[TowerTalk] Baluns...again.

[Roger (K8RI)]

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
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>