Soon the winter comes. I would like to build a Moxon antenna for the 20 meter band. Let’s say I put the antenna only about 2 meters above the ground. Will that be a problem or should that work?
a moxon antenna is a good beam, but for 20m it’s a beast.
Do you intend to put it horizontal at 2 meters above ground? It that is the case you’ll need to adjust its lenght as it will probably become detuned.
The other effect will be a different pattern , probably loosing a low elevation radiation, I guess.
I used it for 20m portable but I managed to put it vertical, on a 10 m pole and worked great. See it here:
I imagine you want to use 4 short poles for your model, but at that low height I don’t think it will be performing like it should.
P.s. I used a vertical Moxon portable for 15m and had a great result.
I also built a horizontal Moxon for 10m, quita small and could hold it by a medium size pole with good results as well.
They both are smaller and feasible for portable use.
I suggest that at 2m above ground, it would require readjustment from its nominal lengths further above ground, as stated by @EA2BD above. In addition, due to the low height, ground losses will reduce its potential gain. It would still have directivity, but I would think a quarter wave vertical with elevated radials would outperform it and not require rotation.
It would be a useful experiment to demonstrate what behaviour it would have at that height. Others would benefit from this experiment.
73 Andrew VK1DA/VK2DA
I am a big fan of Moxon antennas, having made units for portable use at 10m, 6m and 2m.
However I have found them not easy to tune if the dimensions are not quite right; changing wire lengths or tip spacing means changes to the supporting frame.
Perhaps for a low-level directional antenna, a yagi might be mechanically simpler to change wire lengths, and inter-element spacing.
In any case, there is information available on the behaviour of a dipole antenna at low-levels. It has been published by Rudy Severns N6LF in the context of measuring soil conductivity at an antenna farm. “Determination of soil electrical characteristics using a low dipole” QEX, Nov/Dec 2016, pages 5-8, available on-line at Antennas By N6LF: Determination of Soil Characteristics Using a Low Dipole
My interest in this topic is the nature of the ground in the Rocky Mountains. As suggested by the name, there is often very little soil on a pile of rocks! Talk about “poor quality” ground! A low-level directional antenna set-up at the top of a cliff might just provide good performance at low cost, volume and weight.
I did make some measurements using the wire harness from a centre-fed inverted-V, supported on a 6m pole. I rigged it as a horizontal dipole supported different distances above the ground, here in town (mediocre soil quality).
Height above ground, Frequency of Min SWR, Impedance
2.2m, 13.875 MHz, 50.4 + 1.7j
1.7m, 13.870, 50.5 + 0.6j
1.1m, 13.860, 55.8 - 1.8j
0.6m, 13.655, 78.9 + 7.0j
0.31m, 13.186, 84.9 - 4.7j
So in my opinion, definitely an interesting suggestion to look at the performance of a low-level directional antenna.
I look forward to hearing more information.
Very interesting data, Ian. The antenna handbooks suggest that the actual radiation resistance of a dipole goes down like a lead balloon when it is closer to the ground. Your data showing the impedance rising is an indication of the loss resistance in the ground, so a rough approximation to the efficiency can be made by subtracting the theoretical impedance from the measured impedance to arrive at the loss resistance.
At heights below 0.2 wavelength the impedance should be down below 20 ohms, so at 50 it is already no more than 40% efficient. The 0.31m height (approx 1 foot) would be even lower efficiency. Good move to make those measurements.
Some idea of the efficiency of a low rotatable (loaded) dipole can be obtained from the users of those antennas. at 12ft agl it is a sad antenna on 40m and not much happier on 20m.
I’ve not worked any activator from the US using a rotatable (loaded) dipole at 12 ft agl. I’ve heard their chasers but never the activator.
73 Andrew VK1DA/2DA
Agreeing with the others, a Moxon for 20m for portable operation will be very large and complex to set up.
With the higher bands now opening more often perhaps a Moxon for 15 or 10 metres would be a better choice?
I have yet to test them outside of the home garden but I have built vertical wire moxons for 15 & 10m that are supported by two masts, one of which is moved to change the direction of the antenna.
Good luck with your efforts.