Thinking about putting up an OCF

[ke5frf]

I currently have two antennas...an 80m doublet that I feed with 300 ohm twin lead and a wire vertical that is REALLY just a hastily thrown up vertical dipole that didn't make it to the top of the tree when I hoisted it up, so I added hald-arsed radials.
My vertical works "ok" on 15,40,30,10, and 6 meters. I don't waste my time on any other bands with it because it either won't tune well or is too short.

Anyhew, My doublet is supported by the one large tree in my yard and is basically an inverted vee. I have only one other way to install a horizontal antenna, and that is as a sloper perpendicular to the doublet. I was thinking of shooting a line to the very top of the oak tree and hoisting an OCF by its end...then anchoring the other end to a high point on my house, which would be a sloper as I see it in my head.

A few things come to mind. First, the distance isn't long enough for 80 meters, so it would have to be a 40 meter OCF.
Which brings me to this...From what I've read about OCFS, it is REAL CRITICAL that not only the overall length be correct, but also getting the feedpoint just slightly off from the "sweet spot" will put it at a high impedance point. ESPECIALLY on the high bands it works on. A 40 meter OCF is supposed to work well on 40,20, and 10 meters. From my understanding, the reason the higher bands have trouble is because with multiple wavelengths, there are several points along the wire that have a current maximum and thus a low impedance. Alternately, there are just as many low current spots which means high impedance and thus the SWR will be high. Getting the feedpoint wrong by just a few inches affects this greatly. On the band it is cut for, there is only one current maximum spot directly in the middle, however the current at the OC feedpoint is still high enough to be a good impedance match especially with a balun of a certain ratio...6:1 to 9:1 i've read.

Anyway, a few thoughts....I am always an advocate of "rolling your own"....but I've seen real good reviews on some prefabbed jobs. I do not care to build my own balun, so even if I built the antenna myself I'd be dishing out at least $50 for a good balun. Is this maybe the one wire antenna where a prefabbed job might be a better deal? I absolutely have no problem building my own, just asking thoughts. All my current antennas are homebrew.

Next thought....feedline. The popular buckmaster OCF is supposedly good without a tuner and is coax fed. That sure sounds nifty, but coax is always so lossy. I don't have a great distance here, just about 50 feet, but I've also seen balanced fed OCFs...Carolina Windoms I think? Balanced line might help it work on other bands better too, I suppose. I know tuning coax fed antennas is seldom efficient at all.

Next thing...feedpoint. Being that its a sloper idea, seems the long end would be the radiating element and should be the one pulled into the tree. However, that would put my feedpoint quite low. Do any of you know if its crucial where the feedpoint is when configured as a sloper?

Also, here's a potential problem. The ladderline for my doublet will be sort of parallel to the OCF radiators...not right nect to it but it will get closer as it gets to the house....Potential problems with capacitive coupling?

[K1OU]

Heath,

How high is the tree? How much space do you have from the base of the tree to your property lines?

[N9FE]

what bands are you going for here, first take take 300 ohm crap off a pitch it, don't ever run power through 300 ohm, or you will be in for problems, second off center feds work, but what do you want to do local or dx, slopers are very directional, so we need more info here, ok

[K1OU]

I'm thinking that you should try stringing an inverted L. My experience is that a 3/8 L cut for 80 meters and fed with 450 at the base, with as many radials as possible, with at least eight of them being cut for the lowest frequency is well worth it. This concept has been proven time and again to be extremely effective for 80 through 10 meters.

Also, try http://www.cebik.com


It's a great site, well worth a bookmark.

[wd0ct]

I like the inv L idea too but radials are a bit of a pain. In addition to cebik check out w8ji.com
for info about where to feed an ocf versus match on different bands.

No ocf will have a good match on all bands no matter what is claimed.
It is just wire and a balun so just do it and see how you like it.

There are several good hb balun links that will turn up with a google search.

Coax of 50 feet is fine for ocf.

btw, 300 ohm ladder line on the 80 dipole would be mo better.

