Over the last couple of weeks there has been an interesting discussion on rsgbtech@yahoogroups.com (
http://groups.yahoo.com/group/rsgbtech/) on the subject of T&K brackets. The simple question that started the thread was which way round to the two brackets go?
Although one normally sees T&K brackets mounted with K at the bottom, I pointed out that there was some useful information here:
http://www.aerialsandtv.com/polesandbrackets.html#PictureOf12&24T&Ks , which says :
There are two opposing views as to which should go at the top. The K is obviously the more robust bracket and the received wisdom is that this should go at the bottom because, being further down, the wall is stronger there. However the top bracket is probably under the most stress (certainly if the wind is blowing the pole away from the wall) and so some think the K should go there instead ! Basically one has to work out the relative importance of the two factors for the particular install being undertaken. For an install at the peak of a gable I would put the K at the bottom, whereas on a strong flat top wall I`d put the K at the top. Basically it`s six of one, and half a dozen of the other, but,if in doubt put the K at the bottom More information can also be found at
http://www.wrightsaerials.tv/Sound reasoning for the K at the bottom model was provided by Graham G8EWT who said:
The reasoning was that, as has already been stated in this thread, it is the stronger of the two and is better able to carry the vertical load. Hence the U bolt needs to be tightened really well. A tightened bolt reduces the chances of the pole rotating or sliding down but risks deforming the circular shape of the pole and therefore reduces its ability to withstand sideways forces without buckling. As the K is carrying the vertical load the function of the T is simply to provide resistance to lateral forces. It only needs to be moderately tightened and thus the risk of deformation is eliminated.At this point in the debate I remembered a note on the subject written in March 1984 by Pat Hawker drawing attention to BS6330: Code of practice for reception of sound and television broadcast, which has some useful information on the subject. However I had long since lost track of my copy of the text.
Ian GM3SEK noted:
It would be worth tracking down, because that guide was written for Band 1, 2 and 3 aerials which were very similar to yagis for the VHF and UHF amateur bands. That isn't so true of modern broadcast aerials. One tip that you won't see in a British Standard is to insert a length of wood where the tube passes through the top bracket, which is the point of highest bending stress. The wooden filler prevents the thin-walled tube from collapsing, and spreads the bending stress away from the U-bolt. For the typical 50mm/2in tube, use a 600mm/2ft length of 37mm/1.5in square timber with the corners shaved off to make a rough octagon shape. Almost any kind of straight-grained timber will add a lot of strength. Do not aim for a tight fit - make it just firm enough to hold in place when you install the mast, and after a few days outdoors it will swell to a tight fit. (The octagon shape is actually better than a turned cylinder because it leaves room for the wood to adjust as it swells - and very importantly, it leaves gaps for water to drain past.)Unfortunately BS6330 is no longer available, searched my study and tracked down my copy of BS6330 in box of papers, unopened since we moved house 8 years ago. I found quite a few other "treasures" on the way.
It does not have quite as much information as I remembered, but does give some information on types of loading (in terms of antenna type) mast length and dia, spacing of fixing etc. There is a lot of information on distribution systems which is not relevant to the subject in hand.
As to TK, although it always shows T over K in the drawings, all it simply says "where double brackets are used for all mounting one should be a tripod in order to provide proper support
against vertical thrust. The brackets should be spaced at least 450mm apart.
Since most of the text in BS6330 is not applicable to our interests. The key part, which contains valuable information relavent to the type of small to medium sized VHF and UHF amateur radio antennas has been posted here:
http://carc.org.uk/carc_ftp/Extract%20from%20BS6330.pdf.
- Stewart/G3YSX
