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Old 09-06-2014, 08:58 AM   #1
Mr. Ham
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Join Date: Aug 2014
Posts: 95
Default Off Center Fed Dipole Antenna

Antenna Comparisons:

You will always hear someone say - my G5RV work's good with this tuner or that tuner and that they can work everyone that they can hear.
My question is - how much of what they cannot hear - are they missing?

The only way to do a comparison is to do a A/B comparison.
That is to have a resonant half wave dipole antenna and their G5RV up at the same time and switch from one to the other and get signal strength readings.
Also, check noise figures from one to another.
This should be done under more then one Prop conditions.

Most antennas perform best at least 1 wave length above the ground.
40 feet above ground is as low as I would like to go .
Antennas will work at any height but SWR and signal strength will suffer..

http://www.hypowerantenna.com/

An off center fed dipole works on the principal of harmonic's.
The ratio - 1/3 / 2/3 length using a true 4:1 balun causes the short end to act like a reflector..

Why OCF?
The simple answer is: to be able to work more bands with one antenna.

A dipole is basically a mono-band antenna. It is sometimes used on its 3rd harmonic (i.e., a 40m dipole may also be used on 15m), but if it is fed with coax, it should not be used on its even harmonic frequencies. That's because the impedance of the dipole on its even harmonics is too high (typically >2K Ohms) and feed line losses are excessively high.

By moving the feed point away from the center of the dipole,
we enable the antenna to be used on more harmonic frequencies.
We transform the mono-band antenna into a good multi-band antenna.

Warning : THIS IS NOT AN "ALL-BAND" ANTENNA, IT IS A "MULTI-BAND" ANTENNA.

Some will argue that the 'open wire-fed/center-fed' dipole is the most efficient antenna there is, when you are looking to cover 5, 6, 7, or even more bands with just one wire antenna.
I don't dispute this point.
However I do challenge the openwire-fed dipole's "perceived" position of being the "best" solution no matter what.
I suggest we consider the following:

“How much more efficient is it than other alternatives?"
“What disadvantages does it have ?”
"For short feed line lengths, other solutions are more practical."

Openwire-Fed Dipoles REQUIRE a matchbox - on all bands, all the time.

Instead of feeding the dipole in the middle with open wire, let's consider feeding it off-center with coax,  through a 4:1 balun.
It certainly works!

But what is the downside?

The downside is slightly higher losses.

However as long as the feed line is less then 100 feet long, the difference remains less than 1 db “if” you build it right.

So what can be done wrong with an OCF?

Using:
the wrong type or poor quality of balun,
cheap, poor quality coax with excessive loss.
The wrong choice of design, and then compensating with a matchbox.


The problem with openwire-fed/center-fed antennas like the double-zepp is, antennas resonant on one band have a ridiculously high impedance on their 2nd harmonic. It is no different than a normal dipole, as described above. It can be difficult, sometimes even impossible to match the 2nd harmonic band without having to change the feed line length, unless you use huge matchbox.
Antenna tuners that can handle large amounts of power are very expensive.

The problem with using a antenna tuner to match the line impedance to the generator is that it cannot make the antenna resonant. The loss rate is still the same in the coax and the power reflected is turned into heat, which is adsorbed by the coax and the tuner itself.

I generally do not need to use a matchbox on any band!
The exception being 15, 30 and 60 meters...

http://www.hypowerantenna.com/produc...er-fed-antenna

80 Meter Off Center Fed Antenna

7 Band With One Antenna


Our 80 meter off center fed antenna is a very efficient and an effective multi band antenna.
It shows a positive SWR dip on the following bands: 6, 10, 12, 17, 20, 40 and 80 meters.
For a coaxial fed, "One Antenna Do All" you can't beat the off center fed antenna.

