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1/22/2020 12:12:56 PM
Posted: 1/19/2015 11:22:23 PM EST
a few months ago, somebody posted a tutorial on building an OCFD. Since I can only search back 30 days, would it be possible to point me in the right direction? or better yet, sticky that post?

thx
Link Posted: 1/19/2015 11:32:04 PM EST
Link Posted: 1/20/2015 12:09:03 AM EST
Theory, how and why:

http://www.ar15.com/forums/t_10_22/674537_Center_Fed_Dipole_vs__Offset_Center_Fed_Dipole_OCFD_aka_Windom__sic_.html

Construction:

Like this

http://www.ar15.com/forums/t_10_22/677525_.html

except use a 4:1 Current balun in the middle. Best, the Balun Designs 4115et (et means eyebolts)

http://www.balundesigns.com/model-4115-dual-core-4-1-current-balun-5kw-1-54-mhz/

And add wire similar to the centerfed dipole in the article above, except make them 41' & 25' for a OCFD for
40, 20, 17, 15, 10 meters. You will need a tuner to get 17 and 15 m.

Make the legs 93' & 39' for an OCFD for 80, 40, 20, 17, 15, 12, and 10 meters.

A cheaper balun, but will work,

http://www.packetradio.com/catalog/index.php?main_page=product_info&cPath=11&products_id=2478
Link Posted: 1/20/2015 8:49:02 AM EST
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Originally Posted By RyJones:
one of these?
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The sad part is that I've recommended this method to others who were in the same boat as me. Thanks.
Link Posted: 1/20/2015 8:50:35 AM EST
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Originally Posted By Jupiter7200:
Theory, how and why:

http://www.ar15.com/forums/t_10_22/674537_Center_Fed_Dipole_vs__Offset_Center_Fed_Dipole_OCFD_aka_Windom__sic_.html

Construction:

Like this

http://www.ar15.com/forums/t_10_22/677525_.html

except use a 4:1 Current balun in the middle. Best, the Balun Designs 4115et (et means eyebolts)

http://www.balundesigns.com/model-4115-dual-core-4-1-current-balun-5kw-1-54-mhz/

And add wire similar to the centerfed dipole in the article above, except make them 41' & 25' for a OCFD for
40, 20, 17, 15, 10 meters. You will need a tuner to get 17 and 15 m.

Make the legs 93' & 39' for an OCFD for 80, 40, 20, 17, 15, 12, and 10 meters.

A cheaper balun, but will work,

http://www.packetradio.com/catalog/index.php?main_page=product_info&cPath=11&products_id=2478
View Quote


Thanks Jupiter, I was pretty sure it was you. I found your "let's make a dipole" post, but not these.
Link Posted: 1/20/2015 4:18:13 PM EST
[Last Edit: 1/20/2015 4:20:55 PM EST by aa777888-2]
I just finished a complete rebuild of my OCFD over the Christmas holiday. My first attempt was pretty lame and if were not for an antenna tuner it wouldn't have worked very well. The rebuild has got the thing working much better After quite a bit of study, here is what I did for an 80M OCFD:

Step 1--buy a really, really good balun. The long answer: http://static.dxengineering.com/pdf/Choosing%20the%20Correct%20Balun.pdf. The short answer: http://www.dxengineering.com/parts/dxe-bal200h10at.

Step 2--ground the balun! That's what the ground lug is for! Honestly, it pains me to say I don't know why you need to do this, because normally I'm pretty smart about these things, but damn if I know! All I can say is without the ground it's junk!

Step 3--add a good ferrite choke on the coax adjacent to the balun. 6 or 8 Type 31 cores.

Step 4--put up more wire than you need on each end, enough so that you can move the feedpoint between the 20% and 35% points. Just roll up the extra wire in a small, 6" diameter loop at the ends. It will not have any effect on performance. Hang the ends so that you can lower them from the ground for easy, repetitive length adjustment. It needs to be easy, since you'll be doing it a lot during the tuning phase Since I use trees for my end supports I just use enough 550 cord to allow this to happen.

Step 5--throw away an preconceptions you have on how long or exactly what feedpoint you "should" have. There is too much variability associated with ground conductivity and whatnot for the "book" numbers to be anything but an approximate starting point.

Step 6--start with an arbitrary 30% feedpoint location and adjust the overall length, not the feedpoint, to achieve 1:1 VSWR at around 3600KHz for an 80M OCFD.

Step 7--take VSWR readings on all of the other bands other than 60, 30 and 12, those are throw-away's. Then start adjusting the feedpoint along the antenna without changing the overall length until 40, 20, 17, 15 and 10 all come in at under 2.5:1. There is a way to do this with an antenna analyzer that will shorten the process, sadly I don't have an analyzer and just did it by brute force.

Step 8--use your tuner to knock things down from 2.5:1 to 1:1. I'm using a remote tuner immediately adjacent to the balun (my feedpoint is the peak of my house) and this allows me to run 60 and 30 meters, although tuning 60M is very touchy. 12M does not work at all, though.

