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Author Topic: OCF dipole not very "multiband"  (Read 4437 times)
VK5DO
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Posts: 79




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« on: July 24, 2012, 09:43:15 PM »

Howdy all,

I've just put up a OCF dipole for 80m and above.  It's fairly close to the ground so I'm not suprised it's SWR on 80 is a little high but still useable.  On 40 and 20 the SWR is close to perfect.  Above that though - no good at all.

I've used a 4:1 balun at 1/3 along or 17% off centre. 

I thought I should have 15 and 10m working ok with this antenna as well.

Suggestions please.

Yours,

Dene
VK5DO

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KD8DEY
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Posts: 352




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« Reply #1 on: July 24, 2012, 09:55:27 PM »

A little missing info
How long overall
How high at the feed point? you might need to get it up higher

Are you using a voltage or current balun?

Some people like the way the swr falls within the bands using the 80'20 version of the OCF

BTW
if over 40 ft or was it 50 ft? (senior moment)  a 6:1 balun is recommended
« Last Edit: July 24, 2012, 10:05:48 PM by KD8DEY » Logged
W5DXP
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Posts: 3546


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« Reply #2 on: July 25, 2012, 10:02:24 AM »

Suggestions please.

I've modeled a 40' high, 1/3 split 130' OCF with a 4:1 balun. Here are the SWR(50) per band.

80m, 2.6; 40m, 2.5; 30m, 18; 20m, 10; 17m, 6; 15m, 14; 12m, 13; 10m, 11

Seems to me the SWR is too high on 30m, 20m, 15m, 12m, and 10m.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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Posts: 13039




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« Reply #3 on: July 25, 2012, 10:32:10 AM »

Here is a more extensive analysis of the OCFD from the late W4RNL:

http://www.cebik.com/content/a10/wire/iocf.pdf

The "conventional" formulas don't always come out the way we'd like, and the
resonant bands are fairly narrow - small changes in wire lengths can move them out
of the ham bands.

30m and 15m have always been problems with many traditional designs because
the feedpoint is at a high impedance point.  Moving the feedpoint along the antenna
will improve some bands at the expense of others.

Feedline decoupling is often marginal at best with many designs, and the effect
of the coax can shift some of the resonances (depending on the feedline length.)
If you are using a 4 : 1 voltage balun then adding a current balun / choke may
make a difference.


So there are a lot of factors that make an OCFD more challenging than one might
expect at first glance.
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N3JBH
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Posts: 2358




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« Reply #4 on: July 25, 2012, 10:39:54 AM »

This might be a great application for the Andrew Roos, ZS1AN Hybrid Balun
http://f1frv.free.fr/main3c_Baluns_fichiers/A_Better_Antenna_Balun_ZS1AN.pdf
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KL3HY
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Posts: 117




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« Reply #5 on: July 25, 2012, 03:03:25 PM »

Where you say it's fairly close to the ground, that immediately throws a red flag for mw.  When I raised my own OCF dipole (one of the ones offered on Ebay by "Maple Leaf Studios", 4:1 current balun, 1/3 feedpoint) by about 20 feet at the apex, and about 10 feet at the ends of the legs, it had a dramatic difference in SWR.  Something else that seems critical is the leg angle--how steep the "V" angle is.  I've found with mine that best SWR is at about 120 degrees.  When I lifted the legs to flatten out the "V" it raised SWR on most of the bands.

As a side note, the narrow bandwidth at the various points of resonance are why it was important to me to use a low loss feedline (I used LMR-400).  I've thought about using parallel conductors for the legs, or going further and making it a OCF cage dipole in order to flatten out the SWR curve, but I haven't really found the need to do that yet.

I'm a very new ham, and this is my first antenna so take what I say with a grain of salt.  I just thought I'd throw it out there though since I've been having fun experimenting with it quite a bit lately.

Mike
« Last Edit: July 25, 2012, 03:07:14 PM by KL3HY » Logged
K3AN
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Posts: 787




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« Reply #6 on: July 26, 2012, 08:14:55 AM »

OCF was the very worst multiband antenna I ever installed. SWR performance was reasonable using 30 feet of RG-213 as the feedline to a 2nd floor shack, but it just didn't get out very well. If you want to put up a multi-band flattop, feed it at the center with ladder line and use a balanced antenna tuner.
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KG6YV
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Posts: 506




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« Reply #7 on: July 26, 2012, 08:23:03 AM »

Making an OCF antenna sized for 80M as the lowest band is not often straightforward.  Obtaining reasonable SWR on some of the higher bands takes more than a properly cut wire with the feedpoint at the "right location".  If you have not already found this excellent document by the world's biggest expert in multi-band OCF's you should read/study it. 

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=15686

He advocates some additional tweeks including a series resonant circuit at the center point of the 80M wire.

Also, you can join the OCF antenna group on Yahoo and get excellent support from others who have experimented with OCF antennas.  Keep in mind that an 80M wire will become almost omni-directional above 20M due the the # of wavelengths it repreents at higher frequencies.

Have fun,

Greg
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KG6YV
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Posts: 506




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« Reply #8 on: July 26, 2012, 08:27:02 AM »

Forgot one more important thing....

An OCF antenna has unbalanced current on the radiator which will also contribute to feedline radiation.  The feedline becomes part of the antenna, screws up resonance and makes it difficult to achieve low SWR on some bands IF you do not put a choke at the feedpoint.  A current balun or other coaxial choke (line isolator) at the feedpoint will help eliminate the feedline radiation and assist in achieving more deterministic SWR on higher bands.

Greg
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KJ4ADN
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Posts: 39




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« Reply #9 on: July 27, 2012, 08:57:21 AM »

I've heard of, never seen it, might try it....
a "cage" OCF to increase the bandwidth.

