by Pierluigi Mansutti IV3PRK - all rights reserved
IV3PRK Pierluigi “Luis” Mansutti
160 Meters: DXing on the Edge
BOG modeling with Eznec 5 and AutoEZ.
Yes, this is a resonant antenna, nothing to do with Beverages!
I tried to model my BOG with AutoEz starting from the standard “Beverage with 0.25 radials” model found in the EZNEC library. I changed the units to metrical, the ground type to my very poor, almost desert situation, and applied the AutoEz variables. Down here, on the left, we see the original antenna image with the 0.25 radials, and the current distribution. On the right, the same image with shorted radials causing a more uniform current along the antenna wire #1, as recommended by the authors (see below).
At first I tried to find the best F/B varying the length of the BOG wire.
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Freq.: 1.825 khz; Height: 3 cm.; Ground: Real = Sandy (0.002, 10); Load: 250 ohms.
Immediately, it was clear that the BOG is a sharp resonant antenna, but it appeared to be too long than expected - 80 meters - , what’s wrong in the model?
Both the authors of Eznec and AutoEz answered to help me.
Dan McGuire, AC6LA wrote:
«……. I believe the intent of the "0.25 radials" is to simulate a low-impedance connection to ground. This is explained in the EZNEC Help, topic "Connecting to High Accuracy Ground".
As the Help states: "If a low impedance connection is required, make the radials about a quarter wavelength or odd multiples long, and avoid lengths approaching a half wavelength or multiples. These aren't free space wavelengths, however, but wavelengths for a wire at the radial height.
A half wavelength for a wire very close to the ground will be less than in free space, though. You can determine the wavelength by modeling a dipole at the height of the radials and adjusting its length until it's resonant." From looking at the current on the radials as shown in the first graphic of your document, it appears that the radials of your model are a bit too long. For 1.825 MHz at 0.03m above sandy ground I get a resonant dipole length of 56.5m which translates to a "radial" length of 28.25m, much less than a quarter wave lenght, which is 41.07m. » (In fact, 28 m. is the lenght I adopted in ground radials of my Tx antenna).
And Roy Lewallen, W7EL himself wrote:
«Luis, You can see by the current distribution on the radial wires that they are electrically longer than a quarter wavelength. At a quarter wavelength, the current should just reach maximum at the junction. I think you'll find the best efficiency to occur when the radials are an electrical quarter wavelength or shorter, and increasingly poor as they get longer than a quarter electrical wavelength. The electrical lengthening of the radials is due to the low propagation constant of the ground. The amount of lengthening (the effective velocity factor) that happens, depends strongly on the height above ground as well as the ground characteristics. Calculation of the physical length needed isn't simple -- I recommend just shortening them until the current maximum occurs at the junction. 73, Roy, W7EL».
Thus, in AutoEz I put the radials length as a variable from 20 to 30 meters , 3 cm. high, for a BOG length of 65 meters with a load of 250 ohms.
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The antenna is resonating on 1.825 MHz (X = 0) with radials length of 26 m. But what’s about the Front to Back, which is our target in the antenna design?
As Bruce, K1FZ, said:
«The best BOG antenna pattern to hear DX comes with maximum front to back. This is the reason we try to find the length that gives best F/B.»
Quite simple to select another of the several options in the “Custom” sheet of AutoEZ to get this graph ===>
which indicates that’s much better to go towards a shorter length to improve F/B raport.
So moved the sweep a bit down and this is the F/B of a BOG 63 meters long, 3 cm. high, with a 250 ohms load and two sets of radials. ===>>
An impressive F/B of 45 dB is peaking at a radial length of 20 meters.
The azimuth plot shows a nice lobe with a sufficient gain (-16 dB) requiring only a modest preamplifier, and
the elevation one is mainly at high angles (40 degrees), as needed in my present low latitude location.
Now, keeping the same BOG length at 63 m., let’s make a frequency sweep
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A Front to Back of 35 dB from 1.80 to 1.85 MHz, with a peeak of 45 dB on 1.825, and at least 15 dB over a wide 500 khz band.
It confirms that the BOG is a sharp resonating antenna, which has nothing to do with the classic Beverage and the other broadband receiving loops.
But, of course, this is a model and for sure nobody will get a 45 dB of F/B in the realty!
Now let’s see what happens by varying the load
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Nothing: given the above parameters and dimensions, the load is perfect at 250 ohms.
At last, with the same sweep parameters, let’s check R (Resistance) and X (Reactance) at the source:
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As seen before, when the radials length was chosen, the antenna is resonating higher, around 2.1 MHz, and thus on 1.825 MHz - where it happens to be the best F/B - it has, at the source, a resistance of 318 ohms (red scale to the left) with a capacitive reactance around 100 ohms (blue scale on the right).
If it were a TX antenna, we could use an L-Network, but with receiving antennas a precise match is not required and thus a simple binocular transformer would do the trick, and now we see the results.
Modeling with feed system for the Reversible BOG.
In Eznec model, I inserted the transformer to simulate the feed impedance in the FORWARD position with these impedance data: Z of the antenna = 250 ohms, Z coax = 75 ohms.
