Anyone built the Homebrew OCFD and been... disappointed?

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br0adband

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I finally got the parts together yesterday - from Lowes (1/2" copper pipe, PVC T-joint, support, end caps and screws) and Radio Shack (the 15-1230 isolated matching transformer), 6 feet of what appears to be somewhat decent coax from Cox Cable (for testing since this is all inside presently), etc - and put together my first Homebrew OCFD and... well, I'm pretty disappointed in the performance. As for the build itself, if I took a picture of it and posted it you'd be hard pressed to tell it apart from any of the dozens of others that people have built for themselves - I followed the specifics as precisely as I was able including that specific matching transformer instead of the other similar model from Radio Shack.

Now, I realize there's never a guarantee with this kind of stuff, and maybe I've just missed something but I constructed it based on the original plan in the wiki:

- 48" leg, 18" leg
- matching transformer spade lugs attached to screws drilled into the PVC/copper pipe at just about 1/4" in from the end inserted into the T-joint
- 16" PVC support pipe inserted into the middle of the T-joint
- matching transformer lashed to the support using cable ties and attached to the 6 feet of coax (which uses triple shielding as I had another piece I had cut open for another purpose and a solid center conductor that is very very tough to bend)
- other end of coax is using an F to BNC connector which is attached to my BNC to MCX pigtail straight into the RTL stick (I am not using my FM trap on this presently since I'm testing the bare metal antenna, also not using it with the ground plane either)

Now, contrast that with the 1/4 wave ground plane that I built a few weeks ago from an SO-239 chassis mount with 4 pieces of coat hanger (cut to 20") screwed onto the SO-239 (not even soldered, and they haven't been properly stripped of that enamel crap on 'em either) and any one of over a dozen additional pieces of coat hanger inserted manually as required to "tune" the ground plane to a given frequency/range.

For my testing I put a wire cut to 300 MHz (19.7 inches) into the ground plane and then tuned to 172.900 MHz which is a frequency used by the TSA here at McCarran airport for P25 comms. I get a fairly loud and clear signal with just using an RF Gain of 28dB (my typical setting for most everything - I do not use the RTL or Tuner AGC at all anymore) with hardly any errors in the decoding as the signal is passed to DSD+ using the -f1 switch to ignore anything but P25/X2 traffic.

Disconnect that antenna and attach the OCFD and the signal strength is severely crippled, errors in decoding all over the place, etc.

I then decided to test against a known weak frequency - the Boulder City VOR at 116.700 MHz (AM) and strangely I get a scratchy but readable signal from both antennas (can even pick out the Morse ID being transmitted simultaneously). Adjusting the RF Gain doesn't make it any better with either antenna, oddly.

Same results in the 406-470 range, nothing different in the 765-775 range (only thing there presently is an OpenSky system but I still use it for comparison and signal strength), and it's well known that the 800/900 MHz coverage of an OCFD is hampered by design anyway so nothing out of the ordinary there. I guess I just had some higher hopes towards the air band coverage (Civ and Mil Air, both) and so far this ain't squeakin' my sneakers as a friend used to say.

I'll keep playing around with it, probably rebuild it again and I have the parts for a 300 MHz dipole (two 9" sections of 1/2" copper pipe with another T-joint and caps) along with a "shorty" OCFD at 24"/9" and the T-joint and caps so it'll be an interesting weekend seeing what I come up with.

And yes, I'm aware that anything more closely tuned to the given frequency (like my homebrew ground plane) will outperform an antenna that by design will have a far wider bandwidth window (like the OCFD) - I guess I just expected it to perform better across that bandwidth than maybe this ground plane does when I purposely put a wire in it for a frequency/band that's nowhere near the receiving one. I mean, I just put a 1/4 wave 935 MHz wire (3.16" long) into the ground plane and even THAT pulls in the TSA at 172.900 MHz stronger than the OCFD does... :confused:

Was just wondering - on top of all the other threads related to OCFD builds - if anyone had put one together and almost immediately thought, "Ok, that's nothing like I had hoped it would be, especially considering the praise a lot of people offer towards it..."
 

