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At this point in time - Antenna experiences so far
- Thread starter HomerBB
- Start date
No less than a wet lawn, cars and driveway is an indication of it having rained recently, you can't say there was no rain because you didn't see the rain falling while you were inside napping, even though it could have been the neighbor's sprinkler gone awry, the better probability is rain.
If there is voltage showing there probably is a voltage node and therefore NOT a current node, likewise, if there is no voltage, there is probably current, but you can't necessitate it being there, if that's what you were implying, right, I agree, and that's exactly why I have to rethink my Vector test and find a way to visibly show the current radiation points along the antenna, not just the voltage where there isn't current, because that will just lead to more debate.
Now let's carry that just a little bit further.
With a typical 1/2 wave resonant dipole that's center fed, there is high voltages on the two ends and no current. While 90 degrees away at the center of the dipole there is no voltage and high current. The voltage and current are 90 degrees out of phase. That means that the power (EI) is the same at any particular point along the dipole. {The resistance of the conductor making up that dipole will cause a very slight reduction of power, just like happens in the feed line, so the power being radiated will always be less than is being generated by the transmitter. Not much, but some.}
Something to keep in mind with all of this is that it's alternating current not direct current. Don't get 'messed up' with the 'sign' changes and multiplying by zero when figuring power! Think impedance, not resistance.
- 'Doc
(Ain't this sh _ _ fun!
Now think it a little bit further with the above, see if you re-vamp your thinking of the whole thing a little.)
With a typical 1/2 wave resonant dipole that's center fed, there is high voltages on the two ends and no current. While 90 degrees away at the center of the dipole there is no voltage and high current. The voltage and current are 90 degrees out of phase. That means that the power (EI) is the same at any particular point along the dipole. {The resistance of the conductor making up that dipole will cause a very slight reduction of power, just like happens in the feed line, so the power being radiated will always be less than is being generated by the transmitter. Not much, but some.}
Something to keep in mind with all of this is that it's alternating current not direct current. Don't get 'messed up' with the 'sign' changes and multiplying by zero when figuring power! Think impedance, not resistance.
- 'Doc
(Ain't this sh _ _ fun!
Now think it a little bit further with the above, see if you re-vamp your thinking of the whole thing a little.)While the light bulb is quite a rudimentary means for determining what is radiating, it could be used to convince the doubters that the cone radiates. Since we know voltage and current are simply 90 degrees out of phase with one another along a radiator, once you locate a voltage node, it's fairly safe to assume a proportional current node will be just a 1/4 wavelength away.
Energize the Sigma and move the bulb from the base of the cone towards the loop on the cone. When you approach the voltage node towards the loop, the bulb will light. This idea will still leave many important issues unanswered. Where an understanding of the posted computer models would easily remove all doubt. Just because you can confirm the cone is radiating with the light bulb, it tells us nothing about the phase of that radiation and how it constructively combines with the main radiator in the far field.
Energize the Sigma and move the bulb from the base of the cone towards the loop on the cone. When you approach the voltage node towards the loop, the bulb will light. This idea will still leave many important issues unanswered. Where an understanding of the posted computer models would easily remove all doubt. Just because you can confirm the cone is radiating with the light bulb, it tells us nothing about the phase of that radiation and how it constructively combines with the main radiator in the far field.
Since you're claiming that the ring("loop") at the top of the cone/basket is a voltage node, not a current node, then the basket isn't a current node and cannot/does not radiate constructively in phase with the top 1/2 wave, unless you're claiming the bottom of the antenna 1/4 wave below the ring, where the mast mounts, is a current node and part of the radiator, but if that's so then where is the necessary inverse RF current which is at ground potential at the base of the gamma where the shield makes contact?
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Since you're claiming that the ring("loop") at the top of the cone/basket is a voltage node, not a current node, then the basket isn't a current node and cannot/does not radiate constructively in phase with the top 1/2 wave, unless you're claiming the bottom of the antenna 1/4 wave below the ring, where the mast mounts, is a current node and part of the radiator, but if that's so then where is the necessary inverse RF current which is at ground potential at the base of the gamma where the shield makes contact?
I'm claiming nothing. I'm just reporting what both EZNEC and CST have uncovered with respect to currents on the cone because some are still struggling to understand the models. Even EZNEC displays this current distribution on the cone.
The pink current line is bowed away from the base of the 4 radials the most and comes back to nearly zero current along the loop. Common sense tells us the base of the radials carry the current node and the loop is the voltage node.
The only simultaneous inverted current anywhere on this antenna takes place along the first 1/4 wave of the main vertical radiator as displayed in CST. Because it's inverted, this radiation current is undesirable and is exactly why it is prevented from radiating by the cone around it. Of course all of these currents are reversing their phase 27 million times per second and that's why RF radiates and DC does not.
Having said that, I admit there are still certain aspects dealing with antennas and phase inversion that I fail to fully understand. For example my Yagi driven element is currently using a T-Match. Both sides of this balanced element are driven. The interesting part is that they must be driven 180 degrees out of phase from one another.
This clearly demonstrates that one side of the balanced element is completely out of phase from the other. Can anyone explain why equal but opposite radiation currents do not cancel each other in the far field? With these things in mind, I also admit I'm having a hard time finding anything that resembles this phase inversion in the Sigma design.
