OCFD Materials

[tbzep]

I with OCFD's because they are simple and cheap. My first was with a balun and multi-strand wire I had laying around. It made a drastic improvement on scanner reception on the VHF and UHF frequencies I monitor.

I read that larger diameter wire/pipe would increase bandwidth, which in my mind, should give me better reception overall. I tried heavier gauge multi-strand without any noticeable difference and today I pulled some 12-2 solid wire from the sheath thinking the thick solid copper would work better. I made sure to cut identical lengths, used an identical balun and connected with the same section of RG6. I hung it in the same spot, replacing the other antenna but my reception was worse than any of the multi-strands I've made. I reworked with another balun to verify and made sure the RG6 stayed perpendicular in my tests. Testing was WX 162.450 because it's the only frequency that constantly transmits. Once confirming, I tried the solid wire on a couple other scanners with similar results. Testing was with a BC895XLT, then a Pro 2052 and an SC180.

I'm a bit disappointed as I was getting ready to pick up some 1/2" EMT conduit or 1/2" copper tube to see if it would make a noticeable difference. Is there any reason to move ahead with this, or will the EMT or copper pipe only help with extreme?

I was also thinking about trying some dryer vent tubing if the consensus thinks I'd see much difference. They come in flat thin sheets that you assemble and slip together like stove pipe. Would I have to solder this or will a good tight slip fit work?

 

[ladn]

I've used stranded wire and anodized aluminum tubing to make the OCFD dipole listed on RR.
I have not run any tests with instruments, but both seem to be comparable on Rx. The one made out of tubing is more dimensionaly stable. The one(s) I've made from stranded wire are usually easier to set up and seem to perform well for general scanning.

For HF, I also use an OCF dipole mad from insulated, stranded wire, with a choke balun on the coax. Since I can't get it up as high as I'd like, I'm mostly stuck with NVIS propagation.

 

[tbzep]

I dug up some 1/2" EMT conduit out of the barn today and I stopped at the hardware store and grabbed several sections of dryer vent pipe to play with. I will build them this weekend at the 48" - 18" size in the plans. I've already put together a 1/2 size and a 1/4 sized antenna with solid wire, but I haven't tested them yet. I may make a 2X one and try it by hanging it in the bay of the fire station, but with a metal roof, it won't be a good test.

I don't have any kind of meter. All I can do for now is hit the NOAA frequencies and swap antenna to compare on VHF. For UHF I will have to be patient and catch the sheriff or PD on a chase where they will have a lot of chatter over 15 or 20 minutes so I can directly compare them.

 

[ladn]

don't have any kind of meter. All I can do for now is hit the NOAA frequencies and swap antenna to compare on VHF.

Are you close enough to any airports to get the ATIS broadcasts, which like the NOAA broadcasts, are continuous? Also air traffic control can be useful if you are near a busy sector.

 

[tbzep]

I'm near a small ~5000 ft strip, and one big enough for a tower is about 45 miles away. I will check on ATIS, but I think that all the frequencies are on VHF. The mid size airport may have something duplicated in the 200-300Mhz range, but I don't recall it being ATIS.

 

[ladn]

I'm near a small ~5000 ft strip, and one big enough for a tower is about 45 miles away. I will check on ATIS, but I think that all the frequencies are on VHF. The mid size airport may have something duplicated in the 200-300Mhz range, but I don't recall it being ATIS.

The mid sided airport would most likely only have 200-300 MHz coverage if it handled military aircraft. Civilian airports have ATIS on VHF.

 

[krokus]

I am curious about how the vent pipe would work out. I think the seam along the side might affect the RF pattern.

 

[littona]

I've seen tubing used on high power HF transmission line to counteract "skin effect", but dryer vent piping would be interesting! I wouldn't think the seam would matter too much, as it would all pretty much look the same electrically. I agree, without some serious test equipment, it would be hard to determine how effective it is.