dmg1969

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I built one and was pretty happy with the results. For what I monitor (mainly my county's VHF low police, fire and EMS), it did fine. I have since replaced it with an attic mounted Hustler MOR and not noticed a huge improvement over the OCFD. In fact, it is still up in the attic and able to be reconnected if I want to run two scanners...just need to run the additional coax run.
 

popnokick

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I have the copper pipe OCFD in my attic and had it connected to two scanners with a common CATV splitter. It has been fine with both of those scanners on VHF-Low, High, Air (Civ and Mil), and UHF. I don't have any 800 here that's not encrypted, so nothing to listen to there. Recently, I removed the downstairs scanner and replaced it with an RTL stick, HDSDR, etc. Worked way better than the shorty mag mount that comes with the RTL-SDR, as I expected. A few weeks ago I replaced the cheap splitter with an amplified CATV splitter and it made a major difference with the SDR receiver. I've captured screen shots of the amplified versions of several signals. Once I have time to get to the attic and remove the amp, I'll capture some shots of the same signals... there is a measurable difference with the amp on all bands... even VHF Low.
I'm planning to post a separate thread when I have the screenshots ready. I don't believe in using an amp to compensate for a compromise antenna, but this was so cheap and easy to do I couldn't resist. And the effort has been worth it. I don't live anywhere near any strong broadcast or paging transmitters, so I wasn't worried about increasing the possibility of interference from them. Now I just need more time to play with my setup and the SDR.
 

n4yek

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Disconnect that antenna and attach the OCFD and the signal strength is severely crippled, errors in decoding all over the place, etc.
I know this sounds silly, but did you turn the antenna at all? I know the one i built, mounted on the pole, is particular in which way it is pointed. I can actually say that the direction it is pointing can improve or hurt my signal.
Just a thought.

Just fyi, i built mine with the longer leg up even though they say it doesn't matter.
 
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prcguy

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One problem I constantly see with antennas is very few people actually compare them to another antenna or better yet, a known reference like a dipole or ground plane. They simply state it works great based on voices coming out of a speaker. Since you are comparing to a ground plane, which is a known and repeatable reference, you are exposing some deficiencies in the OCFD that others may not have discovered.

I have never built an OCFD because the design is full of lobes high in the air and nulls at the horizon, which is a known characteristic of its larger multiband HF cousin.

I'm confused on a comment where you put a wire cut to 300MHz (19.7 inches) into the ground plane. A 19.7 inch wire would be around 1/4 wave at 146MHz and when trimmed for 173MHz would be about 16 inches.
prcguy



I finally got the parts together yesterday - from Lowes (1/2" copper pipe, PVC T-joint, support, end caps and screws) and Radio Shack (the 15-1230 isolated matching transformer), 6 feet of what appears to be somewhat decent coax from Cox Cable (for testing since this is all inside presently), etc - and put together my first Homebrew OCFD and... well, I'm pretty disappointed in the performance. As for the build itself, if I took a picture of it and posted it you'd be hard pressed to tell it apart from any of the dozens of others that people have built for themselves - I followed the specifics as precisely as I was able including that specific matching transformer instead of the other similar model from Radio Shack.

Now, I realize there's never a guarantee with this kind of stuff, and maybe I've just missed something but I constructed it based on the original plan in the wiki:

- 48" leg, 18" leg
- matching transformer spade lugs attached to screws drilled into the PVC/copper pipe at just about 1/4" in from the end inserted into the T-joint
- 16" PVC support pipe inserted into the middle of the T-joint
- matching transformer lashed to the support using cable ties and attached to the 6 feet of coax (which uses triple shielding as I had another piece I had cut open for another purpose and a solid center conductor that is very very tough to bend)
- other end of coax is using an F to BNC connector which is attached to my BNC to MCX pigtail straight into the RTL stick (I am not using my FM trap on this presently since I'm testing the bare metal antenna, also not using it with the ground plane either)

Now, contrast that with the 1/4 wave ground plane that I built a few weeks ago from an SO-239 chassis mount with 4 pieces of coat hanger (cut to 20") screwed onto the SO-239 (not even soldered, and they haven't been properly stripped of that enamel crap on 'em either) and any one of over a dozen additional pieces of coat hanger inserted manually as required to "tune" the ground plane to a given frequency/range.

