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Author Topic: Mast Diameter Effect On Efficency when coupled with a CAP Hat Question  (Read 6297 times)
GOLDTR8
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« on: August 11, 2014, 04:55:39 PM »

I am working to improve my efficiency (lower losses) of my mobile installation with the combined use of a CAP Hat and mast.

The question is regarding the mast section between the top of a screwdriver antenna (Scorpion Shorty) and the CAP Hat, specifically if you increase the diameter of the mast from 1/2, to 3/4 or 1 inch will it improve the efficiency of the system. 

I already have studied the effects of CAP Hat diameter, spokes or enclosed but I have not spent time studying the effects of the mast diameter.   So anyone with measurements or calculations or other please let me know.

What I need answered is if I fix the antenna length, CAP Hat design, mast length and vary the diameter of the mast section will the resonant frequency of the antenna stay the same, go higher or lower.   

I am looking for everything I can get, even 1/2 of a dB would be worth it. However, before I do the experiments I want to ask the question if its been studied before as it can get expensive to do the trial.

Thanks in advance
KD8NNU
Don 
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KB4QAA
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« Reply #1 on: August 11, 2014, 05:07:05 PM »

Efficiency?  No.

Decreased resistive losses, though given the short distance it would be rather minor.

Slight increase in bandwidth.
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KA0HCP, ex-KB4QAA Relocated to Ks. April 2019.
K0BG
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« Reply #2 on: August 11, 2014, 06:22:24 PM »

The design of the cap hat is much more important than the mast which supports it.

The cap hat mast, hub, and wires that Ron Douglass at Scorpion supplies, is about as efficient as you can get. You could use four loops, but the wind loading is 33% more, with just over a 4% increase in efficiency.

A few other folks make cap hats, but they are both heavy, and have excessive wind loading—at least for driving down the highway as it were!

You might want to read my cap hat article. http://www.k0bg.com/caphats.html
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GOLDTR8
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« Reply #3 on: August 11, 2014, 06:41:57 PM »

Hi Alan.  I am asking about the mast as I am looking for any small gains that I can get independant of the CAP Hat.     I have read and understood your articles.  

The question is mast diameter and its effect.   For me .48 dB matters lookup the 3905 results for 2014 and you will understand why I am looking for more.

KD8NNU
« Last Edit: August 11, 2014, 06:46:41 PM by GOLDTR8 » Logged
W5DXP
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« Reply #4 on: August 13, 2014, 09:24:11 AM »

The question is mast diameter and its effect.

The mast diameter below the loading coil, where the slope of the standing-wave current is close to zero, has more of an effect than the mast diameter above the loading coil, where the slope of the standing-wave current is negative. For a 75m center-loaded mobile antenna, the delta-I is very small between the feedpoint and the bottom of the coil and can even have a positive value, i.e. more current at the bottom of the coil than at the feedpoint. Of course, as one makes the top hat larger and larger, the center-loading coil can be made smaller and smaller - until it disappears altogether and the delta-I in that 12 degrees of current droop in a 75m mobile antenna mast is only about 2%.

One of the most effective 75m mobile antennas that I have ever seen was a 10 ft, 2"dia piece of aluminum tubing with top inductive/capacitive loading. I've always wanted to try a pancake top-loading combination coil/capacitor on top of a 10 ft pole.
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My antenna says, "What makes me happy is when the tuner is adjusted for maximum available current through my radiation resistance!" 73, Cecil, w5dxp.com
K0BG
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« Reply #5 on: August 13, 2014, 02:43:39 PM »

Don, there are two hidden issues here.

If we're after the best we can do mobile, that is to say, in-motion, then what has been said is very relevant. After all, there is a limit to height, and diameter one has to deal with, yet be able to drive down the byways.

If your intent is to win a contest, where in you cannot drive to the event with the antenna erected, then minor parameters can be quite rewarding. In that case, then a solid mast with a large cap hat is the best set up. Arch Doty, W8ACD, wrote an article for QEX with all the information you need in constructing one.

But let me say again! I believe that any mobile antenna shootout worth it weight, must have a provision which says the antenna must be able to transverse the byways and highways of America. Anything else is portable operation, not a mobile one!
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