 

[prcguy]

Going from wire to 1/2" EMT will change the required lengths slightly. Going from that to dryer vent piping will have a serious effect on length like the 18" side might turn out to be only 12" or less. But since this VHF/UHF OCFD project is kind of a joke anyway and a waste of time in my opinion, you can probably make it out of anything and it will receive something.

I dug up some 1/2" EMT conduit out of the barn today and I stopped at the hardware store and grabbed several sections of dryer vent pipe to play with. I will build them this weekend at the 48" - 18" size in the plans. I've already put together a 1/2 size and a 1/4 sized antenna with solid wire, but I haven't tested them yet. I may make a 2X one and try it by hanging it in the bay of the fire station, but with a metal roof, it won't be a good test.

I don't have any kind of meter. All I can do for now is hit the NOAA frequencies and swap antenna to compare on VHF. For UHF I will have to be patient and catch the sheriff or PD on a chase where they will have a lot of chatter over 15 or 20 minutes so I can directly compare them.

 

[tbzep]

I put together the sections of 4" vent pipe. The connecting seam on them is pretty flimsy. The sections are 2 ft long including the part that slips inside the other sections, so my 48" side had to have two full pieces and a very short piece. I fitted them and got the lengths right then stopped to think of how I wanted to permanntly connect...screws? No...duct tape, of course! I've got it ready to mount on a 1x4 or 2x4, whatever I can find. I'll just rig something to hold the coax at 90 deg for a few feet when I set it up to try out. I hope to get it done this weekend and do a quick naked ear comparison with a few NOAA frequencies that are varying distances from me.

I have an SDR dongle...is there (free) software for Android or Linux that will show signal strength? I'm using it right now to play around with our 800mhz DMR public safety system. Our main frequencies are simulcast on our old analog VHF and UHF frequencies, but our jailers and dispatchers talk amongst themselves on a private DMR talkgroup. I don't think they know I can hear them. Lol!

 

[tbzep]

Going from wire to 1/2" EMT will change the required lengths slightly. Going from that to dryer vent piping will have a serious effect on length like the 18" side might turn out to be only 12" or less. But since this VHF/UHF OCFD project is kind of a joke anyway and a waste of time in my opinion, you can probably make it out of anything and it will receive something.

It's just for fun. I've read enough here and on QRZ to know that some HAM guys will try standing on their heads and rubbing their bellies with jello if it will give them a zillionth of a dB better Tx or Rx signal than their buddy. I think my venture is pretty mild in comparison. However, can you point me to some documentation on diameter vs length? It might come in handy if I decide to get a technicians license and start Tx'ing.

 

[prcguy]

I don't know of any documentation that will help going from a thin conductor to something 4" or more, you usually check resonant freqs with the thin conductor then trim the fat one to resonate around the same frequency. My comments about the VHF/UHF OCFD are from the antenna pattern being wrong and degraded for typical scanner use and having other inherent problems. They are fine for HF use.

When I do something that gives me a fractional dB improvement I don't use Jello and it would not be appropriate to describe it here due to children possibly reading it.

It's just for fun. I've read enough here and on QRZ to know that some HAM guys will try standing on their heads and rubbing their bellies with jello if it will give them a zillionth of a dB better Tx or Rx signal than their buddy. I think my venture is pretty mild in comparison. However, can you point me to some documentation on diameter vs length? It might come in handy if I decide to get a technicians license and start Tx'ing.

 

[PRMII]

This is like watching your favorite TV program, and an end of season cliffhanger. Then finding out the series was cancelled!! WHAT HAPPENED???? I'm OLD, so I may not have a lot of time for someone to rewrite and start a new series, with different characters. Come on folks...
I've got a lot of time on the bands, but my favorite thing is antenna work. I get them to work, but they don't always conform to the theories established, and I'm told there is no possible way it does work. Until folks witness it.
I hope you continued your experiment, and hope you see this, and publish your findings.