For my testing I put a wire cut to 300 MHz (19.7 inches) into the ground plane and then tuned to 172.900 MHz which is a frequency used by the TSA here at McCarran airport for P25 comms. I get a fairly loud and clear signal with just using an RF Gain of 28dB (my typical setting for most everything - I do not use the RTL or Tuner AGC at all anymore) with hardly any errors in the decoding as the signal is passed to DSD+ using the -f1 switch to ignore anything but P25/X2 traffic.

Disconnect that antenna and attach the OCFD and the signal strength is severely crippled, errors in decoding all over the place, etc.

I then decided to test against a known weak frequency - the Boulder City VOR at 116.700 MHz (AM) and strangely I get a scratchy but readable signal from both antennas (can even pick out the Morse ID being transmitted simultaneously). Adjusting the RF Gain doesn't make it any better with either antenna, oddly.

Same results in the 406-470 range, nothing different in the 765-775 range (only thing there presently is an OpenSky system but I still use it for comparison and signal strength), and it's well known that the 800/900 MHz coverage of an OCFD is hampered by design anyway so nothing out of the ordinary there. I guess I just had some higher hopes towards the air band coverage (Civ and Mil Air, both) and so far this ain't squeakin' my sneakers as a friend used to say.

I'll keep playing around with it, probably rebuild it again and I have the parts for a 300 MHz dipole (two 9" sections of 1/2" copper pipe with another T-joint and caps) along with a "shorty" OCFD at 24"/9" and the T-joint and caps so it'll be an interesting weekend seeing what I come up with.

And yes, I'm aware that anything more closely tuned to the given frequency (like my homebrew ground plane) will outperform an antenna that by design will have a far wider bandwidth window (like the OCFD) - I guess I just expected it to perform better across that bandwidth than maybe this ground plane does when I purposely put a wire in it for a frequency/band that's nowhere near the receiving one. I mean, I just put a 1/4 wave 935 MHz wire (3.16" long) into the ground plane and even THAT pulls in the TSA at 172.900 MHz stronger than the OCFD does... :confused:

Was just wondering - on top of all the other threads related to OCFD builds - if anyone had put one together and almost immediately thought, "Ok, that's nothing like I had hoped it would be, especially considering the praise a lot of people offer towards it..."
 

br0adband

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I'm confused on a comment where you put a wire cut to 300MHz (19.7 inches) into the ground plane. A 19.7 inch wire would be around 1/4 wave at 146MHz and when trimmed for 173MHz would be about 16 inches.
prcguy
300 MHz 1/2 wave = 19.68 inches, which makes it a 1/4 wave at 150 MHz as well.

I now realize I said 300 MHz (which is a 1/2 wave antenna length) which works as a 150 MHz 1/4 wave as well - I did that on purpose to see how well the OCFD would pull in that signal compared to an antenna that's basically tuned to some other frequency and it doesn't perform nearly as well - yes, again, I understand that using tuned antennas will always result in a better result on a given frequency. My assumption, such as it is, was that since the OCFD is considered to be so broadband in nature (aside from it's natural tendencies at 88 MHz as a half wave by design) that it would at least provide decent reception in the VHF-Hi range which is where that 172.900 MHz signal happens to be. Also, 88 x 2 = 176 (practically a full wavelength) so one might think it's not farm off from being right on a harmonic anyway, but it just doesn't seem to be working that way and it's so weak when compared to the 150 MHz 1/4 - 300 MHz 1/2, I mean really different, like 15db difference in signal levels between the 1/4 wave and the OCFD at 172.900 MHz.

As far rotating it, I've got the feed line coming off at a 90 degree angle for a full 16" inches, can't do much more than that and yes I have repositioned it at various places in my apartment including against the wall as well as even outside the window for a bit hanging off the window sill (long end down). Since the OCFD acts as one big antenna it doesn't matter if it's long side down or short side down, it'll perform the same in either orientation and I've verified that myself already a few times.

