Why not? A "50 ohm" output from a transmitter, whether it's a CB or a ham rig, is a nominal value. I have antennas fed with both RG-213 and RG-11, and I don't notice any difference at all. Don't get hung up on SWR.
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A Dipole for your CB Base Station?
- Thread starter Robb
- Start date
Why not? A "50 ohm" output from a transmitter, whether it's a CB or a ham rig, is a nominal value. I have antennas fed with both RG-213 and RG-11, and I don't notice any difference at all. Don't get hung up on SWR.
So if I happened to have around 1500 feet of RG-6QS (quad shield), that is 75ohm; I could use that for my feedline?? Albeit the RG-6QS is solid core, it is a permanent installation.
I guess I don't understand that statement, how would you tune the antenna? I thought you had to run 50ohm wire with 50ohm radios???
I'm probably getting in way beyond my knowledge base by asking that question!
That 75 ohm feed line isn't as 'wrong' as you might think.
For instance.
Say you are using a 'flat-top' type dipole. The typical input impedance is something like 70-75 ohms, so when using 50 ohm feed line, the resulting SWR att he transmitter will be something like 1.5:1. Now, same dipole but 75 ohm coax. There's a good match -at- the dipole feed point, but the SWR seen at the transmitter is still about 1.5:1. Just depends on 'where' you want that mismatch to be, at the antenna, or at the transmitter. Same difference either way.
- 'Doc
For instance.
Say you are using a 'flat-top' type dipole. The typical input impedance is something like 70-75 ohms, so when using 50 ohm feed line, the resulting SWR att he transmitter will be something like 1.5:1. Now, same dipole but 75 ohm coax. There's a good match -at- the dipole feed point, but the SWR seen at the transmitter is still about 1.5:1. Just depends on 'where' you want that mismatch to be, at the antenna, or at the transmitter. Same difference either way.
- 'Doc
Sorry it took so long to reply, been out of town for the last couple weeks...
So I would still be able to check SWR at the radio (not sure how one would check it at the antenna with a soldered connection) with 75ohm wire??
I've been looking at all the different dipole types...copper tubing, wire, coax (double bazooka), opposed mobile 102" antennas, etc. I'm thinking I may try to build one from copper since it is somewhat self supporting. Should be a simple enough task to solder things together.
Is the length of each leg or is the overall length more important. In other words, if the two pieces can be slide to/from one another (affecting the overall length of the antenna), is that more important than cutting pieces from each end (affecting the length of each leg)? Both affect the overall length, but which would be more desirable to do for tuning?
Also, if the mount is insulated, could a dipole be supported by the lower leg? Looking at this picture:
http://www.geocities.com/n2uhc_2/beacon_ant.jpg
Or this page, near the bottom:
http://verticaldipoles.tripod.com/
This seems to show the center is insulated and the mount is actually attached to the bottom leg of the antenna? If this is the case, this would allow me to get even more distance in the air.
So I would still be able to check SWR at the radio (not sure how one would check it at the antenna with a soldered connection) with 75ohm wire??
I've been looking at all the different dipole types...copper tubing, wire, coax (double bazooka), opposed mobile 102" antennas, etc. I'm thinking I may try to build one from copper since it is somewhat self supporting. Should be a simple enough task to solder things together.
Is the length of each leg or is the overall length more important. In other words, if the two pieces can be slide to/from one another (affecting the overall length of the antenna), is that more important than cutting pieces from each end (affecting the length of each leg)? Both affect the overall length, but which would be more desirable to do for tuning?
Also, if the mount is insulated, could a dipole be supported by the lower leg? Looking at this picture:
http://www.geocities.com/n2uhc_2/beacon_ant.jpg
Or this page, near the bottom:
http://verticaldipoles.tripod.com/
This seems to show the center is insulated and the mount is actually attached to the bottom leg of the antenna? If this is the case, this would allow me to get even more distance in the air.
is it because the theoretical impedance of a dipole, perhaps in space, is ideally around 72 ohms?
and 75/50 is only a ratio of 1.5:1 - SWR?
But perhaps you could compensate with some antenna tuning to bring the combined antenna/feedline impedance darn near close enough to match well with your 50 ohmage radio?
I think so!
and 75/50 is only a ratio of 1.5:1 - SWR?
But perhaps you could compensate with some antenna tuning to bring the combined antenna/feedline impedance darn near close enough to match well with your 50 ohmage radio?
I think so!