* All Stainless steel hardware

* Completely assembled (balun included)

* Overall length: 135 feet

* Power rating: 5KW

* No trimming required

* Uses #12 UV insulated wire

* Works on 7 bands (6, 10, 12, 17, 20, 40 and 80 meters)

* Great Short Wave Listening antenna

This will give you a general idea of how they work:

If you take an 80 meter 1/2 wave antenna and feed it in the middle, the impedance is around 72 ohms and an almost perfect match for 50 ohm coaxial cable. This same antenna is resonant on numerous other bands, such as 40, 20 and 10 meters. If you take this same antenna and use it on forty meters and keep the feed point in the center of the antenna, the feed point impedance will be around 3000 ohms. This is not a good match for coaxial cable. This antenna is a full wave antenna on forty meters but has to be fed at a different location if one wants to use 50 ohm coaxial cable. Move the feed point of this antenna so it is positioned one quarter wave length in from the end of the antenna. The feed point impedance of this antenna will again be about 72 ohms and is a good match for 50 ohm coaxial cable. If you use this antenna on 10 meters, again move the feed point of this antenna so it is one quarter wave in from the end. Again you will have a good match for 50 ohms coaxial cable and now have the makings of a long wire antenna with gain.

To make this same 80 meter dipole usable on numerous bands without moving the feed point for every band, you have to find a common point where numerous frequencies converge at a given impedance. By feeding this antenna one third of the way in from the one end, you will find this so called, "Sweet Spot". The only thing left to resolve is the high impedance of this antenna at that point. The impedance's of the numerous bands, on this antenna, will range from as low as 150 ohms up to around 300 ohms at this feed point. By placing a 4:1 Guanella Current Balun at this point you will have an almost perfect transformation of impedance so 50 ohm coaxial cable can be used. The end result would be an antenna that is usable on seven ham bands.

The overall length of this antenna is 135 feet. One leg of this antenna is 45 feet long and the other leg is 90 feet long. This will place the feed point of the antenna 45 feet in from one end.

40 Meter Off Center Fed Antenna

4 Bands With One Antenna


Radiation Patterns:

Our Hy Power 40 meter off center fed antenna is made with the same material as our 80 meter off center fed antenna.

The only differences are:

1. The over all length of the 40 meter off center fed antenna is 66 feet.

2. The antenna is fed about 23 feet in from one end of the antenna.

3. The OCF40 covers four of the ham bands. They are 6, 10, 20 and 40 meters.

The OCF40, OCF80 and OCF160 all use our, "GU4-HF-5KW-A" Guanella 4 to 1 Current Balun. This balun uses two special ferrite cores and is rated at 5KW.
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Old 09-06-2014, 09:02 AM   #2
Mr. Ham
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Join Date: Aug 2014
Posts: 95
Default QRP / Low Power / Stealth Antennas

This model of antenna is only rated at 150 watts, not 1500 watts.

The standard models are made with the use of 16 gauge insulated wire.
Like all of our Hy Power Antennas we still use all the Hy Quality parts.

We use stainless steel electrical hardware, SO 239 on the center insulator and two end insulators are included.
All antennas are completely assembled and ready to install.

The 40 Meter Off Center Fed Antenna (OCF40Q) is a great multi band antenna. It is a heavy duty QRP/Low Power antenna that uses #14 wire instead of the usual #16 wire used by our other QRP/Low power models.

This antenna uses a 4 to 1 Guanella Current Balun that is rated at 250 watts, all stainless steel hardware and covers the following ham bands: 6, 10, 20 and 40 meters.

The overall length of the antenna is 66 feet, one leg is 23 feet long and the other leg is 43 feet long.

The feed of this antenna is placed 1/3 of the way in from one of the end insulators which would place the balun 23 feet in from the one end.

The 80 Meter Off Center Fed Antenna (OCF80Q) is also a great multi band antenna.
It is a heavey duty QRP/Low Power antenna that uses #14 wire instead of the usual #16 wire used by our other QRP/Low power models.

This antenna also uses a 4 to 1 Guanella Current Balun this is rated at 250 watts, all stainless steel hardware and covers the following ham bands: 6, 10, 12, 17, 20, 40 and 80 meters.

The overall length of the antenna is 134 feet, one leg is 44.5 feet long and the other leg is 88 feet long.

The feed of this antenna is placed 1/3 of the way in from one of the end insulators which would place the the balun 44.5 feet in from the one end.

This is the most highly recommenced antennas to be used for portable QRP and low power operating.