Only thing left to try swapping would be the coax I suppose. I'm not even sure what the hell this 6 foot piece I have is - it's got compression fittings on it and Cox provided it when I got my cable modem a few years ago, I just tossed it in a drawer and never used it till now. Perhaps that's having some effect on it - I have a 6 foot piece of RG-58U attached to the 1/4 wave ground plane I made (screw on PL-259 and BNC connectors, no solder involved anywhere). In fact I haven't soldered anything related to antennas in the past few months and it all works great but I figure at some point I'll make things a bit more permanent. Been hoping to get my hands on some AWG 10 to build it with but that stuff tends to be somewhat expensive per foot so I'll pass for now.

I'm pretty surprised at how well my homebrew 1/4 wave ground plane has been performing, it does much better than I expected actually. Guess I'll just keep using that primarily and chalk the OCFD up as an experiment that just didn't pan out for my particular situation.
 
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popnokick

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There is a major unchecked portion in your OCFD test setup: that "6 feet of what appears to be somewhat decent coax from Cox Cable".... that was used with your cable modem a few years ago. Presumably, it's
- 75 ohm cable
- has no shorts / opens or other partial conductivity problems

Is it possible to replace that chunk of unknown cable with some quality RG-6 that is known good from each connector and end-to-end (checked on a meter or TV set or other antenna)? Also, am assuming you are aware the impedance on the balun transformer is 75 ohms, and you should not use 50 ohm cable with that (like RG-58U that you mentioned).
 

prcguy

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When speaking of ground plane antennas fed with 50 or 75 ohm coax, a half wave vertical element is to be avoided because if its very high impedance and it makes for a really bad antenna. You have to supply some sort of matching on an end fed half wave before it becomes a useful antenna.

Odd multiples of 1/4 wave will tend to match ok to 50 or 75 ohms but then you get gain lobes where you don't want them and a null at the horizon. A 3/4 wave vertical is a typical example where a 150MHz 1/4 wave is operated as a 3/4 wave at UHF.

On the other hand, the OCFD fed with a 300 ohm transformer will work slightly better with an element that turns out to be a half wave on one side.
prcguy

300 MHz 1/2 wave = 19.68 inches, which makes it a 1/4 wave at 150 MHz as well.

I now realize I said 300 MHz (which is a 1/2 wave antenna length) which works as a 150 MHz 1/4 wave as well - I did that on purpose to see how well the OCFD would pull in that signal compared to an antenna that's basically tuned to some other frequency and it doesn't perform nearly as well - yes, again, I understand that using tuned antennas will always result in a better result on a given frequency. My assumption, such as it is, was that since the OCFD is considered to be so broadband in nature (aside from it's natural tendencies at 88 MHz as a half wave by design) that it would at least provide decent reception in the VHF-Hi range which is where that 172.900 MHz signal happens to be. Also, 88 x 2 = 176 (practically a full wavelength) so one might think it's not farm off from being right on a harmonic anyway, but it just doesn't seem to be working that way and it's so weak when compared to the 150 MHz 1/4 - 300 MHz 1/2, I mean really different, like 15db difference in signal levels between the 1/4 wave and the OCFD at 172.900 MHz.

As far rotating it, I've got the feed line coming off at a 90 degree angle for a full 16" inches, can't do much more than that and yes I have repositioned it at various places in my apartment including against the wall as well as even outside the window for a bit hanging off the window sill (long end down). Since the OCFD acts as one big antenna it doesn't matter if it's long side down or short side down, it'll perform the same in either orientation and I've verified that myself already a few times.

Only thing left to try swapping would be the coax I suppose. I'm not even sure what the hell this 6 foot piece I have is - it's got compression fittings on it and Cox provided it when I got my cable modem a few years ago, I just tossed it in a drawer and never used it till now. Perhaps that's having some effect on it - I have a 6 foot piece of RG-58U attached to the 1/4 wave ground plane I made (screw on PL-259 and BNC connectors, no solder involved anywhere). In fact I haven't soldered anything related to antennas in the past few months and it all works great but I figure at some point I'll make things a bit more permanent. Been hoping to get my hands on some AWG 10 to build it with but that stuff tends to be somewhat expensive per foot so I'll pass for now.

