Whitehall
Super Member
So we've seen a number of J-pole designs and methods for calculating lengths, separations, etc and doing the tuning. You'll want to download the attached pdf file since I refer to it often.
(WARNING:
)
When I started to build a J-pole, I was puzzled. Why this dimension and why doesn't that factor affect the performance, why can't this material work, what happens when I change from 75 ohm coax to 300 ohm twin lead. And so on.
Since I'm not one to blindly follow the leader, I set out to learn a bit about RF antennae and how it might affect a J-pole design. Here's what I've found so far.
The basic concept is that radio frequency energy does NOT act like household current. Rather than "flowing" down a line, high frequency energy sets the transmission medium "abuzz" - causing the conductor to have electric and magnetic fields around it vibrate like a plucked string, sometimes with standing nodes and peaks. In antenna, that's what we want - in transmission lines like coax, that's what we want to avoid.
Since the fields are vibrating, the effects differ at various points along the transmission line, in our case, the antenna prongs. The quarter wave length pole has fields that look like figure A at the upper left of the attachment. The lower cross-piece of the J-pole only serves to short out the quarter wave and the half wave prongs.
This shorted quarter wave structure has a number of functions in RF engineering as seen in B and C of the left side. Our J-pole is something like figure C in that we're matching a resonant antenna to a tuner via a transmission line.
Now to get good transfer of energy between the antenna, the cable, and the tuner, each should have the same characteristic impedance - that's called "impedance matching". So if we use 75 ohm RG6 coax, we use the 75 ohm connection on the tuner and match 75 ohms on the antenna. The first two are done for us - just buy and connect correctly. (Impedance matching transformers would work too but with some insertion loss.)
The last task, getting a 75 ohm antenna connection, was a mystery to me but figure A made it clear. The graph shows zero ohms at the shorted end and infinite ohms at the open end. When we slide our connection along the prongs, we are finding the point on the prong where the curve shows 75 ohm (assuming RG6). Since a half wave prong is like two quarter waves hooked together, the same position works for both prongs. This assumes that we built it correctly and that the half is twice the length of the quarter.
If we wanted to use 300 ohm twin lead, we would just connect a bit farther out on the prong where the impedance curve rises to 300 ohm.
Now one problem is that free field RF waves have a higher velocity than RF waves in a medium, in this case our antenna prongs. What we want to know is what is the velocity of RF in our prongs is relative to vacuum - first cut is say, 70%. We would then have to cut our prongs to the correct "electrical" length which is not the same as a free field length but longer since electromagnetic radiation is slower in any media, like copper.
Have any real idea what the velocity of light is in copper pipe? Aluminum tape? Twin lead? Me neither but it makes a difference in how long the prongs need to be for a specific frequency.
One way to find out is to use the gadget on the right the "Lecher-wire system." Using a shorted pair prongs at least one free field wave length long, move your connection to your tuner, set at the frequency of interest, run your connection points up and down until you get a MINIMUM signal strength. Keep moving and when you hit another MINIMUM, you've found one half wave length in the material under test.
Note that I haven't tried this yet but the US Navy recommends it so it must be super. It might be overkill for our purposes though.
Another question is how close can the prongs be? I wanted to use aluminum tape back-to-back. That might not work - the book says that one needs some space between the prongs just as one needs space between the central conductor and the shield in coax or between conductors in twin lead. Alternately, I might try a dialectric between the aluminum tapes, like styrofoam or polyethylene.
So you want to tune your new J-pole to another frequency? Rather than cutting or soldering extensions, one can tune the length electrically by adding capacitance or reactance between the antenna and the transmission line as shown in the center sketch. I suspect that one would insert equally for BOTH connections in the J-pole.
Another issue is grounding. The J-pole does not seem to NEED to be grounded although the place to do so would be on the shorting connection. Using coax, you're grounding at the tuner or at the RF shield ground in any case. This would be helpful for an outside antenna for lightning protection but for indoor use, only the RF shield ground would be helpful to ditch the transmission line reception of RFI. Don't understand the need for the coax "choke" yet seen in some designs. Probably twin lead doesn't need it.
