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Author Topic: Off-Center Fed Dipole question  (Read 4341 times)
K6RQR
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Posts: 364




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« on: September 15, 2017, 05:19:48 PM »

Hello All -
 Last year I put up a homemade OCF antenna and had very good luck with it on 80-6 meters. The feedpoint is 33 feet off the ground and I am using a 4:1 current balun which is connected to an 88 foot piece of wire on one end and a 44 foot piece of wire on the other end. It's the classic dimension for the OCF. I was using an Icom 756Pro with an internal tuner and it worked very well with low SWR on all bands, even 6. I recently sold the Pro because I will be needing a smaller rig for HF and also so I could go mobile with it in my small car. I chose the Yaesu FT-891 and have been trying it out on the various bands using this OCF. The results are mixed. On 20 and above the SWR is good - 1.7:1 or less. However, on 80 and 40 the SWR is most acceptable on the higher parts of both bands but still over 2:1. It would seem that they are not long enough but if I added a few feet to each end I'm concerned I will negatively effect the other bands.
 I know that I could deal with this by getting a modest antenna tuner and I'm looking at a few of them to buy but I was also wondering if anyone had any ideas about what I could do to make the antenna work better on 80 and 40? Thanks.

73,
Bruce  K6RQR
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K6RQR
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Posts: 364




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« Reply #1 on: September 15, 2017, 06:20:31 PM »

 I wanted to add that I just looked at the SWR again and I have to correct something. 40 is actually pretty good with an SWR near 7,290 of 1.3:1 and rising to about 2:1 on 7,130.  However, 80 meters is basically 3:1 or more across the whole band. It looks like the most adjustment, if possible, will have to be for 80 meters.

73,
Bruce  K6RQR
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KE6EE
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Posts: 1855




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« Reply #2 on: September 15, 2017, 06:31:10 PM »

Evidently you were using a rig with an antenna tuner and you are now using a rig without one.

What you do depends on what you enjoy doing. If you have an antenna analyzer you can spend a day or two trimming the antenna and adjusting the length of the feedline so that you get an acceptable SWR. Or not since it may not be possible given the limit of the OCF design.

Without an antenna analyzer you might allow another day or two.

You could also redesign the antenna so that you are feeding it at another point. The off-center feed can be at 20% or 25% as well as 33% of overall length.

Going in this direction may not bring you the results you want.

Another alternative is to buy an autotuner which connects to your new rig and operates similarly to a built-in tuner. That should work as well as with your old rig. Might cost you a couple of hundred dollars.

A third is to buy a good wide-range manual tuner. Lots of used ones out there at reasonable prices. Will allow you some room for further experiment with antenna design. That would be my kind of fun. I like turning knobs.
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K6RQR
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Posts: 364




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« Reply #3 on: September 15, 2017, 07:49:20 PM »

Hello -
 Thanks for the reply. I guess the best solution is to get a good basic manual tuner and, as I mentioned, I'm looking at a few. The OCF dipole has already shown how well it worked with the limited internal tuner of the 756Pro.

73,
Bruce  K6RQR
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K0UA
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Posts: 1380




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« Reply #4 on: September 15, 2017, 07:52:21 PM »

I shortened the short end and lengthened the long end to get the SWR down on 80.  I was able to achieve 1.2 to 1 at 35 feet high with about 39 feet on the short end and over 90+ not sure exactly how much on the long end..  But it the 40 meter doesnt get below 1.8 to 1.  And it was better before I started chopping and adding,  but I wanted 80 to be better so I am accepting it.  You really need a cheap auto tuner.  I use the LDGz11pro with my FT891.  It works well for what it is.
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K4SAV
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Posts: 2389




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« Reply #5 on: September 16, 2017, 06:49:39 AM »

Most 80 meter OCF antennas will have an SWR of 3 or more on 80 meters.  The 4 to 1 balun causes the feedpoint impedance to be too low on that band, although that balun helps a lot on the other bands.  A few manufacturers of OCF antennas have begun selling OCF antennas that use a voltage mode balun.  That allows common mode currents to exist on the feedline and with the right length of feedline you can get a low SWR on 80.  That comes at the expense of lower gain and maybe some RFI problems.  Most people can't measure the gain and they just wonder why they have RFI problems, but they can see the SWR and that makes them happy.  That's probably why manufacturers are building them like that (along with the fact that it is cheaper to build a voltage mode balun than a current mode balun).