I'm pretty surprised at how well my homebrew 1/4 wave ground plane has been performing, it does much better than I expected actually. Guess I'll just keep using that primarily and chalk the OCFD up as an experiment that just didn't pan out for my particular situation.
 

br0adband

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The 6 foot compression-connector coax works fine, came in the box with the cable modem (a DOCSIS 3.0 one I got last year from Cox) and it works, that much I know - it has CommScope "Brightwire" listed on the cable itself so I'm guessing that it's RG-6; the specific number on the side is 9900693 but I can't find that anywhere doing a search for that specifically - it has F6SSVV listed as well which does come up in a search as a variant of RG-6 however. As stated, it was in the box with the cable modem (a Netgear CG3000-RD cable modem/Gigabit router/etc) so I can't imagine there's anything wrong with it and it should be 75 ohm.

As for the RG-58U that's what I have attached to the homebrew 1/4 wave ground plane. When I got the chassis mount part at Fry's a few weeks ago they only had a 6 foot piece of RG-58U on the shelf - for whatever reason they were either out of stock of pretty much everything else in terms of coax or they'd moved everything to another area and I wasn't going to go digging for it, I just grabbed what they had in front of me. Got a screw on BNC connector for one end and the PL-259 screw on for the other to attach to the SO-239 chassis mount and that was that. I know I should be using 75 ohm cabling end to end, but if it's 50 ohm so be it, all I know is that homebrew thing I created is pulling in stuff I can't hear with anything else and that signal is getting to the RTL stick so it works fine for me. ;)

At some point I'll most likely go back and redo all this and get some proper RG-6 with the proper ends on it or just a length that I'll cut up myself and do whatever with. I have a 15 foot piece of what I'm assuming is RG-6 (another piece that Cox left with me) but the outer sheath has absolutely no markings or printing on it of any kind - when attached to the OCFD the same way the signal levels are pretty much equal as the 6 foot piece so, again it's just not what I was hoping for or expecting in terms of performance.
 

popnokick

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Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 7_0_4 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11B554a Safari/9537.53)

Not what I would have expected either.... Which is why I'm thinking something else is wrong.
 

br0adband

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Cheap copper, maybe? :D Paid $10.41 for a 10 foot section of it, I figure copper is copper in most respects, who knows. 48" section (verified), 18" section (verified), proper matching transformer attached to the sections within 1/4" of their respective ends inserted into the T-joint using a sheet metal screw through the PVC into the copper pipe (it was a nice tight twist with the screwdriver so I know absolutely that contact is being made). End caps on the copper pipes, 16" PVC beam inserted into the side of the T-joint, matching transformer cable tied to the support beam, etc.

I can't see what might be wrong myself. I just did some testing with the OCFD vs the 1/4 wave ground plane I made using only the 150 MHz cut piece alone (meaning I didn't swap wires for the specific bands). I used frequencies where I know signals are somewhat strong save for the two weaker ATIS frequencies here in my area that are farther away than the one from Nellis AFB which I get pretty strongly. RTL AGC is off, Tuner AGC is off, and the RF Gain is set to 28 dB (again, my typical setting). I'm "measuring" the signal strength by what's shown on the spectrum so it's an eyeball measurement, give or take a few dB.

Also, squelch will be wide open (disabled/unchecked) for all AM signals, for FM squelch is enabled at 60. I do have an FM trap but it's not in use at this time and I don't bother with FM broadcast signals at all. Frequencies are also centered in the spectrum and not off to the edges/ghosted.

At 116.7 MHz (AM) - Boulder City VOR:

The OCFD shows -35 to -40 dB typical, noisy but readable speech. The 1/4 wave shows maybe -38 to -42 dB so a slight improvement for the OCFD there, same noisy signal but readable.

At 132.400 MHz (AM) - McCarran Airport ATIS:

The OCFD hovers around -44 dB, pretty severe change in readability (voice is buried in noise). The 1/4 wave shows about -37 dB solid, very little variation in signal strength, maybe a MHz up or down, far more readable in terms of voice quality.

At 162.550 MHz (FM) - NOAA Weather from McCarran Airport:

The OCFD shows a fairly level signal at just shy of -30 dB when voiced. The 1/4 wave offers an almost identical signal level at about -30 dB when voiced.

