Yesterday I installed my first BOG (Beverage On Ground) at Hilltop. Russ, NN3Q, was kind enough to loan me two 200-foot BOGs with transformers that he had taken out of service on 160 meters. So the BOGs are a bit short for 630m and 2200m, but i figured that I would give them a try.
The impedance of an above-ground Beverage is generally on the order of 400-450 ohms, but that of a BOG is lower, on the order of 200-250 ohms. A beverage should be terminated with a total resistance to ground equal to its characteristic resistance. So the sum of the ground rod resistance plus the terminating resistance for a BOG should be 200-250 ohms. I used a 220 ohm resistor, which is likely too high, as ground rod resistance is likely on the order of 50-350 ohms, depending on soil conductivity, according to ON4UN’s Low-Band DXing.
I wanted to run the beverage along an azimuth of approximately 50 degrees, to cover Europe, and I ended up with a heading of about 45 degrees, I think, as you can see on the satellite map above.
Russ suggested that I first try the BOG without a preamp, but I wanted to have a preamp available if it was needed, so I hooked up a DX Engineering RPA-1 which has the feature that when it is powered down it is bypassed, making it easy to take it in and out of the circuit remotely. I had turned it on initially to make sure it would power up, and so it was turned on when I had my first look at how the BOG behaved. The table below shows what I saw in the first 5 minutes or so after activating the BOG:
| Station /band kHz | Signal Level – BOG with Preamp On | Signal Level – BOG with Preamp Off | Signal Level: 40 foot wire |
|---|---|---|---|
| K3RWR 0.136 | -27 dB | -9 dB | -15 dB |
| W3LPL 0.4742 | -17 dB | -2 dB | -6 dB |
| AA1A 0.4742 | -17 dB | -8 dB | -9 dB |
| WA3ETD 0.4742 | nil | -15 dB | -18 dB |
The numbers with the preamp on were surprising. I figured that they meant that either the preamp was being overloaded or the strong signal it was putting out was causing overload further down the receive chain and thus the worsened numbers, or something was wrong with the preamp. The preamp had been working fine previously, so my bet was that overload was the problem. I took the preamp out of the circuit and checked it, and it IS fine…slight increase in noise floor and 20 dB increase in signal strength at 0.474 MHz as viewed on my 141T when it is placed in the circuit and powered up.
But you can see from the table that WITHOUT the preamp, the BOG appeared to be 1-6 dB better than the 40 foot inverted L, depending on the azimuth.
I ran WSPR receivers overnight on both the BOG and the 40 foot inverted L, and I accumulated 745 simultaneously received datapoints. You can see them on the graph below:
The graph may be easier to see if you open it in a separate tab. It shows the signal difference between the Inverted L and the BOG with this difference expressed as inverted L signal strength in dB minus BOG signal strength in dB. The horizontal green line is the x axis. So points above the green line are those where the Inverted L was superior, and those below the green line are those where the BOG was superior.
You can see that most of the points are below the green line, indicating that most of the time the BOG was better. The two antennas were not statistically different at 63 degrees or 272 degrees. The Inverted L was better at 284 and 290 degrees. At all other azimuths the BOG was better by 5-15 dB ( 25, 32, 194, 200, 219, 225, 228, 261, and 357 degrees).
It is difficult to know what to make of the azimuth data given that we don’t know the pattern of my inverted L. The axis of the horizontal element of the inverted L is 123 degrees, so broadside would be 33 and 213 degrees, so one might guess that those directions would be favored for the inverted L. But the length of the inverted L’s horizontal element is such a small fraction of a wavelength (18.8/630 = 0.029) that the inverted L likely has little if any directivity (see for example figure 18a and 18b of L. L. B. Cebik’s paper, “Straightening out the Inverted L”).
The axis of the BOG, as you can see in the image at the top of this page, is about 45 degrees. So the main lobes of the inverted L and the BOG would be expected to have similar azimuths (33/213 degrees for the inverted L and 45 degrees for the BOG) but this is not apparent in the data. There are likely several reasons for this. First, the take-off angles for the two antennas are likely different but complex functions of azimuth for the two antennas. Second, the pattern of the secondary lobes and nulls for the two antennas is likely very different. Third, the 200-foot BOG is extremely short from the standpoint of its wavelength, and so like the Inverted L, it may actually have little or no directivity.
It would be advantageous to have a longer BOG, but in the direction of my first BOG I do not have enough room, if I start the BOG at my fenced-in compound. Also, from 50 degrees to 200 degrees azimuth I don’t have enough room for a BOG if I start it at the fenced-in compound. But from 200 degrees through 30 degrees I have room for beverages of 400-500 feet starting at the fenced-in compound; perhaps not ideal, but significantly better than at 50 degrees. If I run a Beverage from one end of the property to the other at 45 degrees or so, I can get about 800 feet of Beverage, although that would require a feedline length of 400 feet. If I run a Beverage from one end of the property diagonally at 250 degrees (good for Australia and New Zealand) I can get about 600 feet of Beverage.
So the question is, how long should a BOG be for 630m? Generally 1 wavelength is said to be the minimum “ideal” length for an above-ground Beverage. But BOGs can be shorter than this, because while the velocity factor for a Beverage in air is on the order of 95-98%, the velocity factor for a BOG is on the order of 50-60%. So BOGs can be (in terms of their physical length, ignoring the velocity factor) 0.5 to 0.6 wavelength long and have an electrical length of one wavelength. Thus, in the 630m band a BOG would ideally be 315-378 meters or 1033-1240 feet long. So even at the maximum length that I can accommodate my Beverages will be less than ideal. But I could get close, if I used the full extent of my property. The downside to that would be disruption of the Beverages by (illegal) hunters, trespassing hikers, and critters. And my current BOG is less than 0.2 wavelength long, assuming a velocity factor of 50%. So it is not surprising that I did not detect any directivity along the axis of the BOG.
I also evaluated this short 200-foot-long BOG on 135 kHz, but that is a subject for another day.
References: There is a good reference on Inverted L antennas here. I had previously referenced W8JI’s excellent receiving antenna notes as well as his section on Beverages. Here is another web page with some hints on BOGs, by K1FZ.
73,
Roger
W3SZ