End fed di(mono?)pole construction

[kd0fx]

I could use a little advice on construction of an end fed dipole. I'd like to make a 1/2 wave 80M end fed horizontal wire antenna, fed by 50 ohm coax. I'm guessing that the center conductor becomes the radiating wire of the antenna. Since there's no counterpoise or 2nd leg, I guess the coax shield just goes unused? Trim it back to the insulation?
It looks really simple so I must be missing some details.

Future question, if I trim it accurately enough to become resonant on 80, can I add a 40M trap later and use it as a dual band? More traps for more bands? I've never thought about traps before, so I'm ignorant to their use. I'd like to build my own if possible but I don't even know what they are made of.

 

[RickC.]

I could use a little advice on construction of an end fed dipole. I'd like to make a 1/2 wave 80M end fed horizontal wire antenna, fed by 50 ohm coax. I'm guessing that the center conductor becomes the radiating wire of the antenna. Since there's no counterpoise or 2nd leg, I guess the coax shield just goes unused? Trim it back to the insulation?
It looks really simple so I must be missing some details.

Future question, if I trim it accurately enough to become resonant on 80, can I add a 40M trap later and use it as a dual band? More traps for more bands? I've never thought about traps before, so I'm ignorant to their use. I'd like to build my own if possible but I don't even know what they are made of.

Welllll....

You cannot feed an end fed half wave directly with 50 ohm coax. I don't want to talk down to you, but that is a high voltage and therefore high impedance point. Instead of 50 ohms the impedance is on the order of around 4K, give or take a couple hundred ohms.

There are a few ways around this. Rather than clutter up the board here, I'll just suggest this-- and you'll probably answer your own questions (which would probably be more effective anyway).

Do web searches on:

1. Zepp antenna
2. End fed half wave
3. Tank circuits

I don't have any links handy, or I'd post them here-- but none of it is hard to find. I've built a few end fed antennas and they can work very well, but you have to figure out which method of matching works best for your application. Also, they're single-band affairs unless you can adjust the matching network. Just curious-- why end fed?


Rick

 

[kd0fx]

OK, I knew I was missing something.

I'm going for an end fed simply because that will match the lot shape and shack location best.

 

[Beetle]

A dipole, by generally accepted definition, is a half-wave, center-fed antenna. "End fed dipole" is wrong terminology.

You can end-feed a half-wave wire on any band, but you need more than a piece of 50-ohm coax to feed it with. You'll need a good radial system (for a vertical), or a very good counterpoise. Then you'll need a low-loss feedline and a good matching network to transform the high impedance which the antenna system will present at the feedpoint into something the transmitter can handle.

 

[HomerBB]

I have just hung an "end fed sleeve dipole", as I've heard it called, for CB band up. It is directly fed with 50ohm coax with a coax choke in line at the point where it runs into the end of the lower (shorter) end of the antenna. It is mounted vertically, and has a 1.1:1 SWR at 27.405 and 1.2:1 SWR at 26.965. I made it out of 1/2" tubing with the coax fed up through the lower tube to the feed point where the the upper and lower tubes are separated by about 1".
apparently, the antenna is not technically a dipole that is end fed by the descriptions given in the foregoing posts. I had no calculator for the dimensions for an 11 meter antenna of this sort, so I took dimensions I found online for a different wavelength and worked it out the best I could from there. I came up with 122" for the longer tube, but went with 120" because that was the length of pipe I already had, and 84" for the bottom tube but trimmed to 83" because of the shortening of the other end.

I have no real idea what I've actually made if not a dipole, but it works well so far, it seems.

I'm not sure if my experience helps in any way, but there it is.

 

[W5LZ]

What you've made is a center fed dipole, the feed point is in the center. The feed line can certainly be 'fed' up through a tube tot hat center feed point. You can do the same thing by peeling the braid back over it's self for a 1/4 wave length. That'd give you a 1/4 w of center conductor, then the feed point, then a 1/4 w of braid back down and over the feed line. That's fairly common of the higher bands, not so common on the lower ones (requires more coax).
Since there's very high voltages at the ends of a 1/2 wave antenna (end effect), a choke located there is a very good idea.
- 'Doc

 

[RickC.]

I'm going for an end fed simply because that will match the lot shape and shack location best.

Good reason, I've done the same. Well, using an open wire feedline match could be a solution but you'd have to bring about 65' of it off at as close to a right angle as you could manage and avoid contact with metal, etc.

A tuned circuit is more compact but not exactly small at 4 MHz.


Rick