ll probably include my answer text in the post here.

Below, the WBA section as built. Note that the output RF connector is floating, not grounded to the chassis. The box is not especially weather proofed. In my next version I’ll do a better job with that.

Below, a photo showing the entire assembly, with batteries and the battery management circuitry.

The actual wire antenna can be any loop. This leaves lots of room for experimentation. I plan on testing different size loops, to compare their signal to noise ratio for different bands. Also, I’ll be trying some twin-lead loops. At present, the loop is a vertical one, about 10 feet on a side, square, with the WBA mounted on a fence at the bottom of the loop. Note the grounding strap going from the chassis to the 1/2 inch pipe, pounded into the ground:

The practical results with the amplified loop antenna on the 80M band are quite dramatic: I see noise reduction of about 12 db over the phone bands. Some conversations which were detectable but not readable are now quite easily readable. Improvement for weak signals on the 40M band seem to be about 10 db, with some very weak CW signals seeming to “emerge” from the noise when the loop is switched on. In general, I’m quite happy with the unit, and now I’ll begin experimenting with it to see if I can improve it. Remember: when running the unit, turn off the charging supply for best results. Then turn it back on once you are done, to bring the batteries back up to full charge. Charging current will automatically limit to about 0.25 amps. When you see the charging current fall to about 0.12 amps, that means that the batteries are fully charged, but there is no need to disconnect the power supply because the circuit will not overcharge the batteries. Use about 11-12 VDC as your charge voltage. I built a small charger interface box to use with the amplifier box — see the schematic for the details.


I looked around the web and found some more reading on the subject, if you’d like to see designs by others.

I found a ham who published a WBA design, single ended, using only one J310 transistor.

Someone is selling a dual J310 circuit on Amazon, but it has a single ended input, which is not as good a choice for a loop input. It looks like a near copy of this circuit.

This ham experimented with a J310 circuit in his Youtube video.

This ham had a circuit which was the most similar to mine (using a NPN rf transistor to drive the 50 ohm cable), though again, it was single sided only.

For a theoretical analysis of the optimum low noise amplifier design problem, this article by VA3IUL is a good introduction. It emphasizes the noise in the amplifying component and is an alternate to the mostly-experimental approach I used here. The collection of links on his web site is a treasure trove.

The MFJ-1886 receive-only loop antenna is a double ended input like mine, and it uses the GALI-74, so it’s a more modern device. I would love to compare this commercial unit’s performance to mine but I’d have to buy one, and I’d rather not do that. I think it’s possible that the J310 FET might do as good or better job when compared. But see David Casler’s video on it. He has good things to say about dedicated receiving loops in general, though MFJ’s loop is tiny compared to my 10 foot square vertical – signal strength increases with loop area and number of turns.

So, this quick look at the web did not find a close match to my circuit. There are dozens of good ways to design with these parts, so perhaps this is one of them, but I have no illusions that this circuit is highly optimized. If you use my circuit as a starting point, please mention my call sign NK6Y and leave a link back to my article here.

Next, I’ll try to figure out how to measure the performance of the loop. My purpose is to improve on the signal to noise ratio. I want to try a 2-turn loop, using 450 ohm twinlead wire, and see how it does.


Below is the LTSpice drawing file. Copy the following into a text editor and save it as a file with extension “.ASC” and open that in LTSpice. Because of limitations of this hosting service, I can’t put the file here any other way.

