So if my 75 ohm cable has less loss than 50 ohm cable, I should just use the 75 ohm cable from antenna to scanner?
My homemade Off Center Dipole Antenna
- Thread starter dmg1969
- Start date
- Status
Yes... scanning receivers work fine with 75 ohm coax, and the lower loss cable is better at higher frequencies. I would think it highly unlikely you would be introducing susceptibility to internodulation products into the receiver front end by using 75 ohm coax. Not impossible, just unlikely unless you are currently experiencing internod problems. If so, try both types of cable to see if there is a reduction in your radio's susceptibility to intermod with either cable.
An alternative to using a 300 / 75 ohm transformer is to use a short length of coax as a transmatch, like this:
http://www.n-lemma.com/calcs/dipole/balun.htm
A 25" section of coax should do the trick.
http://www.n-lemma.com/calcs/dipole/balun.htm
A 25" section of coax should do the trick.
I just discovered this little OCFD project a day or two ago. It seems pretty awesome and a fun project! Before I start, just a couple of questions if ya don't mind:
Did you put screws on both sides of the T-joint or they're all just on one side? For the rg-6 coax cable, will it fit onto my BNC antenna port on my pro-668 or will I have to get a BNC adapter to connect? And regarding the signal and all- I am using my pro-668 with the diamond rh77ca antenna. I live in central NJ and currently scan NYPD/FDNY in brooklyn and manhattan and was wondering if this OCFD would improve the signal.
For brooklyn NYPD about 35 miles away I get anywhere from 1-3 bars of signal (usually 2) and with NYPD in manhattan about 40 miles away I get no signal at all to 1 bar (usually no signal). I also have FDNY fire/ems programmed in there but I never listen to them because there's always extreme static (the same 35-40 miles away). Both NYPD in brooklyn and manhattan that I want to listen to are in the 476 MHz frequency. NYPD transit (35 and 40 miles away) is in the 160 and 161 MHz frequency. And FDNY fire is in the 153, 154, 482, and 485 MHz and ems is in the 482 and 483 MHz.
So would this OCFD improve the signal given what I scan and how far away it is? I just wanted to know whether or not it would help before I go ahead and start making it.
Last edited:
paulears
Member
I've come to this topic quite late, and didn't know people were downsizing the old Windoms, and using them on VHF and UHF bands. What I'm wondering is 'why'? There were always a compromise antenna when I did them on HF. UK gardens were always far smaller than the space available to people abroad, and a simple horizontal dipole that worked really well on one band was a bit rubbish on others, even with a tuner. The Windom allowed you to make the thing resonate on multiple bands, depending on how you arranged the split point, but had a very odd polar diagram, and they were a bit unpredictable. Turning them on end and using them to receive vertically polarised signals seems a bit odd - your main lobes will be at many angles to vertical, depending on the operating frequency, so at best, you have the performance of a dipole and worse, the performance of a random bit of metal in the sky at others. At VHF and worse at UHF, the tubing has a width as well as a length, so you have the impact of that too - doing things to the performance. I'd be surprised if a dipole, centre fed didn't work a lot better than these designs. People do seem to like them, but I'm not sure why. When they were on 40 or 20m, people used them simply to be able to work 2 bands. They knew they weren't very good as aerials, but they worked. For a scanner or wide band receiver, a centre fed dipole with multiple elements each cut for a specific band might work even better?
Yes, you have come to this topic quite late....and apparently haven't read the multiple threads about the OCFD scanner antenna that have been posted here on RR as far back as 2008. Here's but one of them:
http://forums.radioreference.com/bu...3-off-center-fed-vhf-uhf-vertical-dipole.html
The OCFD (Windom as you described) behaves very differently at VHF/UHF frequencies than it does at HF . The "why" people build them are multiple:
- They are very simple and inexpensive to make
- They cover a VERY broad range of VHF and UHF frequencies, unlike a dipole
- They offer some gain over a dipole or discone
- They are easily made from wire, rolled up for transportation, and hung in a window or attic
- They are easily disguised behind a curtain or hung on a balcony.... no radials or other stuff sticking out
The reason you do the "odd" thing of "turning them on end" is that the VHF / UHF signals you are receiving are vertically polarized and do not arrive at multiple angles because unlike HF signals they are not being bounced off the ionosphere. A centre fed dipole does not offer the wide bandwidth or gain that the OCFD affords.