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l can't find the picture of the model, can you repost it?
OK, thinking this through, as the phase of the top 1/2 wave comes across the zero line and crosses over (the high voltage node) into the inverted phase bottom 1/4 wave of the main radiator, the current at it's base is necessarily 180 degrees inverted from the current in the top 1/2 wave doing the radiating, and as you state is undesirable, but if the current which is desirable is at it's 1/4 wave peak node at the base of the antenna where the radials connect then two equal but opposite currents are present at the same point on the base if you also have constructive current there!
And that's where you stated simultaneous inverted current takes place, but that's not possible without cancellation of both currents, leaving none for radiating RF.
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One can see the complete CST model at this link: http://fmbroadcastantenna.com/images/Dominator NWE-34 in CST.gif
It displays the design throughout one complete 360 degree cycle of applied drive.
It displays the design throughout one complete 360 degree cycle of applied drive.
Now that model shows the current node, not the voltage node, at the ring.
It appears "Common sense" is mistaken.
I had imagined that it would have to have the current node at the ring if it were to constructively radiate with the upper 1/2 wave, like the model shows, but how does the RF get there in phase?
That analysis is incorrect. Perhaps this still photo of the current in EZNEC+ will help identify what's really going on here. Excuse the old model as I'm on my laptop and don't have access to all my models here. In any event, pay close attention to the pink current lines around the radials and loop. The further the pink line deviates away from the actual radiator, the stronger the current is.Now that model shows the current node, not the voltage node, at the ring.
It appears "Common sense" is mistaken.
I had imagined that it would have to have the current node at the ring if it were to constructively radiate with the upper 1/2 wave, like the model shows, but how does the RF get there in phase?
Notice the pink line bows away from the radials at the base and just about perfectly traces over the actual radiator in the loop. This means maximum current is at the base of the radials and the voltage node is at the loop. Exactly where it should be 1/4 wavelength away from the source.
Attachments
I see the eznec pink line, and now I'm wondering if that's what I've heard about, relative to Cebic & others stating that eznec doesn't work with this design.
The current is obviously peaking at the ring in the cst model but just the opposite in the eznec picture.
Can you post the entire eznec picture so I can also see the pink current line on the top 1/2 wave?
I've noticed discrepancies between the two programs when working with this design for some time now and they just add to the confusion. I've done several experiments that lead me to strongly believe EZNEC mistakes this design as a coaxial end fed 1/2 wave.
I base this on the fact if you try to improve the design in ways that require the program to understand the correct radiation phase angles, the attempt fails every time in the field. Treat the design as a 1/2 wave over a 1/4 wave and some success can be found.
When I compare what's going on with the cone in both programs, I don't see the exact opposite placement of currents. To me it looks as though the radiation currents on the cone are concentrated more towards the middle of the structure in CST. That confuses me even more while both programs report the same current distribution along the main vertical radiator.
PS: The current on the main radiator is identical in both programs as far as I can tell. EZNEC shows the main radiator starting off at maximum current. It reaches minimum current 1/4 wavelength up, right around the loop. Then begins another 1/2 wave cycle where it starts off with minimum current, reaches a peak another 1/4 wave up and diminishes to no current at the top. I cropped the picture because it was too large to post.
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It appears that eznec is only displaying the main radiator current. I wanted to see the whole eznec model to see if it's showing inverted phase from that pink line in the cone to the pink current line on the top 1/2 wave.
I know what you mean about the current peak appearing to be at the center of the cone in the cst model, but I think that's just the after-peak image of the current declining as it moves down the cone toward it's opposite phase.
In the cst it appears to peak bloom at the top of the cone in phase with the middle of the top 1/2 wave bloom, energy which would not be there due at all to the top 1/2 wave since it's voltage node is at that point where the ring surrounds the main radiator and should show null if it was radiating by itself, without additive current from the cone.
...but how does it get there? :headbang
But does it cross over at the top of the cone area and become inverse current relative to the in-cone current? If so, it's showing only the main radiator current.
I'm starting to think this design might be an off-center-fed full wave with the inverse current on the cone righted back to being in phase with the top by inverting it (folding it) up.
I was able to reformat the picture size so it can be posted here. Unfortunately I don't think it's much help in determining the phase here. You'll notice the pink current line is on the same side of the main radiator at the top and bottom nodes. I know that's not right because anytime the radiator is longer then 1/2 wave, a phase inversion must begin towards the feed point. At one time I thought I had EZNEC displaying this current correctly.
I think this mysterious "energy" on the cone is really not much different then the currents we would find on the radials of a typical ground plane. The key difference is due to the longer main radiator which in itself creates a condition where it is beneficial to allow them to radiate in nearly the same vertical plane as the main radiator. Furthermore, by sweeping them upwards against the main radiator we confine the out of phase portion of the lower main radiator.
This allows the vertical radiator to go through a 90 degree phase shift BEFORE it's allowed to begin radiating. That key factor is what allows the 1/4 wave cone to be in phase with the top 1/2 wave. It's not the phase on the cone that is being shifted. The phase that has been allowed to radiate on the vertical radiator is what has been altered by the cone. I hope this helps.
