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Author Topic: End Fed multi band "dipoles"  (Read 1739 times)
KJ4ING
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Posts: 9




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« on: June 26, 2009, 12:28:59 PM »

Anyone have experience with the EFAB end-fed multi band "dipole"?

The reviews on the PAR ef-10/20/40 have been very good, but I don't find anything on the EFFAB.

I travel in an RV a lot and both of these sould like an easy solution to the RV problems.

Thanks for any input.

Bill
kj4ing
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W3LK
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Posts: 5644




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« Reply #1 on: June 26, 2009, 01:59:21 PM »

There is no such thing as an "end-fed dipole", multi-band or not. An antenna is either end-fed OR a dipole; it cannot be both.

I am suspicious of this antenna, although I have never tried one. It sounds too much like another smoke and mirrors contraption called the MaxComm tuner - a 50 Ohm resistor across the output with a piece of wire connected to one side.

There are some good end-fed antennas out there - the PAR antennas, for example - but they are pretty much all single-band antennas.

73,

Lon - W3LK
Naugatuck, Connecticut
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KV9U
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Posts: 166




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« Reply #2 on: June 27, 2009, 09:12:33 PM »

Dipoles (half wave antennas) can be fed at the center, offset from center (Window type), or end fed "j-pole or zepp type.

In general, I would recommend a centerfed antenna. If you want multiband capability, you can add additional parallel dipole elements or purchase a commercial design if you do not want to build.

For a good overview of antennas, and if you are interested in public service/emergency communications, I have an article on the HFDEC (Hams for Disaster and Emergency Communications) yahoogroup, but you have to be a member to access the files section.
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G3RZP
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Posts: 4365




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« Reply #3 on: June 28, 2009, 02:54:42 AM »

Can it truly be a dipole if it isn't centre fed?

The J is an end fed half wave, as is the original Zepp. The Hertzian dipole is of necessity centre fed, as otherwise you don't get the phase inversion between halves

I have seen some VHF 'dipoles' that were apparently end fed. In reality, the coax went up inside the lower element (these were always vertically mounted), with a quarter wave 'bazooka' choke on the outside of the coax, and then the dipole proper was fed in the centre. It was just a convenient method of manufacture.
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W3LK
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Posts: 5644




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« Reply #4 on: June 28, 2009, 01:59:08 PM »

KV9U:

<< Dipoles (half wave antennas) can be fed at the center, offset from center (Window type), or end fed "j-pole or zepp type. >>

Sorry, but you are wrong. Just because an antenna is a 1/2 wavelength long does NOT make it a dipole. By definition, a dipole is two equal lengths of wire fed in the center. Anything else may (or may not) be a 1/2 Wl antenna, but that does not make it a dipole.

Off-center fed antennas (despite the incorrect nomenclature), end-fed, Zeep and J-poles all fall into the "category" of non-dipole antennas.

And off-center fed antennas are commonly referred to as "Windom" antennas, not "window" antennas. Smiley

73,

Lon - W3LK
Naugatuck, Connecticut
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KV9U
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« Reply #5 on: June 28, 2009, 06:16:51 PM »

Many antennas are based on the "dipole" or having two poles. In fact, when I teach my classes, I go over this two sided concept (or two poles).  

Typically, half wavelength antennas are considered dipoles by most of us and it does not matter if they are fed in the center or the end or in between.

For more information, I might suggest the ARRL Antenna Book. As they point out, " A fundamental type of antenna is the center-fed halfwave dipole", but they go on to say that "there are also versions of dipoles that are not fed in the center. These are called off-center-fed-dipoles."

As you move the feedpoint out from the center, matching can be a challenge, but in some cases the design has other values that are worth the tradeoff, but when you feed the dipole on the end, you can expect very high impedances which can be difficult to match. Also, an end fed half wave dipole, has another side (pole) which often becomes the outside of the feed line unless major effort is made to decouple.

And dipoles don't have to be half wavelength since we have many variations on the basic theme with extended designs.

The main advantage of the half wave center fed dipole is the simplicity and ease of matching nominal coax impedances to a non-reactive load.
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N0YXB
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Posts: 302




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« Reply #6 on: June 29, 2009, 09:05:30 AM »

Thanks KV9U for being a voice of reason.  With antennas, reason sometimes takes a back seat to antenna religionists and self-appointed antenna evangelists.  

N0YXB
Vince
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Vince
W3LK
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Posts: 5644




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« Reply #7 on: June 29, 2009, 09:31:07 AM »

Richard, the ARRL antenna manual definitions notwithstanding, I guess we'll have to agree to disagree on the definitions. I was taught differently by my elmers back in the 60's. Thanks for your response.

