are you asking eastside who started this thread or me or? what do you get when you model a 5/8 with and without a top-hat? does the resanance frequency change if its 10" both ways?
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- Thread starter Eastside
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are you asking eastside who started this thread or me or? what do you get when you model a 5/8 with and without a top-hat? does the resanance frequency change if its 10" both ways?
Removing a "top-hat" or capacity hat changes the resonance, since a crucial Xc (capacitive reactance) is now missing. I'm not sure what the exact effect will be, but I believe that it would then be difficult to achieve a 50 ohm match at any frequency. Some experimenters have removed the capacity hat and lengthened the antenna to achieve resonance. This makes the antenna significantly longer.
B
BOOTY MONSTER
Guest
Any number of logistical reasons: easier to ship a shorter antenna, back in the CB hay-day, some users didn't like very tall/long antennas, or with some antenna design techniques, you need that capacitive reactance that the capacity hat provides; the antenna just won't match otherwise. As well some antenna designers believe that the spikes from a capacity hat drains off static (which it does).
ok but i thought shortning an antenna made it resonate higher and isnt that more capacative? i wrote about helping a friend a while back asemble and install a sigma25/8 and it measured just right fpr 5/8 on channel20 if we add 2 inches to the actual length to compenaste for the average diameter of the main tube. that antenna doesnt have a factory top-hat and it works perfect with flat swrs and no static problems
i just read about it helps to change the q lower for wider bandwith when its a shorter antenna then a 1/4
The purpose of a capacity hat is to compensate for the added Xl (inductive reactance) that a matching system provides. The hat's Xc counteracts the matching systems Xl. That Sigma 2 5/8ths matching system was already designed not to need a hat. You tune that antenna by shortening or lengthening it, just like the P500.
Yes, short antennas are capacitive which is why they have larger coils in the matching system. More Xl.
ok but i dont see why the p500 would need one without a inductive coil in the match and wth the balancing connection it has to ground. i think it should just need to be 10 inches longer if the top-hat was removed and why would it get any more static then a sigma25/8 or a macov5/8 that dont get much static at all? i think it was more for looks and to fool cbers into thinking it is better because it has a top-hat
B
BOOTY MONSTER
Guest
im of the impression that spikes/sharp points on antennas actually focus the static to a finer point , actually creating noise and having more potential for a lightning strike .
here's a thread on top-hats if anyone is interested .
http://www.worldwidedx.com/cb-antennas/40536-tophats-such-top-antennas.html
here's a thread on top-hats if anyone is interested .
http://www.worldwidedx.com/cb-antennas/40536-tophats-such-top-antennas.html
No NB, I asked the questions of our Nemesis and problem child. Look back and check out his current status.
I stopped, because the top hat subject died, there was a battle of bad words, and we went on to the dimensions for the new antenna.
You ask a good question however. I think I was curious about the same thing as you, but I didn't want to show my work until he had his chance to give us his opinion. I was trying to put him on the spot, more or less. I had done that with him before, but he never cooperated.
I think you might be surprised at what my Eznec models show happens with a small top hat on a .625 wavelength radiator vs. no top hat and extending the length of the radiator, and also adding even more radials than 4.
If you look back somebody made a reference to what happens with a hundred radials and MrS did give us his wisdom on that topic. He is not wrong on every thing he claims, but check below and see if Eznec supports his claim or is even close on what happens if many radials are used or what happens when a top hat is removed and the length is adjusted.
My models are not iron clad proof, but they might be considered in your thinking on the subject. I did also use a model based on Cebik's dimensions for a 5/8 wave antenna at 28.500 mhz.
View attachment MrS SP500 Top Hat.pdf
The P500's hair-pin match is the inductor. I haven't tried removing the hat and lengthening the antenna. That might work. The spines of the hat help drain static when the air is electrically charged, like in a T-storm. You may be right about Hy-gains marketing back in the '70's.
All I know is that the antenna works well.
if havng a rod go from the coax to the antenna is indcuctive then why wouldnt the rod be that goes to ground? i think it balances it without a top-hat. but booty made a point about static and the static goes to ground through the ground rod in the match but if you run alot of power then i think you need a top-hat so there isnt a coromna effect. i keep thinking about Daves sigma25/8 and how it hasnt got a top-hat and doesnt get static iether
it works well - good. it looks good to. i like the look of the top-hat but i like the old one when they are bent down at 45 degrees
Both the hair-pin and the shorting rod are inductive. That is to say that the shorting bar provides a DC short to ground but at RF frequencies, at some point in it's length, it exhibits a relatively high impedance to RF. If you examine the matching coil in the Super Magnum, or CLR2 antennas you will notice the the coil is tapped to ground at about three turns from the bottom (feedpoint). That tap to ground occurs at the 50 ohm point of the coil. It's a DC ground, but not grounded at RF.
VA3ES, I tend to agree with you, but if the matcher is designed fixed, and is set for 50 ohms resistance in the middle of 11 meters due to its dimensions, which I think it is, would it also do the same if the resonance (tip) was tuned at 10 or 12 meters?
I think the SP500 will probably work in either area OK, but won't the sweet spot for the coil be off a bit from the desirable 50 ohms you note above?
Isn't this what Jay did to his I-10K's tuner to allow it to move more effectively?
Yes. My original P500 was tuned for the CB at about 27.200 +/-. I put the analyzer on it and it read 51 ohms, X=0 or very close to 0. I simply shortened the top section while looking at the analyzer, until I hit my new target, 28.800 MHz. Of course, I had to make small adjustments to the analyzer for it to follow the excursion. At 28.800 MHz, it read 50 ohms (I think 50.5 or 51 ohms) and X=1. Close enough. Somehow, that hairpin is carefully matched to the length to get proper resonance. Since the hair-pin is fixed, and the antenna is designed to provide resonate from 24.8 through 29 plus MHz, it's obvious that the "sweet-spot" is where ever you tune the antenna to.
See above.