So I would still be able to check SWR at the radio (not sure how one would check it at the antenna with a soldered connection) with 75ohm wire??
Since you have to put that meter in-line to do any checking, you'd have to break that soldered connection to do the measuring at the antenna. there's a catch though. A typical/common SWR meter is designed to work based on 50 ohms input impedance, not 75 ohms. The readings gotten with 75 ohms coax and a 75 ohms input antenna would not be 1:1, I have no idea what it would read, but it would not be accurate at all. Do that measuring from the transmitter end, since it has a 50 ohm output. (50 ohm output to a 50 ohm input on the meter, through a 50 ohm jumper works fine.)
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I've been looking at all the different dipole types...copper tubing, wire, coax (double bazooka), opposed mobile 102" antennas, etc. I'm thinking I may try to build one from copper since it is somewhat self supporting. Should be a simple enough task to solder things together.
I don't know about where you live, but here, copper is more expensive than aluminum. Aluminum is lighter in weight, as strong as, if not stronger, than copper, and works just as well as far as making antennas with as copper. There's no particular benefit in using copper, other than in wire form, to make an antenna. The only benefit to using copper is that it's easier to solder to. So use a nut-n-bolt. Or go get that aluminum soldering stuff, it works too.
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Is the length of each leg or is the overall length more important. In other words, if the two pieces can be slide to/from one another (affecting the overall length of the antenna), is that more important than cutting pieces from each end (affecting the length of each leg)? Both affect the overall length, but which would be more desirable to do for tuning?
The overall length AND the length of each 'leg' is important. Each leg should be the same length. The total length depends on the frequency you want the thing to work on. Does it make any difference where you do that length adjustment, at the center, or the ends? No, not really, either will work. Which would be easiest for you? That's where I'd do that adjusting. How about moving the legs further apart or closer together? Not a problem, but you still have to make the connections to those ends of the legs, and those lengths of wire/whatever count as length too. See where I'm going with that? If you move those legs closer together and leave those connecting wires sort of looped, twisted, just hanging there, they are still going to count as 'leg' length. Not that big'a deal, but still a deal.
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Also, if the mount is insulated, could a dipole be supported by the lower leg? Looking at this picture:
(URLS deleted)
This seems to show the center is insulated and the mount is actually attached to the bottom leg of the antenna? If this is the case, this would allow me to get even more distance in the air.
That's not the case, exactly. The feed line runs up through that bottom leg of that vertical dipole, it isn't connected at the bottom, and yes, the center is still insulated and the feed line connects there. It will work. There's a 'catch' though. The ends of a 1/2 wave dipole have very high voltage on them (very little or none at the center insulator). that high voltage can play heck with feed lined. can make then radiate, etc. So if you run a feed line through the bottom leg of that dipole figure on putting a coax choke at thea point, Actually a few feet below the end of that dipole. That 'few feet' doesn't have to be a 1/4 wave, but that wouldn't hurt. (Also wouldn't hurt to drop down a few more feet and put in a second choke either.)
Can a dipole be supported by it's bottom 'leg'? Sure. If that leg is string enough to stand on. It also helps if the pole/stick/whatever that's doing that supporting is an insulator, or at least a non-conductor of some kind. Remember that high voltage on the ends of a dipole, it affects stuff around it. (Trick: Tape a small fluorescent bulb near the bottom end of that vertical dipole on the support. When you transmit it'll light up. Lets you know things are working. Use a -small- bulb! Cuz you're letting everyone else know it's working too, so they'll know who to complain to.
)
- 'Doc
Since you have to put that meter in-line to do any checking, you'd have to break that soldered connection to do the measuring at the antenna. there's a catch though. A typical/common SWR meter is designed to work based on 50 ohms input impedance, not 75 ohms. The readings gotten with 75 ohms coax and a 75 ohms input antenna would not be 1:1, I have no idea what it would read, but it would not be accurate at all. Do that measuring from the transmitter end, since it has a 50 ohm output. (50 ohm output to a 50 ohm input on the meter, through a 50 ohm jumper works fine.)
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I've been looking at all the different dipole types...copper tubing, wire, coax (double bazooka), opposed mobile 102" antennas, etc. I'm thinking I may try to build one from copper since it is somewhat self supporting. Should be a simple enough task to solder things together.
