http://elecraft.85.s1.nabble.com/Single-Band-40-Meter-Dipole-tp7141541p7142311.html
Thanks for all the responses both on-line here in the group and off-line. Several people pointed me to Jim Brown's (K9YC) paper, including just a few minutes ago, Jim Brown himself (thanks).
I grabbed the paper last night but was too tired from a half-bottle of champagne to do anything but briefly scan the parts about the common mode chokes. Also, I found a power-point talk by K9YC that I will also go through this morning.
So, I am pretty sure I will end up doing the Jim Brown solution. That would be an RG8X feed using one of his common mode toroid chokes (though, I do need to read the paper in detail). I guess finding a source for the ferrite toroids is next on my list although I think I saw one referenced in one of the responses (digikey).
I suppose best placement for the choke is at the center feed point of the dipole but I am a little concerned about the weight. Maybe this is not an issue. By the way, I make my own center insulators with nice screw on brackets and SO-239 connectors. The plastic I use is very tough and I buy it from Tap Plastics.
> Don,
>
> goals:
>
> 1. Lowest loss.
> 2. Full 40-meter band coverage but resonant at 7.1 MHz.
> 3. Assume using a tuner (like K3 or KAT500).
> 4. Choke common mode currents.
>
>
> Total feed line length will be on the order of 100 feet for this particular dipole. For the 30 meter dipole I also plan to put up, total feed line length is about 75 feet. My 80 meter dipole will be about 100 feet of feedline.
>
> Before I posted this question to this forum, I would have gone ahead and used the same configuration I used previously when I had my 80 meter doublet up. That antenna was fed by 450 ohm ladder line to a 1:1 balun and then RG-8X to the shack.
>
> But one motivation for my question is weighing the difference in loss of tuner + 450 ohm ladder line and balun versus a direct run of RG8X right to the feed point with a choke balun at that location.
> The RG-8X loss is about 2 dB on that 100 foot run. I could switch to LMR-400 (at 0.7 dB loss) but I am curious about other solutions.
>
> 73, phil, K7PEH
>
>
> On Jan 1, 2012, at 5:39 AM, Don Wilhelm wrote:
>
>> 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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