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Author Topic: OCF dipoles  (Read 3327 times)
WX7G
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« on: July 12, 2012, 12:59:53 PM »

Why all the OCF dipoles recently? It seems that the majority of folks asking about RFI issues have OCF dipoles.
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K5LXP
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« Reply #1 on: July 12, 2012, 01:32:42 PM »

It's the G5RV of the current ham generation.  An antenna that "covers" all bands.


Mark K5LXP
Albuquerque, NM
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K4SAV
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Posts: 1851




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« Reply #2 on: July 12, 2012, 01:43:15 PM »

Either that or end fed antennas of some kind.  It's probably due to marketing, promise everything thats good but don't mention anything that's bad.  I thought one comment by one of the manufacturers who did mention RFI and uses a voltage mode balun at the antenna feedpoint of their OCF was interesting:
"Problems of RF on the exterior (common-mode feed line currents) of the coaxial feed line have not been noticed or reported. "

Come to think of it, there also seems to be a trend of manufacturers reporting claims that they get from some of the users, regardless of how ridiculous those claims may be.  Things like "this dipole has 10 dB more gain than my old G5RV", or other such nonsense.  They don't actually say the antenna does that, but say that "some users have reported", so that they can imply something without actually saying it.

Jerry, K4SAV
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K0ZN
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« Reply #3 on: July 12, 2012, 06:47:04 PM »

 AMEN !!  The fact is most hams no longer will pick up a book and STUDY antenna theory.  They read internet blurbs, read commercial antenna manufacturer's
              exaggerated claims and jump on the band wagon..... with predictable results.  To wit: a typical "over the counter" OCF is not a balanced antenna
and it is not a properly fed UNbalanced antenna......so why is it a surprise that they have "issues"?  The fact they appear to be "plug and play" coax fed antennas
is also part of the mythical attraction.  If you get on here and write a post explaining that OCF's are  ALWAYS  a compromise in some condition, you get flamed.

Few people seem to TRULY understand the inevitable issues with off center feed of a wire antenna. Since they do radiate pretty well (with *most* of the radiation coming from the antenna!! ), the conclusion is they "work".....but this of course does not negate the unsymmetrical feed issues.

If you go back into the archives on here you will find a laundry list of people having various problems, typically RF in the shack or "SWR issues" with OCF's....and since they can be built/made in several different configurations there is no simple solution to the issues.

My guess is that if the average OCF purchaser would spend a couple of hours studying antenna and transmission line theory I would bet sales of OCF's would drop significantly !

73,  K0ZN
« Last Edit: July 12, 2012, 06:50:07 PM by K0ZN » Logged
N3JBH
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Posts: 2358




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« Reply #4 on: July 12, 2012, 07:41:55 PM »

Quote
To wit: a typical "over the counter" OCF is not a balanced antenna
and it is not a properly fed UNbalanced antenna......

Ah yes to which i have asked numerous questions about and still mystified by what i read and seen.  It is a unbalanced antenna fed with a balun. Some thing there just seems wrong to me. But then i am the typical thick sculled kraut too  Undecided Kind a makes me think of non alcoholic beer it just don't make any sense to me.
« Last Edit: July 12, 2012, 07:48:55 PM by N3JBH » Logged
KC4MOP
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Posts: 764




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« Reply #5 on: July 13, 2012, 03:59:15 AM »

It will take a little time and some empirical analysis for the OCF folks to see the light. As there is light being shed on the magical 43 foot antenna that operates on all bands and no tuner.

Fred
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AA4PB
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« Reply #6 on: July 13, 2012, 05:25:18 AM »

Everybody wants a "free lunch". They want a physically small, easy to install, all band antenna that will work all the DX they can handle. There are plenty of mfgs who will happily supply an advertisement for such an antenna but when you get one you always find it has issues of one type or another. Nobody wants to tell them that there is no such thing as a free lunch - somebody always has to pay. Physically small, easy to install, and all band always cost's efficiency. Many times, like mobile or perhaps an apartment, we are forced to trade off efficiency. That's okay, just don't let some mfg convince you that their high dollar antenna includes efficiency too.
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K3SF
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« Reply #7 on: July 13, 2012, 07:03:20 AM »

 I have designed, built and now use a 139ft long OCF hanging at 75ft high. Meets my expectations as a gain antenna for 40m and decent 80m antenna and backup on 30 thru 10m (way to many lobes on these bands)

BUT
 
I got there from some experimenting with a full wave 40m wire(139ft) that was feed 1/4 wave from one end. This gave a nice 50ohm match...no balun needed.