[N9FE]

Yes thats why we need more info on what is trying to be done here, you can put the ends higher than the center on a windom and it will tighten up your pattern, more local, inverted, ends lower, longer pattern, dx, theres all kinds of things that can be done with limited space, i have about 100 feet up and like 12 feet on each end hanging stait down, works great, so we can help you when you figure out what you want to do,,,,

[ke5frf]

I'll add more about the antenna this evening when I get home. Spent too much time online today already.
It really isn't all that hard to explain.

[ke5frf]

http://img354.imageshack.us/img354/4726/ocfdoubletdq1.jpg

OK this is about the dimensions of my home and yard and how my current doublet looks.
The doublet is roughly 130 feet long give or take...it is actually fed with 300 ohm ladderline, not true twin lead. Sorry for the confusion. It is the good dxengineering stuff.

I didn't draw my vertical because I plan on getting rid of it, it only works "OK" as it is and the feedline gets in the way.

OK, the dimensions aren't exact as the doublet leg to the bottom of the picture isn't quite so "long looking" as compared to the other side. They are actually pretty equally spaced considering.

The height of my doublet is only about 35 feet in the tree. One of these days I'm going to manage to get the feedpoint higher. I have shot a line up into the tre 5 times to hoist it up, but ever time it seems to get "snagged" at about 35 feet or less and won't budge without snapping my pull up line...so for now I'm settled on where it is.

However, in the scenario of putting up the doublet in a sloper shape, I will be hoisting one end of the antenna into the tree by the end of the wire rather than the center insulator. I think I can pull it up closer to 50 to 60 feet high in the tree this way. The other end I plan on securing to a point on my roof.

I have no worries about building the antenna myself...I suspect it wouldn't be too tough. I just don't want to hoist it into the air only to find the feedpoint impedance is way off and unusable. Its going to be a chore to get it up anyway. Thats why I was considering a prefabbed job that should be close to resonant from the factory.

Anyway, The doublet I have works great on 20 meters, 15 meters, 30 meters...really it works pretty well on every band I tune it for. It makes local contacts on 80 meters. But It COULD do better IMHO on 40 meters. I hear DX on that band that often has trouble hearing me. It was better in the winter than it is now.

I was looking at a 40 meter resonant designed OCF at a half wavelength...that SHOULD be easily accomodated by my tree and house as supports.
I would really like a good performing antenna on 10 meters too, and I suspect that the OCF would work. I also suspect it would tune well on 15 meters. OCFS are supposed to be good WITHOUT a tuner on even multiples like 20, 10 given the resonant 1/2 wavelength is 40 meters.
I would be happy getting rid of my half-arsed vertical for an OCF given the above circumstances.

The other side to that is the various OCF designs...of course a coax fed OCF won't tune up as well on other bands and will be lossier through the coax...so would a balanced line OCF (windom?) be better? I have indeed read all the websites I can find on OCFs and also looked at the antenna handbook. But coming down on "what is better" is a matter of experience of which I have none.

[wd0ct]

"The other side to that is the various OCF designs...of course a coax fed OCF won't tune up as well on other bands and will be lossier through the coax...so would a balanced line OCF (windom?) be better? I have indeed read all the websites I can find on OCFs and also looked at the antenna handbook. But coming down on "what is better" is a matter of experience of which I have none."

The deal with ocf is a reasonable impedance [with balun and coax] on several bands. They do this quite well and the pattern is almost exactly the same as a center fed of the same length on the same bands. I see no point at all in deliberately making an unbalanced antenna [ocf] and feeding it with balanced line.

If you are familiar with a modeling program [I'm not], model the impedance of the new ocf and run coax loss for the impedance, coax type, length, etc. If it is within 1db or so from what ladder line/tuner will do then you are set [with coax]. Human ears can barely tell 1db under ideal conditions.

What I should have said: it is just a 4:1 balun and some wire. Quit over analyzing it and just put it up. I don't think store bought is any better than a regular hammie can do.

