[SixClub] dipole orientation?

[Christopher Boone]

Bill et all,
it's time for antennas 101....

A dipole cut for normal 1/2wave resonance has a normal or typical 
feedpoint impedance of 75 ohms...(give or take depending on height above 
ground in wavelengths of freq in question).

Now looking at what you describe below, I have some issues...

1) 1/2wl dipoles are 75ohm....Your feedline is 450 ohm...see a problem 
here? HIGHER SWR means higher loss especially on 6mtrs...coax with a 1:1 
balun would be BEST in this case

2) use of a tuner is ok on lower bands....at 6mtrs, I HIGHLY discourage 
a tuner (Actually I despise them on 6 :) ...WHY? Tuners tune all the 
feedline and antenna combo to make it look like 50ohms at the 
radio....BUT this means the SWR on the coax is probably much higher than 
without the tuner in the 1st place...now on HF, this is not a problem as 
additional loss in coax due to SWR is low..
on 6 though we are now in the VHF range...the rules are different here! 
Best to tune the antenna at the antenna.....the rest of the system 
should be a pipe down to the radio (50ohm pipe if possible but again, 
this is a dipole...thats 75!)

3) the use of 450 ohm line can be beneficial if the tuner has a balanced 
output and thus this can reduce feedline radiation...but again, at freqs 
above 28Mhz, I don't care for open line feed anymore.....its bulky, hard 
to handle...and coax is easier to deal with...and lower loss at higher 
SWR levels on VHF.

All this is probably the cause of the antenna not matching as you hoped 
it would...going to coax feed and a 1:1 balun (you could easily make a 
choke mode balun or use clamp on ferrite beads near the end of the coax 
at the antenna to do this! and it will work on 6 as well!)

Also a 17meter dipole will resonante at 6mts (3rd harmonic)...so will 
80/75 mtr dipoles depending on their cut....a multidipole antenna works 
in theory AND if the lower freq wires are not resonant at higher freqs 
as well (thus the 15mtr wire is not needed since the 40 will work there 
as well!) but if you forget about certain harmonic issues, you can get 
bit (as you are evidently finding out; also a 10Mhz antenna will be on 
its 5th harmonic on 6m....another low Z point for the antenna...remember 
EVERY ODD harmonic is a match!)


Chris
WB5ITT


William Baker wrote:
> 
> Dave,
>      This is a dipole.  This is an 80-40/15-30-17-10-6 meter resonate
> dipoles with a single 450 ohm feed from a tuner. Each dipole is cut for
> the center freq of it's band except for the 40/15 since 40 and 15 are
> harmonically matched. 
> 
> The way this thing is suppose to work is that only the leg that is
> resonate will actually radiate.  I am almost sorry I started this
> project.  I am finding out that I will need to stabilize the spacing of
> the wires. So I have to hit homeless depot for some wood dowels and the
> Agway store for some more insulators.  HINT!!! You can use Fence
> insulators in place of dog bones. A bag of 10 was $2.79..!
> 
> If my friend's modeling is correct @ 90' this should play very well. I
> am just a tad leery of the sway in the trees. But I think I have that
> solved too.  I have weighed all the components and then added 20 lbs to
> each end and will let the whole thing float in the breeze. The 80m leg
> is the main support and is #10 wire. all the others are #12.  
> 
> Bill
> kc2lix