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Author Topic: End-fed dipole -- does it work ?  (Read 1826 times)
VA7CPC
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Posts: 2386




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« on: December 05, 2003, 03:30:00 AM »

In the ARRL Handbook, there's a brief discussion of a "resonant feed dipole".   It's an end-fed dipole.  There's a common-mode choke on a length of coax, then roughly 1/4 lambda of coax, with the end of the shield _open_.  Then, a 1/4-lambda length of wire is soldered to the center conductor.

So the bare conductor is one leg of the dipole, and the shielded section of coax is the other.

I've found one person who used one on 2 meters and says "it works well", and one person (on the Web)who says "it doesn't work _nearly_ as well as a center-fed dipole".

For a sailboat owner (me), it would be really neat if it _did_ work -- the fed end at deck level, and the free up up toward the masthead.  Just one wire, no need for a ground plane or counterpoise.

Does anyone have any experience with these things?  I don't have enough "RF intuition" to figure out whether it should be a good antenna or not.

There is a strange antenna called a "PAR End-Fedz" which _looks_ similar.   That one, I'm almost sure _should not_ work -- it's a long wire with no counterpoise, 1/2 lambda long, end fed.   But "RF intuition" is often wrong.

Any help unraveling this mess would be appreciated.  Thanks.
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WB6BYU
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Posts: 13238




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« Reply #1 on: December 05, 2003, 05:23:33 PM »

There have been a number of variations of these end-fed
half wave antennas (and others) over the years. Let's
look at them individually.

The "resonant feedline dipole" is simply a center-fed
dipole that uses the outside of the coax for one of
the dipole legs.  (Normally we would add a balun to
prevent current flow on the outside of the coax, but
in this case that is the only place we give it to flow.)
The key is the efficiency of the RF choke that is
supposed to keep all the current on the 1/4 wave part
of the coax and not on the rest of it.  The impedance
at the end of a dipole will be over 1K ohms, and a
good choke should have at least 5 or 10 times this
reactance.  I've seen these built using 6 or 8 turns of
coax through a large ferrite core, but this comes
nowhere close to the ideal impedance for such a choke.
On 20m you'd need about 20uH of inductance with less
than 6pf of stray capacitance:  I suspect you will have
better luck winding a coil with lower inductance and
connecting a capacitor across it to tune the whole
assembly as a trap at the operating frequency.  (Use a
high voltage capacitor.)

However, if you are over salt water, even an imperfect
vertical antenna should put out a good signal!

The various "single wire feed" half-wave antennas date
back at least to the Zepp, where only one side of a
balanced feedline is connected to a half wave wire.
The idea being that the input impedance of a half wave
antenna is very high, so the current is very low.
Connecting this to one side of the feedline and an
infinite impedance (open circuit) to the other side
isn't too bad of a match.  And certainly thousands of
hams have used such antennas over the years, usually
with good results.  (This is the basis of the J-pole
antenna.)  Instead of a shorted quarter wavelength of
open wire feedline, some commercial antennas use a
parallel-tuned circuit.  I suspect the coax shield
becomes a significant part of the antenna in many cases
but again this may still work.  Personally I'd try to
ground the other side of the parallel tuned circuit to
the back rail or other good metal object.

If you are looking for a temporary antenna to hang
from a halyard while at anchor, then a wire J-pole
may be a better choice, though either of these could
be adequate if well built.  You can also hang a vertical
dipole of the standard type sloping down from the mast
to the stern, with the coax pulled forward before it
drops into the cabin.

One of the best antennas is to install some insulators
in the backstay and connect it at the bottom to a well-
grounded automatic antenna tuner.  this keeps everything
pretty much out of they way when under sail.
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N3CJN
Member

Posts: 34




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« Reply #2 on: January 07, 2004, 02:09:48 PM »

I don't own a PAR end fed dipole (yet), but I believe that it is a half-wave radiator end fed through a matching network to match the high impedance at that point to a 50 Ohm coax.  In the case of that commercial design, I don't think feed line radiation is
part of it.

My supposition is that the matching is set for the band it is cut for, and that it would not generally be usable on other bands.  However, an ad does say that it is designed so that you can replace the radiator with a wire of a different length.  

You've probably noticed the favorable reviews here on eHam.  They have me intrigued enough to consider trying one.
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KF4DX
Member

Posts: 2




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« Reply #3 on: March 03, 2004, 04:47:04 PM »

Also you might try a J type antenna.  Just find the formula for a 2 or 6 meter version and substitute values for the desired frequency.  Toss a half wave for 80 or 160 meters over a remote tree and pull the matching/feed toward the shack.  Have used them and they work great.  
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AA5CB
Member

Posts: 1




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« Reply #4 on: January 20, 2009, 07:19:55 AM »

I've used several Resonant Feed-line Dipoles over the years, including 2-meters and 40 meters.  I recently replaced my 'quickie' 40 meter RFD with a more traditional 40 meter dipole at the same location and I am somewhat disappointed with the results.  The 40 meter dipole seems to have more baseline noise and the band seems less active.  I gave my old 40 meter RDF to my dad and he's having a ball working the world. Oddly enough the 40 meter RFD tunes well on most bands with the auto tuner, including a rather long list of good DX contacts on 80 and 20 meters.  Go figure.  Needless to say I'm planning to build another, more permanent, 40 meter RFD in the near future.  

I constructed it entirely from RG-6U (cheap and available from the hardware store): ~1/2 wavelength total length with the shield cut in the middle then far end shield shorted to center conductor, and 18-feet of feed line coax wound on a 4" PVC pipe with the remaining RG-6U running to the rig.  Simple, cheap, easy to install (one end attached to the house with the rest laying across the top of trees) and works the world on 80-10 with a tuner.  

Others have reported poor results with RFD's, but looking into some of these reports shows that they either scramble-wound the choke (a big no-no) or used only 6 or so turns on a small form.  
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