Beverage Transformers for HF Receiving Antennas

Matt Roberts - matt-at-kk5jy-dot-net

Published: 2023-04-10

Updated: 2024-07-11

Some of the most common questions I receive relate to how to build a Beverage transformer for use in one of my receiving antenna designs. Such a home-made transformer was described in my loop-on-ground article, but I use them in most of my receiving antenna projects.

This article is a brief FAQ about the transfomers I use, how to build them, and where to buy them.

Q: How do I build my own transformer?

The isolation transformer used in my receiving antenna articles is not my original design. I used the transformer described by W8JI in his own article about beverage antennas (w8ji.com). Scroll down to the Transformers section of his article for all the information you need to build your own transformer, including the model number of the core he used.

Q: Where do I buy transformer cores?

I get them from Mouser.

As of the writing of this article in springtime 2023, they are available from Mouser for $0.36 each in quantities of ten or more. I recommend you buy several of them, in case you break a couple. They are made of sintered materials, and are somewhat fragile if dropped or mishandled.

Q: What kind of wire should I use?

My transformers are wound with #28 enameled solid coper wire.

The cores are rather small, so you will want thin wire. W8JI used insulated wire instead of enameled wire. As long as the insulation is thin enough, that is fine, too.

Q: What ratio should I use?

For a real Beverage antenna, you should use a ratio that matches the Z₀ of the antenna to the Z₀ of your coaxial cable. The Z₀ for a Beverage antenna is usually the value of the terminating resistor, if one is used, typically 450 Ω. W8JI describes how to choose a ratio for such antennas in his article.

For a LoG or SRL or similar single-element receiving antenna, the ratio is not critical. This is because the antenna element itself is not resonant. So pick a reasonable turns ratio such as 5:2 (6.25:1 Z ratio) or 6:2 (9:1 Z ratio), as described in the W8JI article.

Q: How do I match the LoG or SRL to my coax cable?

You don't.

The transformers in those antenna designs are used for isolation, and not for impedance matching. The antenna elements themselves are not resonant on the bands where they are used, so impedance matching is pointless.

I have used and modeled many ratios for these antennas, and there is little difference in transfer characteristics between them, even for long coaxial cable runs.

The W8JI design is very broadband, and has good energy transfer characteristics over the MF and HF spectrum. For a LoG or SRL antenna, either of the ratios he described will work fine.

Q: Do you have a schematic of the transformer?

Yes, there is an extended discussion of my hand-built transformer in the loop-on ground article. Pictures and a schematic of the device are included near the top of the article.

Q: Can I use a 9:1, 6:1, or 4:1 transmitting Unun or Balun?

No. These aren't the right kind of transformer. Many of them have the same impedance ratio as a typical beverage transformer, but they offer no isolation.

Nearly all transmitting transformers have continuity between the coax shield and one or both terminals on the antenna side. Some are autotransformer designs, with a single winding connecting every terminal to every other terminal.

You want a true transformer design, with two, independent, isolated windings — one for the antenna, and one for the coax cable.

Q: Do I have to use a transformer in my LoG or SRL?

No, but you should.

When feeding loop antennas with coaxial cable, something needs to isolate the outer surface of the coax cable shield from the antenna element, so that the coaxial cable doesn't become part of the antenna. Otherwise, the coaxial cable will pick up signals and convey them to the antenna and then to the receiver, which can spoil the benefits of the antenna.

Some people call this "common-mode pick-up" and some call it I₃ current, but the basics are well-known.

There are two ways I have used to isolate the cable from the antenna. The first is using a Beverage transformer for isolation, as described in the LoG and SRL articles. The other is to use a so-called common-mode choke. This can be used as an alternative to the transformer, if you want.

The reason I don't like chokes for receiving antennas is that effective chokes for 160m or longer wavelengths can be large and pricey, and require a lot of turns of cable, even on a proper core like FairRite #31. By comparison, the Beverage transformer is quite small and inexpensive.

The transformer design is often used for directional Beverge antennas where unwanted current suppression is critical for a clean pattern. There is no reason why it can't serve a similar purpose for smaller receiving antennas.

Q: Can I just buy a transformer instead of building one?

Sure.

The W8JI design was used by DX Engineering for their BFS-1 Beverage transformer kit (dxengineering.com). That transformer is virtually identical to the ones I hand-build, and has been used by many people for LoG and SRL antennas. See the reviews on their webiste. The BFS-1 includes a nice aluminum case with an F-style connector. I have used this transformer, and its performance is identical to my hand-built ones. It is one of the few commercial designs intended for use outdoors. Unfortunately, the price of this device has recently doubled, which makes hand-building a lot more attractive.

There are other vendors who make Beverage transformers, and they are all very simple devices. There are even a few vendors offering transformers specifically intended for loop-on-ground antennas. The only commercial model I have tried myself is the DXE BFS-1.

Q: Can I buy a transformer from KK5JY?

No. I'm not set up to do fabrication, and DXE has exactly the part you want. Maybe someday they'll send me a gift certificate for recommending their product. ;-)

— Copyright (C) 2023 by Matt Roberts, KK5JY. All Rights Reserved.