[Antennas] Question reg. multiband dipole - balun

[Charles Greene]

At 04:26 AM 2/21/2002 +0200, Deon Erwin wrote:

Deon and Christian,

As a general rule, the use of coax in a non-resonate antenna has problems, 
some of which can be attributed the lack of the antenna tuner to have 
sufficient impedance range.  However, there are exceptions.  I currently am 
using two non-resonated antennas and both have coax as part of the feed 
line.  One is the G5RV ;which is a non resonate dipole and in the 
configuration designed by Varney, it has 30' of twin lead from the antenna 
then coax to the antenna tuner.  The other is the W3EDP antenna, which is 
85' wire and a 17' counterpoise.  I added two additional counterpoises, one 
for 80 and one for 10, and put a 4:1 balun at the intersections of the 85' 
wire and the three counterpoises and ran coax to the rig.  It tunes great, 
and the balun has very little loss, so most of the power  is getting to the 
antenna.  As I said in a previous post, put a 4:1 balun at the feed line 
before it comes into then run coax to your antenna tuner.  You have to make 
sure the length of 1/2 of your antenna (if a dipole) plus the open wire 
feed line is not a 1/2 length on any operating frequency at the point you 
install the balun.  Otherwise, you have a current node at that point and 
the balun will not work and could burn up if you give it any substantial 
power.  You can even use these antennas by connecting the coax directly to 
the feed line at the above two points and dispensing with the 
balun.  That's what Varney recommends.  On 10, the G5RV  twin wire feed has 
a very high impedance point and that would cause problems for any type of 
balun.

  One thing to try if you have trouble sorting out who is right and who is 
not quite right, is to put up a one of the well documented and extensively 
used antennas like the G5RV antenna or W3EDP antennas that have a length of 
coax  and then a length of open wire feed line to the antenna.  The proof 
is in the doing.

Nice chatting with you,

73,  Chas, W1CG

>Hi Christian
>
>You are bound to hear many conflicting opinions on the subject of balanced
>feedline versus unbalanced feedline and antenna tuners.
>
>The golden rule is that one cannot tune a non-resonant antenna with an
>antenna tuner via any length of unbalanced feedline.
>
>In other words, you cannot tune your dipole as a multiband antenna if any
>part of the feedline is coax.  Your dipole must have a balanced feedline all
>the way from the tuner to the dipole.  The person who told you not to add
>coax is correct.
>
>But, how to solve your metal window frame problem?
>
>Replace a window pane with Plexiglass or similar transparent plastic
>material.  Construct a waterproof feed through the plastic window pane,
>which is similar in dimensions to the balanced feedline, and use that
>feed-through to take the balanced line to the tuner on your desk.
>
>As long as your window pane feed-through is more than about 300mm from the
>metal frame and your balanced line has a clear run to the indoor tuner, your
>antenna should work well.
>
>Regards
>Deon ZR1DQ