Version 4
SHEET 1 2668 680
WIRE 1056 -912 656 -912
WIRE 1056 -848 976 -848
WIRE 2112 -768 496 -768
WIRE 2112 -736 2112 -768
WIRE 496 -720 496 -768
WIRE 496 -608 496 -640
WIRE 656 -608 656 -912
WIRE 656 -608 496 -608
WIRE 688 -608 656 -608
WIRE 800 -608 768 -608
WIRE 848 -608 800 -608
WIRE 976 -608 976 -848
WIRE 976 -608 928 -608
WIRE 2112 -608 2112 -656
WIRE 2112 -608 976 -608
WIRE -48 -544 -944 -544
WIRE 336 -544 -48 -544
WIRE 496 -544 336 -544
WIRE 1568 -544 496 -544
WIRE 1952 -544 1568 -544
WIRE 2112 -544 1952 -544
WIRE 2272 -544 2112 -544
WIRE 336 -480 336 -544
WIRE 1952 -480 1952 -544
WIRE 496 -432 496 -544
WIRE 2112 -432 2112 -544
WIRE 496 -256 496 -352
WIRE 2112 -256 2112 -352
WIRE -48 -208 -48 -544
WIRE -48 -208 -192 -208
WIRE 400 -208 208 -208
WIRE 432 -208 400 -208
WIRE 1568 -208 1568 -544
WIRE 1568 -208 1440 -208
WIRE 2032 -208 1824 -208
WIRE 2048 -208 2032 -208
WIRE -192 -160 -192 -208
WIRE -48 -160 -48 -208
WIRE 208 -160 208 -208
WIRE 1440 -160 1440 -208
WIRE 1568 -160 1568 -208
WIRE 1824 -160 1824 -208
WIRE 400 -144 400 -208
WIRE 2032 -144 2032 -208
WIRE -192 -48 -192 -80
WIRE -48 -48 -48 -80
WIRE -48 -48 -192 -48
WIRE 1440 -48 1440 -80
WIRE 1568 -48 1568 -80
WIRE 1568 -48 1440 -48
WIRE 208 -32 208 -80
WIRE 336 -32 336 -400
WIRE 336 -32 208 -32
WIRE 1824 -32 1824 -80
WIRE 1952 -32 1952 -400
WIRE 1952 -32 1824 -32
WIRE 336 0 336 -32
WIRE 400 0 400 -64
WIRE 400 0 336 0
WIRE -48 16 -48 -48
WIRE 1568 16 1568 -48
WIRE 1952 16 1952 -32
WIRE 2032 16 2032 -64
WIRE 2032 16 1952 16
WIRE -512 96 -800 96
WIRE -496 96 -512 96
WIRE -304 96 -416 96
WIRE -160 96 -224 96
WIRE -96 96 -96 80
WIRE -96 96 -160 96
WIRE 1104 96 912 96
WIRE 1120 96 1104 96
WIRE 1312 96 1200 96
WIRE 1456 96 1392 96
WIRE 1520 96 1520 80
WIRE 1520 96 1456 96
WIRE -512 160 -512 96
WIRE 1104 160 1104 96
WIRE 336 176 336 0
WIRE 1952 176 1952 16
WIRE -1120 240 -1232 240
WIRE -944 240 -944 -544
WIRE -800 240 -800 96
WIRE -352 256 -400 256
WIRE -160 256 -160 96
WIRE 496 256 496 -160
WIRE 496 256 464 256
WIRE 592 256 496 256
WIRE 1264 256 1216 256
WIRE 1456 256 1456 96
WIRE 2112 256 2112 -160
WIRE 2112 256 2080 256
WIRE 2208 256 2112 256
WIRE 208 288 208 -32
WIRE 1824 288 1824 -32
WIRE -400 304 -400 256
WIRE -352 304 -352 256
WIRE 464 304 464 256
WIRE 1216 304 1216 256
WIRE 1264 304 1264 256
WIRE 2080 304 2080 256
WIRE -1120 320 -1120 240
WIRE 592 320 592 256
WIRE 2208 320 2208 256
WIRE -1120 448 -1120 400
WIRE -944 448 -944 320
WIRE -944 448 -1120 448
WIRE -400 448 -400 368
WIRE -400 448 -944 448
WIRE -352 448 -352 368
WIRE -352 448 -400 448
WIRE -160 448 -160 320
WIRE -160 448 -352 448
WIRE -48 448 -48 112
WIRE -48 448 -160 448
WIRE 208 448 208 352
WIRE 208 448 -48 448
WIRE 336 448 336 256
WIRE 336 448 208 448
WIRE 464 448 464 384
WIRE 464 448 336 448
WIRE 592 448 592 384
WIRE 592 448 464 448
WIRE 800 448 800 -608
WIRE 800 448 592 448
WIRE 1216 448 1216 368
WIRE 1216 448 800 448
WIRE 1264 448 1264 368
WIRE 1264 448 1216 448
WIRE 1456 448 1456 320
WIRE 1456 448 1264 448
WIRE 1568 448 1568 112
WIRE 1568 448 1456 448
WIRE 1824 448 1824 352
WIRE 1824 448 1568 448
WIRE 1952 448 1952 256
WIRE 1952 448 1824 448
WIRE 2080 448 2080 384
WIRE 2080 448 1952 448
WIRE 2208 448 2208 384
WIRE 2208 448 2080 448
WIRE -944 480 -944 448
WIRE -800 512 -800 320
WIRE 912 512 912 96
WIRE 912 512 -800 512
WIRE -1232 576 -1232 240
WIRE -512 576 -512 240
WIRE -512 576 -1232 576
WIRE 1104 576 1104 240
WIRE 1104 576 -512 576
FLAG -944 480 0
SYMBOL voltage -944 224 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 4.8
SYMBOL voltage -800 224 R0
WINDOW 123 24 124 Left 2
WINDOW 39 24 152 Left 2
SYMATTR Value2 AC 120e-6 0
SYMATTR SpiceLine Rser=50 Cpar=22e-12
SYMATTR InstName V2
SYMATTR Value SINE()
SYMBOL res 944 -624 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 25
