With the lull in tower and antenna work until spring arrives this is an opportunity to experiment with one or more low band receive antennas. My first is a unidirectional Beverage antenna pointed at Europe. I have the space for several moderately long Beverage antennas should I wish to do so.
This antenna is a trial run to see how it works and how much effort it'll be to build Beverages through bush, bog and forest. I will not run these antennas over open fields since those have better uses. The land I've chosen has no use other than for antennas like these.
Since I'm coming from a position of ignorance I have relied on several experts in the field of Beverage antennas. One is Low Band Dxing by ON4UN. The other is W8JI. Those are my primary sources. I have also spoken to and read material written by other hams with theoretical and practical Beverage experience. I especially like ON4UN's book because it is a compendium of material from many reliable sources. One of best sources he heavily relies upon is W8JI.
There are, of course, many hams with experience and opinions, but be careful. Too many of those voices are not reliable. As with other antenna topics there is a great deal of myth being promulgated about low band receive antennas. I strongly suggest sticking with reliable sources.
As W8JI, ON4UN and others have demonstrated a Beverage can be successfully modelled with NEC2 despite being so low and connected to ground. The ground proximity is not a problem for NEC2, however the ground connections require special provision in the model. These connections are simulated with two equal length radials that run orthogonal to the antenna and close to the ground. The source and load are placed on wires that connect the antenna wire to the radials.
I was originally intending the antenna to be 120 meters long, which is enough for good directivity and not so long as to make construction in the bush too onerous. It turns out that is a very poor choice since there is a significant minor lobe directly rearward. So I went longer, trying to get reasonably close to ON4UN's suggestion to make the Beverage an integer multiple of 89 meters.
The following azimuth pattern is for a 170 meter long and 2.5 meter high Beverage at 3.5 and 1.8 MHz with a 500 Ω termination resistor. Although the modelled source impedance is closer to 550 Ω the pattern is stable for small deviations.
The 2.5 meter height was chosen to ease construction and to reduce the risk of wildlife incidents. One local top band enthusiast (VE3QAA) says his Beverages are 1.5 deer heights so that a running stag does not break the wire with its antlers. Mine is perhaps not quite high enough. I'll deal with that if the antenna becomes permanent.
As the Beverage length approaches 178 meters (2 x 89 meters) the most rearward lobe shrinks to a very small value. This is useful to attenuate QRM from that direction. The other minor lobes change only a little for small length excursions.
The length of my Beverage is approximately 175 meters, close to the ideal, and which is the longest I could fit in the location I chose. The F/B should therefore be 20 db versus the 16 db shown above for 80 meters.
The chosen 45° bearing covers all of Europe, from northern Scandinavia to the Mediterranean, within the fairly broad main lobe. On 160 meters the lobe is even wider. The azimuth plot is an overlay of the patterns for 160 and 80 meters.
Location, location, location
When I first laid out my plans for antennas and towers on my property I looked at the easternmost bog and forest as the best location for receive antennas. This area has no other use and it is furthest from dwellings and power lines, particularly in the direction of Europe. This matters for contests since that is the most productive path for QSOs and multipliers. I want to catch as many European signals as possible.
The scaling of my 50 meter graphical "measuring rods" is not perfect. When the wire is extended at both ends right to the property line (upper left) and the southwestward large tree the Beverage is ~175 meters long per the map scale -- I used a higher magnification of the satellite view for my calculation. Only the most eastward 10 meters is bog. The rest is bushes and scattered trees. It is not that easy to walk and work in. Snow cover and winter clothing makes it easier and safer. The countless thorn bushes are a serious hazard. I did not bother to with an actual measurement since the length is not overly critical for my purposes.
I originally considered the bog area at the lower right for a northeast Beverage. Although there are stunted trees that can support the Beverage wire it is unpleasant to work in and requires a far longer transmission line in order to avoid the hay field. Although bulk RG6 is cheap the routing of such a long cable is more work is because it won't be buried there is additional wildlife hazard.
The trees in the selected field are sparse enough to allow stringing Beverage wires without being so dense that it becomes unreasonably difficult. The large trees lining the field boundaries are not as handy as it might appear since, close up, there is dense underbrush. Even so I may try to use those boundary lines for north-south and east-west Beverages.
|45° bearing corrected for magnetic deviation|
Blazing a trail
Satellite views are comfortably clean. You don't see the reality of what it's like on the ground. Starting from the source I found it impossible to walk in the northeast direction without routing around the many obstacles and losing my way. It really is that difficult.