At 172.900 MHz (FM - P25) - TSA at McCarran Airport:

The OCFD shows peaks of around -34 to -36 dB, decoding from DSD+ is wrought with errors which makes the result unreadable in terms of voice quality. The 1/4 wave shows peaks of -27 to -30 dB when a signal is being received, no errors in the decoding from DSD+ are shown, sounds like typical P25 traffic.

At 270.100 MHz (AM) - Nellis AFB ATIS:

The OCFD shows peaks of about -22 to -25 dB, solid signal here so that's a plus, crystal clear as far as AM is concerned. The 1/4 wave shows about -25 dB solid, same voice quality.

At 385.625 MHz (FM - P25) - Nevada Test and Training Range Control Channel:

The OCFD here is useless, signal peaks are barely -58 to -60 dB (-60 dB is where I have my FFT range capped), Unitrunker gets something like 2-5 for the levels. The 1/4 wave shows a somewhat steady signal -43 to -48 dB, using Unitrunker I get levels in the 65-68 range.

At 407.350 (FM - P25) - Nellis AFB:

The OCFD shows -33 to -38 dB, Unitrunker shows levels around 40-45. The 1/4 wave has a consistent signal around -26 to -30 dB, Unitrunker shows 85-90 for decoding levels.

At 464.100 (FM - DMR/MOTOTRBO) - Fremont Street Experience Security

Both antennas here are basically almost -0 dB on the scale since I'm like 5 blocks from the transmitter so not much difference at all. That signal is so powerful in the downtown area it causes a lot of intermod and de-sense, somewhat irritating to be honest. DMR/MOTOTRBO signals seem to do that across the bands when they're in use, very powerful stuff in my experience.

The following are well into the upper UHF range and as known the OCFD has issues with the higher frequencies because of design but I figured I'd include some results anyway. I'm on the 3rd floor of my apartment building with a window to the immediate rear (studio apartment, rear meaning south) which basically looks out and I see a brick wall only 5 feet away that is one side of a parking structure attached to an 11 story brick/steel/glass structure.

At 771.0125 MHz (FM - OpenSky) - Las Vegas Metro Police Department

Can't monitor this obviously but it's excellent for a 700 MHz signal for testing. The OCFD gets a -10 to -12 dB signal, solid as expected. The 1/4 wave gets almost exactly the same and it's most likely because the transmitter in use is in the downtown area on top of the new LVMPD HQ about 1.5 miles from me line-of-sight.

At 854.3125 MHz (FM) - S.N.A.C.C. trunked public safety system (this is a voice channel)

The OCFD shows -28 to -33 dB approximately, signal is still there, readable, not fully quieted but very close, not bad at all actually. The 1/4 wave shows peaks of -15 to -20 dB, fairly consistent, excellent voice quality with full quieting, no scratchiness at all.

At 939.475 MHz (FM - DMR/MOTOTRBO) - Boyd Gaming Corp Control Channel w/Voice content

The OCFD shows a solid -30 dB without issues, decoding with DSD+ is clear of errors. The 1/4 wave shows a similar strength of -28 to -30 dB, decoding with DSD+ is fine with this one too.

All in all the OCFD ain't that bad I guess, and I know this impromptu testing doesn't really do anything to make the OCFD seem to be any better than expected given the shortcomings it has. For me I suppose the issue is whenever I find a weak signal of most any kind: with the OCFD when I tune to it and I hear noise, scratchiness, static, etc, I can then switch over to the 1/4 wave (just using the 150 MHz 1/4 wave element) and it'll immediately make a noticeable difference in quality.

Now I understand that the difference could be attributed to many things, most notably having a "proper" ground plane, and so on. I suppose I was thinking that having the OCFD would just be better in some respects, especially for weaker signal reception - I mean, with all that copper and all one might be silly enough to think "Hey, there's more material here to gather radio energy so it should improve things..." and so far it really hasn't, at least as compared to the homebrew ground plane which again surprises me at how well it works especially for weak signals.

