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Author Topic: OCF dipole: doubts...  (Read 1937 times)
PY1VHF
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Posts: 14




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« on: September 21, 2008, 11:49:05 AM »

Hi, guys/gals!

I am building an 80/10m OCF dipole, and would like it to also resonate on the 15m
band. I know the “New Carolina Windom” antenna was designed to resonate on 15m,
by means of controlled feed line radiation (wishful thinking?) installed 22ft below the antenna`s feed point.

I was wondering: If I choose to install the RF choke balun (W2DU style), how do I
calculate the distance from the antenna’s feed point to the choke? I mean, as the W2DU
balun I am building is about 34cm in length, does it start "choking off" the braid
currents right at its beginning? In the middle? End?? I will be using 12 large beads
(Length: 28.5mm; Diameter, external: 14.2mm) over RG-58.

Question 2: How come the 4:1 current balun (at the feed point) allows this radiation?
Isn’t it supposed to create a barrier to feed line radiation?

So many questions, so little time... Smiley

Thanks for any input.

Andy - PY1VHF
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K0ZN
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Posts: 1563




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« Reply #1 on: September 21, 2008, 12:31:26 PM »

Hi,

I will be honest here, and say right up front, that I am not a fan of OCF dipoles. Here is the reason and some food for thought: An Off Center Fed dipole is kind of like a man without a country. Ultimately, you are trying to fit an "elliptical peg" into either a round or square hole! i.e. it is not round and it is not square!

 It is not a balanced antenna but it is typically fed like it is. This doesn't make great sense if you think about it. To wit: an OCF is not optimized ANYWHERE, design wise; it is a compromise everywhere.

Now this is not to say they don't "work"; obviously, they will, and I am sure you will get a bunch of posts from "happy customers", but that doesn't eliminate the fact that you have all kinds of issues related to mismatch and unbalance and radiation from the feed line. Almost any good antenna design attempts to eliminate feedline radiation, especially on the shield of coax, however, in the case of various OCF designs, an attempt is made to "use" this radiation. The problem is that you basically can only "control" the feedline radiation on one band....*maybe*. Frankly, with a significant mismatch at the feed point and unbalance in the system, I suspect there is a lot of wishful thinkng about how you might "calculate" a length to "control" that feedline radiation on 15 M. Personally, suspect that marketing efforts by commercial antenna companies have more to do with OCF popularity than physical radiation characteristics.

There are a number of proven Balanced antennas that will work efficiently without feedline radiation issues on all bands such as the Center Fed Zepp (also called a Doublet), multi-wire dipoles and some more complex wideband designs. Respectfully, maybe you want to do a little more digging in the ARRL Antenna Book or ON4UN's Low Band DXing (which is an excellent antenna book) before making a final choice on an OCF.

I am sure you will get a lot of comments to this post.....it should be interesting, if nothing else.

Good luck with you antenna project!

73,  K0ZN
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PY1VHF
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Posts: 14




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« Reply #2 on: September 21, 2008, 01:23:08 PM »

Hi, Jon, thanks for the reply.

Yeah, I am  aware of all the OCF's actual and potential pitfalls. But OTOH I like the
idea of having one and only antenna radiating eficiently on multiple bands.
I read a lot about this type of antenna (ARRL Antenna Book, RSGB Radio Handbook,
 the works of Cebik, Devoldere etc).
I know one has to be careful of feedline radiation, multiple lobes on higher frequencies,
choosing the correct feedpoint position, antenna height, balun's material  
choice/construction methods...
This is not an antenna for beginners. And it is not a panacea antenna, as I see advertised
here and there (the same thing occurs with the G5RV...).

But I would like to see what it works like in the real life.

Besides that, building and testing antennas is a great pastime for me.

Thank you!



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WB6BYU
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Posts: 13574




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« Reply #3 on: September 21, 2008, 01:57:26 PM »

Perhaps the simplest method is to build an OCFD for 30m and connect
it in parallel at the feedpoint.  It will resonate on 30 and 15m (or so.)

As a starting point try 5m on the short leg and 9.5m on the long leg.  
According to my EZNEC model that puts the resonance in 15m and a
bit off for 30m, but still usable.
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AC5UP
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Posts: 3956




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« Reply #4 on: September 21, 2008, 01:59:48 PM »

Until it was knocked down by a tree that went Sybil after a storm (split personality), I worked the world with an OCF and had no complaints. I kept it simple. 136' of wire fed 45.5' from one end through a 4:1 dual-core Guanella balun. Pruned only the long end until it lined up in the band segments I preferred. No RF on the feedline, no 15 Meters, no problems with the phone line or TVI / RFI.

If I really, really wanted 15 I'd have put up a separate wire instead of jacking the OCF around. It worked too well as it was and there's an old saying that begins with: "If it ain't broke, don't......"

When it comes to some antennas like the OCF and G5RV there is a tendency to "improve" the design until it becomes a dummy load. Resist that urge. Wink

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W8JI
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Posts: 9296


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« Reply #5 on: September 21, 2008, 03:36:23 PM »

Andy,

The 4:1 voltage type balun will not isolate common mode currents. It tries to feed each dipole half equal voltages as it "pushes" against the cable shield. Since one leg of the dipole has a different impedance than the other does, the unequal loads cause common mode current that would otherwise feed the high impedace dipole leg to flow down the coax shield.

Then, some distance down the cable, you would need a very high impedance isolator to stop the common mode. It would have to be many thousands of ohms, something a W2DU balun would never do. I doubt any balun or isolator would have that high of an impedance, so you may need to use two isolators or baluns.