This full wave 40m wire antenna gave me four major lobes with gain over a standard 1/2 wave dipole. I was looking to improve my 40m signal for dx'ing.

it worked well and as designed. Also worked on 15m but way too many lobes and besides i have a LPA that covered that band.

well as with any wire antenna mother nature takes her toll..a tree branch came crashing down and took down this full wave 40m wire antenna.

i had read a lot about ocf that were feed not 1/4 wave  but more like 1/3 of the wire length...and how the impedance varied as function of frequency which could be compensated for by using a balun. I still wanted my 40m full wave wire and its gain but with a slightly different feedpoint i could get a standby antenna for other frequencies  but now i would now have 1/2 wave antenna on 80m...i like the added 80m capability

so i went and designed, built and tested 139ft long wire feed 1/3 from one end using a 4:1 balun and rf isolator... yep..still had my 4 main lobes with gain for 40m, and it even worked fine on 80m just like my 80m dipole and still have backup for all the other bands...

The OCF can take many forms...from 1/4 wave offset feed of full wave wire but pick a band for desired 4 lobe gain pattern...to 1/3 offset feed that still give the same original 4 gain lobes and other bands that 'work' to some degree...

This is all accomplished  without the need for any antenna tuner

I have NO experience with G5RV design...hence wont comment other than it doesnt seem fit MY definition of an OCF (off center feed) full wave wire antenna.

My next antenna project is to replace my spare 80m dipole with double Inverted-L for 80m and 160m.
can hang it from the same trees the 80m dipole is on. Still doing design work (antenna modelling) to fit my site.

My OCF works as designed and does indeed meet my expectation..it is a good two band antenna..one with gain over a dipole and one with standard dipole performance. it does match impedance on other bands BUT a dipole on those bands would provide better and more consistent performance.


my 2 cents which have been severely eroded by the banks and wall street

paul K3SF










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W5DXP
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« Reply #8 on: July 13, 2012, 08:02:39 AM »

I got there from some experimenting with a full wave 40m wire(139ft) that was feed 1/4 wave from one end. This gave a nice 50ohm match...no balun needed.

Such an antenna has an advantage over a 33%/67% OCF split. The impedance looking into a 1/4WL wire is fairly close to the same value as the impedance looking into a 3/4WL wire so the feedpoint currents tend to be better balanced (lower common-mode) and closer to 100 ohms than the 33%/67% split. The 25%/75% split feeds the 1WL wire at a current maximum (minimum impedance) point. That changes the current phase in the other half of the wire from the center-fed configuration and turns the wire into an end-fire for the radiation pattern with very little broadside radiation.
« Last Edit: July 13, 2012, 08:05:23 AM by W5DXP » Logged
WX7G
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Posts: 6322




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« Reply #9 on: July 13, 2012, 11:13:48 AM »

I built and used a traditional Windom fed with a single wire feedline. I wrote an article on it for AntenneX and analyzed it (a 66' Windom) for use on 160-10 meters.
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KC4MOP
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Posts: 764




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« Reply #10 on: July 14, 2012, 04:42:38 AM »

The guys who are experimenting and or using modeling software will have benefits of the OCF.

I think the thread is directed to plug-n-play folks who are experiencing difficulties.

How many Hams can afford to live on a "measly" 1 acre lot? A 1 acre lot is just the beginning of play room for antennas and not having to compromise.
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W6WO
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Posts: 44




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« Reply #11 on: July 15, 2012, 10:04:07 AM »

OCF designs are popular because they provide feed-point options which can be more convenient than the mid (50%) point of  common wire antennas. The extreme case is the historical end-fed Zepp but today most designs fall into the 25-35% range. I have designed and built mono and multi-band versions with feed points in the range of 10- 20% that are described as near-end-fed NEF in QEX issues Mar/Apr 2009 and Nov/Dec 2010.  All unbalanced antennas are prone to feedline radiation and many practical center-fed projects have the similar issues-the same mitigation measures apply. Claims made for several products sold as OCF, Windom and especially G5RV antennas are beyond my belief.
Ron W6WO
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