There is much more to read here on RR about the simple scanner OCFD here on RR, including EZ-NEC modeling, pattern testing, pictures, and user reports. Back to the books for you....
ANY antenna that is not a telescoping whip or rubber duck mounted directly to your scanner, but rather connected via a coax to a higher location will be an improvement. So yes, if you use the OCFD in an upper story window or attic it will perform better than your RH77CA antenna, assuming you don't have a metal roof, foil-backed insulation, or other obstruction to incoming RF. If you go stand on the roof with your RH77CA and the scanner in your hand above the level of your OCFD.... well then yes the scanner-mounted antenna will do better. But I'm betting that is not how you want to do most of your listening....
paulears
Member
YEP - but it's those angles that interest me - for it to have useful gain over a dipole, those lobes need to be pointing somewhere sensible - a dipole has the strongest emissions at right angles to the dipole, the toroidal polar pattern that's damn handy. Unless you are listing to aircraft at exactly the right height, or listening via reflection - I'm surprised scanner users find them useful at all? As these lobes change with frequency, isn't this kind of antenna a bit unpredictable? I can see some gain being achieved at some angles, but this design seems a bit PT Barnum as a useful kind of device? No doubt fun to make, but just not that useful, compared to more modern designs that are more suited for shorter wavelengths. I think it was Tonna who in the 80s discovered that their 16 element 2m beams worked better with thin aluminium directors rather than ones the same thickness as the driven element. Gave them a bit more gain at the expense of bandwidth?
I'm not saying these Windom designs are bad, because they clearly work, but a toroid as a gain model makes sense, whereas something more spikey and uneven would seem to be less good? Just a thought, that's all. Slim Jims performed better because the angle of strongest field was more close to 90 and less was wasted going upwards at an angle. They were rubbish out of band though. Interesting this design has risen again though.
In the antenna modeling that was done (I believe with EZ-NEC) and documented here on RadioReference by either the member "hertzian" or "prcguy", it was found that the OCFD for VHF / UHF scanning started to "point" skyward when you went higher in frequency... beginning at about 500 mHz and becoming more acute as frequency increased. So the OCFD's effectiveness for 700-800 mHz may be limited. However, it performs very well toward the horizon at frequencies below 500 mHz. And in fact it works well down to 29 mHz in the VHF Lowband range.... where the feedline actually becomes part of the antenna. Can you do better with a purpose-built, cut to frequency, narrow-bandwidth antenna? Absolutely. Can you get wide frequency coverage (29 to 500 mHz) at very low cost and low effort in a portable antenna? Not so simple to do. Can you afford NOT to try an OCFD... which takes 15 minutes of your time and about $10 in parts (wire version) to make?
paulears
Member
That's indeed worth a go, and I'll investigate as soon as I'm home after christmas - as a wideband design, it could be quite useful - in my head, I'd never even considered the downscaling to the VHF/UHF bands and using them vertically. Thanks for the pointers.
I wouldn't be mounting this on the roof, if I was going to put anything on the roof I would just go straight to a yagi, but the roof needs some work and I'm a bit apprehensive when it comes to mounting anything up there. But as for the OCFD- where I'm scanning is at the highest level of the house (besides the attic-I don't believe there's room up there) so I would be putting it at the same level as the scanner and rh77ca is now. Would the OCFD still outperform what I have now and give me a more clear signal in the frequencies listed above and at the distances they are if used at the same level?
Last edited:
Possibly.... And it would let you hang the antenna in a window away from the radio, computer, and other sources of interference. Placed right next to the RH77 in the center of the room, maybe not. Try it.
This is the OCFD with the copper pipe I'm talking about, not the wire version- I apologize... should have mentioned that in my original post. I would be using this instead of the rh77ca scanning the above freqs in the same room that I usually use the rh77ca.
And how should I screw together the t-joint and copper pipe- screws on both sides or just one side is necessary?
Last edited:
The T-joint is a press-fit... not threaded. The two elements should NOT contact each other inside the tee. The screws are self tapping sheet metal and are used to connect the TV transformer leads to each of the two pipe elements.
Regardless whether the wire type or copper pipe version.... the antenna will work better if you place it in a window or attic / crawlspace.