What I did was carry a bunch of wood stakes under one arm and a compass in the other and blaze a trail. As I reached an obstacle (often doubling as a wire support) I would pound a stake into the ground, walk around the obstacle and use the compass to orient myself. I would then continue onward to the next obstacle.
Through the camera lens the path doesn't look too bad. That is deceiving. Many of those innocent looking twigs are covered in thorns, some 2" (5 cm) long and needle sharp. Old clothes are recommended! Doing this in winter helps since you can comfortably wear heavy, thick and warm clothing.
You can see the imprint of my snowshoes in the snow. Snowshoes can easily get caught in the brush so some care is needed. The termination is in the trees at the edge of the bog behind the final stake. In the background can be seen the rotting logs of the fence marking the eastern boundary of my property. There is more bog and bush towards the northeast for at least 1 or 2 km. Man-made noise is not an issue.
Labelling the ground in this area as "EZNEC medium" is perhaps optimistic. For a Beverage ground quality is not critical so I didn't fret about it in the model.
Running the wire
My equipment for running the wire is shown in the photograph: ladder and a crate of electric fence insulators, gardening tools, plastic ties, nails, porcelain insulators, rope, ground rods and wire clamps. It was very strange being out in the bush and carrying all this stuff with me. I discovered that it is difficult to climb a ladder with snowshoes.
An example of a tree attachment can be seen in the picture. I took the picture while walking back from the end of the line. This was one of the easier attachments. Most required extensive pruning of branches and twigs that covered the targetted tree trunks and branches so that nothing would touch the wire in windy weather.
I took along two types of fencing insulators: those that could be screwed in and those that took a nail. The nailed variety were used for the smaller branches and were attached with UV-resistant plastic ties rather than nails. The screw in type were easier to attach on tree trunks.
The wire is AWG 17 aluminum electric fence wire. It is quite inexpensive. The downside is that if it breaks the splice repair cannot be soldered. For an experiment it is perfectly suitable. Should I go all in with Beverages my wire choice will change since I want something more robust, and preferably twinned for reversible Beverages.
I tried a few mechanical termination techniques until I settled on one that allows tension to be finely adjusted and which does not interfere with the electrical terminations. Large trees serve as anchors at both ends. The wire ends at a porcelain egg insulator, securely twisted and dangles down for electrical connection. A short rope ties the egg insulator to the tree.
Rather than wrap around the tree and tie a knot I hammered two 3-½" galvanized nails into the tree at Beverage height and used them as an improvised rope cleat. No knots required and the tension is easy to adjust.
The electrical termination at the far end is simply a resistor connecting the Beverage wire to ground -- the ground rods are 4' copper clad steel and were hammered in with a small sledgehammer. On the near end there is a transformer to match the RG6 transmission line. The high-impedance primary attacjes to the Beverage wire and a ground rod. The low-impedance secondary attaches to an F connector. I used a barrel connector since I could not find a suitable bulkhead female connector locally. I wasn't prepared to wait for a mail order delivery.
Both were put into small plastic "hobby" boxes. The hardware for wire connections is stainless steel. The boxes are not waterproof so (not shown) I lined the rim of the boxes with good quality electrical tape to reduce the map between box and cover. The screws pierce the tape to add some protection there as well. This amount of precaution is sufficient for an experiment. I did not protect the screw heads from rusting.
The transformer is the very same one I built as an experiment last year, in preparation for this very project. It better matches 50 Ω coax than the RG6 I am using, however the SWR and consequent transmission line loss that is not a serious concern.
|Sorry that the display isn't readable (see text)|
On his Beverage page (link at the top of this article) W8JI demonstrates how to calculate the radiation resistance with a range of analyzer readings. Ideally you should insert a termination resistor of that value. My (very old stock) carbon composition resistor measures 495 Ω, a little higher than its nominal 470 Ω value.
I was happy with that so I attached the transformer box and again measured the impedance. The SWR was fairly constant at around 1.7 from 1.5 to 4.0 MHz. Of course that is relative to 50 Ω. The actual resistance component of the impedance varied between 65 and 75 Ω, with a small amount of reactance. This is higher than expected for the 9:1 transformation ratio. The good match to RG6 is a fortunate accident.
