The Loop on Ground Antenna

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

Published: 2016-04-10

Updated: 2017-01-29


Antennas made from conductors installed flush to the ground are not a new idea.  The Beverage antenna is an example of an antenna that is typically installed close to ground.  As a special case, the antenna can be installed on the surface, an arrangement called a "Beverage on Ground" or "BoG."  These are also sometimes called "snake" antennas.

The Beverage antenna is not the only antenna type that can be installed close to the ground.  Other more conventional antennas can be installed on the ground in order to obtain specific performance characteristics.  Horizontally oriented antennas in particular can benefit from installation on the surface.  Such installations can be used effectively for the reception of HF skywave signals at all arrival angles.

When a horizontal antenna is installed on (or very near) the surface, its primary response is vertically-polarized, rather than horizontally, because the ground reflection cancels essentially all of the horizontally-polarized wave response.  The elevation-plane response becomes nearly uniform from the horizon to directly overhead.  While a dipole antenna mounted on the ground is the simplest such design, other shapes can be used effectively for reception.  In particular, the loop antenna can be a very space-efficient ground-mounted antenna for reception on the low-bands.

The proximity of the wire to the ground makes such an antenna very lossy, rendering it rather useless for transmission.  For reception, however, a lossy antenna can be a serious asset, and the pattern and directivity very helpful, especially at longer wavelengths.

The Prototype

I have recently started to experiment with such an antenna, a corner-fed square loop, 15' per side, mounted at ground level, and constructed with insulated wire.  For now, I am taking a cue from the Beverage community, and calling this a Loop on Ground, or "LoG" antenna.  The EZ-NEC model for this antenna is shown in Figure 1, with an overhead view in Figure 2.

LoG Antenna
Figure 1: EZ-NEC Antenna Model (3D View)
  LoG Antenna
Figure 2: EZ-NEC Antenna Model (Overhead View)

6.25:1 Transformer 6.25:1 Transformer
The antenna is not grounded anywhere in its RF path, and it uses an isolation transformer at the feedpoint.  This provides a near-short circuit at DC on both sides of the transformer, while DC-isolating the antenna from the cable.  The coaxial cable shield is grounded at the building entry point, providing lightning and static immunity similar to that of a properly constructed Beverage antenna.  I originally used a 9:1 transformer designed for Beverage antennas, which provided an effective match across the lower HF bands.  I have recently replaced it with a home-made transformer, mounted in a custom enclosure which is easier to install flush with the ground.  The new transformer is a 6.25:1 (75-ohm to 470-ohm), wound around a Fair-Rite 73 Material binocular core, which is about 1.5 cm square.  The datasheet for this ferrite material shows that it is optimized for frequencies roughly in the range of the 160m through 30m amateur bands.  The transformer uses two turns connected to the coax, and five turns connected to the antenna.  That core is quite small, so I used teflon-coated wire-wrap wire, because it was thin but still well-insulated, and all seven turns could fit comfortably on the core.  The transformer is essentially the same design used by W8JI for his Beverages, which is the design used by DXE for their very nice commercial version.  The performance of the homebrew version is very similar to the DXE model across the lower HF bands.  The open enclosure, shown at right, measures about 2" square and less than an inch thick.  Unlike the DXE design, I chose to mount the F connector on the side of the enclosure so that the 75-ohm cable can be run completely flush with the ground.

This antenna has demonstrated very effective reception at both high and low angles.  This makes it particularly useful on the longer wavelengths, where both domestic (high-angle) and DX (low-angle) signals can be received at the same time.  The S/N observed is excellent, even in the presence of convective weather, while DX and domestic stations appear with similar S/N.  The azimuth response is bidirectional, similar to a dipole or small vertical loop antenna, or an unterminated Beverage.  The aperture of the loop is more than sufficient to allow the antenna to set the noise floor of an average receiver, without the use of an outboard preamplifier.  This is an important feature, because each additional amplifier in the receiver chain, including each outboard preamplifier, raises the noise floor a bit.  Fewer receive amplifiers mean better overall S/N ratio, which is particularly helpful with longer wavelengths, where environmental and atmospheric noise levels are already elevated.

The Model

EZ-NEC elevation plots of this antenna are shown below.  Note that the -3dB and -6dB points in the elevation response are at very low angles, even lower than many full-sized verticals achieve over similar ground.  For each band, the left-hand plot highlights the -3dB elevation angle (indicated by the green dot on the trace), and the right-hand plot highlights the -6dB elevation angle.  The -3dB angle is where the gain is one-half of an S-unit down from the overhead peak.  The -6dB angle is where the gain is expected to be one full S-unit down from the overhead peak.  The frequency, elevation angle, the absolute gain (dBi) and the antenna-relative gain (dBmax) are highlighted in a green box on each plot.