Our first HF antenna was a 160m Carolina Windom, not high enough to work the lower bands effectively.
There was a website, don't remember when I last saw it, years ago, they took hoola hoops and made a 6 or 8 wire cage Windom.  one of these days...  I'm gonna try it (cage) with a 160-80m SS Zapper and see what happens.

At some of the HAMfests, these other guys have been selling OCF and Dipoles made with woven fencing wire, saying it's wide band - about 3-4" wide, SS wire, I don't know if that would *really* make enough difference... interesting idea, no comparision results ever given.

People swear by fan dipoles...

KJ4ADN - Bill
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AE5JU
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Posts: 223




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« Reply #10 on: July 29, 2012, 12:36:58 AM »

First, you need to tune the lowest band, 80 meters, low, in the 80 meter portion of the band.  Let me suggest 3.55 mhz.  This way, 40 meters will fall at about 2 x 3.55 = 7.1 mhz, 20 meters will be about 4 x 3.55 = 14.2 mhz, 17 m will be about 5 x 3.55 = 17.75 mhz (and still good SWR across 17 m band), and 12 m will be about 7 x 3.55 = 24.85, again, still good across that band. 

A 1/3-2/3 80 m or 40 m OCFD will not work on 15 meters because there is a high impedance node right at that 1/3 feedpoint.  You could move the feedpoint over a bit, from 33.3% to 28.9%, lose 12 m, but obtain 15 m. 

Back to 80 m.  You will still need a tuner to work the 75 m portion of the band.  If you were to tune the lowest band to, say, 3.900, it would push all of the higher bands resonance up above the ham bands. 

Another thing I have just recently found out, for a more accurate reading you need to make sure, if using an analyzer, to have it grounded to earth ground.  If you just screw on the coax the antenna and analyzer are floating and it will read higher SWR than you will get if grounded.  And your antenna will be used that way, screwed to your radio, the radio chassis grounded, right?  It will make a difference in the measurements.  For our club OCFD, on 80 m it measured 2.5:1 at 3.57 mhz floating, but when I connected a ground, it measured 1.5:1. 

A tuner is still useful for final touch up on some bands.

73,
Paul - AE5JU

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AE5JU
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Posts: 223




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« Reply #11 on: July 29, 2012, 12:45:50 AM »

"People swear by fan dipoles...

KJ4ADN - Bill"

Mine works well.  I have three sets of elements, for 75 m, 40 m, and 20 m. 

The 75 m portion is tuned for 3.900 mhz, right in the middle of where I want it.  I can work a large part of 75 m without tuner.  With the tuner I can cover all of 80 m, do some PSK31 down at 3.58.

The 40 m portion is tuned for 7.150 mhz, and I can work the whole band without tuner.  This also allows me to work 15 m with a little touchup from the tuner.

And the 20 m portion is tuned for 7.200 mhz.  I can work the whole band without tuner.

My tuner allows me to work 17 m and the SSB portion of 10 m, too.  With the tuner bypassed, the 17 m band is about 3.5:1 SWR, and 10 m is 2.2-2.5:1 SWR, not great, but not bad.  Easily tunable.

73,
Paul - AE5JU
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KJ4ADN
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Posts: 39




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« Reply #12 on: July 29, 2012, 06:40:24 AM »

"People swear by fan dipoles...

Mine works well.  I have three sets of elements, for 75 m, 40 m, and 20 m. 

The 75 m portion is tuned for 3.900 mhz, right in the middle of where I want it.  I can work a large part of 75 m without tuner.  With the tuner I can cover all of 80 m, do some PSK31 down at 3.58.

The 40 m portion is tuned for 7.150 mhz, and I can work the whole band without tuner.  This also allows me to work 15 m with a little touchup from the tuner.

73,
Paul - AE5JU

Exactly - probably the easiest multi-band, tuner-less antenna to build.
Playing around with a 40m a bit, if you could "fatten" that element, 15m might come in without any help from a tuner.
I was thinking another 40m wire, horizontal axis to the existing 40m one might increase it's bandwidth and gain 15m.

Considering the transmitter's RF converted to heat (or Huh) by tuning, it seems like a guy would be further ahead starting with a resonate antenna than a one-antenna-does-it-all-tuner-required....  I was stunned how much better a resonate antenna "hears-the-band-you-want" than one that's not. 

KJ4ADN - Bill
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W5DXP
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« Reply #13 on: July 29, 2012, 07:25:46 AM »

I was stunned how much better a resonate antenna "hears-the-band-you-want" than one that's not.

And why not? A resonant system is known to increase the voltage and current at resonance to a maximum value. What you may not realize is that a properly tuned antenna tuner also resonates the antenna system as does series matching sections. (Unfortunately, resonating an HF antenna system does not appreciably change the signal/signal+noise-ratio so all one needs to compensate on receive is a good AGC system.)

Example: take a 102' dipole used on 40m. Its feedpoint impedance is 557+j1170 ohms according to EZNEC - clearly non-resonant. But, also according to EZNEC, we can add a 35' section of VF=0.9, 450 ohm twinlead, to bring the antenna system to resonance, i.e. the impedance looking into the ladder-line on 40m is a resonant 60+j0 ohms. Through the use of a series-section matching section, we have resonated the entire system. For those who don't recognize what we did, it's a G5RV.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W9KDX
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Posts: 770




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« Reply #14 on: July 29, 2012, 07:56:07 AM »

I have an OCF dipole that works very well on 6,10,20, and 40.  This is what it was designed for.  I found it much easier and more efficient to add a fan dipole below it for 12,15.and 17.  All three bands have remarkably low native SWR, close to 1.2:1 or better and once I had the OCF up, adding the fan with such short lengths was not much trouble.  Tuning the lengths was interesting, but doable.
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Sam
W9KDX
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