How to make an end fed dipole?
I was thinking, Im getting set up for a camping station. While I will have a dipole, It sure would be nice to have a antenna when I have only on support. Is there a way to make one easily? I have some 50 Ohm coax and 16 guage wire. Is there an easy way to make one with parts from radio shack only?
I would like to have 100watts but 50 would be bearable. Im fine with a counterpoise system also.
Dipole = two wires ("di") pole.
No such thing as an end fed dipole. You can have an end-fed half wave antenna that works like a dipole, though.
For a single band, use 1/4 wavelength of open wire or ladder line to feed it, and it becomes an "end fed Zepp," which can work pretty well.
If you have one single elevated support for an antenna, a real dipole (center fed) installed as an inverted vee is less fussy.
An alternate definition is that a dipole is two opposite charges, in which case and end fed dipole is not inconsistent.
However, I have to answer the OP's question with a question. Why does the physical configuration (only one elevated support) mandate end feeding?
You could centre feed a sloping dipole, or centre feed an inverted V.
Yes, if it's fed at the end, then by definition it's not a dipole.
There are two easy antennas you can make, however. The first is what 'WIK suggests, and make an inverted vee. Put the center of the dipole on your support, and run the two ends to any available location. That can be the ground--just use a tent stake or something to secure it.
Another easy antenna is a quarter wave wire, which is best if it's more or less vertical. But it need not run straight up and down. A 45 degree angle, or even shallower, will probably work fine.
There are two ways you can do that. One is to run your coax to a convenient point on the ground and go up from there. For an even easier setup, you can simply run the wire directly to the center pin of your rig's coax connector. A "banana plug" is helpful for doing this. This will work better with a lower power level. It works great for QRP. With higher power, you'll have a lot of RF floating around the rig, which might possibly cause problems. On higher frequencies and/or higher power, it might also cause RF exposure issues. Right off the top of my head, I would say that 50 watts on 20 meters won't cause any such issues (but you should check it yourself, since you are then one foot away from the antenna).
With the quarter-wave wire, you do need as good a ground as possible. Wherever the wire starts, the ground should be attached at that point. Since you probably don't have ground rods with you, just run any available wire along the ground. Multiple radials is even better. Attach these radials to the braid of the coax. Or, if you plug directly into the rig, attach them to some convenient bolt on the chasis of the radio.
Just a nit from The IEEE Dictionary: "dipole antenna - any one of a class of antennas having a radiation pattern approximating that of an elementary electric dipole." Nothing said about the conductor being broken in the middle.
Originally Posted by WB2WIK
As Owen implies, the word "dipole" does not refer to two physical fishin'-type poles. It refers to the two (and only two) electric (voltage) poles at each end of a dipole.
Hint: a gamma-matched 1/2WL single element is still a dipole. The single element directors and reflectors on a Yagi antenna are dipoles. An end-fed Zepp antenna is a dipole.
The professional definition of "dipole antenna" certainly includes 1/2WL end-fed dipoles. It is the colloquial ham lingo use of "dipole antenna" that is different from the professional definition. I vote for the IEEE definition.
73, Cecil, www.w5dxp.com
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AA5TB has a nice discussion of end fed antennas with recommendations for construction. I haven't tried his plan yet.
Why would someone desire an end fed antenna? Simplicity of deployment, e.g. throwing over a tree limb, dropping out an apartment window. We apartment dwellers can get pretty desperate!!
I think you are quite right, except that votes don't determine facts... a concept a bit alien to ham online fora! Voting often shows some misconceptions to be very popular, perhaps even mythical.
The misleading things about most "end fed half wave dipoles" is that the description ignores the path for current at the feedpoint other than that on the dipole conductor. Many of the configurations are three quarter waves, fed one quarter wave from one end, eg J Pole, the common End Fed Zepp.
It is the acceleration of charge from one side of the dipole to the other that gives it its radiation characteristic, even if it is a lot less than a half wave in length (the latter causes difficulty with a lot of hams who insist that a dipole by definition is a half wave in length).
To the OP, needing to connect one side of your feedline to dirt introduces a variable and almost always lossy element to the system. The concept that you can lay a "wire counterpoise, tuned or not' on soil doesn't fully address the problem as the fields due to current in that wire will heat the soil more than if they were not in such close proximity.
The disadvantage of unbalanced antennas is they typically create all sorts of problems with RF induced into the equipment. Of course, that can also happen with a balanced antenna that's simply "too close" to the equipment, or balanced antennas fed with unbalanced feedlines, without baluns.
Originally Posted by KB4QAA
Lots of bugaboos are possible, but end-fed ones seem to introduce more of them.
I know that you mentioned coax fed end fed wire, there is an end fed half wave wire that is fed with ladder line. Trouble is in certain situations the feed line is very susceptible to radiating and picking up lots of noise. I used to think that they were panacea but more trouble than they are worth many times.
If you absolutely have to have an end fed half wave with coax feed then I suppose, controversies aside, the easiest would be the J - Pole.
Last edited by N5YPJ; 11-20-2010 at 03:25 AM.
I thought you guys might bring it up, Yes I can and plan to include this in my go kit, but if it is short term Im not really interested in throwing up a rope, pulling up the wire, staking it down and the other end.
Say for 20m how would I make it?
Coax taking off the shield leaving the coax and insulated weight? What should I do to the shield? Should I roll up the coax a couple of times? Im looking to be able to work (almost) every station I hear, so no perfect rx.
I guess im realy not going to run 100watts or even 50 continually, but if im in the middle of a qso I would like to know that I can turn it up if conditions get a little worse.