[N9FE]

it looks like you did a good job snaking that doublet in your yard, you will get better results with 450, or better yet 600 ohm, at 135 feet that doublet presents two half waves on 40 to the tuner, which will be very touchy on 40 meters, what i would do is get that doublet playing better, and maybe go with a good vertical, the reason is your doublet tweeked up with 600 ohm all the way to the tuner will stomp the living snot out of a windom, the only windom that i have ever heard perform well is a friend of mine stripped the goofy iso-res balun and the coax off the thing and went with 600 ohm all the way to the tuner, keep asking around, see what you come up with, but please don't blow your hard earned money on something that just won't do it for you,

[WV6Z]

In addition to the dipole that Mike sent to me, I am going to be going up with an OCF, similar to Radio Works 80 Special shortly. BTW Mike, I don't think you have heard the antenna you gave me yet..... where ya hidin'?

[N2NH]

it looks like you did a good job snaking that doublet in your yard, you will get better results with 450, or better yet 600 ohm, at 135 feet that doublet presents two half waves on 40 to the tuner, which will be very touchy on 40 meters, what i would do is get that doublet playing better, and maybe go with a good vertical, the reason is your doublet tweeked up with 600 ohm all the way to the tuner will stomp the living snot out of a windom, the only windom that i have ever heard perform well is a friend of mine stripped the goofy iso-res balun and the coax off the thing and went with 600 ohm all the way to the tuner, keep asking around, see what you come up with, but please don't blow your hard earned money on something that just won't do it for you,

At 600 Ohms, wouldn't you need a 12:1 balun for a balanced line if you want to run the last few feet with 50Ohm coax to the rig, amplifier or tuner?

it looks like you did a good job snaking that doublet in your yard, you will get better results with 450, or better yet 600 ohm, at 135 feet that doublet presents two half waves on 40 to the tuner, which will be very touchy on 40 meters, what i would do is get that doublet playing better, and maybe go with a good vertical, the reason is your doublet tweeked up with 600 ohm all the way to the tuner will stomp the living snot out of a windom, the only windom that i have ever heard perform well is a friend of mine stripped the goofy iso-res balun and the coax off the thing and went with 600 ohm all the way to the tuner, keep asking around, see what you come up with, but please don't blow your hard earned money on something that just won't do it for you,

At 600 Ohms, wouldn't you need a 12:1 balun for a balanced line if you want to run the last few feet with 50Ohm coax to the rig, amplifier or tuner?

Indeed yes you do need an impedance transformation to make this work. If you are looking at only one band, you can use a stretch of 50 ohm coax to feed the 600 ohm ladderline that would transform the impedance and properly match it. Also, you could use a stub. But a broadband transformer would work over several bands.

[WV6Z]

The home brew OCF Carolina Windom is up and is performing like a charm so far. The TS-570's internal tuner can tune her up anywhere almost instantly with ease, with the exception of course, of 160, 60 and the top end of 10m. I thought about posting more on it, like pix n stuff but as there seemed to be no interest, I figured why bother?

For those who might be some what curious, I went with a 2kw 4:1 balun and a 2kw 1:1 feedline isolator, a 22' RG-8X vertical radiator and 12g wire for the flat top to improve it's structural integrity and it's ability to survive Carolina ice storms. She is up right at 50'.

[WV6Z]

Update, shifted the antenna around a bit today now the trees are completely naked (man ya gotta love vacation time) and it's better than before. It is now pretty well flat topped and oriented north and south..... but, VK land was booming in this afternoon (22:30GMT) on 20m. Man I am loving this antenna and it performs like there's no tomorrow! ;)

[N3ATS]

Good on ya man. Now that my trees are naked once again, I can get my two 10-40 doublets up, N&S and one for E&W.

[WV6Z]

Excellent and you really should put up something for 60m if you can.

[kc9kow]

This is quite off - topic to the keyboard QSO, but I gotta ask where WV6Z got the avatar of the angry priest. It came from a movie, and I can't remember which one.

[WV6Z]

It's Father Jack from the old BBC sitcom 'Father Ted'.

Here is a short example you may enjoy..... http://www.youtube.com/watch?v=WAtb48io6do