Wednesday, June 04, 2008

A vertical half-moxon for the 40m band

The MMANA model: Not obvious here, but there is a 20cm gap between the 2 horizontal parts.
Following is the description of the 40m array we used at SY3M. The antenna was designed "on the spot" and constructed very quickly using 2x 10m fiber masts. It is composed of 2x inverted L's facing each other with 6m spacing between the 7.4m vertical parts and 20cm separation between the 2 ends of the horizontal parts. We used two sets of 10x10m ground mounted radials for each monopole. As we did not know the "velocity factor"of the plastic insulated electric wire, we used the same technic as we used for constructing the wire beams at CN2WW for the ARRL. Suppressing the reflector on the model gives X=0 for 7.372Mhz.
The MMANA simulation, using MININEC "average" ground.
We cut the driven element accordingly, which gave us the velocity factor of the wire (0.96). The R at X=0 was measured at 33 ohms; exactly what was predicted by the model inserting a 1 ohm resistance, simulating the ground losses at the base of the monopole. We then cut the reflector taking into account the 0.96 velocity factor. After a few minor adjustments, consisting in inserting small pieces of wire at the base of the elements, we obtained X=0 for 7.025... this was kind of useless, as the bandwidth is extremely broad, as predicted by the simulation. In fact the only major difference with the model, was that the R value, was found at 50 ohms, and not at 36 ohms as predicted by the simulation using mininec ground, but it did not worry me, as I know that Mininec is not very accurate for the R-value as this calculation is done on a "perfect ground" basis.
The MININEC simulated SWR response. The very broad bandwidth (about 300kHz at 1.5), was confirmed on the real antenna. The impedance was found very close from 50 ohms on the real antenna.
On top of this, the practical antenna was somewhat different from the model, due to the flexion of the fiber masts (we had actually 6.5m between the 2 masts bases).
First "on the air" tests were done on the Russian C-beacon (Moscow) on 7.039 a the end of the afternoon; compared to the 16m vertical, tuned on 7Mhz, the difference was constantly over 2 S-points in favor of the half-moxon (about 6dB on the pro2 S-meter). We found this difference to be quite constant for all stations in the North sector, the difference becoming minimal on the EA stations located West of us.

MININEC simulated Gain and Front to Rear ratio vs frequency.
Our 40m score during the contest and the fact that we often won the "power battle" and got first in the pile-ups, confirmed that the antenna was working fine. The simulated gain using mininec average ground and 1 ohm resistor simulating ground losses, gives 3.3dBi at X=0 and near to 20dB F/B, the gain being -0.1 dBi for the radiator alone. This value might be a bit pessismistic, as the simulated gain for the radiator alone, using NEC2 finite ground and 10x10meters radials is about 1dB higher than the model using mininec ground. Unfortunately NEC2 finite ground does not allow to specify 2 sets of ground mounted radials with different centers. Later (on the ship) we simulated a NEC2 model using a Sommefeld-norton ground and 2 sets of 4 elevated radials, which confirmed an impedance close to 50 ohms, a bandwidth matching the measured one,a gain slightly over 3dBi and a F/B ratio of 25dB.
MININEC patterns from 6.8 to 7.25 Mhz.
Of course we could have gone for a 2 vertical monopoles passive array (driven+reflector), but the horizontal pattern would have been sharper and the array more difficult to match, due to the lower radiation resistance. We liked the cardioid pattern of the half Moxon and his very broadband characteristics, making the construction somewhat uncritical.
Dimensions (for 1.6mm diameter bare copper wire with vel.coeff 1)
and Fo= 7.025
vertical parts height: 7.40m
Driven horizontal part: 2.7m
Reflector horizontal part: 3.20m
Gap between the horizontal wires: 20cm
Separation between the verticals: 6.10m

Above, the NEC2 simulations using a model with 2x4radials, over a "Sommerfeld-Norton" average ground.


Anonymous said...

Would it be possible for you to send me the antenna modeling files for the 1/2 moxon. Tks. 73

Viliam OM0AAO OM0A said...

I have tried twice this antenna in two QTHs. The SWR is really flat and I found some directivity (5 to 10dB) but no gain.

In first QTH I have only 8 radials, in second 20 radials.

Comparing to the dipole 9m above ground there wasn't any (local or DX) station louder.

Don't know why...