Step 9--enjoy your multiband antenna! I love mine and get the best signal reports of any wire antenna I've ever had, but I spent two entire days tweaking it. OCFD's are twitchy beasts. Of course like any long dipole it will generally work better on the lower freq's due to take off angles and all that. If you are brave, you can trim the excess wire now, but I left mine in case I ever want to re-tune it.

Link Posted: 1/20/2015 5:04:09 PM EST
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Originally Posted By aa777888-2:
I just finished a complete rebuild of my OCFD over the Christmas holiday. My first attempt was pretty lame and if were not for an antenna tuner it wouldn't have worked very well. The rebuild has got the thing working much better After quite a bit of study, here is what I did for an 80M OCFD:

Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Step 7
Step 8
Step 9
View Quote


You are coming to my house when I build mine for 160 meters.
Link Posted: 1/20/2015 9:51:32 PM EST
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Originally Posted By SandHillsHillbilly:


You are coming to my house when I build mine for 160 meters.
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Discussion ForumsJump to Quoted PostQuote History
Originally Posted By SandHillsHillbilly:
Originally Posted By aa777888-2:
I just finished a complete rebuild of my OCFD over the Christmas holiday. My first attempt was pretty lame and if were not for an antenna tuner it wouldn't have worked very well. The rebuild has got the thing working much better After quite a bit of study, here is what I did for an 80M OCFD:

Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Step 7
Step 8
Step 9


You are coming to my house when I build mine for 160 meters.



Link Posted: 1/21/2015 8:50:33 AM EST
Wait a minute!

There is a Windom Yahoo Group?

Can someone send me the link or post it here please?
Link Posted: 1/21/2015 9:41:10 AM EST
Step 2--ground the balun! That's what the ground lug is for! Honestly, it pains me to say I don't know why you need to do this, because normally I'm pretty smart about these things, but damn if I know! All I can say is without the ground its junk!
View Quote


I'm always the stickler for reality matching up to principle and theory and modeling, so don't let my grumpy-ness getcha down.

However,..... If the match changes or improves with the addition of a ground wire, that necessarily means that current is flowing on that wire, rendering all the modeling data mostly pointless! (And is almost certainly a source of inefficiency and noise ingress...)

If you are running a ground wire to the balun at the vertical section feed point then that means you no longer have a proper OCFD. Again, that's fine, ya gotta do what ya gotta do to get a match, but it means that something specific to your installation (some nearness to massive metal or a wierd resonating structure?) is throwing off the calculations.

So, it's fine if that wire works for you, but one should always start with the modeled data or the reference designs that others have posted above - chances are that those designs will work well and chances are that adding the ground wire will actually hurt the match.

Again, congrats on making your system work for you, but ham radio sometimes gets bogged down with patches and fixes that turn into internet legend that can do more harm than good (they often show up on *uhg* hamuniverse - the isle of misfit antennas). It's always good practice to stick as close as possible to good antenna principle and modeling, and track down the reason for the departure from the model or reference design before resorting to patches.
Link Posted: 1/21/2015 10:26:44 AM EST
@aa777888-2 - thanks for a bunch more stuff to do to get a better antenna.

As an aside - anybody have any bright ideas for the 'easiest' antenna setup for a QRP that does 40/30/20 CW?
Link Posted: 1/21/2015 11:03:47 AM EST
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Originally Posted By spclk:
@aa777888-2 - thanks for a bunch more stuff to do to get a better antenna.

As an aside - anybody have any bright ideas for the 'easiest' antenna setup for a QRP that does 40/30/20 CW?
View Quote


Check out these guys, LNR End Fedz. They may not have exactly what you want but they are end fed efficient antennas.
Be sure to follow the arrows through the available antennas.
73,
Rob
Link Posted: 1/21/2015 12:04:44 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By robmkivseries70:


Check out these guys, LNR End Fedz. They may not have exactly what you want but they are end fed efficient antennas.
Be sure to follow the arrows through the available antennas.
73,
Rob
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Discussion ForumsJump to Quoted PostQuote History
Originally Posted By robmkivseries70:
Originally Posted By spclk:
@aa777888-2 - thanks for a bunch more stuff to do to get a better antenna.

As an aside - anybody have any bright ideas for the 'easiest' antenna setup for a QRP that does 40/30/20 CW?


Check out these guys, LNR End Fedz. They may not have exactly what you want but they are end fed efficient antennas.
Be sure to follow the arrows through the available antennas.
73,
Rob


It looks like an option would be a 40M EFHW - this will work for 40 & 20. Then, add a link @ 30M Y/2 so disconnect part of the antenna.
But, this all seems to me that a clothesline reel with 66 ft of wire, reeled in to 30M and 20M lengths as needed would do the same thing........

Or, ~66' with breakable links @ ~33' and ~45' too.
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