It also works well as a stealth type antenna for the primary station.

http://www.hypowerantenna.com/products/qrplowpower
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Old 09-06-2014, 09:37 AM   #3
Mr. Ham
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Posts: 95
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Performance results:

At the shack, at the same height, at the same time of day, on the same frequency, there was no perceived difference between the Barker& Williamson BWD 90 - Folded dipole antenna and the OCF 80 dipole on all bands.

The Barker&Williamson being a much more expensive antenna, that acts somewhat like a 2 element beam antenna on 20m and a true di pole antenna on 40m. The only advantage to the B&W being that it is an all band antenna - will work everything from 6 meters to 80 meters with a built in antenna tuner on most transceivers...

Field Day results - Ft. Venango Mike & Key club -

80 meter off center fed dipole - QRP version - 100 watts

2014 - 360 QSO's on 40m in 6 hours
- 60 qso's on 80m in 1 hour...

2012 - 600 QSO's in 12 hours of operation 20 / 40 / 80 meters.

All honest signal reports were near 5/9 for everyone I worked with just 100 watts....

In 2014 I shut down early due to band conditions - noisy ( static crashes ) - and a imminent threat of lightning.

I lacked covered transportation for my gear - ( the trailer was broke down with a bad wheel bearing ), and I had no tent to operate from.

Had I stayed, I would have easily surpassed 1000 QSO's - even while operating on a band with two other operators on the same band.

One operator at our field day site was using CW - QRP, while another was operating digital modes on 40 meters during the daytime.
I had a lot of QRM in my receiver from the other two operators..

We shut down at 7 PM for supper.

Due to a lack of help unloading and setting up my equipment, I did not get on the air until 3:30 PM Saturday, which caused some problems finding a quiet frequency to operate from - away from the other operators..

I was limited to the upper portion of 40m - 7.250 - 7.290 MHz...

This was with my antenna being 30' off the ground!.....

I would wholeheartedly suggest to anyone that is looking to operate remotely - emergency communications - to have at least one of these antenna's in their Go Bag.

The Off-Center Fed Dipole antenna - "QRP" will operate the whole way up to 440 Mhz without an antenna tuner...
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Old 12-13-2014, 11:48 AM   #4
Lamewolf
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Join Date: Dec 2014
Posts: 5
Default

Quote:
Originally Posted by Mr. Ham View Post
Antenna Comparisons:



An off center fed dipole works on the principal of harmonic's.
The ratio - 1/3 / 2/3 length using a true 4:1 balun causes the short end to act like a reflector..

.
The short end is not a reflector, its just part of the overall length that makes the antenna resonant on the lowest bands. If you look at the voltage and current of the off center fed you will notice that it is identical to a center fed dipole on the lowest band of operation with an increasing amount of voltage and current peaks as the frequency increases. The off center feed just changes the feed point impedance to a higher value that basically stays the same on harmonic frequencies allowing the use of a current balun to convert it back down to something near 50 ohms so it can be fed with 50 ohm coax. But anyway, the OCFD is a great multi-band antenna !
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Old 12-14-2014, 10:44 PM   #5
ke0me
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Location: Ft. Collins, CO (mail forwarding)
Posts: 303
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Just bought a 40-10 OCFD, appx 66 feet long which I will install in an inverted V configuration. I am full time RVer so we stay at private parks a lot, much smaller sites than state parks.
Any thoughts on minimum height I should try to shoot for? I see recommendations are 30+ feet, but doubt I can get that high in RV park setting.
What are the trade offs?
Thanks
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Old 12-15-2014, 06:23 AM   #6
Lamewolf
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Join Date: Dec 2014
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Quote:
Originally Posted by ke0me View Post
Just bought a 40-10 OCFD, appx 66 feet long which I will install in an inverted V configuration. I am full time RVer so we stay at private parks a lot, much smaller sites than state parks.
Any thoughts on minimum height I should try to shoot for? I see recommendations are 30+ feet, but doubt I can get that high in RV park setting.
What are the trade offs?
Thanks
30 feet is good to aim for but I have seen them work at 20 feet. But the lower you go, the lower the feed point impedance is so a tuner may be needed to get a good match.
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