Posted by
Don Wilhelm-4 on
Jan 01, 2012; 1:39pm
URL: http://elecraft.85.s1.nabble.com/Single-Band-40-Meter-Dipole-tp7141541p7141982.html
Phil,
What do you want to accomplish (other than just an antenna for 40
meters)? Lowest loss? Broadband? Easy- up? Etc? Those parameters will
determine your best choice.
Easiest IMHO is to just feed it with coax with a current mode choke at
the feedpoint. That should present a less than 2:1 SWR over the entire
40 meter band.
If you have an extremely long feedline (multiples of a half wave), then
you might consider using balanced line cut to some multiple of a half
wavelength (electrical length) and the rest of the way to the shack with
coax - put the common mode choke at the transition between the balanced
line and the coax.
This latter suggestion takes advantage of the fact that an electrical
half wave transmission line has the same impedance at both ends - it
does not matter what the characteristic impedance of the line may be.
But unless you have a very long distance to the antenna feedpoint, the
loss of practical coax runs is rather low on 40 meters, so the easiest
is just to use coax.
73,
Don W3FPR
On 1/1/2012 12:14 AM, Phil Hystad wrote:
> OK, lots of discussion on open wire feeders and other ideas so I thought I would throw out a question to soak up some of the Elecraft community knowledge and wisdom.
>
> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed configuration for single band usage to minimize loss and minimize common mode currents and radiation from the feed line?
>
> I am thinking of building such an antenna and I would have no plans for using it on any other band but I would like to use it on the full spectrum of 40 meters with primary focus on the low end for CW.
>
> My current idea is to cut the antenna for 7.1 MHz but I am not sure about the best plan to feed it. I am thinking of 450 ohm window line (aka ladder line) to a 1:1 balun and the coax the rest of the way into the shack. Or, how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way into the shack? Would this be a significant difference? Or, some other combination?
>
> Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?
>
> Any comments or suggestions?
>
> I am thinking of also raising up two other dipoles. I have room to put up 80, 40, and 30 as separate dipole antennas -- it is actually a little bit more awkward to do a fan dipole so I am leaning on not doing that.
>
> 73, phil, K7PEH
>
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