40m Elevation Plane  40m Elevation Plane
Figure 3: 40m Elevation Plane

80m Elevation Plane  80m Elevation Plane
Figure 4: 80m Elevation Plane

160m Elevation Plane  160m Elevation Plane
Figure 5: 160m Elevation Plane

Experimenting with the model, it appears that the best DX pattern, with lowest -3dB elevation angle, is obtained with a loop whose total length is approximately 15% of the target wavelength.  The 60' overall length of the prototype loop was a rough estimate of a loop that would be a good compromise across the 40m, 80m and 160m bands.

The direction of maximum azimuth response is normal to the feedpoint.  For example, if the square loop is arranged such that its sides face the cardinal directions of the compass, and fed at the NW corner, the strongest azimuth response is to the NE and SW.  Experimental results confirm this behavior.  The loop can be fed at a corner or on a side, whichever gives the desired response.  This allows a single fixed loop installation to be selectively fed for at least eight different orientations.  The loop doesn't even need to be a square.  The square tends to be simplest to set up, but an octagonal or circular loop should also work just as well.

The -3dB azimuth beamwidth at low angles is approximately 90° in each of the two main lobes, as shown here for the 80m response at 8° elevation, fed at the 12-o-clock position of the loop.

80m Azimuth Plane
Figure 6: 80m Azimuth Plane at 8° Elevation

If a unidirectional pattern is desired, this antenna can be deployed in a pair, separated by as little as a single loop diameter, to form a phased array.  An example pair for 80m is shown in Figure 7, constructed with a ~130° delay line on one element.  Such an antenna can have the directivity (RDF) performance of a large beam antenna at significant height, or a long terminated Beverage antenna.  This antenna however, can be completely invisible in the yard, and consumes a fraction of the space.  Note that the lower -3dB elevation angle is around 6°, which should be excellent for DX reception.

80m Elevation Plane
Figure 7: 80m Elevation Plane for Two-Element Phased Array

A phased array can be fed with a specific phase angle, as shown above, to achieve unidirectional pattern, or the elements can be fed in opposite (180°) phase, to produce a sharper bidirectional pattern than is provided by a single LoG element.  The latter is extremely easy to feed, and can be done without the buffer amplifiers needed for refleciton isolation with smaller phasing angles.

Testing the Loop-on-Ground with Real Signals

In a recent RTTY contest, I watched as a friend did a CQ run on 40m.  He was using a full-size 33' 40m vertical at his station, which has similar ground elevation to mine.  Since our stations are also within a few miles of each other, this allowed me to watch him work several stations in quick succession, while I observed his run on the LoG antenna.  In order to compare the receive performance of the two, I watched to see how many stations he could hear on the vertical that I could not hear on the LoG, and vice versa.

The vast majority of stations that he worked were also perfectly copied on the LoG.  There was one station that he heard that was marginal copy on the LoG.  Surprisingly, there were several stations that I could copy perfectly on the LoG that he never heard on his vertical.  He called CQ, and they answered him, but he just couldn't hear them.  I looked up those stations to find their location, and none of them were inside the skip zone, or anywhere near it.  In fact, the stations he was missing were all on the order of 1,000 to 1,500 miles away from him.  Because of the pattern differences at high angles, you would expect the LoG to hear close-in stations better than the vertical, but these were not NVIS signals.  These were low-angle skywave signals that the LoG simply heard better than the vertical.  So at least for this one test, the 15' square LoG antenna actually pulled in more stations on receive than his full-size vertical antenna on 40m.  When we both switched to 80m, I was able to copy several stations, while he couldn't hear anybody.

More recently, I used the 15' square loop-on-ground as a dedicated receive antenna for the ARRL 160m Contest.  The antenna was pointed roughly ENE/WSW.  I used a much larger vertical loop antenna as the dedicated transmit antenna for the event, similarly oriented to the compass, and separated from the LoG by roughly 50' to the broadside.  Bear in mind that I live near the center of a town of roughly 50,000 people, and my lot is approximately 100' by 60', located in a subdivision of similarly-spaced houses.  A resonant vertical is unusable as a 160m receive antenna here, for obvious reasons.  If the RF surroundings weren't already busy enough, the weekend of the contest was filled with an active weather pattern, with nearby storms and showers during the entire event.

Despite these challenges, the LoG antenna performed beautifully.  The recieved S/N ratio on 160m looked better than 15m does on my hexbeam, and a screen capture of the panadapter can be seen at right (click to enlarge).  The LoG served as the sole receive antenna for the entire contest, driving both the TS-590SG, and an RSP-1 used as the pandapter.  The antenna produced enough signal to "set the noise" in both the panadapter and the transceiver, even with the 3dB splitter loss.  Not only was I able to work the contest despite the challenging RF environment, I greatly enjoyed listening to very clean and low-noise CW for over ten hours of 160m operation.  The LoG easily heard stations at all distances, from a few miles to many hundreds of miles, all with very similar S/N levels.