Bleh, I'm just rambling on I suppose, it's not like there's a shortage of OCFD related posts and threads around here. ;)

It works, make no mistake on that, it does work and I still have to construct the shorty OCFD as well, that might offer some improvements towards the airbands which I was hoping would have seen more improvement - that was the primary reason for the OCFD in the first place. I found a Maldol AL-500H a few days ago from some guy in Brussels for $40, been searching for one for months now and finally located one so that should be arriving in a few weeks (if he ships it soon, that is). Can't wait to see what that one can do just because it has a fantastic reputation and it's nearly impossible to find anymore - I will keep that one for whenever I do get another handheld to be mobile.

So, there's my ramblings about the OCFD I just built, I doubt anyone will learn anything new in all this but who knows... :D
 

popnokick

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And after your latest post, I'm convinced it's built correctly, the coax and connectors are all good, etc. It's definitely working.... no fundamental problems with it.
Have you ever noticed with your cellphone or handheld scanner and rubber or whip antenna that when you move the cellphone or radio a couple of feet (or sometimes inches at higher frequencies) the signal improves? This is especially noticeable at UHF freqs, but happens even with FM broadcast. It's usually due to multi path fading (Rayleigh fading) from reflected signals off buildings and other structures. Based on the description of your location, I'm guessing there would be a LOT of that going on inside your apartment. So....
Is it possible that the ground plane is in a spot a few feet (inches maybe) different from the OCFD? And that for some signals you are receiving, the OCFD is in a multi path "null"... while the ground plane is in a "peak"? Hard question to resolve, because it's tough to move an OCFD a few inches/feet in any direction as compared to the ground plane, which can easily be moved around to find the "sweet spot" for a given signal.
 

br0adband

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Yah, I know, it's fine for now. Wish I could get it or the 1/4 wave outside but that's not going to happen so this will just have to do for the time being. I'm not "radio deaf" at least, I figure with my centralized location here in this area which is a valley surrounded by mountains on all sides I'm doing better than most, especially being on the 3rd floor so I'm not stuck on the ground. Still can't quite get the ground traffic from Nellis or McCarran but I'll survive with what I'm hearing.
 

wkm

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I used the RS fm trap to clean up the air band with the ocfd. I didn't realize how much interference there was till I was monitoring acars. Through the computer speakers I could hear the songs being played.
 

nanZor

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Sure sounds like you did everything right and as a compromise antenna, it is just not working for you *in that situation*.

This is one reason I always suggest just cobbling up the simpler wire version hooked to a 300:75 ohm block and hung temporarily. That will save you time, money and aggravation if you build one out of tubing first. :)

The good part is that like prcguy mentioned is that you now have the makings for a standard reference - a half-wave dipole if you cut down the tubing for equal lengths on both sides, and remove the transformer. ie, for a milair dipole, then about 8 to 9 inches equal length on each side ( and no transformer, but direct coax connection) would at least get you centered near 300mhz.

Without a standard for comparison, like a dipole, you'll just be pulling your hair out trying to make any sense out of other antennas.

Note that the ocfd is NOT just a scaled down amateur-hf antenna turned vertically. Typically those are fed at exactly 1/3 of the way in, and excel at even harmonics, except for the 6th harmonic. The dimensions for the scanner ocfd feedpoint placement is actually tweaked to provide a compromise impedance coverage since we are rx-only.

If you like, make a "shorty" version by cutting the existing element lengths in half (9 inches on one side, and 24 inches on the other) and see how that plays.

Yep - the standard version even though fed off center is primarily a broadcast fm antenna half-wave so FM overload can be a very serious issue. Radio shack traps have their notch near the middle of the upper third of the band, so if you can identify which station is causing the trouble, you may need to get more serious with a tuned filter from PAR and the like. Do NOT just stack two radio shack filters together, as the notch will only be 3db greater. Ie, if the RS notch doesn't work, you need to get serious. And, you don't always have to hear FM broadcast bleed through to be affected by overload, so there is that possibility.

I use the ocfd in a lot of portable operations, and like it a lot if it works well in a particular environment, but I also have learned to throw it in the trash really fast when it doesn't, and not waste too much time trying to optimize it. This may be one of those times.
 
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2guntom

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Greeneville, TN
Thanks to all who posted...

I'm trying to get my head around the "basic" antenna building principles... probably some questions to follow
 
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