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Author Topic: 80 meter dipole antenna  (Read 6067 times)
N7GCO
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Posts: 142




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« on: April 24, 2011, 08:58:11 PM »

I am planning to put up a full length 80 meter dipole. Now I have a Alpha Delta 40/80 but want better performance on 80.

Are there any advantages to an off-center dipole, windom or other wire antenna. From what I have read it seem a plain dipole is better.

I do not need multiple bands, except for 160, but I do not have enough property to put up a full size 160 dipole
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WB6BYU
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Posts: 13045




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« Reply #1 on: April 24, 2011, 09:39:15 PM »

The only advantage of the OCFD is that it can provide a better match on other bands besides 80m.
If you aren't interested, the standard dipole is simpler and it shouldn't need a tuner (at least to cover
150kHz or so of the band.)

If you have room for an 80m dipole, you probably have room for a 160m antenna as well.  I just
tested one out yesterday, where I start with a 40m dipole and add loading coils and extension wires
to resonate it on 160m in about the same length as an 80m dipole.  (If you make the 40m wires
about a foot or so long it will work on 40m also.)  Putting a pair of these wires across the same
feedpoint as the 80m dipole and spreading them apart at the ends can, with a bit of pruning, give
3-band operation with a single coax feedline.  (Bandwidth is somewhat narrow on 160m - around
40kHz or so.)  Actually the version I was testing for the ARES team used manual switching at the
end of the 40m wire:  color-coded Anderson connectors to choose 40, 80 or 160m, but the other
portable antenna kit will have the two pairs of elements to give 3-band performance.  If you aren't
worried about 40m, you can use whatever wire lengths are convenient and just wind a coil of the
appropriate size.  (100uH or so is a good starting point for the 40m extension.)
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W4VR
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Posts: 1190


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« Reply #2 on: April 26, 2011, 01:45:45 PM »

If you have plenty of tall trees, as apparently you do, you can feed the 160 and 80 dipoles with one transmission line (fan dipole), but make sure you separate the legs sufficiently as to reduce coupling.  I find that 50 degree separation works adequately.  Keep the legs horizontal...stay away from inverted vees.
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K0ZN
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Posts: 1538




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« Reply #3 on: April 26, 2011, 09:20:19 PM »


You are correct that a plain dipole is *usually* better than an Off Center Fed system.  Several reasons:  mainly that the dipole is truly balanced and there are many less issues with RF on the outside of the shield causing TVI/RFI, "RF in the shack", picking up noise, making the antenna less SWR stable, etc. This is not to say that OCF's don't work....they do, but the cold fact is that an OCF is a COMPROMISE in ALL electrical conditions on not optimum in any! 

Part of the fascination with OCF's seems to be that many manufacturers advertise them as the "do all, be all" antenna and they are coax fed (which looks "easy") so many hams who are not fully versed in antennas bite into this. Again, OCF's WILL radiate and may have good signals but that doesn't eliminate their disadvantages related to compromise.  You NEVER get something for nothing in the antenna game!!!

One of the nice things about a dipole is that it is a straight forward antenna and you know what you have and how to adjust or change it and they
are very reliable and trouble free....both mechanically and electrically. The down side is that when fed with coax, they are usually mono-band antennas.
Fed with ladderline or open wire, they are excellent and efficient multi band antennas, but require a good tuner, etc. to accomplish this.  Actually, most
OCF's really require a tuner too because they rarely have low SWR on all bands....so kind of off setting penalties on that one.

I am sure you will get a laundry list of posts telling you about how happy people are with their OCF's...... and that should not be terribly surprising....they do
radiate...and a great deal of the radiation is from the antenna!...and a significant amount is from the outside of the coax shield!.... no question: it is all radiation and
puts out a signal.

73,  K0ZN
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W9GB
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Posts: 2600




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« Reply #4 on: August 31, 2012, 05:53:18 PM »

Gene Preston, K5GP version of OCF for 80 and 160 meters
Uses coils wound on forms and coaxial capacitors
http://egpreston.com/K5GP_broadband_80_meter_antenna.pdf
« Last Edit: August 31, 2012, 06:00:31 PM by W9GB » Logged
KC4MOP
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Posts: 729




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« Reply #5 on: September 01, 2012, 05:04:47 AM »

The plan is to get your dipole up about 70 feet in the air, and then things happen. Legal limit helps too.
The dipole becomes directional at a 1/2 wavelength above ground for the band in question so, even at 60 or 70 feet it is still a cloud warmer, but very effective coverage over a pretty large area.
The full length 80 M dipole will be nice and as WB6BYU mentions, with loading coils, you can reach 160M. An antenna analyzer will help a lot in the tuning process.

Fred
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K3VAT
Member

Posts: 701




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« Reply #6 on: September 01, 2012, 08:29:10 AM »

I am planning to put up a full length 80 meter dipole. Now I have a Alpha Delta 40/80 but want better performance on 80.
...

Should you consider a loaded dipole from a manufacturer, then this website seems to have the best products at reasonable prices:
http://www.hypowerantenna.com/products/dipoles
there are several (short length) versions to choose from.

GL, 73, K3VAT
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KU3X
Member

Posts: 141




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« Reply #7 on: September 01, 2012, 10:31:21 AM »

Will a 154 foot long dipole fit on your property? If so, Google search, "2b7516S". This antenna will work on both the 75 and 160 meter bands. Sounds like a simple dipole will fit your needs.

To just say that you need a tuner with an OCF antenna is a kind of misleading statement. Today’s solid state rigs cut back on power once you go over a 1.5 to 1 SWR. You will not cover the entire 75/80 meter band on a simple dipole and not need a tuner.  Either way, half wave center fed dipole on 80 meters or an 80 meter OCF, you will at one point need a tuner.

If you take a half wave length of wire, cut for 80 meters and feed it 1/3 of the way in from the end, the radiation patter and performance will be the same on 80 with both antennas. A half wave is a half wave, it’s just fed at a different place along the same wire. If done properly you will be able to work numerous bands as the OCF is in fact resonant on numerous bands.

The following web page shows why a properly built OCF is resonant on so many bands.

http://www.universal-radio.com/catalog/hamwire/1164.html

If the outside shield of the transmission line is radiating on an OCF, it’s not built properly and/or installed improperly. If the OCF is built properly with the use of a true 4:1 Guanella Current Balun, the transmission line WILL NOT RADIATE ! I have a GU4-HF-5KW installed on my OCF80 and have checked for feed line radiation with a RF Current Probe. I’ve transmitted on all of the bands the OCF80 is designed to work on and there was ZERO RF on the outside shield of the transmission line. If you run your coax, to your station improperly, you can have common mode issues but that’s not fault of the balun.

There is one thing you have to understand about an OCF antenna, "The radiation patterns change from band to band." Other than that, the OCF antenna is in fact a great coaxial fed, "One Antenna Do All."

Barry,KU3X

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