I suppose, depending on balun impedances, you could make the SWR low on 15 meters by where you place the sleeve isolator or balun.


Personally I think the antenna is more an example of a vivid imagination than any kind of good engineering, but that is how it would work in theory.

73 Tom
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AA4PB
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Posts: 13032




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« Reply #6 on: September 21, 2008, 03:40:21 PM »

Almost any good antenna design attempts to eliminate feedline radiation
-----------------------------------------------------
Can we say that again :-)  Mama says "feedline radiation is like a box of chocolates, you never know what you are going to get". One person puts up the antenna and swears by it. Another person puts up the same antenna but his feedline length, routing, and surrounding environment are different so he gets a completely different result. The goal is to have the antenna element do all the radiating - its usually in a more controlled environment.

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K0ZN
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Posts: 1563




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« Reply #7 on: September 21, 2008, 06:52:23 PM »

Hi, again, Andy....

OK... if you are into the experimenting and "playing" with antennas, that is another game. I do a lot of that too, and build and test some antennas that I certainly would not want to depend on for emergency communications, but I am curious to see how they will actualy work and usually you learn something....and some times the work better than they should and other times they seem to be huge, unmatchable, aerial dummy loads!...but at least you get to see what they will do. If you are of that mind set, you probably will have a intersting experience with the OCF. I have messed with a couple of designs and candidly, they were "OK" and "worked", but they didn't give me results that I would call "great" and were kind of quirky in some areas. Interestingly, results I had with various OCF's, all things considered, indicated that the original, classic single wire feed (against ground) as designed by Loren Windom, W8GZ worked about as well as any other design. It is also dirt cheap and stone simple mechanically; certainly a good competitor in the "bang for the buck" category. It kind of looked to me like he got pretty close to the target in the 1930's. The biggest problem with it was that it was pretty noisy on receive, but this was likely a pro and con, since it probably indicated a good low angle radiation component. Maybe you can find the OCF holy grail !! Good luck....  

73,   K0ZN
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PD2R
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Posts: 131




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« Reply #8 on: September 22, 2008, 02:50:48 AM »

Take a look at this website: http://www.pa0fri.geerligs.com/

Go to "ANT" and then look for the FD-3 and FD-4 windom antennas. You will find a load of info on the antenna you wish to make.

73, Maarten
PD2R
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G8JNJ
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Posts: 496


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« Reply #9 on: September 22, 2008, 05:15:00 AM »

Hi All,

I've tried Carolina Windoms, Single wire fed Windoms, ladder line OCFD's and Doublets. All hung from the same supports at the same height.

The Carolina Windom was about 1 to 1.5dB worse than a ladder line fed OCFD of the same dimensions. It also suffered from a higher noise level on receive.

A ladder line doublet with each leg the same length as the shorter leg of the OCFD or Carolina Windom produced even better results.

A single wire fed Windom is OK on the lowest operating frequency where it is mainly horizontally polarised. However it doesn't work much better than a vertical of similar length to the vertical section of the Windom on higher frequencies.

In all cases I used an auto-tuner at the base of the antenna to achieve a match for the coax feedline to the radio.

Best compromise is to use a ladder line fed doublet with the ability to strap the feeders for use as a T.

See

http://g8jnj.webs.com/lowbandwireantennas.htm

and

http://g8jnj.webs.com/hfantennaswitching.htm

for some suggestions.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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PY1VHF
Member

Posts: 14




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« Reply #10 on: September 22, 2008, 12:51:14 PM »

Dale: Thanks. I will model it and see what it looks like.

Nelson: I know what you mean... From the moment a G5RV is
modified, it is no longer a G5RV. The only "improvement" I like in the G5RV is ZS6BKW's.

Tom: I was suspecting that: a voltage balun, instead of a current balun. Thanks for the info and insights.

Bob: "Almost any good antenna design attempts to eliminate feedline radiation...". Tell that to the EH antenna builders... hi hi hi... Thanks.

Jon: I tried once the original Windom antenna, but had lots of stray RF problem, and at the time I did not try to solve these problems. Maybe now that there is more literature about CMC available I will try again. Thanks.

Maarten: Dank u wel! I will read everything there. Dag!

Martin: Nice site, good stuff you have there. I will read it all. Thanks.




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W9DJ
Member

Posts: 7




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« Reply #11 on: September 23, 2008, 07:43:49 PM »

I've been using a homemade OCF dipole very successfully for a number of years.  It's resonant at 1.9 MHz, and fed at the 2/3, 1/3 point by about 30 feet of 450 ohm ladder line, which is connected to 16 feet of low loss coax through a 4:1 "current" balun.  The antenna is in a rough "V" configuration, with the apex at around 40 feet, and the ends at around 20 feet. The apex angle is around 130 degrees.  While feedline radiation could be a problem, I have not had any issues with this particular configuration. The reason I moved to the OCF design vs. a balanced dipole was to be able to get reasonable matching conditions on all HF bands.  This antenna loads quite easily on all HF ham bands and gets out well, with no RFI in the house or shack with the exception of a tendency to ring the doorbell on 160 a.m. with power. I modeled it, and my experience agrees pretty well with the model. The SWR is within reason (5:1 or something worst case -- I have forgotten) on all bands --  it's much lower on some. When I was using a 160 meter center fed dipole the high power tuner (which is very hefty) would often arc with the linear on, and on many bands the low power autotuners would not match at all. Now arcing is a thing of the past, and even one of my wimpy low power autotuners can handle the matching job at 100 watts.  It's not as efficient as a single band center fed dipole fed correctly, but I wanted the multiband capability.
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