Matching transformer for “loop on ground” and Beverage antennas

This is a companion piece to the post “Hearing through the noise on the low bands“.

Why bother?

After years of thinking about it, in early 2021 I finally got serious about building some proper matching transformers for my short Beverage wire antenna along the property line and a loop on the ground laying on the back lawn.

Over the years I have puttered around with a short Beverage — about 280 feet of wire along a fence line and down into the wooded area of our lot, running due east-west.

In “experimenting” mode (my default mode, unfortunately) I fed the short Beverage with a simple, hastily wound matching transformer then alligator-clipped the leads to the antenna wire and a ground rod on the antenna side, and to 75-ohm RG6 coax on the radio side.

It worked, but the experimental nature of the construction was substandard as it was never intended for longer-term use. Over time, things deteriorated to the point that the Beverage was no longer in service. I wasn’t all that serious about low-band performance as I had enough on my plate building a station for the higher HF bands.

I’m back to being keen about contest scores on the low bands. Perhaps more important for me, I have DXCC on the higher bands so finishing DXCC on 160M would give me nine bands completed. As of this writing, I have stalled at just 25 DXCC entities on Top Band, mostly due to my inability to hear DX.

From the West Coast of North America, being able to hear and work Europe occasionally would be a game-changer.

Audio recordings

Before we get into the nuts and bolts of the receive antenna transformer boxes, let’s see what the end result sounds like.

These recordings are made on an Icom IC-7610 — they start with the loop-on-ground antenna then switch to an 80M vertical. You’ll notice the difference in noise — it jumps up with the vertical and drops out with the loop on ground, dramatically improving the signal-to-noise ratio of the signal.

In a contest or DXing, this can make the difference between working them and not making a contact.

80M CW — TI5/N3KS | Feb. 1, 2021 — 0152Z
80M CW — KN6EIF | Feb. 1, 2021 — 0149Z
80M CW — W6 unidentified | Feb. 1, 2021 — 0143Z

I made two recordings during the CW 160M CW contest on January 30. These are the low-noise loop on ground, and a much noisier inverted-L antenna. Clearly, the loop on ground isn’t as good on 160M as it is on 80M, but it’s still very useful:

160M CW — K0UK | Jan. 30, 2021
160M CW — KV0Q | Jan. 30, 2021

About the loop on ground pattern

I should note that the experimental loop on ground’s “peanut-shaped” pattern is oriented with its main lobes pointed due north and south. The 80M recordings are of stations located in California and Costa Rica due south of me, so they’re in the main lobe.

The 160M signals are from Colorado (W0-land), so they are in the east-side null of the antenna, hence somewhat reduced performance.

Here’s the layout of the existing antenna, and the future double-wide version that will be switchable to cover most of the horizon, using a single matching transformer and a DPDT relay.

Handy resources

Before I started building the receive antenna matching transformers, I spent hours reading everything I could find on the topic of low-noise receiving antennas — what antennas to use and how to feed them, what kind of ferrite core to use, what kind of wire, how many turns to match the antenna impedance to 50-ohm or 75-ohm coax, and so on.

No sense reinventing anything here. I’ve compiled a list of some handy resources that helped me decide what I need to try for my installation.

Eventually, I ended up with the following recipe for making the matching transformers now in use at VA7ST.

The goal: matching antenna to feedline

In this case, our objective is to construct a receive antenna matching transformer suitable for single-wire Beverages, Beverages on ground, and loops on the ground.

Typically, we’ll be trying to match the impedance (Z) of the antenna — say, 450 or 500 ohms — to the characteristic impedance of our feedline coax — say, 50 or 75 ohms.

While a receive antenna that works well on several bands is nice, the matching transformer should be purpose-built for the band of greatest interest.

For me, the target is 160M so I want the best possible match from a 75-ohm cable to a 450-ohm antenna at 1.8 Mhz.

Gathering the bits and pieces

The matching transformer box consists of five components, and for ease of access I decided to see if I could acquire everything from the junk box or order it online (we have virtually nothing available from local suppliers here).

I almost succeeded — the little project boxes, binding posts and magnet wire were easily found on Amazon, and the ferrite cores were ordered from Digi-Key, but you can also order them from Mouser or Amidon Associates (links below).

Components List

  • Small plastic project box
  • Two binding posts — you can also use bolts and wingnuts, which I would consider if doing this again
  • One female F-type (cable TV) chassis-mount socket. I “harvested” five of these from an old cable TV four-way amplifier that was in the junk box.
  • One 73-material binocular core ferrite (Amidon #BN-73-202 or Fair-Rite #2873000202).

    These are under $1 each, plus shipping. I ordered 25 of them from Digi-Key (US$0.43 each when ordering 25), so have a lifetime supply. They arrived the next day, shipped from the US warehouse to my home in Canada. Amazing service!

    Sources: Digi-Key | Mouser | Amidon Associates
  • #30 (or #28) enameled magnet wire or equivalent — three feet is plenty. In one of my transformers, I used #30 insulated wirewrap wire from the junk box , which from a durability and ease-of-handling perspective, I prefer over enameled magnet wire.

Putting it all together

The first iteration of the transformer — a 2:6 turn version.

There’s nothing magical about assembly. I drilled a 3/8-inch hole on one side of the box for the F-type female coax connector, and two 1/4-inch holes on the other side of the box for the binding posts.

Then I wound the actual matching transformer. I my case, I wanted to match 450 ohms (high-Z side at the antenna) to 75 ohms (low-Z side at the coax connector).

Measuring with an antenna analyzer I started with a turns ratio of 2 turns : 6 turns. That got me close. Then I tried 3 turns : 9 turns, and that was a little too much — so I removed one turn on the high-Z side, for a 3 turn: 8 turn model that matches 450 ohms to 75 ohms at 160M (1.8 Mhz).

I made notes of the transformer measurements as I adjusted the turn counts. Here are the low-Z “coax side” readings, using a test load of 450 ohms:

2 turns : 6 turns76.594.5148.5
3 turns : 9 turns60.854.038.3
3 turns : 8 turns74.369.854.0
Table 1: Impedance matching measurements into 450-ohm test load

Once you’re happy with the transformer, just make the soldering connections. Small wire diameters make the job a little touchy, but with good lighting — and a magnifier if you have one — you will get it done.

Now, as I often say when finished with a build, run out and hook it up then run back to the radio and see how it works!

Measuring at the receiver

In real-world practice, here’s how things look at the radio in the shack (using about 50 feet of RG6 to get from the loop-on-ground antenna on the back lawn):

3 turns : 8 turns — measured in the shack160M80M40M
Impedance ( |Z| )69.2100.176.7
Table 2: Real-world measurement at the radio — Loop on Ground antenna

As you can see, antenna impedance varies by band and so does the matching transformer’s characteristics. The best you can aim for is getting it close for one band, and anything beyond that is “gravy.”

Next step for me: move the short Beverage wire (currently about 5 feet off the ground) onto the ground — making a “Beverage on ground” to see if it performs any better than the minimalist length for an in-the-air Beverage. I will post audio recordings of that antenna once the changes have been made in the spring.

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