Bulk RG6 is cheap. I bought a 500' (150 meter) roll at a local big box store along with some twist on male F connectors. I pulled the coax from the feed point west along the tree line until I was north of the Trylon tower. I then crossed the stone wall and ran the coax toward the tower and laid it next to the cables running from the towers to the house. Cables will eventually be buried or elevated by messenger wire from the tower to the house.
I used trees and carefully selected weights at the two corridors where people are likely to walk and elevated the RG6 over them. This reduces the chance that it will be accidentally stepped on or tripped over. Apart from that I'll take my chances with the wildlife. Other hams have varied reports whether animals will gnaw at coax lying on the ground. Now I get to find out.
The total amount of RG6 I played out was ~400' (130 meters). This is perhaps 50' (15 meters) longer than necessary. The extra length is to wind a common mode choke at the feed point and for future consideration in regard to cable routing and Beverage switching. As I said, the cable is very cheap.
Once routed into the shack I attached a UHF connector and remeasured it on the analyzer. Apart from some attenuation from the long transmission line it was fine. Since it was nearly sunset I connected it to the FTdx5000, pressed the RX antenna switch and proceeded to tune the low bands.
It works very well. In fact it works very, very well. As soon as I had it connected I heard European and west Asian (4X, 4J, EY) stations on 80 meters, despite being before sunset and a disturbed geomagnetic field. The gain is low so it is necessary to use the highest pre-amp setting on the rig to ensure that the antenna noise exceeds the receiver noise. An external pre-amp would only be required to equalize gain with the transmit antenna for operating convenience when switching between antennas.
On strong signals it is not easy to tell if the Beverage is helping at all. It is on weak signals that it shows its effectiveness. Many signals that are inaudible on the inverted vee are solid copy on the Beverage. I could listen in on European stations working each other and Asians long before they were workable here. Of course they might be workable with a better transmit antenna and more power.
A couple of other indicators are when an American and a European are on the same frequency and copied on both antennas. On the inverted vee the European signal is heard but covered up. Switching to the Beverage reverses the situation so that the European is solid copy and the American signal is knocked down enough that it is no more than an annoyance. The other indicator is SSB. The wider bandwidth makes many signals a chore to dig out of the noise. Even unheard SSB Europeans on the inverted vee are perfectly copied on the Beverage.
The beam width is noticable for stations that are somewhat off the main lobe. EA8, for example, is 88° bearing from my QTH. This is 43° off the peak direction of 45° bearing. From the pattern this should only reduce the signal by -7 to -8 db, yet that is enough to make the Beverage little better than the inverted vee. It is for this reason that hams I know with Beverages of similar length (2λ on 80 meters) opt for 8 directions rather than just 4. Often this is accomplished with 4 reversible (2 wire) Beverages.
On 160 meters the antenna also performs well. The most significant difference is that the beam width is broader -- see the pattern comparison above -- since the Beverage is only 1λ long on that band. A proper comparison will need to wait for a full-size transmit antenna. The 80 meter inverted vee is not an ideal basis for comparison. For now all I can say is that I can copy more stations and copy them better on 160.
What comes next
What the Beverage teaches me is that there are many more stations out there that I cannot work. I can hear them but they do not hear me. I expected that. This is why the Beverage is not especially needed at this time. Apart from experimenting with it I can copy more stations with far less QRM and QRN. It isn't merely directivity. The antenna is so much further from local noise sources that there is (so far) no evidence of the electronic and power line noise that occasionally make an appearance on the inverted vee.
After I build a high performance transmit array later this year I will have to decide on permanent receive antennas. If I choose to go with Beverages I will likely need to emulate others and go with 4 reversible Beverages (8 directions). They could all fit in the same field, crossing each other (this is perfectly fine) and with transmission lines brought to a single point for switching. ON4UN has a lot to say on this topic in his book.
Although Beverages work on both 80 and 160 it is desirable to have two receive systems for SO2R and multi-op contests so that 80 and 160 antennas can be separately steered. The secondary receive array can be simpler and only 4 directions -- operators can have a friendly fight over who get the Beverages. Of course if I build a directional array on 80 that reduces the need to use a Beverage antenna except for the weakest signals and to reduce QRM with its superior F/B.
This weekend is the ARRL DX SSB contest. Since I do not plan to be too serious in the contest it is an opportunity to play with the Beverage under conditions of heavy activity and QRM. I have high expectations for it.