Vincent:

It's too bad people cannot have an honest disagreement without someone not even  a part of the discussion jumping in and start name calling.

73,

Lon - W3LK
Naugatuck, Connecticut
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WW5AA
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Posts: 2088




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« Reply #8 on: June 29, 2009, 10:49:43 AM »

"A dipole antenna, developed by Heinrich Rudolph Hertz around 1886, is an antenna that can be made by a simple wire, with a center-fed driven element for transmitting or receiving radio frequency energy."

Now let's all look up what the real zeppelin (Zepp) antenna looked like.

73 de Lindy
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G3RZP
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Posts: 4365




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« Reply #9 on: June 30, 2009, 03:18:14 AM »

An interesting discussion. So I figured I'd have a look in my textbooks.

'Radio Engineering', F. E. Terman, McGraw-Hill 1936 doesn't even mention a dipole. The 3rd edition, (1951) is the same.

The Royal Navy in the 'Admiralty Handbook of Wireless Telegraphy, 1938' defines a dipole as 'any symmetrical aerial the two ends of which are at opposite potential with reference to the central earthy point'

That doesn't help much.

Lamont V. Blake, in 'Antennas', Wiley, 1966, speaks of analysing wire antennas by considering them to be made of a number of elemental dipoles in series, which suggests that they don't need to be centre fed.

The UK 'Services Textbook of Radio, Volume 5, Antennas and Propagation', Glazier and Lamont, HMSO London 1958, in talking about Hertzian dipoles makes the point that the current at the ends must be zero, and uniform along its length. That doesn't help, but short dipoles have a linear distribution of current along them, being zero at the ends, and so must be centre fed.

H. Page in 'Principles of Aerial Design' Iliffe, London and D Van Nostrand, Princeton, N. J., 1966, says 'A linear antenna energised at the centre is usually called a dipole'. That seems pretty definite at last.

The ITT 'Reference Data for Radio Engineers, 6th Edition doesn't mention how the dipole is fed.

'Antennas for all applications', Krauss and Marhefka, 3rd edition, 2002, McGraw Hill doesn't define the feed point, but doesn't consider under 'dipoles' any other feedpoint than the centre.

I haven't looked into the 'amateur' books, although wikipedia (for what that's worth) defines it as being centre fed. So it seems that a dipole may need to be centre fed if you take one definition, and not if you take another - depending on which professional textbook you take. I'm not sure what the IEEE definition is, so I've signed up for a free trial to IEEEXplore. You'd think that as a member of the IEEE Antennas and Propagation Society, it would be easy to find this info!

I'll let you know how the IEEE define it...

73

Peter G3RZP
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N3JBH
Member

Posts: 2358




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« Reply #10 on: July 07, 2009, 04:22:28 PM »

"I am suspicious of this antenna, although I have never tried one. It sounds too much like another smoke and mirrors contraption called the MaxComm tuner - a 50 Ohm resistor across the output with a piece of wire connected to one side."

Well Lon i do understand the reason you wonder.. But it is not a maxcom and sure is a lot less money too. The web site pretty well explains there is no silly stuff in there to make it a dummy load.... No resistors etc...

The object that makes this work is a TLT - Transmission line transformer That pretty well is it in a shell.
There now being used by several hams and  just starting to be used by a few goverment agenices for emergency prepardness and response work. They will also see extensive field testing at a drill involving none other than Northwest airlines this fall.

Now skeptical or not of these antenna's i say that is a pretty darn good group of customers that are using them... Jeff --- I don't own the company but sleep every night with the owner Smiley
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WX7G
Member

Posts: 5917




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« Reply #11 on: July 07, 2009, 05:24:23 PM »

From the description and the reference to the Comet CHA-250B antenna the EFAB end fed antenna uses a lossy ferrite or powered iron core transformer to work the 25' EFAB wire against the coaxial feedline shield.

The Comet does cover the HF bands with a tunable VSWR but with radiation efficiency as low as 1%. The EFAB, if similar, will suffer the same low radiation effciency on some frequencies. But like the Comet one can make contacts with it.

I'm tempted to order one, analyze it, and write a review for AntenneX.
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N3JBH
Member

Posts: 2358




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« Reply #12 on: July 07, 2009, 07:10:17 PM »

WX7G If your email on QRZ is good well then i sent you a email with some information about getting one to test it... I personally would welcome it and would love to see a real and honest  comment about it...

We dont have anything to hide or trying to pull a shady dealing... It is a good product that done what it claims to do. What it is not is a dx'ers dream antenna nor is it ever going to kick a yagi's hind end. But it will get you on the air and let you make contacts on 10 bands and offer you the most bang for your money compared to any thing else out there....
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