Wednesday, June 25, 2014

The Moxon-Beam


The Moxon-Beam was introduced by L. Moxon (G6XN) in his book "HF Antennas for all Locations" (RSGB- Publications, Great Britain 1993). This beam is a 2-Element-Yagi with radiator and reflector and reduced size to about 75% of a normal beam. The 2-Element-Yagi with reflector has normally a 0,2-lambda-boom and an impedance of 50 W. The Moxon-beam has a 0,18-lambda-boom and still 50 Ohm. This is a good impedance for wire- beams.The ends of the two elements are bended backward (radiator) or forward (reflector) and act as a capacitive load. That is much better than inductive loading with coils. So we have greater bandwidth and lower losses.Through the reduced size we get a 0,5-0,7 dB lower gain than with a fullsize beam.

This type of a 2-Element-Yagi has an unbelievable F/B-ratio on the design frequency of >= 30 dB. That is higher than with any other 2-Element-Beam.

The gain is higher on the beginning of the band and lower at the end. The bandwidth for a SWR < 1,5 is great enough for the range of 28,0-28,7 and 21,0-21,45 MHz if the beam is built up with aluminium tubes. Wire-beams of the Moxon-type have a smaller bandwidth.

The design frequency should be for a frequency 1/3 from the beginning of the band, because the SWR raises more below the design frequency. For example look for the SWR of a tube-Moxon for the 15-m-band:

Designing a Moxon- Beam is very easy with a useful little program by D. Maguire, AC6LA with the name "Moxgen".

This freeware can be downloaded at:

Moxgen generates an output file for "EZNEC" for modifications (e.g tapering) and shows you the dimensions for  building a wire-Moxon.



This ATU was designed in about 1990 to allow QRP rigs to be used with portable antennas. The unit had to be small and light weight. Generally I am against using ATU's at all regarding them as a cop-out for poor antenna design. However, when portable operation is considered, it is not always possible to erect the ideal antenna. This ATU was designed with idea of achieving a better match to an antenna that was nearly right.

A later version was tried which had an additional coil switched in and also a balun to accomodate twin feeder. I was never happy with this version, as it didn't seem to work as well as the simple version. When it was deconstructed in 2007 to do some loss measurements, it was discovered that a constructional error during the mod may account for this.



Hi to all! Today I found a very interesting project that using a small whip for hear the long VLF waves. Adrian Knot says that with this simple and inexpensive system you have good results, but suggests the using outdoor far from electric sources.
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