Off-Center-Fed vhf/uhf vertical dipole

[VegasMike]

The wires are from the balun

[d_phekt]

Sorry if this has been covered already, but does anyone know what lengths would be best for 800MHZ?

[hertzian]

It is pretty good as is for 800 mhz, except for the fact that at that frequency it looks straight up for the most part, or catches a strange reflection. You could tilt it a bit I suppose for 800 mhz reception and see if that improves it a little bit. Also at these high frequencies, the connections to the tubing / wire themselves are a major part of the antenna, so that should be taken into account.

(Honestly, a simple dipole fed in the middle for 800 mhz would do a lot better for single-band use. typically, just use 468 / f Mhz, ie 468 / 850 mhz for example to get your length in feet. Multipy by 12 and the total overall length is about 6.6 inches - now just cut that in half and attach to coax - mindful that your connections need to be extremely short, AND the coax run to the scanner also has to have very low cable attenuation.)

I love these OCFD antennas for their simplicity, but they have a lot to be desired at UHF and above due to the high look angle.

Essentially, we have a 66" inch overall off-center fed dipole. The length of the dipole is basically a half-wave at 85 mhz, and we rely upon offset-feeding to obtain a match on several bands.

IF this was classically fed at the 1/3 point (22 inches) we'd have something like this:

85 mhz - native half-wave
170 mhz - 2nd harmonic
340 mhz - 4th harmonic
510 mhz - 6th harmonic (WARNING! the 6th harmonic is a bad match)
680 mhz - 8th harmonic
850 mhz - 10th harmonic

With this OCFD being fed at 18 inches in, instead of at the classic 1/3rd point of 22 inches, there are many more peaks and dips for a reasonable match.

(Note: OCFD's provide a reasonable match on EVEN harmonics, except for the 6th, whereas a center-fed dipole can work on ODD harmonics.)

I suspect that long ago, someone was actually trying to make a long-wire antenna for 800mhz, and maybe didn't realize the look angle aspect - but it worked anyway being in a strong signal area, or picking up a strange reflection in an urban setting even with a high look-angle.

[hertzian]

Got a stash of ferrite-chokes that work up to at least 300 mhz or so? Here's another version:

Attach 4-feet of wire, the thickest you can find to the center conductor of the coax. 18 inches down from the end of the coax, snap on AT LEAST 4 of the rf-chokes onto the coax. Preferably more.

In this instance, we are purposely using the common-mode of the coax shield to serve as one of the antenna elements, and trying to choke it off 18 inches away.

Makes a nice stealthy ocfd easy to hang.

[hertzian]

GASP! Forget the previous post about the "stealth" ocfd. It has very poor matching with the dimensions I gave, and is a truly poor performer. Stick to the original plans.

What was I thinking? I was looking at my full-wave air-band antenna fed a quarter-wave in that has a high look angle on purpose. Something most won't want.

Nothing to see here with the dumb stealth ocfd - everyone move on.

[webley445]

I made a ghetto version for the heck of it to see what it would do. Had a short length of RG59 with a BNC on one end. Stripped back the other end and attached lengths of 48" and 18" to center and shield, no balun. Then hung it vertical in the kitchen with the scanner on the table indoors.

Significant increase in reception on marine bands. I have a PS500 and use just the duck. Also live in a coastal area. Can always hear the CG station clearly with the duck but now I am picking the other side of the traffic. Also getting the other marine channels that used to be silent. Also better reception on aircraft. Local 800 Motorola system still coming in, though with just a bit more static on the transmissions than with the duck.

[popnokick]

Built mine just like the pics in the beginning of this thread. El cheapo, with simple hardware store 300 to 75 ohm transformer, and two lengths of wire. Hung it in the attic vertically, with about 30 feet of RG-6 that was already installed in the house in an unused CATV jack. WOW! Major difference. I am now picking up signals that I normally only hear when mobile with my BCT-15 and "hilltopping". I know that the NY ARTCC (controllers... not aircraft) is easily heard from high points of terrain when I'm in the car. But I've never heard them from any scanner in the house... not even a handheld in the attic. This antenna brings in the ground side of the transmissions (controllers), no problems. Hearing a lot of UHF Medcom I was not getting before, and state police dispatch more than 60 miles to the north in a neighboring state. Still testing it on various UHF ham repeaters (receive only). For about $12 in parts, this antenna rocks. Next step: put a splitter in the attic and connect it to the rest of the CATV distrib system in the house so I can use it on a scanner downstairs. That will probably mess it up due to line losses, but I just have to try it! Easily reversed if it doesn't work. And if that doesn't, I could slap together another one and put it at the other end of the attic and feed it into the old CATV cables.

[BaLa]

crap delete

[BaLa]

I hate to bump an old thread like this.

I have one put together with copper pipes and it seems to work good.


Would this work with good with a Solid Steel Rod?>
Obviously it would be 2 pieces.

[popnokick]

Unless it's stainless steel, your issue with steel would probably turn out to be rust. That would make the connections from the feedline subject to varying resistances and even diode-effects from the possible corrosion. Can you solder to stainless steel (I don't remember)? If you can't solder to it then I'm assuming you'd have to use some metal clamps to attach the feedline. If the clamps are a dissimilar metal, it increases the possibly of a bad electrical connection/diode effect. I'm certain steel would work; the bigger question is how long will it withstand Mother Nature?

[BaLa]

Well see I guess.
The Steel was free anyways, since I'm not Tx I don't think it would do any damage to the equipment.

It's 1 inch in Diameter.

[SWeiss]

I know this thread is ancient, but a buddy of mine pointed it out to me. Sorry to resurrect it, but this is great information here fellas!

I love the OCFD design, and I may have solution to achieve resonance on a particular frequency.

I designed a balanced doublet antenna that when fed with balanced line (twin lead, ladder line, etc) can be adjusted to resonance based on the length of the twin lead, in your case, the flat 300 ohm stuff.

I haven't experimented with it yet, but it worked on my antenna, and there is no reason it wouldn't work on yours.

First of all, run your 300 ohm flat TV line from the antenna's feed point ALL the way to the scanner, and put your balun at the antenna port of your scanner. Don't run coax at all. Not only can you trim the twin lead to a certain length to achieve resonance, but you will also have next to no SWR loss because you're not using an unbalanced feed (coax) in the system.

You'll have to use a F to BNC Male adapter (or whatever type of antenna you have) to mate the balun with your scanner.

I am going to build your antenna, and use my SWR analyzer (which covers HF - UHF by the way) to see how long the twin lead needs to be to achieve resonance on a certain frequency. I haven't done this yet, but I plan to, and report back here.

Here is a link that basically explains what I am doing....

http://www.w5dxp.com/notuner.htm

Once I get this done. I can post a list of common freqs/bands so you guys can cut your twin lead to the exact length you need for yours to work where you really want it to.

If anyone wants the plans to build my 44 foot 40-10M doublet that is resonant on 20M, drop me a line at

swweiss at epix dot net

Cheers for now!

Steve
N3ATS