SYMBOL ind2 512 -736 M0
SYMATTR InstName L1
SYMATTR Value 29e-6
SYMATTR Type ind
SYMBOL ind2 480 -448 R0
SYMATTR InstName L3
SYMATTR Value 140e-6
SYMATTR Type ind
SYMBOL njf -96 16 R0
SYMATTR InstName J1
SYMATTR Value J310
SYMBOL res 320 -496 R0
SYMATTR InstName R6
SYMATTR Value 12000
SYMBOL res 320 160 R0
SYMATTR InstName R8
SYMATTR Value 10000
SYMBOL cap 192 288 R0
SYMATTR InstName C4
SYMATTR Value .01e-6
SYMBOL res 448 288 R0
SYMATTR InstName R9
SYMATTR Value 32
SYMBOL ind2 -64 -176 R0
SYMATTR InstName L2
SYMATTR Value 330e-6
SYMATTR Type ind
SYMBOL cap 576 320 R0
SYMATTR InstName C5
SYMATTR Value 220e-12
SYMBOL ind2 224 -176 M0
SYMATTR InstName L4
SYMATTR Value 33e-6
SYMATTR Type ind
SYMBOL ind2 -320 112 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 4 56 VBottom 2
SYMATTR InstName L5
SYMATTR Value .12e-6
SYMATTR Type ind
SYMBOL cap -176 256 R0
SYMATTR InstName C1
SYMATTR Value 15e-12
SYMBOL res -528 144 R0
SYMATTR InstName R1
SYMATTR Value 500
SYMBOL res -400 80 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 5
SYMBOL diode -368 304 R0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode -384 368 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL npn 432 -256 R0
SYMATTR InstName Q2
SYMATTR Value 2N5769
SYMBOL ind2 2128 -336 R180
WINDOW 0 36 80 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName L6
SYMATTR Value 140e-6
SYMATTR Type ind
SYMBOL njf 1520 16 R0
SYMATTR InstName J2
SYMATTR Value J310
SYMBOL res 1936 -496 R0
SYMATTR InstName R2
SYMATTR Value 12000
SYMBOL res 1936 160 R0
SYMATTR InstName R3
SYMATTR Value 10000
SYMBOL cap 1808 288 R0
SYMATTR InstName C2
SYMATTR Value .01e-6
SYMBOL res 2064 288 R0
SYMATTR InstName R7
SYMATTR Value 32
SYMBOL ind2 1552 -176 R0
SYMATTR InstName L7
SYMATTR Value 330e-6
SYMATTR Type ind
SYMBOL cap 2192 320 R0
SYMATTR InstName C3
SYMATTR Value 220e-12
SYMBOL ind2 1840 -176 M0
SYMATTR InstName L8
SYMATTR Value 33e-6
SYMATTR Type ind
SYMBOL ind2 1296 112 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 4 56 VBottom 2
SYMATTR InstName L9
SYMATTR Value .12e-6
SYMATTR Type ind
SYMBOL cap 1440 256 R0
SYMATTR InstName C6
SYMATTR Value 15e-12
SYMBOL res 1088 144 R0
SYMATTR InstName R10
SYMATTR Value 500
SYMBOL res 1216 80 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R11
SYMATTR Value 5
SYMBOL diode 1248 304 R0
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL diode 1232 368 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL npn 2048 -256 R0
SYMATTR InstName Q1
SYMATTR Value 2N5769
SYMBOL res 784 -624 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R12
SYMATTR Value 25
SYMBOL ind2 2096 -640 M180
WINDOW 0 36 80 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName L10
SYMATTR Value 29e-6
SYMATTR Type ind
SYMBOL res 384 -160 R0
SYMATTR InstName R13
SYMATTR Value 390
SYMBOL res 2016 -160 R0
SYMATTR InstName R14
SYMATTR Value 390
SYMBOL res -208 -176 R0
SYMATTR InstName R15
SYMATTR Value 5000000
SYMBOL res 1424 -176 R0
SYMATTR InstName R16
SYMATTR Value 5000000
SYMBOL voltage -1120 304 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value -.8
TEXT -816 -216 Left 2 !.ac oct 8 .1e6 300e6
TEXT -808 -152 Left 2 !K1 L4 L2 1\nK2 L3 L1 1
TEXT 1200 -696 Left 3 ;5 to 2 turns ratio
TEXT -24 -248 Left 3 ;3 to 1 turns ratio
TEXT -808 -72 Left 2 !K3 L7 L8 1\nK4 L6 L10 1
TEXT 1592 -248 Left 3 ;3 to 1 turns ratio
TEXT 1064 -880 Left 2 ;Output to 50 ohm cable

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2 thoughts on “Receive-only Loop Antenna with Wideband Amplifier (Part 1)

  1. Pingback: Reconditioning a 1973 Wavetek 3000 RF Generator | barbara4tech

  2. Pingback: A Compact High Current AA Battery Pack For Mobile or Outdoor Applications | barbara4tech

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