Homebrew OCFD Issues

[mciaio]

I made the Homebrew OCFD from the wiki out of cpvc, 1/2 copper pipe and a TV matching transformer. I connected RG6 quad shield with an f connector on one end and a BNC on the other. I cut the tubes 48 and 18 inches. This things suck worse than the rubber duck antenna that the pro-194 comes with stock. The NOAA wether stations come in better with the rubber antenna. I can barely get stuff from the next county over. How are people getting 75 miles with this thing? What did I do wrong?

 

[nanZor]

Are you putting the copper tubes inside pvc as a housing, or just using a joint-coupler? Placing it inside pvc as a housing detunes it.

How is the quality of your transformer? I prefer to use those with isolated transformer couplings, like the RS #15-1230.

Note that the transformer should be very close to the feedpoint - short enough that the additional hookup wires from the transformer to the tubing doesn't become part of the antenna themselves, and change the overall length electrically. This is a common error if one uses the 15-1230 with its twinlead for the 300 ohm port. Try not to separate them very much when you attach it to the tubes. If you can get the tubes only about an inch apart, and run the small 300 ohm twinlead section directly to the tubes without much separation, you'll do much better.

Are you running the feedline straight away from the antenna horizontally for at least 4 feet - preferably more before bringing the coax down vertically? If you run the coax near the elements, it will definitely skew the pattern, mostly upwards.

Note that the OCFD will actually work, BUT at UHF or above, the directional pattern really starts to head upwards of 45 degrees or even more by the time you reach 800mhz. But it shouldn't be that bad at VHF - although it won't win any awards. I like the OCFD a lot, but I'm always aware of it's limitations.

One last possibility is that it is actually working too well - desensing your scanner with overload. This can easily come from FM station overload, maybe overload on the NOAA signal itself, or some other source. Does your signal improve when you use the attenuator - at least temporarily for testing?

 

[mciaio]

Not inside, just using the t-fitting as a coupler. I am using a RS 15-1230, changed it out for a new one just in-case my old one was bad. No change. There are no extra wires between the balun and the copper tubes; just what the balun comes with. The coax runs 90 degrees to the antenna for about 2 feet, then actually goes under the antenna off the roof. That maybe the issue. I also have a repeater tower very close to me. Close enough that I can get a strong signal with no antenna on. That might be the issue. With the squelch all the way down, I swear I can hear AM/FM radio stations coming from the scanner. There are no AM/FM/TV stations on that hill with the repeater antenna, just a bunch of UHF/VHF/800 and Cell phone antennas. That's probably the issue.

 

[nanZor]

Sounds like you did everything right!

One of the bad things about being so broadband is that it can expose the scanner to signal overload from out of band signals that narrowly-tuned antennas don't. It is almost like the case of someone putting up an nice new discone, and then having to put a filter on it. The OCFD is no discone, that's for sure.

Another problem with the OCFD is that it is guaranteed to have common-mode coax coupling due to the imbalance of an off-center feed. Along with that, the TV transformer is usually a voltage-balun, which does nothing to help with unbalanced currents. Even current-baluns have a problem with such an offset feed. The only good point about this is that it makes for a good VHF-LOW random wire.

You may want to try choking it just after the balun/transformer. Wind about 18 inches of coax into a 3-4 inch coil with the turns zip tied next to each other - not just a random wind. Try not to allow the coax entry point and exit point of the coil get near each other. Or you could use a bunch of RS snap-on ferrites, but unless you have 5 or more on hand, that could be VERY expensive. This choking is extremely important if you run the coax down a metal mast.

This OCFD has a pretty healthy response in the FM band, so if you think you are hearing broadcast stations, it could be an FM overload issue. You may want to try putting an FM broadcast trap on it - even RS has one. Not the greatest, but it might let this contraption play.

How is the reception on 800 mhz? If it is already weak, it is probably due to the high angle of radiation, and this might be a signal to stop with the OCFD and go for something a bit more conventional.