Regardless whether the wire type or copper pipe version.... the antenna will work better if you place it in a window or attic / crawlspace.
okay. I'll probably start this tomorrow. Thanks for your response and help, I appreciate it!
Have a good day.
Nick
If you're wanting to monitor NYPD from 35 to 40 miles away, why not make a small vertical yagi from coat hangers and PVC and suspend it from the ceiling? For 476 Mhz, it would be quite small...
From the website Amateur Beam Antenna Calculator...
Calculated Driven Side Element Length: 0ft. 5 - 31/32in. or 0.151 M
Calculated Total Driven Length: 0ft. 11 - 15/16in. or 0.303 M
Calculated Reflector Length: 1ft. 0 - 21/32in. or 0.321 M
Calculated Element Spacing: 0ft. 4 - 3/4in. or 0.120 M
Calculated Director Length: 0ft. 11 - 7/32in. or 0.285 M
The antenna would be smaller than the dimensions of a computer screen
From the website Amateur Beam Antenna Calculator...
Calculated Driven Side Element Length: 0ft. 5 - 31/32in. or 0.151 M
Calculated Total Driven Length: 0ft. 11 - 15/16in. or 0.303 M
Calculated Reflector Length: 1ft. 0 - 21/32in. or 0.321 M
Calculated Element Spacing: 0ft. 4 - 3/4in. or 0.120 M
Calculated Director Length: 0ft. 11 - 7/32in. or 0.285 M
The antenna would be smaller than the dimensions of a computer screen
Probably not a bad idea, but besides regular NYPD, theres NYPD transit and FDNY both fire and ems frequencies which go into the 480s, 160s, and 150s MHz. I plugged those different freqs into the antenna calculator and the dimensions changed around pretty drastically. I'd rather just stick with the copper pipe OCFD for now being that it seems to be so highly praised and used by many people across this forum. After we get the roof done I'm probably just going to put up a yagi anyway....
majoco
Stirrer
You have to remember that it's not just the antenna - what may work in one place and get good signals may not work in another. It all depends on lots of variables - height off the ground or roof - other antennas nearby - other reflecting surfaces such as another house roof - a proper match to the coax changes the directional pattern - and so on. It is impossible to give an answer to the many questions such as "if I put this antenna here, will I be able to pick up this station that is xxx miles away". The best answer is - make the antenna - put it up, and see. One test is worth a thousand opinions.
For what it's worth, I have an HF horizontal OCFD with a 9:1 unun feeding 50ohm cable. In the evening I regularly receive a 500watt station in Brazil on 5940kHz. At a rough guess that's about 11,000 km/6,600miles. So OCFD's do work.
For what it's worth, I have an HF horizontal OCFD with a 9:1 unun feeding 50ohm cable. In the evening I regularly receive a 500watt station in Brazil on 5940kHz. At a rough guess that's about 11,000 km/6,600miles. So OCFD's do work.
Last edited:
Well I am going to definitely try it regardless. I know there are many factors involved in how well the signal will come in and those factors just as equally affect my rh77ca and the signal isn't that terrible for it so I'm willing to say that the OCFD will show improvement. I'm excited to get the materials and build it tomorrow, hopefully I won't have any trouble finding the 75-300 ohm balun at my local radioshack, if so I'll just have to order it online and build it another day. Maybe I'll order 2 of them and some copper wire and try out the wire version of the OCFD as well some other time....
So I just went out and got the materials for the copper pipe OCFD. Did anyone else have trouble when it came to the part of the T-joint with the pvc pipe, and copper pipe and fitting everything? Every pipe I got and the T-joint as well is 1/2" in diameter as it states in the materials list at the beginning of this forum thread, but the copper pipe seems to be too small to fit into the 1/2" T-joint and the T-joint just slides right down the pipe and off the other side. I double, triple, and quadruple checked the sizes of everything while I was at home depot and I am 110% sure it's all 1/2" diameter. So how is putting this together all possible if the copper pipe and the pvc pipe are 2 different thicknesses? The only thing that fits is the 1' long pvc pipe into the bottom part of the pvc T-joint. The copper pipes go right through and there are no pvc end caps that fit either.
I tried different variations of sizes while I was at home depot, but nothing seems to work at all. Any ideas?
I tried different variations of sizes while I was at home depot, but nothing seems to work at all. Any ideas?
Last edited:
- Status
