On 12/28/2010 10:04 AM, Jim Thomson wrote:
> Looking through some old notes plus the info on Jim Brown's site.... there
> was a question about using
> an electrical 1/4 wave or electrical 3/4 wave of coax. The 1/4 [3/4]
> wave of coax was run between the
> DE of the yagi and the tower... with the braid of the coax bonded to the
> top of the tower.
As a personal opinion and in practice, I do not like to use stubs unless
there is a necessity for something being frequency specific. Any other
time I'd use one of the current baluns in Jim Brown's tutorial which are
broad band, efficient, relatively small and weigh less than a quarter
wave stub on HF
73
Roger (K8RI)
> The theory is... if a 1/4 [3/4] wave of coax has it's braid bonded to the
> top of the tower... the Z at the DE
> should be sky high..and there should be no requirement for a balun.
> Apparently there was an article out
> about doing just that. The ant was a quad... and individual 1/4 wave [ and
> 3/4 wave] pieces of 50 ohm coax
> were brought to a remote switch box at the top of the tower..where of course
> all the braids are bonded to the tower
> top. The author claimed that no baluns were needed on the various loops
> used on each band. I have not seen the article,
> but it was brought to my attention by a local ham.
>
> W8JI also mentioned the use of 1/4 and 3/4 wave lines from DE to tower
> top...and bonding the braids to tower top. Seems
> to me that W8JI mentioned that if a balun was used in conjunction with the
> 1/4 [3/4] wave coax, that the balun would makes things
> worse. There was some discussion on one of the other refelctor's years ago
> about the 1/4 and 3/4 wave concept. It had something do do
> with the height of the tower. If the tower was not the 'correct height'....
> the concept would not work. Does anybody remember this ?
>
> In one of Jim browns write ups.... there is a depiction showing feedlines
> of various lengths... from 0 up to... but not quite 1/4 wave. Then
> the chart starts up again... just a bit longer than a 1/4 wave..... and up
> to... but not quite a 3/4 wave. Then it starts up again.. just past a
> 3/4
> wave. OK, what gives? Is an electrical 1/4 wave and odd quarter waves
> the real answer to a high Z on one band ? In the case of
> something like monoband yagis.... this may prove to be a huge benefit... if
> the theory actually works. If I remember correctly, the main coax
> braid was bonded to the base of the tower as well. IF the theory does
> work... and a high Z is presented to the DE... how high is this Z? Is is
> mainly reactive... mainly resistive..or a bit of both ? Why would a bead
> balun or torroid balun, when used in conjuntion with a odd qtr wave
> of coax makes things worse ?
>
> later........ Jim VE7RF
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