The curves are the same as before, but look at the scales:
with Xfmr insertion, on 1.825 MHz we got
R= 96 and Z = - j31 ohms, with an SWR of 1.55 on a 75 ohms system, not bad for a receiving antenna.
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Next, I tried to simulate what happens in the REVERSE direction. I moved the load at the beginning of the BOG wire and inserted a 150 ohms transmission line with its VF of 0.73. I changed also the transformer to the new relations of 150/75 ohms and got these results:
The pattern is exactly as before, just correctly reversed, but the source impedance worsens to R 121 and X – j73 ohms at 1.825 MHz with an SWR of 2.4, not very well. Probably, the model of Rev.BOG circuit and transformers calculation do not take into consideration the VF of the line in the “transmission line” mode.
With the insertion of an L-Network we could achieve a perfect match, but that’s going to be a complication too difficult to manage for such a simple efficient antenna. Anyway, just for a funny exercise, let’s use another of the beautiful weapons of AutoEZ.
In the “Objects page”, let's hit on the “Create Impedance Matching Network” button, and automatically an L-Network is added without the need to go on a separate TL program.
In my case, a series 1.095 pF capacitor with a shunt 8 µH inductor would give the following perfect match, but not necessary for a receiving antenna.
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Raising the BOG at the typical Beverage height.
When Gary, KD9SV, and Bruce, K1FZ, introduced me to seriously consider a BOG in my difficult situation, I had already tried it without any success: it was simply too long!
I thought “longer is better”, but the BOG does not behave like a Beverage. K1FZ said:
«A BOG has too much capacity to ground over its entire length to behave like a traveling wave above ground Beverage does. A BOG becomes a tuned circuit».
As a countercheck, I went again on AutoEZ varying the height of the 63 m. BOG from 3 cm. gradually to 0.5 meters, and then up to the classic 2.5 m. high of a Beverage. The Front to Back drops from 45 dB to less than 10 dB.
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OK, if we raise the BOG we should also lengthen accordingly the radials.
With longer radials, the F/B slowly rises at 2 m. heigth, but still far from the 3 cm. heigth of the starting configuration.
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Now, keeping our BOG at 2 meters heigth, like a Beverage, let's investigate on wire length variations from 60 to 75 meters.
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The best Front to Back occurs with a length of about 68 meters, but the curve is much broader: only 6 dB of difference from max to min. and 3.5 dB above the F/B of the 63 meters BOG.
Here is the modest azimuth pattern of this short Beverage (yes, it is NOT a BOG) 68 m. long, 2 meters high with two 40 m. radials at right angles at each end in the model.
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This is the frequency sweep showing the Front to Back of both antennas.
We can realize the difference between a resonant BOG and a broad Beverage:
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- Red line is the BOG 63 m. long and 3 cm. above ground, with 20 m. radials
- Blue line is the Beverage 68 m. long and 2 m. above ground with 40 m. radials.
And finally, here is the frequency sweep showing the most important parameter, the RDF of the same antennas
- Red line is the BOG 63 m. long and 3 cm. above ground with 20 m. radials ====>>
- Blue line is the Beverage 68 m. long and 2 m. above ground with 40 m. radials.
Important note added on January 2017 by IV3PRK back in Italy:
The RDF - Receiving Directivity Factor - became in the last years the benchmark for receiving antennas, and its variations are appreciated in the order of dB fractions. Here we see that the BOG is about 1 dB better than a short Beverage - 9 vs. 8 dB - but, to get an idea, the RDF of my new Waller Flag in Italy is 11.7 dB - comparable with a long Beverage or a big four-square array.
A modeling final note:
This is just modeling with NEC2 and not NEC4 engine, so the results must be taken with caution, but at least the general trend, showing a tuned antenna, should be correct.
We know that NEC2 engine does not handle wires on ground or very close to ground, so I performed a last test to investigate about the minimum height for two different grounds.
In this graph we see:
- the red line shows that on the very poor sandy ground (0.002, 10) of HC1PF in Ecuador, the peak of F/B is at 3 cm high of the BOG wire;
- the blue line shows that on a very good ground (0.0303, 20), like at IV3PRK in Italy, it should be better to keep the wire at a lower height.
This is the conclusive comment received by EZNEC’s author about my huge work, many thanks Roy:
«Hello Luis, Yes, you understood and applied what Dan and I said.
Even slightly elevated radials will behave differently than buried ones, but in most cases (this one included), the difference will be a small one in efficiency and perhaps a moderate impedance change.
With a sharply tuned antenna like this, you can't expect the resonant frequency to be exactly like EZNEC predicts, but after you adjust the antenna, it'll behave very much like EZNEC shows.
But even NEC-4 makes the assumption that the ground is homogeneous to an infinite depth -- which real ground isn't -- so it's not terribly accurate whenever ground effects are important. And the only difference between NEC-2 and NEC-4 in this application is in the modeling of the radials.
Otherwise the results will be the same.
73, Roy, W7EL»
Quito, June 19 2015 Luis IV3PRK / HC1PF