My MiniVNA3 program shown in these images will be available for sale shortly.

The MiniVNA3 program is not compatible with the miniVNA Pro version as yet.

MFJ-1785 Rotatable multiband dipole antenna

Assembly

The assembly took two whole days. The instructions was a bit messy and not so simple and logical to follow. In addition, one plastic bag was missing so I had to buy some screw and nuts at my local hardware shop. I could have complained to MFJ to get it from them but that would have taken at least two weeks shipping from USA. There was also a missing aluminium latch to connect the 80m segment to the rest of the dipole (only one was supplied). This was neither a big problems, as I made one of isolated copper wire instead.

The spoke assembly was very tedious since there is so many of them and the whole spoke system was difficult to handle. The two spoke systems are like a Christmas tree!

 

Testing the antenna

I have a large enough elevated veranda / porch facing my back yard which runs on two sides along my house, so I decided to place the antenna on the railing to make the final adjustments.

 

Initial frequency response

The initial frequency and SWR response was like this (click on the image to zoom). Pay attention to the narrow 80 m band-width. As the MFJ-1785 overall length is 10.5 meters it's nothing more to expect from this antenna.

Fig. 1

 

 

80 meter band :

After adjustments, the 80m band is now starting to be acceptable with a lower frequency of 3.2 MHz and SWR showing 1:1.35. I had to remove 2 pair of spokes and and addition trimming of another two spokes.

Also note the extra "dip" at about 4 MHz !

Fig. 2

 

 

40 meter band

This band is wider in bandwidth, covering most of the 40m band without problems. The resonant frequency is now at 7.14 MHz with an SWR showing 1.17:1 which is very good.

 

 

20 meter band

This band needs an additional trimming to move the lowest dip of these two to be acceptable. I have also ticked the checkbox to show the series resistance Rs in this diagram, which shows that the resistance is practically 50 Ω at each resonant frequency, and hence the impedance is 50 Ω as well since |X| is examined to be almost 0. The series resistance between the bands is in the order of 160-300Ω.

The MFJ-1785 is a full length dipole on the 20m band.

Fig 4

 

 

Conclusion

This antenna performs about 3-6 dB better than my trap 5-band Diamond W-8010 does, even at the test site on my veranda. The signal/noise ratio is about 2 dB better than W-8010 at the test site.

The MFJ-1785 is rated to 1.5 kW, which should be plenty enough working with a 1 kW linear amplifier.

In the end it will replace the W-8010, hung between two 10 meter (=30") Spider Beam masts. The distance between the masts will be approximately 6-7 meters (= 20-23") to support this antenna. A problem with my W-8010 is the sag, since this antenna is not a rigid one. The two Spider Beams are actually 12 m tall but those upper 2 meters (=6") are too weak to support a 15 kg rigid MFJ-1785.

A review of this antenna may be found on /www.eham.net/reviews/detail/6695 .It seems that I am not the only one to complain about missing parts.

 

June 11, 2011

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