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Balanced Loop Antenna Preamp/Tuner

K0FF (K0FF) on January 8, 2001
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KFF Homebrew Tips

Balanced Preamp and Tuner for Frame Antenna

by KFF

The original design for the electronics for this project are credited to Lyle Koehler, and detail can be found on his webpage at:

Simply stated, four Varactor diodes located right at the antenna are used to resonate a frame or other loop, controlled from an indoor box. An outside rated unit houses the tuning diodes, as well as preamp of the familiar JFET input/ bipolar output design theory. Lyle has gone a couple of steps further by including a balanced design, and also stabilizing the FETs with a current source power supply, and a transformer output.

These are the construction details of the example that I built for use with a 10 foot frame loop with 10 turns of #16 Ga wire, owned by N1LF. Les' main interest is LF but there is no reason this same circuit can't be used for BCB or 160 meters using smaller loops. If your interest is in shielded loops, or SLOOPS, this circuit can also be used to remote tune them as well.

Using 4 of the Varicap diodes gives a capacitance swing of 70 to 770 pF, and more or less could be used to tailor the capacitance swing needed.

Of course the frame loop and the Shielded Loop or SLOOP are receive only antennas and should not be transmitted into under any circumstances.

Refer to Lyle's webpage for the electronics, but I will continue to describe the mechanical details of this example.

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PHOTO #1 shows an overview of both the control box and the outside unit. In this case, I could not find a suitable cabinet for the indoor unit, so one was fabricated out of heavy gauge aluminum stock.

KFF Homebrew Tips

The outdoor unit utilizes an electrical box made for wet environments. Two H.H. Smith terminals connect to the loop wires

(This version does not use the center-tapped loop; otherwise there would be 3 terminals). Direct mounting to a groundrod is accomplished via the socket and screw seen on the bottom panel. If circumstances prohibit this, then a brass screw for direct grounding is utilized

Photo #2 shows the backside of the outdoor box, and the method of attaching the SO-239. Here a silver plated/ Teflon SO-239 is secured with stainless steel screws into the heavy aluminum boss at the center point, and a homemade rubber gasket seals against moisture.

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KFF Homebrew Tips

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Photo #3 gives some details of the circuit board and internal wiring of the outdoor box. The circuit card is one of my Proto Boards and uses a grid pattern on the back, and a groundplane on the topside. Only one jumper was needed in this layout and then only because I wanted to keep both halves of the preamp balances in every practical way. Ferrite beads on the control wires help to eliminate RF ingress via the leads.

KFF Homebrew Tips

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Photo #4 gives a different angle and here you can see the desiccant capsule and neon-bulb spark gap.

Using a spark gap across a transformer-coupled output is a very effective hedge against lightning surges. If a surge does come in, then the transformer winding itself is a shunt to ground. What the gas tube is for is to dampen any ringing voltage actually generated by the inductance of the coil itself, if excited with a fast rise time current. There is little energy ever dissipated in the gap when used in this manner.

KFF Homebrew Tips

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Photo #5displays the simplicity of the control box, and allows fabrication detail to be examined. This cabinet is very simple and made with non-sophisticated tools. All bending was done on a $15.00 brake that gets clamped tighter with C clamps.

Control of the diode bias and therefore the capacitance is accomplished via a simple voltage divided and a 10 turn linear pot of 10 k Ohms.

Single point grounding techniques were followed.

Coming off the coaxial power socket is a leaded component that is a PicoFuse, to protect the power source if something shorts out the control wires. Additional fusing should always be provided when small wires are used to feed a voltage to some device, as a fire precaution. If a short, or worse, a high resistance short occurs, the wires can overheat and catch on fire, or cause a fire, long before the 50 amp fuse in the shack power supply blows out. Always fuse the circuit according to the wire size in use.

KFF Homebrew Tips

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Finally #6 shows some more details, and the ubiquitous Do Not Eat The Batteries label.