 

[popnokick]

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Something that you may not have tried: did you check the basics? Specifically, checking for opens and shorts in your feedline and connectors at each end of the feedline using a multitester or continuity tester? Be certain to check end to end continuity on both the center conductor and outer shells of the connectors, and that there is no short (continuity) between center and outer conductor at each end of the cable's connectors. I know, it's like being asked by the Help Desk "Is it plugged in?". But if not tested you won't know for certain.

 

[mciaio]

Thanks for the help guys. I did check my cabeling annd connectors. Thy all tested good. I made the coil out of the coax as suggested which made some improvement. It is more like 36 inches long rather than 18. 18 inches only gave me 1.5 a coil. I still need the fm trap and still not getting the performance I would like. I still can't get the next county over. A city with a 900 turned system 12 miles away is still unreceivable.

Here is some pics of the coil. Good or bad?

Also, the only way I can get this antenna to work is with global ATT. On. I that right?

 

[majoco]

Vertical or horizontal polarisation?

 

[mciaio]

Vertical

 

[nanZor]

I'd ditch that metal pipe as the support bracket first of all. Can you live with a short piece of PVC as the horizontal support structure?

The leads coming from the transformer should not cross each other - they should run straight out as possible from the output to get as close a possible to the center of the tee, and then split to each leg. Try to keep them as parallel to each other as possible to simulate twinlead, I'd also put the "twinlead" from the transformer on the other side of the TEE, as you don't want the leads near the metallic screw.

When you do split the "twinlead" apart to reach each of the tubes, you may want to take into account that even those 1-inch length twinlead wires when they separate from no longer being in parallel to each other are now part of the antenna itself. If the leads are long, you may want to compensate by trimming down the end of the tubes by an inch or so. But worry about that later - not a showstopper.

Nice job on the coaxial choke - it's an improvement although it is a pain to wind RG-6 tightly without damaging it internally.

What scanner is this being used with again?

 

[mciaio]

Pro-164

I bought a small magnet mount antenna this week. It seems to be working OK. I still get that radio station on that too.

I will remove the copper pipe mount and change it to CPVC and try the other suggestions you offered. I'll let you know how it comes out.

 

[nanZor]

Great! If it doesn't work out, you have the makings of a great center-fed vertical dipole by cutting the tubes to equal lengths, and directly attaching the coax to the tubes without a transformer. Keep the coiled chokes and filter.

What frequency(s) is the next county on? Depending on the frequency, it might be far enough away from the FM broadcast band that it may no longer be too big of an issue - and you'll have superior performance to boot by cutting it for a specific band. (ie, 468 / f Mhz = total length in feet, or 234 / f Mhz for each tube).

 

[mciaio]

Did as you said.

1. Ditched the copper support in favor of CPVC; glued into T.
2. No copper = no screw on center of T.
3. Un-crossed feed lines, and kept parallel until turning 90 degrees to be connected.

I am happy with the performance now. I am getting some low power ATIS signals from two airports on either side of me about 20 miles.

I do have a rather strong pager on 152.200 that blasts the front end of my scanner out, but oh well.

 

[nanZor]

Well, you did a great job with the ocfd project!

Is your main interest the VHF airband? If so, I think a regular dipole would be the best bet instead of the ocfd. Just take what you have now, and cut each tube to be 22 1/2 inches long. Remove the transformer, and attach the coax directly to the tubes. You could use an F-chassis connector to make soldering the short jumper wires easier instead of stripping the coax.

With the dipole being narrowbanded, unlike the ocfd, it might even help with the FM broadcast overload, and even the pager to some extent.

If you still have some RG-6, you could try to cut an open quarter-wave stub hanging off a tee near the radio. Just attach 18 inches to the tee, and start snipping back in 1/4 inch increments to see if you can attenuate the 152.2 pager. You'll have to decide if the insertion loss of the filter itself is worth it:

http://forums.radioreference.com/antennas-coax-forum/212129-notch-filters-effects.html

If that pager is blowing through near 130.8 mhz, it is likely an image from the 1st 10.7 mhz if of your scanner if it is a standard dual-conversion receiver.

At the very least, you have a baseline to judge from when you construct other antennas!