CROSSED YAGI for BAND II

For FM DXing in western Europe, it is very beneficial to be able to select between horizontal and vertical polarisation for the DX antennas. Vertical (VP) is needed for reception from Eire and Holland, and for many local stations as well as nearly all pirate transmissions. HP is preferred for other DX, because a) tropospheric refraction favours HP, b) local electrical noise is lower with HP, c) considerably better directivity is available when a small Yagi is used horizontally - just think about the directivity of a single dipole!


Many small FM Yagi's could be paired up for dual polarisation, but the Triax FM5 is recommended. I have found the 'straight' FM5 to be an excellent DX antenna, performing well throughout the band. For HP, the deep nulls at about 45 can be exploited to null out interference. The FM5 is reasonably strong, and includes a balun, desirable for weak-signal reception as well as for rejecting the unintended polarisation.

2 x Triax FM5

Made from 2 x Triax FM5 antennas (the 'straight' 5 element Yagi type, not the FM5 with dual reflectors)

Modification is trivial, needing only the bodging of a few 5mm holes. There is room on the boom to place one set of elements 17mm further forward and the other 17mm backwards. However, the minimal compromise in the position of the front director was considered unimportant, so I placed the VP elements 35mm forwards of the original positions and 'fudged' the front director.

Note that the connection box should be turned round for the VP section, looping the feeder round the adjacent director, and take note also to mount the drainage holes in the boxes facing downwards.

close-up

Close-up view of driven-element feeds, before making off the blue grommets for the two cables

As with any VP antenna, there is a conflict between the element positions and the supporting mast (and not forgetting the feeder). Cantilever mounting from the rear - fine for UHF TV - is not really practical for a Band II Yagi, but distortion of the directivity will be minimised if the boom is mounted at the top of the mast, and positioned with the mast about mid-way between the (VP) dipole and reflector. The resultant unbalance makes it essential to use an additional support bearing with the rotator.

The achievable directivity will be compromised by local reflections, from surrounding structures and local houses. It will probably be found that the ability to null out unwanted signals will improve with every additional foot of elevation above the roof level of the surrounding houses.

Two examples of this design, mounted on light-duty rotators

 

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JH Apr.2008. Last updated Nov.2010

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