Rather than explain it here, you can find a nice section dedicated to Capacity hats at K0BGs website
The goal of todays experiment: see how much a Cap Hat lowers frequency
(As usual, you can click on any of the photos to enlarge them)
I started with a quarter wave on 10m made up of two screw together sections plus the base of the hotrodz sytem :
As you can see below, it was a bit long and resonated aroud 27 MHz, but since this is just for comparison, it doesnt really matter.
I then added two 24" sections at the top to make a "T" shaped top loaded antenna.
As you can see, it brought the resonance down to 20.4 MHz, a 6.5 MHz drop in frequency.
I then took the bottom 4' section off, leaving the antenna at around 5' long
I ws unable to measure the resonant point since my analyser only goes up to 30 MHz but we can asume maybe 31 or 32 MHz.
The next step was to increase the capacity by installing another 4 24" rods
This brought the frequency down to around 25.6 MHz.
To see if I could raise the frequency I tried "shortening" the tips. After a few attempts using trial and error, I shortened each tip by approx 6 " (sorry, I didn't measure them)
So, am I going to start using cap hats from now on to reduce the height of my antennas without losing much efficiency?
The answer in my case is no, and the reason is because of the weight/stability of the antenna system. The mast and hub is heavy duty and makes a pretty top heavy system for using with my puny ball mount on the roof. Not to mention that in my case where the antenna is always installed and taken off every time I park up, this is too long and unwieldy.a process.
Where I might use this information is if one of these days in the future I might decide to make a shortened 80m vertical on a spiderpole, for a contest or something. Or, to make vertical dipoles like the old Force 12 Sigma or XR series antennas for using in phased arrays or 4 squares on the dock, where I'd like to keep wire on the ground to a mínimum.