A little over a month after the ARRL 160m contest, I used the LoG again to work the CQ 160m CW Contest.  The antenna again performed quite well and contacts were made across a similar range of distances as during the ARRL event, resulting in a similar QSO count and final score.  During this event, I also experimented with making QSOs both with and without the 590's preamplifier enabled.  When run on 80m or 40m, this antenna needs no preamplifier, as long as quality feedline is used.  Even though the pattern gain on 160m is nearly 15dB below that on 80m, a preamplifier still wasn't necessary for making any 160m contacts.  However, using a preamplifier did seem to raise some of the weaker signals a bit, without adding too much extra noise in the preamplifier itself; this is probably a matter of taste for each operator.  One thing that I would note is that if a preamplifier is desired for 160m operation, the preamplifier built into any modern transceiver is more than enough.  Coaxial cable loss on 160m is tiny, and a remote preamplifier is not needed.  Given that the preamplifier is definitely unnecessary on 80m or shorter wavelengths, the choice becomes whether the operator prefers the sound obtained by pressing the PRE button on the rig during 160m operation.  If more signal is desired on 160m, another way to get it (and arguably a better way) would be to simply make the loop a bit larger.
Practical Considerations

As low-band receiving antennas go, a 15' square is hard to beat for space efficiency.  There is nothing special about the 15' figure, either.  I have modeled this antenna at several different side lengths, from 5' per side to 60' per side, and the trade-off appears to be pattern shape on shorter wavelengths, versus signal strength on the longer wavelengths.  Generally, longer wavelengths tend to have better low-angle pattern coverage, possibly due to the increased skin depth of the ground at these wavelengths.  This makes the loop antenna a more natural option for the lower bands than the higher bands.  In practice, the 15' square antenna produces generous signal levels on the lower contest bands: 40m, 80m, and 160m.  It produces similar gain levels on 40m and 80m, and similar noise floor level across all of the bands where I have used it.  The gain increases with increasing frequency, which means that very little adjustment needs to be made to receiver gain when changing from one band to another.

Location and Colocation

Effective Beverage antennas are completely out of the question for many of us, but to work the low bands, Beverage-like performance would be particularly helpful when operating in our noisy urban environments.  In fact, urban operators need effective receive antennas for the low bands even more than those who actually have sufficient space for Beverages.

Many hams use a vertical antenna on the low bands, particularly on 80m and 160m, in order to fit the yard space they have available.  The loop-on-ground makes an excellent companion to a vertical, using the LoG as a dedicated recieve antenna and the vertical as a dedicated transmit antenna.  The vertical can be installed in a back yard away from view, while the LoG can be installed in a front or side yard, where it will be completely invisible, safe, and well-separated from the transmit antenna.

Even for people with a generous piece of property, this kind of antenna allows the yard containing the receive antenna to be used for other purposes.  When installed carefully, the kids can play soccer right over the top of it and never know the difference.  As a receiving antenna, it is completely RF-safe.  Since the size and shape are not critical, otherwise unavailable pieces of a yard can be purposed to hold a low-noise receive antenna.  Such areas could be the corners of a yard that surrounds a pool, or the area around a large tree that is otherwise unusable for antenna projects.  This kind of device would be ideal for any landscaped area that is maintained for aesthetic purposes.

A loop antenna covered by the lawn is the very definition of a stealth antenna, which should make it very attractive for people living with antenna restrictions.  Combined with its small size, the loop-on-ground design might make an effective low-noise receive antenna for people who live with a combination of a small lot and land use restrictions (particularly the family-imposed restrictions ;).

Advantages

When compared to a classical Beverage antenna, the LoG antenna can offer similar RDF performance, but in a much smaller space, and without obstructing other use of the affected property.  This can be a real asset to people who have had their Beverage antennas destroyed by deer antlers, or who have had enormous areas of property made otherwise unusable because of the dangling Beverage wires.  A single LoG element approximates the performance of an unterminated Beverage antenna, while a two-element phased array of LoG elements approximages the performance of a terminated Beverage.

As with all receive antennas, the main advantage is the simultaneous use of separate transmit and receive antennas, each optimizing its respective operation.  That is, use a receive antenna for the best signal-to-noise ratio while receiving, and use a transmit antenna that provides the best signal at the distant station, given the available real-estate.  If the prime real-estate for your low-band antenna is tied up with a vertical and its radial fan (as is the case at my location), antennas like this give you the opportunity to have improved performance during reception, as well, without giving up your preferred transmit antenna, or its location.  In my case, the location of the loop is such that it is actually a better location for a receive antenna, but the location isn't suitable for an above-ground antenna.

The Bottom Line

Now that the antenna has been in the yard for a while, I have been able to test it on some real-world signals, as described in the sidebar at right.  This is where the rubber meets the road for me, because my home is located in a less-than-friendly RF environment.  Nonetheless, I am very pleased with the results of the 15' square loop-on-ground, and it will probably become my primary recieve antenna for most HF work.  Since we are well into the low-band years of the sunspot cycle, this antenna appears to be well-sized for this kind of on-air work.  In the end, a loop of wire in my yard has turned out to be one of the best pieces of gear I have ever used for HF, as well as one of the least expensive.  That's a slam-dunk combination.

As I spend more time with this antenna during major on-air events, I will post the results here.


Copyright (C) 2016,2017 by Matt Roberts, All Rights Reserved.