Monday, January 28, 2019

Short Beverage Antenna for 160 Meters

One of the many projects I did not get done in 2018 was putting up more receive antennas for the low bands. The delay is due to its relative priority being lower than transmit antennas, such as my still not quite complete 80 meter vertical yagi.

Up to now I have just the 175 meter long Beverage antenna pointed northeast at Europe. It's a wonderful antenna, although one that requires periodic maintenance. That's a common complaint about Beverages strung through bush and forest. The area was selected because it is far from sources of potential interference and it is only populated by wildlife. The farm fields are easier to work in but would require taking the Beverages down during spring and summer.

With the upcoming CQ 160 meter contest as motivation I spend some time last week putting up a temporary Beverage pointing west. It is shorter and simpler than what I have planned for a permanent antenna. What it therefore lacks in performance is compensated by the reusability of its components (even the wire) and ease of construction. By "ease" I mean relatively easy compared to alternatives.

The Beverage is ~89 meters long, about the shortest a Beverage for 160 meters can be and still deliver acceptable performance. That length is not randomly chosen. Like yagi booms there are lengths that exhibit peaks in F/B, and therefore directivity and noise attenuation. To demonstrate this I selected three lengths -- 70, 89 and 110 meters -- and modelled them with EZNEC. The impetus for the modelling comes from ON4UN's book Low-band DXing, in which optimum lengths are discussed.

Notice the differences. F/B is significantly better with an 89 meter length. However gain increases (beam width decreases) with length. There is a peak in F/B approximately every multiple of 89 meters; gain continuously increases as the length increases. Gain is not too critical since, if needed, it can be provided with a pre-amp. The length does not have to be precisely 89 meters. A few meters either way has little impact on performance. The small difference in F/B is likely swamped by other variables such as ground quality.

The gain difference is noticable. The modelled gain of the 175 meter Beverage on 160 meters is -10.5 dbi versus -15.1 dbi for the 89 meter Beverage, or -4.5 db worse on the shorter and lower antenna. About -0.5 db of that is due to the lower 1 meter height of this antenna. (There is another -1 db of coax loss, as we'll see later.) The additional loss is enough to make comparisons between the Beverages difficult. Of course the gain is not indicative of the antenna's directivity, and therefore its effect on copy of signals in the target direction.

Feed point

In the spirit of keeping it simple for this temporary antenna the feed point is built into a discarded plastic food container. The clear plastic helpfully shows all of the components, inside and outside, but does confuse which side each is on. The inside only contains the matching transformer. Stainless steel hardware provides studs to attach the Beverage wire and groung wire. I am using UHF connectors since I have them and I don't have type F bulkhead connectors.

The ferrite core and construction are the same as for the northeast Beverage. The turns ratio of 5:2 is a good match between RG6 (70 Ω) and 500 Ω for the antenna. This was tested with a 470 Ω resistor which measure 495 Ω. A better match is possible with a turns ratio of 8:3 but I got frustrated trying to thread more turns through those tiny holes in the BN73-202 binocular core. This is despite using thin AWG 24 insulated wire scrounged from old Cat5 cable.

With the cover on the container it is pretty well watertight when the lid faces up. Otherwise water could seep in around the studs and SO239 connector. Sealant is near useless due to the flexibility of the thin plastic.


Driving the ground rods into frozen ground is not fun. After a couple of weeks of extremely cold weather and thin snow cover the frost has penetrated deep. I used a pick axe to remove the top 10 cm of frozen soil. That was enough to hammer the rods through the remainder of the frozen soil into the warmer ground below. However I did encounter rocks that required repeating the process several times until I could get down the full 4'.

I did not use a 470 Ω resistor for the termination. After reviewing material on Beverage antenna construction I realized that I had to compensate for the serial resistance of the ground loss. With a single ground rod this can be exceed 100 Ω even in good soil. I opted for a 330 Ω carbon composition resistor that measure close to 350 Ω (carbon composition resistor increase in value with age). This worked out well when I measured 495 Ω at the feed point from 1.5 MHz through 7.5 MHz.

I kept the construction simple. The resistor is clamped directly to the ground rod. A wire nut connects it to the Beverage wire. Since the wire swings a bit in the breeze I dropped a piece of scrap lumber on it to avoid breakage due to metal fatigue.

The wire tensioner uses a thin nylon rope and a cleat made from a couple of nails pounded in a tree. The feed point end of the wire is secured to a wood fence rail with a nail driven through the insulator. It's simple, cheap and effective.

Beverage wire and siting

I used what was available and long enough: AWG 14 THHN solid copper wire. I have a 300 meter reel from which I've been building my wire antennas. When the Beverage is removed in the spring the wire will be reused for other wire antennas. The insulation allows the wire to sit directly on trees and wood fencing without risk of performance degradation.

The Beverage wire runs along the wood fence line about 1 meter off the ground. There is little to no risk of deer or humans tangling with the wire, other than yours truly. I climb the fence to get to work on the 80 meter array. Because the coax and control cable for the 80 meter antenna cross the fence the Beverage passes close to them. While the Beverage performance should not be affected by the proximity there is a risk 80 meter transmission into the Beverage when operating SO2R on 80 and 160 meters concurrently. I have not tested this yet.

