HF OCF Question

[WB4CS]

As I'm getting ready to set up an HF station at my new home, for right now I'm looking at just getting some wire in the air so I can get back on the air. In time I'll put up a more efficient antenna.

One issue I have is a bare lot with NO antenna holders, AKA Trees. Until I can get some masts in the air, I was thinking of going with a Radiowavz DX 80 OCF.

In the image below is a drawing of the proposed setup viewing top down. Apologize for the crudeness of it but I just quickly jotted it down on a whiteboard. My idea is to use the chimney as the feed point, that's probably up about 30 feet. Then take the short leg (S) and long leg (L) of the OCF and bring them down to the top of my privacy fence, which is about 8 feet tall. Problem is, in this configuration the antenna would be in a sort of L-Shape.

Those of you with more antenna experience than me, how do you think the height and shape would effect the OCF performance? I know it's not an ideal antenna, but it gets some wire in the air for now until I can get something better.

Ideas? Thoughts?

 

[sloop]

Have you thought about setting it up as an inverted V? Check out Windom Antenna Home Page, and Handbook this site has tons of info about the windom from math behind the ant to set up possibilities.

 

[WB4CS]

Yes, I have thought about it and may possibly do an inverted V at some point. Right now I don't have anything to use as the mast or feed point for an inverted V, except my chimney in the drawing above. So technically the OCF in the drawing (or any other wire antenna I used) would be an inverted V if installed as I've drawn. Except it would have an L shape as viewed above.

In the next few months I'll have the material I need to erect 4 masts that I plan to use for an 80 meter skywave loop, but until then I don't have much in the way of masts to mount to.

 

[AK9R]

I have a Buckmaster OCFD which I believe is similar to the Radiowavz antenna. I used it for a while and then took it down. I think my biggest problem with it was that I didn't get the feedpoint high enough off the ground. I think Buckmaster recommends something like 30 feet and I was only around 15 feet.

You may experience some pattern changes and SWR weirdness by not running it in a straight line, but I don't think it will be a big deal.

FWIW, I replaced the Buckmaster OCFD with a home-built fan dipole for 80, 40, 20, and 10 meters. The center is still only about 15 feet off the ground with ends dropping down to 6 feet. It works OK, but I could sure use some more height. Nonetheless, I worked the W1AW/KP4 station in Puerto Rico on 80m just before dawn this morning from central Indiana.

 

[GSPD]

I recently installed an OCF dipole similar to the Buckmaster OCF antenna that replaced my G5RV. Your configuration should work and not be affected by the slight L shape. Of course many things come into play, center height, end height, balun, etc.

Some key issues have been RF getting back in the shack which was a concern on an OCF antenna. This can be resolved by using a good balun. I constructed my own OCF to save money since the high power rated Buckmaster price couldn't be justified for a wire antenna. I researched baluns and settled on a Balun Design model used for OCF antennas. The center is mounted just under 30 feet so I used a 4:1 balun for my installation. I had to position the legs similar to your layout and have the whole setup in some trees. I assembled the antenna for about $120.00.

I'm very satisfied with it's performance so far for a multi-band antenna. With the nicer weather coming I'll try some fine tuning. Good luck with your decision and remember there is no perfect multi-band antenna.

 

[vagrant]

You should be fine with that. I have put my OCF into a true L configuration "to test" and I had no ill effects on the meter, analyzer and transmitter. The apex is 35' AGL on my inverted V and a concern with an OCF antenna is keeping the legs apart at least 120 degrees.

At the height I'm at a 4:1 balun at the feed point does the job. I presume a 4:1 should be fine for your height as well. The OCF antenna I'm using is good down to 40 meters with 40' and 20' something legs as I have a limited area to work with.

The one thing that might help you is a 1:1 current choke below the feed point 4:1 balun. I would not order this right away. You may not have any RF issues in the shack, but if you do, that should work to resolve it. A run of a few feet of coax from the 4:1 to the 1:1 choke is fine. You can even build a 1:1 choke vs buying one.

 

[LtDoc]

With an OCF antenna, an unbalanced antenna, why would you need a balun at all? An 'unun' maybe, but not a balun.
- 'Doc

 

[WB4CS]

Thanks all for the suggestions and ideas!

Next weekend if the weather cooperates I plan on taking some rope and rigging up a mock-antenna to see if this will even work. After I got to looking at the house I'm not sure if the height of the chimney will allow me to slope down to the privacy fence.

 

[N1BHH]

With an OCF antenna, an unbalanced antenna, why would you need a balun at all? An 'unun' maybe, but not a balun.
- 'Doc

A guanella (or current) balun is required for an off center fed dipole. The feed point impedence will be around 200-250 ohms. Pretty poor match fo 50, or even 75 ohm coaxial cable. A current balun forces currents equal in amplitude but opposite in sign (180° out of phase) to flow through its output terminals. A current balun may simultaneously act as an impedance transformer, changing the voltage-to-current ratio of the output with respect to its input. The feed point height will determine what value is needed for the balun. A 4:1 ratio is good for antennas up to about 45 feet high, 5:1 for those up to around 60 feet and 6:1 for higher heights. If you get that antenna feed point in a tall tree it will perform quite well no matter how you lay out the wire elements, as long as they are not parallel.

 

[LtDoc]

I'm sorry, but a balun is not height dependent. It is dependent on the type of antenna being fed with it, it's impedance and if the thing is 'balanced/unbalanced'. An OCF antenna is not a balanced antenna, and that's why I recommended an 'unun', which can also adjust impedances.
- 'Doc

Antenna height can affect it's input impedance but it's not the 'determining' factor.

 

[prcguy]

For OFC dipoles its common to change balun ratios depending on height. When they are near or below a 1/4 wavelength a 4:1 is common to match the 200-250ohm impedance and when up around a half wave a 6:1 might give a better match since the impedance is closer to 300 ohms.

They also usually use a balanced to unbalanced transformer and a current type to better isolate the antenna from the feedline.
prcguy

I'm sorry, but a balun is not height dependent. It is dependent on the type of antenna being fed with it, it's impedance and if the thing is 'balanced/unbalanced'. An OCF antenna is not a balanced antenna, and that's why I recommended an 'unun', which can also adjust impedances.
- 'Doc

Antenna height can affect it's input impedance but it's not the 'determining' factor.

 

[KC8ESL]

Edit: Irrelevent.