I don't know about where you live, but here, copper is more expensive than aluminum. Aluminum is lighter in weight, as strong as, if not stronger, than copper, and works just as well as far as making antennas with as copper. There's no particular benefit in using copper, other than in wire form, to make an antenna. The only benefit to using copper is that it's easier to solder to. So use a nut-n-bolt. Or go get that aluminum soldering stuff, it works too.
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Is the length of each leg or is the overall length more important. In other words, if the two pieces can be slide to/from one another (affecting the overall length of the antenna), is that more important than cutting pieces from each end (affecting the length of each leg)? Both affect the overall length, but which would be more desirable to do for tuning?
The overall length AND the length of each 'leg' is important. Each leg should be the same length. The total length depends on the frequency you want the thing to work on. Does it make any difference where you do that length adjustment, at the center, or the ends? No, not really, either will work. Which would be easiest for you? That's where I'd do that adjusting. How about moving the legs further apart or closer together? Not a problem, but you still have to make the connections to those ends of the legs, and those lengths of wire/whatever count as length too. See where I'm going with that? If you move those legs closer together and leave those connecting wires sort of looped, twisted, just hanging there, they are still going to count as 'leg' length. Not that big'a deal, but still a deal.
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Also, if the mount is insulated, could a dipole be supported by the lower leg? Looking at this picture:
(URLS deleted)
This seems to show the center is insulated and the mount is actually attached to the bottom leg of the antenna? If this is the case, this would allow me to get even more distance in the air.
That's not the case, exactly. The feed line runs up through that bottom leg of that vertical dipole, it isn't connected at the bottom, and yes, the center is still insulated and the feed line connects there. It will work. There's a 'catch' though. The ends of a 1/2 wave dipole have very high voltage on them (very little or none at the center insulator). that high voltage can play heck with feed lined. can make then radiate, etc. So if you run a feed line through the bottom leg of that dipole figure on putting a coax choke at thea point, Actually a few feet below the end of that dipole. That 'few feet' doesn't have to be a 1/4 wave, but that wouldn't hurt. (Also wouldn't hurt to drop down a few more feet and put in a second choke either.)
Can a dipole be supported by it's bottom 'leg'? Sure. If that leg is string enough to stand on. It also helps if the pole/stick/whatever that's doing that supporting is an insulator, or at least a non-conductor of some kind. Remember that high voltage on the ends of a dipole, it affects stuff around it. (Trick: Tape a small fluorescent bulb near the bottom end of that vertical dipole on the support. When you transmit it'll light up. Lets you know things are working. Use a -small- bulb! Cuz you're letting everyone else know it's working too, so they'll know who to complain to.
)- 'Doc
I dont think anyone has mentioned that according to L.B. Cebik's computer models, over real ground a horizontally polarized dipole should have about 4db gain over a vertical 5/8 antenna. I use a dipole on 10m, I used 2 102" stainless whips I had laying around.
I used a RadioWavz Bazooka 11 meter dipole for years, hit almost every state and England, Alaska, New Zeland, Australia to name a few......
Most of the watergates of me on various radio web sites around the net were from that dipole. I had it up around 50 feet, in an inverted V stretched out on a pvc frame join to a plumbing vent, click the thumbnail to see it:
I just purchased a couple of these to play around with. As far as tuning SWR on 11-meters, what exactly is it that I have to trim if required?
Oh, you're gonna love this!
A bazooka is a sort of 'multi-element' antenna, meaning that there are really two parts to it, the whole center conductor is the radiating part of the antenna, then there's the two tuned sections that do the feeding of the antenna (outer shield that the feed line connects to. To tune for resonance you'd do it like you would any antenna, make the radiating part's length (center conductor) right. Doing that on each end isw the simplest way. Shorten the thing to raise the resonant point, lengthen it to lower it.
Normal so far, right? Well, here's the 'catch'.
Those two tuned circuits, the shielding the feed line attaches to, need to be lengthed or shortened too. Otherwise, they don't stay "centered" at a particular frequency. If you raise/lower the resonant freq of the center conductor, you ought to raise lower the shiled's resonance point to match it. Otherwise, you won't be able to go as far in one direction (frequency) as you will in the other direction. How do you do that? You change where the 'short' is at the outer ends of those shield sections. Raising the resonance point is no really big deal, just means shorting that shield closerto the center of the antenna. Not all -that- easy, but not all that difficult either. What if you want to lower the resonant frequency? That get's a bit harder to do. How do you 'un-short' the shield to make it longer? I don't know either. So, it's a pretty good idea to start with the thing resonant at the lowest frequency of use, so you can raise that that resonant point later.