73, Geo > KFF


Member Comments:
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Balanced Loop Antenna Preamp/Tuner  
Anonymous post on January 9, 2001 Mail this to a friend!
6 photos, each over 100k bytes each, and just a link to another site to give the details of the project. I'm sorry that I waited an unecessarily long time to load this page.

PLEASE be more sensitive to us users that have dialup connections.
Balanced Loop Antenna Preamp/Tuner  
by K1XT on January 9, 2001 Mail this to a friend!
Great photos George. I need to get one of these built. LF listening can be very exciting. Remote tuning is a must. Lyle has done some great work in designing preamps and LF transmitters. His knowledge has been very beneficial to the LF community. He is always willing to help.
K1XT Bill
Balanced Loop Antenna Preamp/Tuner  
by W0LPQ on January 9, 2001 Mail this to a friend!
I too am a dial in. Great article. Well worth waiting for the download time. Sorry there are those who thought this was a waste of is not..!!

73 and Thank You..!!

Bill, W0LPQ
RE: Balanced Loop Antenna Preamp/Tuner  
by N5NJ on January 10, 2001 Mail this to a friend!
I considered reducing the size of the photos in this article, but the quality of them is outstanding and as such, decided to leave them in. As you have seen in other articles, we have reduced image sizes, and it renders the images somewhat unclear which then requires you to click on each image to then download the larger, clearer image.

Either way, someone is not happy.

Unless you're in an area that does not yet offer DSL, cable modem or other high-speed Internet access, you should seriously consider these alternatives. The economics are beginning to make sense.

In my case, the cost of ADSL (768k download / 256k upload) with ISP is the same as a second phone line and a separate ISP. The increased performance makes downloads of high quality images such as these a non-issue. The full-time connection is wonderful also.

No dialing up, no busy signals.

Highly recommended. It's a good thing.

Bob N5NJ Articles Manager
Balanced Loop Antenna Preamp/Tuner  
Anonymous post on January 10, 2001 Mail this to a friend!
Perhaps the image can be set for 256 color or less
along with tighter cropping can reduce file size.

I need to use 640 x 480 due to my eyesight so
to view the images I scroll the pages.

I have access to the backbone at the University
where I work and the images did take a few moments
to load up.

Yes, the economy of faster connections is within
reach of many. However, the larger population cannot
afford this luxury, myself included !

This is a neat project and is now on my future build
list !

RE: Balanced Loop Antenna Preamp/Tuner  
by N5NJ on January 11, 2001 Mail this to a friend!
I found that I am able to reduce the size without compromising the high quality of the photos using a photo editor that came with a digital camera. Hopefully, the reduced size, as well as the change in formatting will allow all users to view this article more efficiently.

Bob N5NJ Articles Manager
Balanced Loop Antenna Preamp/Tuner  
by K5MA on January 11, 2001 Mail this to a friend!
Geat design. Can anyone recommend a similar receiving loop antenna/preamp/tuner design that would cover 160 meters? How about a dual band 160/80 meter design? I don't have room for effective beverage antennas at my QTH. I'll bet similar designs have been published. Link info would be greatly appreciated.
RE: Balanced Loop Antenna Preamp/Tuner  
by N5NJ on January 12, 2001 Mail this to a friend!
Re: K5MA's request

Stand by for the next article from K0FF !
Balanced Loop Antenna Preamp/Tuner  
by K0CS on January 13, 2001 Mail this to a friend!
Bob, W4DR and his XYL, Rosalie are participating in the upcoming YK9A (Syria) DXpedition. Lance Johnson Engineering sent K0FF an "Elbert County Preamplifier" ECP-1 and 18:1 Pennant Transformer that will be forwarded to W4DR for lowband use on the DXpedition. Thanks K0FF for matching the ECP-1 to W4DR's YK9A Pennants! A very nice contribution to the low band community! You can look at the ECP-1 System at:
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