Home |
Search |
Today's Posts |
#9
|
|||
|
|||
NM5K:
At the base, there is not much difference, just like in cosine of the angle corresponding to the electrical length of radiator at that point. I'm not sure what you mean by this. Do you mean the current across the coil would be more steady than if the coil were higher? Does this include using the top loading wires? It is roughly like this: Consider quarter wave vertical (90 degree radiator), no coils or loading, you will get current max at the base and then diminishing towards the end to zero. Distribution is in the form of cosine function, nice cosine curve. Now if you would insert the coil anywhere in that radiator and shorten it and tune it back to resonance, the current distribution accross the coil would correspond to the "missing" portion of the radiator that coil replaces. Efficiency of the antenna is roughly proportional to the area under the curve. ON4UN pictures show that. Appears that the best compromise position for the loading coil is about 2/3 from the base. Having top loading, it "stretches" the high current carrying portion of the radiator. The lower the loading coil is, the less difference in current drop accross the coil (cosine function) but shortening of the more current carrying radiator - less efficiency. Again, this subject of current distribution is important in optimizing the antenna design by fine tuning the position of the loading coil in the antenna, combination with top loading etc. Morew current flowing in the radiating part of the antenna - the stronger the field and louder signal. Sure. But this is old news. I beat Reg's vertload program to death finding the best overall coil height for my mobile antenna. Basically I ended up putting it as high as I could. Which ended up a center load at 5 ft up, with a 10 ft whip. Or 8 ft up on a 13 ft whip when parked, and using the "Super" mode... That perhaps points to some error in validity of the formula, and confirms our findings. Experience and W5DXP reported shootout results point to reverse dimensions, 10 feet mast and 5 ft whip, coil about 2/3 up the antenna. You want to have as much as possible the mast length and then best compromise between the coil inductance (properties) and remaining whip (and hat). The "linear" current distribution mentioned in ARRL Compendium and Antenna Book is the simplification propagated from Belrose's 1955 QST article. It is close, but not exact and introduces confusion as it is demonstrated by the flat earth society. Dunno, I've never read it. Maybe calling the distribution as "linear" is the wrong term, but the current is still fairly steady along the radiator in the case of the vertical with the large top hat. I'm looking at a model of one now. The current distribution is almost like a twin tower standing next to the vertical. There is a slight decrease from bottom to top, but it's very small. It's still my view that the difference in current at each end of the coil used in such a case is fairly small. No matter where the coil was mounted. I've never said they would be exactly the same. Seems to me I started off by saying they could vary a bit depending on the antenna. So if you are saying they would be close, but not exact, you seem to be saying the same thing I said to begin with, which seems to be the same thing Cecil is saying. ??? Crap, I'm becoming confused.... I'm not good at playing these type games. That's why I didn't even bother reading the "current war" over on e-ham. *Sounds* like a 598 thread nit pick contest just judging from what I've heard... It's not "that" important to me, being I don't see what it would do for me, even if I found there to be a fairly large difference from each end. If you have pertinent info which shows the current is not even close to being constant across the coil, please enlighten us. "I guess you are attempting to" But at this point, I think everyone is starting to chase their tales and bark at the moon. I'm not really seeing the point, being the art of improving current distribution in short verticals using coil placement is old news. MK Well, looking at fairly simple example of typical 40m loaded mobile model antenna, as W9UCW used, having current vary 40 to 60% is significant, measured differences in field strength are in order of 10 dB and that is significant. I guess it must be like religion, you believe what you want and if the reality doesn't matter, than let everybody be happy. But this has tremendous impact on modeling especially in loaded parasitic arrays. If W8JI showed that Eznec calculated current to be different by fractions and the measurements show around 50% difference, then we have huge discrepancy and warning not to rely on results like that. There is too much reliance now going on modeling program results, ignoring some realities. Some people are becoming "experts" on antennas based on modeling results, without building one. But, even lightbulb can radiate and make some people happy, but it is not my intention to argue with those. My goal is to maximize the performance of the antenna and take advantage of propagation modes for maximum results in the contests, where every fraction of dB counts. It just amazes me that some people go to great length to speculate, calculate, rather than go and verify the measurements and see what it REALY is. You can see that in the threads after the articles. What I was looking for is to see 1. if anyone else MEASURED the current in loading coils, and what results they arrived at (and if we are wrong, then where did we go wrong). 2. If this is right than to have modeling software implement it with least error. I would like to use that for optimizing, say, loaded elements for receiving arrays on low bands, optimizing mobile antennas, loaded multielement beams, etc. Yuri, K3BU/m |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Inverted ground plane antenna: compared with normal GP and low dipole. | Antenna | |||
Smith Chart Quiz | Antenna | |||
QST Article: An Easy to Build, Dual-Band Collinear Antenna | Antenna | |||
Eznec modeling loading coils? | Antenna |