Anyway, that's what I think I know. Anyone is welcome to set me straight or amplify what I've posted here.
(WARNING:
)When I started to build a J-pole, I was puzzled. Why this dimension and why doesn't that factor affect the performance, why can't this material work, what happens when I change from 75 ohm coax to 300 ohm twin lead. And so on.
Since I'm not one to blindly follow the leader, I set out to learn a bit about RF antennae and how it might affect a J-pole design. Here's what I've found so far.
The basic concept is that radio frequency energy does NOT act like household current. Rather than "flowing" down a line, high frequency energy sets the transmission medium "abuzz" - causing the conductor to have electric and magnetic fields around it vibrate like a plucked string, sometimes with standing nodes and peaks. In antenna, that's what we want - in transmission lines like coax, that's what we want to avoid.
Since the fields are vibrating, the effects differ at various points along the transmission line, in our case, the antenna prongs. The quarter wave length pole has fields that look like figure A at the upper left of the attachment. The lower cross-piece of the J-pole only serves to short out the quarter wave and the half wave prongs.
This shorted quarter wave structure has a number of functions in RF engineering as seen in B and C of the left side. Our J-pole is something like figure C in that we're matching a resonant antenna to a tuner via a transmission line.
Now to get good transfer of energy between the antenna, the cable, and the tuner, each should have the same characteristic impedance - that's called "impedance matching". So if we use 75 ohm RG6 coax, we use the 75 ohm connection on the tuner and match 75 ohms on the antenna. The first two are done for us - just buy and connect correctly. (Impedance matching transformers would work too but with some insertion loss.)
The last task, getting a 75 ohm antenna connection, was a mystery to me but figure A made it clear. The graph shows zero ohms at the shorted end and infinite ohms at the open end. When we slide our connection along the prongs, we are finding the point on the prong where the curve shows 75 ohm (assuming RG6). Since a half wave prong is like two quarter waves hooked together, the same position works for both prongs. This assumes that we built it correctly and that the half is twice the length of the quarter.
If we wanted to use 300 ohm twin lead, we would just connect a bit farther out on the prong where the impedance curve rises to 300 ohm.
Now one problem is that free field RF waves have a higher velocity than RF waves in a medium, in this case our antenna prongs. What we want to know is what is the velocity of RF in our prongs is relative to vacuum - first cut is say, 70%. We would then have to cut our prongs to the correct "electrical" length which is not the same as a free field length but longer since electromagnetic radiation is slower in any media, like copper.
Have any real idea what the velocity of light is in copper pipe? Aluminum tape? Twin lead? Me neither but it makes a difference in how long the prongs need to be for a specific frequency.
One way to find out is to use the gadget on the right the "Lecher-wire system." Using a shorted pair prongs at least one free field wave length long, move your connection to your tuner, set at the frequency of interest, run your connection points up and down until you get a MINIMUM signal strength. Keep moving and when you hit another MINIMUM, you've found one half wave length in the material under test.
Note that I haven't tried this yet but the US Navy recommends it so it must be super. It might be overkill for our purposes though.
Another question is how close can the prongs be? I wanted to use aluminum tape back-to-back. That might not work - the book says that one needs some space between the prongs just as one needs space between the central conductor and the shield in coax or between conductors in twin lead. Alternately, I might try a dialectric between the aluminum tapes, like styrofoam or polyethylene.
So you want to tune your new J-pole to another frequency? Rather than cutting or soldering extensions, one can tune the length electrically by adding capacitance or reactance between the antenna and the transmission line as shown in the center sketch. I suspect that one would insert equally for BOTH connections in the J-pole.
Another issue is grounding. The J-pole does not seem to NEED to be grounded although the place to do so would be on the shorting connection. Using coax, you're grounding at the tuner or at the RF shield ground in any case. This would be helpful for an outside antenna for lightning protection but for indoor use, only the RF shield ground would be helpful to ditch the transmission line reception of RFI. Don't understand the need for the coax "choke" yet seen in some designs. Probably twin lead doesn't need it.
Anyway, that's what I think I know. Anyone is welcome to set me straight or amplify what I've posted here.
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