The 150' tower is as close as 25 meters to the Beverage. The tower and antennas are not resonant on 160 meters and the separation should be enough to avoid Beverage pattern distortion even if the tower were resonant. The 80 meter array elements are closer but are not resonant on 160 meters. However if the Beverage is used on 80 meters there is a risk of interaction. First listening tests on 80 meters don't demonstrate cause for concern. The southwest radials get close to the Beverage, although those of the 160 meter vertical (gray circle to the south) are 20 meters from the feed point.

Coax for the northeast Beverage (feed point at the northeast corner of the site map) runs along the fence line on the ground, as far west as the Trylon tower in the house backyard. Therefore they overlap for ~30 meters. This is generally poor practice due to the potential for noise incursion but difficult to avoid in this instance. The RG6 coax for the west Beverage angles south into the hay field, running midway between the 160 meter radials and the Beverage.

Terminating the Beverage further west to reduce all these interaction issues was inadvisable because it would bring the antenna close to the power line that runs through my property (indicated by the gray line). The drop to the house runs underground along the forest of young tree south of the driveway.

These pictures illustrate how the fence is being employed for the west termination (left), supporting the wire and the east feed point. It's as simple as it looks. Laying the line on the fence rails took a couple of hours of walking through the undergrowth with a wire reel, cutting branches and thorns along the way. The deep snow and slush didn't help. The bush on the verges of the hay fields has grown thick over the years due to not being maintained. I am unlikely to change that.

Initial on air testing

I had one evening with the antenna before the CQ 160 contest. Testing was limited due to finding few stations far to the west to properly. I compared it to the vertical itself and the northeast Beverage. At least the large number of Europeans active in preparation for the contest made it easy to establish that the F/B of the west Beverage was reasonably good, perhaps as much as 20 db. That indicates the wire length is performing in accordance with the model.

Later in the evening I heard a W6 and a couple of W7's. The Beverage definitely improved the SNR on those stations. While promising, this was only a tentative assessment. I would have to wait for the contest to have lots of activity and a range of signal levels to better assess its performance. The directivity is better on 80 meters, but again there was a dearth of suitable stations available to get reliable comparison data.

Since I used the full 150 meter roll of RG6 there is an estimated transmission line loss of ~3 db. The extra length was coiled up outside -- it will later become a bidirectional Beverage. The loss is not a great inconvenience other than to make antenna comparisons more difficult. A lesser impact is that the periodic impedance variation with frequency is mostly smoothed away when measured in the shack.

Contest performance testing

Hundreds of stations audible at any time from all over North America and around the world is a fantastic environment in which to conduct an antenna test. Mediocre to poor DX conditions on Friday night were helpful in providing many weak signals for testing, although it was damaging to my score. Late in the evening when west coast activity peaked was the best time to test. It gave me something to do after largely working out the east coast and being unable to work many Europeans.

The TLDR version of its performance: meh! This is about the shortest Beverage you can have on 160 meters and get anything approaching good performance. Although I have not done a direct comparison it is visually apparent on looking at the far field patterns of this and a compact antenna such as a Flag or K9AY array that performance is comparable. One important difference is that the compact antennas require a pre-amp, which in some cases can create noise during SO2R and multi-op contesting.

Despite having a broad azimuth pattern the antenna performed best on VE7 and northern W7 and W0. Reception of W6 stations was slightly improved but very little on W5. I doubt that I would have missed any QSOs during the contest without this antenna but it made easier copy. No Pacific stations were heard. That test will have to wait.

One disadvantage of the overly broad pattern is that the F/S is poor. W2, W3 and W4 stations showed little improvement with this Beverage and were not notched as they are with the longer northeast Beverage. Removing QRM, not just noise, is desirable in a receive antenna. It did reject signals well off the back, but all I have off the back is Europe and, sadly, there was no QRM from that direction.

Initial tests on 80 meters are promising. Directivity is superior to that on 160 meters due to the antenna being twice as long relative to wavelength. It has been used just once in daily DXing, during a sunrise Pacific opening (I don't often wake up early enough for these!). It made a modest improvement on VK3 and E5 signals, which are more southwest than west from here. I suspect that the modest result is due to atmospheric QRN predominantly coming from the southwest. But I could be wrong.

There are upcoming contests where I expect it to assist with adding multipliers on 80 meters. As yet I have nothing conclusive to report about performance on 40 meters. Another reception tool would be welcome on 40 due to the poor directivity of the XM240.

Future plan

This experimental Beverage antenna will come down in the spring. As an experiment is has already proven fruitful and I expect to get more out of it for the rest of the winter. The wire and coax will be recycled into other antennas and the transformer will likely see service in another Beverage. The plastic container will be discarded (recycled) since it won't survive the weather for long.

Many low band enthusiasts have replaced their Beverages with switchable vertical arrays, which can achieve equal or better directivity with a smaller footprint. But they're more complex, requiring hybrid combiners, phasing harnesses and amplifiers among other specialized components. Most opt for commercial products. As I said earlier, these arrays need to be well separated from transmit antennas to avoid the potential for grief from the pre-amps.

I am on the fence regarding Beverages versus vertical arrays. This is a choice I'll have to made by the fall. Regardless of that decision I am very likely to twin the northeast Beverage to make it bi-directional. Ultimately that may be my only Beverage.

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