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Sunday, September 11, 2016

I accidently came across a post on radioreference and thought this lone post was very interesting. Maybe a good idea for someone with very little real estate. thanks Hertzian!

  End-Fed OCFD

I wanted to put some items that weren't being used to good use, and came up with a general purpose end-fed OCFD for hf. I'm currently using it for rx-only.

Essentially, it is a 4:1 UNUN in the center of the ocfd dipole, and at one end of the transmission line, is an 1:1 RF choke balun, and after that another run of coax into the shack. Being basically a non-resonant antenna, I'd keep jumper 2 no longer than about 50 feet to help keep the swr losses down.

Schematically it looks like this:

[ Receiver ] ----- jumper 2 ----- [1:1 RF Choke] ---- jumper 1 ------ [4:1 UNUN] ------------- wire -----------------

*The coax jumpers are 50z LMR200. Jumper 2 is about 25 feet from the receiver to the choke just to get it outside.
* The 1:1 RF choke is an MFJ 915
* Jumper 1 is 12 feet lmr200. (serves as transmission line AND antenna element)
* The 4:1 UNUN is from LDG. Nothing connected to ground side.
* The end wire is 22 feet long, a little less than twice the length of jumper 1. Attached to ant connector of the UNUN.

Essentially all I'm doing here is utilizing the common-mode of jumper 1 to serve as one side of an off-center dipole, and as a transmission line at the same time. This is a non-critical antenna for me, so I just make the end wire twice the length of jumper 1. Jumper 2 just gets it from the antenna to the shack.

Subsitute your own components if you like. Critical however is the RF choke. The MFJ was sufficient for now, although I'll probably end up with some stuff from Balun-Designs LLC.

The antenna is relatively broadband, and matching is not critical, although if you want to, you could use a tuner. I found no need for this non-critical application. Works well from 10mhz up to 28. If I wanted better low-band coverage, I'd increase the length of both jumper 1 and the end wire - but generally trying to keep a 1:2 ratio (about 1/3 off center) to make it easy on the 4:1 unun. Again, not entirely critical.

Mount it any way you like, and see what the reception lobes are like. In my case, I needed an impromptu stealth single-wire antenna mounted away from the house using only a wooden garden lattice for support. Jumper 2 runs out the house, jumper 1 runs along the ground, and the wire is run up vertically along the lattice for about 8 feet high, and the rest horizontal. Envision an inverted-L with only 1 radial wire on the ground. Mount yours any way you like.

Not the best, BUT it is outdoors, away from my shack noisemakers, and the rf-choke help prevent rf-ingress from those noisemakers, as well as making jumper 1 do double-duty as a transmission line and one side of the ocfd.

I'm not going to over-analyze this thing. I put it up, and let the lobes fall where they may. From about 10 to 30 mhz, my swr as read on an antenna analyzer is about 4:1, which is ok for reception, BUT swr is not the whole story!

Fun for sure. It goes into my antenna bag of tricks notebook.

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