Best way to feed off-center fed dipole?

[Highlander_821]

I want to build an off-center fed dipole for 80 meters and up. I will be using an LDG AT100Pro II automatic tuner.

Possibilities I have thought about are:

a) feed with coax with a 4/1 balun at the feedpoint.

b) use coax just to get outside the shack and place a 4/1 balun there and go the rest of the way to the antenna with 300ohm ladderline. Maybe with this product: http://www.ldgelectronics.com/c/252/products/2/13/1

Thoughts?

 

[543_Dallas]

An off center fed antenna is not a balanced antenna so I wouldn't feed it with twin lead. The more the load is unbalanced the more radiation you'll get from the feedline.

I think you'd be better off with the balun at the feedpoint and running coax.

 

[camaro1]

feed it in the center with your 300ohm twin lead all the way to your tuner and have a 80-10 dipole

 

[Highlander_821]

feed it in the center with your 300ohm twin lead all the way to your tuner and have a 80-10 dipole

The only problems I see with that are for one, the LDG tuner has only a coax connector for antennas, so I need some way to transition to the ladder line; and two, I need to get out of the shack without the twin lead getting disturbed by any other feedlines like the 4 runs of coax already exiting the shack through a feed-through, no? In other words, the twin lead isn't going to like being all packed up against the coax in that feed-through, right?

 

[firerunner]

Do as Camaro said use a balun with short piece of coax. You will be glad you did.
Buy this kit if you still need to get you antenna parts, 10.00 extra and you have extra wire to make another antenna.
https://www.dxengineering.com/parts/dxe-wa-260

 

[KC9Q]

Feed the OCF with a 4:1 Current Balun made using two cores. This is made by winding two ferrite cores with 100 ohm, 1:1 windings. Wire one side in parallel (100/100 = 50), and the other side in series (100 + 100 = 200). This is the best balun choice to reduce Common Mode Current (CMC) from coming down the transmission line. Don't use a voltage balun as this will cause RF down the transmission line and will require a 1:1 isolation transformer to block it. (This is the Carolina Windom design). Like others have stated Do Not use twin lead/open wire line as this will also lead to RF down the line.

The only question is what split you want? The typical (common) split is 33%/66%; which will give decent VSWR on 80/40/20/17/12/10, and the lower part of 6 meters, (notice that 15 meters is out since it is not "harmonically" relayed to 80 meters). It may sound strange, but make the antenna resonate around 3.55 MHz -- this will provide low VSWR on the other bands. There are other splits, but they will just add some bands and eliminate others. You will have to google those.

73,
Mike, KC9Q

 

[Highlander_821]

I'm still wondering how to get the 300 ohm line out of the shack (assuming I go with Camaro's suggestion to feed the 80 meter dipole in the center).

Am I right to think that pushing it through the existing 3" PVC pipe that already has 4 runs of coax going through it is going to be a problem? (Not physically, I'm sure there's enough room to get it through, I mean RF-wise). I really don't want to put another hole in the house if I can help it.

 

[Robb]

600 ohm real ladder line with a 10-80m dipole and 4:1 balun.

 

[Road Squawker]

I'm still wondering how to get the 300 ohm line out of the shack....

run a short piece of coax thru the wall and use something like this outside

https://static.dxengineering.com/global/images/prod/norm/pas-b4k-4-1_yj_m.jpg

 

[Highlander_821]

run a short piece of coax thru the wall and use something like this outside

https://static.dxengineering.com/global/images/prod/norm/pas-b4k-4-1_yj_m.jpg

I think that's what I want to try doing. I only need about 10 feet of coax to get to the outside, and, once there, I can mount that balun or one like it where it will be out of the weather.

 

[543_Dallas]

I'm still wondering how to get the 300 ohm line out of the shack (assuming I go with Camaro's suggestion to feed the 80 meter dipole in the center).

Am I right to think that pushing it through the existing 3" PVC pipe that already has 4 runs of coax going through it is going to be a problem? (Not physically, I'm sure there's enough room to get it through, I mean RF-wise). I really don't want to put another hole in the house if I can help it.

My solution was to build a tuner and park it at the base if the tower. Having a true balanced tuner like a balanced L or link coupler will serve you better than an unbalanced tuner with a balun on the output. It's not as complicated as it sounds at first either.

For what you pay for a tuner and balun from the store you can build a bullet proof tuner that will match anything and have low loss.

I do have to go outside do switch bands because I used copper tubing instead of roller inductors to save money. The tubing will handle a lot of current too. A cheap gear reduction motor on the variable cap let's me fine tune it from inside.

If you go that far built the ladder line and doublet from the same length of wire so there's no splices. #8 or #10 thhn and 4 inch electric fence spacers works great. If you're not running legal limit AM through a short antenna smaller gauge wire is more than adequate.

 

[Road Squawker]

The real question is, Why do you want to build an OCF instead of a doublet?

 

[Highlander_821]

The real question is, Why do you want to build an OCF instead of a doublet?

I'm actually now leaning towards tje center fed doublet for 80 meters fed with 300 ohm line and connected to my LDG tuner with a balun and short length of coax to get inside with.

 

[543_Dallas]

I'm actually now leaning towards tje center fed doublet for 80 meters fed with 300 ohm line and connected to my LDG tuner with a balun and short length of coax to get inside with.

Good man. I'd use a robust 1:1 current balun in that case.

 

[camaro1]

I have a 80m doublet fed with 600ohn "true ladder line" I put up last year and followed all the guidelines for running open wire feeder right into the house and to my johnson matchbox and it works great,,,, so a couple months ago I decided I wanted another "ladder line" fed antenna for another radio here, I had a bunch of wire and some 300 ohm twin lead, I will say on this one I broke most of the "rules" when it comes to running ladder line style feed line. I have the 300ohm twin lead feeding a 40m dipole about 18-20 feet off the ground in the back yard, the feed line then enters the house where I used to have a coax running in, it goes into my basement and runs along other electrical wires and through all the holes in the floor joists with the other house wiring, then it runs along the heating duct work for a little ways then through the floor to behind my radios and to another Johnson matchbox or the MFJ-974hb. guess what,,, it works great, no problem making contacts on 40-10m , even qrp with my Bitx40 at 4-5 watts

No RF in the house on either the 80m doublet fed with 600ohm wire or the 300ohm twin lead 40m dipole,, just amazing how well they work

now a little bit about tuners,,, I have been playing around with tuners for the ladder line and open feed line antennas, I have a few

*Mfj-986 with built in 4:1 current balun,, this one worked fine tuning all bands and seemed to work fine, just takes a long time to change bands with the roller inductor, had to spin it forever to get from one end to the other

Dentron Super Tuner with built in 4:1 balun, worked ok , faster to tune when changing bands but I did have some feedback on some of the bands in my computer monitor

then I started doing some looking into a true balanced line matchbox(tuner) and picked up a couple of the Johnson Matchbox's the design doesn't use any balun and is a true balanced tuner,,, this is a keeper if you can find a good one, I have a few fully functioning ones and a parts one.. these work great, easy fast to change bands ,, only thing they only cover 80,40,20,15,10 bands

the modern version of the Johnson Matchbox is the MFJ-974hb , it is based off of the Johnson matchbox but uses a 12 position inductance switch so you can tune all the bands, im using this one now with the 40m dipole with 300ohm twinlead


so you never know,, just run the 300ohm twinlead into the house and to the tuner and give it a try,, I was really surprised on how well it works even though I didn't follow any of the "rules" on that one.