n4yek
Member
With a balun hooked to the antenna, it doesn't matter. The question is do you want the long end up or the short end. Personally I installed mine with the long end up, but that is just a personal choice.
My homemade Off Center Dipole Antenna
- Thread starter dmg1969
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With a balun hooked to the antenna, it doesn't matter. The question is do you want the long end up or the short end. Personally I installed mine with the long end up, but that is just a personal choice.
rudyizcool
Member
I'm still debating on how long my coax should be from the antenna to the receiver. I'm "hanging" the dipole about 10 feet on my wall inside my house, and listen to it as I'm going to bed. Sometimes I will take it outside and prop it up about the same height, and listen. So about how long should the coax be in general if I'm using it for portability? I still have about 25 feet of the RG6 on it. And yes, 25 feet of any wire can be annoying when carrying around, especially if you have big feet lol. By the way, I'm hearing most shortwave bands very clear with this dipole. Some interference from the electrics can be heard in the higher meter bands though. I'm still hearing the Brazilian pirates very well on the milsat frequencies!
I have another quick question. Can I use hot glue to keep both ends of the copper pipe inside the PVC Tee, rather than screwing them in? I had a hard enough time drilling in the copper pipe for the screws for the balun connectors.
I have another quick question. Can I use hot glue to keep both ends of the copper pipe inside the PVC Tee, rather than screwing them in? I had a hard enough time drilling in the copper pipe for the screws for the balun connectors.
dmg1969
Member
I don't see why not. Expansion and contraction of the metal in hot and cold weather could cause the glue to crack and fail though. Screws will make it sturdier.
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In a portable situation, a 6 to 12 foot jumper of RG6 would have lower loss, would be better than 25 feet especially on 800mhz. But you want it long enough to be practical, and you want to run the coax away perpendicularly from the antenna elements for at least a few feet. Don't let the coax hang down near the elements.
Reminder about 300 mhz and above - the antenna while being a decent match for rx only, still has lobes that look upwards, rather than outwards.
If one really wanted to have some fun, while keeping in mind the pattern compromises, you could turn it into an ocfd groundplane. That is, the long section would be vertical of course, and at least two, preferably 4 "radials" 18 inches long horizontally. That way you could bring your coax down vertically instead of having to run it away perpendicularly. Still using the balun of course. And still a compromise, but this antenna is about fun, not perfectly engineered.
Reminder about 300 mhz and above - the antenna while being a decent match for rx only, still has lobes that look upwards, rather than outwards.
If one really wanted to have some fun, while keeping in mind the pattern compromises, you could turn it into an ocfd groundplane. That is, the long section would be vertical of course, and at least two, preferably 4 "radials" 18 inches long horizontally. That way you could bring your coax down vertically instead of having to run it away perpendicularly. Still using the balun of course. And still a compromise, but this antenna is about fun, not perfectly engineered.
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I am going to build this antenna and was wondering if a larger diameter pipe would hurt, rather than help the reception!!!!???? Seems like I read somewhere that larger diameter would increase the bandwidth! Can anyone confirm this or would larger be a waste of $$$$$? Thanks
rudyizcool- You know there is a "portable" version of the OCFD, right? Pics and discussion in this thread:
http://forums.radioreference.com/scanner-receiver-antennas/283233-seeking-better-travel-antenna.html
http://forums.radioreference.com/scanner-receiver-antennas/283233-seeking-better-travel-antenna.html
A larger diameter pipe tends to smooth out the rapid impedance fluctuations as you cross the bands, but it may not be significant enough for you to notice.
This antenna is a compromise from the start, and trying to perfect it, means pretty much a waste of money. And I like them for what they are.
About the only improvement I would make would be to use a bunch of snap-on ferrites around the coax near the feedpoint, to help reduce the common-mode coupling of the feedline. These should be at least vhf rated like the Radio Shack #273-105 chokes on RG-6. Problem is, you need a LOT of them to do much good, like 8-10 or more. Unless you have these laying around, that gets pretty expensive for a compromise antenna. Still, if you got some, put them on.
That's about as far as I'd try to customize the ocfd.
This antenna is a compromise from the start, and trying to perfect it, means pretty much a waste of money. And I like them for what they are.
About the only improvement I would make would be to use a bunch of snap-on ferrites around the coax near the feedpoint, to help reduce the common-mode coupling of the feedline. These should be at least vhf rated like the Radio Shack #273-105 chokes on RG-6. Problem is, you need a LOT of them to do much good, like 8-10 or more. Unless you have these laying around, that gets pretty expensive for a compromise antenna. Still, if you got some, put them on.
That's about as far as I'd try to customize the ocfd.
Speaking of chokes... if you add them, will that reduce/impair VHF low-band reception? Reason I'm asking is that I've read that the coax feed line actually becomes part of the antenna in the 30-50 mHz range (approx). If you applied chokes to the feed line would it affect the low band receiving capability? And I know low band is an unpredictable compromise with the OCFD, depending on factors like the length and position of the coax. But I also know that mine works quite well on low band (about 75 feet of RG6 running down from the attic). So if I apply chokes to reduce common mode RF, what happens to VHF low?
Yes, the braid of the coax is part of the antenna at ALL frequencies. Which makes the reception pattern even more unpredictable everywhere.
The balun/transformer, usually being a voltage-type balun, is effectively doing nothing as far as balance goes as it can't handle the severe asymetrical-ity. But the transformer part does it's job. (300:75 ohm, aka 4:1 ratio).