Jerry, K4SAV
« Last Edit: September 16, 2017, 06:52:51 AM by K4SAV » Logged
K3LRH
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Posts: 87




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« Reply #6 on: September 16, 2017, 02:30:35 PM »

...have you tried 90/45?  That works for me with a 4:1 current balun.
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K6RQR
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Posts: 364




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« Reply #7 on: September 20, 2017, 11:34:20 AM »

Hello All -
 Thank you again for the helpful comments. I seem to remember that the man who works at Balun Designs recommended the same thing - to increase the long end and shorten the other end. I just swapped the FT-891 for a transceiver that has an internal tuner with the usual 3 to 1 range. I'll see if that works on 80 meters and, if not, I will start to experiment with the lengths of the OCF.

73,
Bruce  K6RQR
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KC4ZGP
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Posts: 1637




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« Reply #8 on: September 20, 2017, 12:04:50 PM »


That's the spirit, experiment.

And I'm sure you and K0UA have many FT-891 stories to swap. He's Mr. FT-891.

So I'll back outta here.

Kraus
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WI9MJ
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Posts: 10




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« Reply #9 on: September 20, 2017, 12:27:29 PM »

I have used a similar OCF for many years with both a manual and LDG 100 watt autotuner. It works very well on both amateur and MARS frequencies above 2 Mhz.
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KC8Y
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Posts: 469




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« Reply #10 on: September 20, 2017, 05:05:51 PM »

Have an OCF 7-band (80-6m) antenna, that I've used for the past several years.  

It's ratio is 90 ft-to-45 ft in length and the voltage balun (it's 6:1 ratio) is rated at 300-watt (never go above the 100 watt level).  Feed the system with cable and use a Palstar ATK1P antenna tuner.

The tuner operates great, on any band at power levels below 100 watts; never any feedback or radiations.

I operate mostly, digital (95%) with some CW (5%).  

Ken KC8Y
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K6RQR
Member

Posts: 364




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« Reply #11 on: September 28, 2017, 08:00:14 PM »

Hello All -
 I talked with the man who builds the baluns for Balun Designs and he said that they have a new balun called a "Hybrid" which combines the best characteristics of a voltage balun and a current balun. It sounds like a real good unit to use with an OCF. It's a bit pricey ($92) but I can attest to the quality of his baluns. I've owned 2 of them.

73,
Bruce  K6RQR
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K4SAV
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Posts: 2389




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« Reply #12 on: September 29, 2017, 07:01:39 AM »

Any adjustment to length of an OCF will affect all bands.  If all you are interested in is good SWR on 80 you can tune it for that.  Also a balun that allows feedline currents can lower the SWR on 80 if you have the right feedline length.  That sounds like maybe what a hybrid balun will do.  The penalty you pay for that will be lower gain and maybe some RFI problems.

Kerry, K4SAV
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WB6BYU
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Posts: 17072




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« Reply #13 on: September 29, 2017, 09:32:24 AM »

I think a "hybrid" balun usually refers to using a "voltage balun" (or un-un) to achieve the
impedance transformation, following by a current balun to reduce common mode current.

Baluns typically work best in square ratios, usually 1 : 1, 4 : 1 or 0.25 : 1.   Transformers
that don't act as a balun can be built for many different ratios.  So for a 6 : 1 balun for an
OCFD, a 6 : 1 transformer followed by a 1 : 1 current balun is probably one of the better
approaches.
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K4SAV
Member

Posts: 2389




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« Reply #14 on: September 29, 2017, 09:50:29 AM »

I think a "hybrid" balun usually refers to using a "voltage balun" (or un-un) to achieve the
impedance transformation, following by a current balun to reduce common mode current.

Thanks for that info.  I never looked it up.  An unun followed by a choke should be at least as good as a standard current mode balun.

Jerry, K4SAV
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