Converted wire OCFD to copper tubing

[popnokick]

Inspired by dmg1969's copper pipe OCFD, I converted the hookup-wire version of the OCFD in my attic over to 3/4 inch copper pipe and PVC fittings. I'll post pics this week of the before and after. But first, a signal report -
I've read here on RR that Off-Center Fed Dipoles are not that great for 800 mHz, so I wanted to see if switching to the copper pipe version from the wire version made any difference. I have a strong control channel for some unidentified service on 851.450 that runs all the time. It was at 1 bar on the signal meter of my BC895XLT on the wire version of the OCFD. After I made the 3/4 inch copper pipe version and placed it in exactly the same place as my old wire OCFD, the 851.450 signal has gone from one bar to two bars. So there was an improvement. Other services (VHF air, VHF high band, and UHF) seem to be about the same. Still waiting to hear the CW IDer's that PPL (power utility in NE PA) uses to see if any difference on VHF low.

 

[nanZor]

Yes, using copper tubing for the OCFD (with 18" and 48" elements) will result in wider bandwidth *swings* as you sweep the spectrum, ie the impedance at the balun output will typically be seen going from about 50 to 200 ohms. With wire, it will rapidly peak and dip. With tubing the peaks and valley won't swing as wildly.

But that is just an impedance issue. The problem with the OCFD is that it is a very long wire above vhf, just fed off center, and that means many high-angle shallow directional peaks and lobes. At 800mhz it is essentially looking straight up in the sky. But if you are in an rf-saturated area, this kind of directional attenuation may not be a big issue. One option is to *purposely* tilt it, or even place it next to some other metal plane to rely on reflection to pick up a signal better due to its poor directional characteristics at uhf and above. This is sometimes the reason it seems to work well when thrown into a corner.

Also note that balun performance at 800mhz can differ among manufacturers. Like the ST2 antenna, some afficianados like to run with higher performance baluns like those from Channel Master instead of radio shack etc. Still look for isolated windings if you can.

Like any other uhf / 800 mhz antenna, you want to use the lowest loss cable you can - especially because you are likely to run into additional swr loss - at 800mhz you could easily be at 3:1 or more at the output of the balun as it swings through the 800mhz band. Normally I'd recommend people put up specialized antennas for uhf/800 mhz instead of spending big bucks on low-loss cabling for an antenna that at 800mhz is looking straight up - but if you want to make the ocfd play as well as it can keep the cable losses down, and maybe look into low-loss baluns at uhf and above.

Again keep an eye out for FM broadcast overload - the ocfd's primary resonance is 88 mhz - and might be what I suspected back in the 90's on usenet postings that someone was trying to find a way to make an 800-mhz long wire.

The other issue is that you will never really be able to get rid of the common-mode of the coax shield, further skewing the pattern. This is because the radical physical offset makes it hard for any low-power tv-type balun to perform it's job properly, especially because the 300:75 ohm balun/transformers are voltage-type and not current types. So we just accept it. You may be able to choke it off for low-band up to about 150mhz with the proper ferrites, but above that it is hard to divorce the common mode. Sometimes being perfectly vertical is not the best solution with this antenna - the mystique is that no installation is the same or predictable so move it / tilt it at uhf and higher because of the funky directionality.

In your case, just make sure you are running very low-loss coax, and perhaps tilt it away from dead center vertically, and see if that helps any.

 

[popnokick]

My situation is therefore ideal for an OCFD:
- Farmland, open fields, and woods in all directions to the horizon
- Nearest tower/transmitter of any type (other than my GMRS repeater) is many miles away
- Little or no possibility of FM broadcast or other signal overload
- The only 800 mHz systems used are proprietary, fully encrypted, and unmonitorable (state police). (PASP OpenSky system often is unreliable, and they frequently use old VHF channels anyway.)
- Fire, EMS, rescue are all VHF HI or UHF
- VHF LO is utility company and comes in well
- High angle of reception is very favorable for VHF air band and MilAir- both of which I receive very well
So I have a very low cost, high performing attic antenna ideal for my location.

 

[popnokick]

Pictures - better late than never. Yes, there are a few AC power wires in the attic. No, they have had no noticeable effect on either the wire or the copper tubing antenna. The RG6 down runs to each scanner in the house were from existing/unused CATV wiring (have satellite TV and not using cable TV at all).
Have heard all the PPL VHF low band CW IDers that I was hearing before... same signal strength. XYL is happier now... wire version was in the way of attic storage. With copper tubing version right up against wall/window, no complaint.

 

[737mech]

I'd get that thing away from the white romex wires for sure. Also the coax connectors you have going to the splitter need replaced. One looks like a compression style that is not compressed? The other looks like a crimp style that was improperly crimped with something other than a f crimper. If possible try a stand off piece of tubing about 2 feet long right into that pvc fitting. Use a 2 foot coax jumper and then do the splitter. Balun at the antenna then jumper then splitter. By the way you half your signal by using that splitter. Always be sure to double check the connections, use proper crimpers or compression pliers. I hope this info helps, (fun project wasn't it? )

 

[popnokick]

Interesting thing..... with all the issues and deficiencies cited, it works great. Including the horrible, unthinkable loss of 1/2 the signal to each scanner. There is absolutely no discernible difference with the splitter in place or the antenna wired to only one scanner. Same stuff I always hear and want to hear at the same strength on either/both scanners.
Actually, there is one big difference if I remove the splitter... I am then forced to use the back-of-set cheap telescoping antennas that come with the scanner.... and then lose nearly half of what I can hear, particularly on the downstairs scanner. "But think how much better it could be if you fixed the issues."
Doubtful. Tested everything with multimeter and continuity is fine. No shorts either. I will probably put a short section of PVC to ensure a 90 degree angle at the feed point, but this was intended as a MacGyver antenna lashup.... and it works really well for what I need.
Two things I will never do with it in this condition: 1) put it outdoors; 2) transmit with it.