Vhf low being the easiest to choke (the chokes have the most resistive impedance here), means that it would be pretty effective. Not so much above 300 mhz. That would effectively leave just the elements doing vhf-low, which is below the primary resonant freq of 88 mhz, and by itself is not going to do a good job.
HOWEVER, on vhf low you may actually get an IMPROVED pattern, rather than the random one from the feedline.
In the end, it's a toss up as to if it will help or harm your vhf low monitoring.
The balun/transformer, usually being a voltage-type balun, is effectively doing nothing as far as balance goes as it can't handle the severe asymetrical-ity. But the transformer part does it's job. (300:75 ohm, aka 4:1 ratio).
Vhf low being the easiest to choke (the chokes have the most resistive impedance here), means that it would be pretty effective. Not so much above 300 mhz. That would effectively leave just the elements doing vhf-low, which is below the primary resonant freq of 88 mhz, and by itself is not going to do a good job.
HOWEVER, on vhf low you may actually get an IMPROVED pattern, rather than the random one from the feedline.
In the end, it's a toss up as to if it will help or harm your vhf low monitoring.
In my situation, going from 1/2" copper pipe to 3/4" copper pipe made a noticeable difference in reception. My OCFD is inside my house because I don't yet have an outdoor mast installed. The 1/2" copper wasn't much better than the whip on the back of my 197, but the 3/4" copper helped quite a bit. However, as hertzian said, you may not notice a difference in your situation. The only way to find out is to give it a try!
rudyizcool
Member
In a portable situation, a 6 to 12 foot jumper of RG6 would have lower loss, would be better than 25 feet especially on 800mhz. But you want it long enough to be practical, and you want to run the coax away perpendicularly from the antenna elements for at least a few feet. Don't let the coax hang down near the elements.
Reminder about 300 mhz and above - the antenna while being a decent match for rx only, still has lobes that look upwards, rather than outwards.
If one really wanted to have some fun, while keeping in mind the pattern compromises, you could turn it into an ocfd groundplane. That is, the long section would be vertical of course, and at least two, preferably 4 "radials" 18 inches long horizontally. That way you could bring your coax down vertically instead of having to run it away perpendicularly. Still using the balun of course. And still a compromise, but this antenna is about fun, not perfectly engineered.
Thank you for the reply about the length of the coax! Today, I shortened it to about 8 feet, just enough to reach from my wall (where the dipole hangs), to my bed. This gives it proper length, and so far, it does sound like it is receiving MUCH better! And my coax does come down from the balun, down the "handle", and then over to my bed. I have it tied to the pole to keep it from moving and causing the cable to come loose from the balun as it has in the past.
Thanks again!!
trailblazer354
Newbie
Built antenna yesterday. Works great thanks for the info
Xicaque
Member
Well done! Thanks for sharing!
It might work, but not as well as it would if you use the specified dimensions (18 and 48 inches). Many have tried, and the reports are that shortening it does NOT help VHF at all... in fact, it hurts it.
Well after a couple days use I think I have succeded . I went from 1 noisy noaa channell to 1 strong channel and 2 that are able to be heard and understood. It is doing well hearing uhf fire dispatch at 35 crow flys miles. I am using it on a pro-433. It is sitting in the window of my basement apt. I am using a half wave dipole made of 1/2 copper pipe cut for 453.950 on my hp1.
Would it be better to use RG6 quad shielded 75ohm cable for a run since the transformer is 300 to 75 ohm? I know the scanner is 50ohm impedance, so you'll have reflection anyway...
Is it worth it to use impedance matching pad or simpler to create your own impedance matching pad from 300ohm to 50ohm? Last item on the page Matching 50Ω to 75Ω
Is it worth it to use impedance matching pad or simpler to create your own impedance matching pad from 300ohm to 50ohm? Last item on the page Matching 50Ω to 75Ω
prcguy
Member
A minimal loss pad is broad band but has 5.7dB of loss! The L network shown in your link is very low loss but also very narrow band. The only good solution for broad band 50 to 75ohm impedance matching is a broad band transformer but they are usually limited to 300MHz or maybe 400MHz max.
Quad shield RG-6 has the same loss as regular RG-6 but its sometimes used where lots of cables are bundled together and the extra shielding helps reduce signals from migrating between cables. For regular home use it doesn't buy you anything unless your cables run very close to some RF interfering stuff.
prcguy
Quad shield RG-6 has the same loss as regular RG-6 but its sometimes used where lots of cables are bundled together and the extra shielding helps reduce signals from migrating between cables. For regular home use it doesn't buy you anything unless your cables run very close to some RF interfering stuff.
prcguy
Which part of last item on page isn't clear? "Matching 50Ω to 300Ω"
Update:
Yes, I know all that. I am interested in figuring out which combination will provide the lowest loss.
Update:
Yes, I know all that. I am interested in figuring out which combination will provide the lowest loss.
I had looked for a broadband 75 to 50 ohm matching transformer, UNUN, and did not find one.
The match difference is not that great, but sometimes the mismatch makes the receiver more subjective to intermod in the frontend.
The loss in the restive 'pad', 75 to 50 ohms, is not that much, IIRC.
The match difference is not that great, but sometimes the mismatch makes the receiver more subjective to intermod in the frontend.
The loss in the restive 'pad', 75 to 50 ohms, is not that much, IIRC.
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