Ain't that a bummer?
There are 'tricks' that can make that a bit easier if you build your own. They aren't the 'best' way of doing it, but they do work. Ever think that "pinning coax" would have a useful purpose?? It works. It also works better if the whole antenna is made from coax, instead of there being a 'solid' wire/ladder line/whatever on the ends.
Having said all that...
A bazooka antenna sometimes doesn't have to be 're-tuned'. It's sort of 'broad' to start with, right? If that 'broadness' fits where you want it to fit, then it isn't absolutely required that you re-tune it. Will it be as efficient as it possibly could be? Nope. But a bazooka antenna isn't exactly the most efficient antenna in the world to start with. (You'll get arguments about that, take whichever side you'd like.) How much efficiency is lost? I haven't a clue.
- 'Doc
A bazooka is a sort of 'multi-element' antenna, meaning that there are really two parts to it, the whole center conductor is the radiating part of the antenna, then there's the two tuned sections that do the feeding of the antenna (outer shield that the feed line connects to. To tune for resonance you'd do it like you would any antenna, make the radiating part's length (center conductor) right. Doing that on each end isw the simplest way. Shorten the thing to raise the resonant point, lengthen it to lower it.
Normal so far, right? Well, here's the 'catch'.
Those two tuned circuits, the shielding the feed line attaches to, need to be lengthed or shortened too. Otherwise, they don't stay "centered" at a particular frequency. If you raise/lower the resonant freq of the center conductor, you ought to raise lower the shiled's resonance point to match it. Otherwise, you won't be able to go as far in one direction (frequency) as you will in the other direction. How do you do that? You change where the 'short' is at the outer ends of those shield sections. Raising the resonance point is no really big deal, just means shorting that shield closerto the center of the antenna. Not all -that- easy, but not all that difficult either. What if you want to lower the resonant frequency? That get's a bit harder to do. How do you 'un-short' the shield to make it longer? I don't know either. So, it's a pretty good idea to start with the thing resonant at the lowest frequency of use, so you can raise that that resonant point later.
Ain't that a bummer?
There are 'tricks' that can make that a bit easier if you build your own. They aren't the 'best' way of doing it, but they do work. Ever think that "pinning coax" would have a useful purpose??
It works. It also works better if the whole antenna is made from coax, instead of there being a 'solid' wire/ladder line/whatever on the ends.Having said all that...
A bazooka antenna sometimes doesn't have to be 're-tuned'. It's sort of 'broad' to start with, right? If that 'broadness' fits where you want it to fit, then it isn't absolutely required that you re-tune it. Will it be as efficient as it possibly could be? Nope. But a bazooka antenna isn't exactly the most efficient antenna in the world to start with. (You'll get arguments about that, take whichever side you'd like.) How much efficiency is lost? I haven't a clue.
- 'Doc
Anyone that has been on here for awhile knows I used an RadioWavz dipole in an inverted v for quite awhile. AND - people heard me on that General Lee and that Varmint XL 250 ALL OVER THE COUNTRY. I was not the biggest gun by no means but fun per $ spend? I had a blast and held my own.
I use a Dipole for 11 meter. I have it in the rafters of my basement and it gets out pretty well. Around 5 miles. If it is mounted north and south, then your signal will go east west. I made mine from #12 wire. 468 divided by frequency, (27.205) in 11 meter.
maybe something is sinking into my thick head because it made sense to me . thanks doc.
odd 1/4 wave bad !!!!
even 1/4 wave good !!!!!
even 1/4wave = 1/2 wave . good !!!!!
as frankenstein might say .
i "think' the point he was trying to make is: if you use an odd 1/4 wl, you are feeding from either 90 or 270 degrees on the sine wave where current is min and voltage is max.
at 0(360) and 180 degrees it flipflops.... voltage is min and current is max
I've read every post here and there is a wealth of information. In the early 80's a "ham" friend of mine helped us build a diapole antenna we could use in an apartment. We used a single piece of coax with a PL-259 on one end. If I remember right it went out left to right 8' 8" each way . It came out the radio up the wall as 1 piece the spit like a T and run along the wall. These posts brought back alot of great memories. David (Triple L) was the ham guy. Anyway I'm going to purchase one of the mentioned diapole antennas and try one. Thanks for all the great reading.
Why buy? Build your dipole (not "diapole") and save at least half over what a commercially-built one would cost (plus the shipping).
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