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The Texas Bugcatcher and capacity hats
It seems that nobody has pointed out the third advantage of capacity
hats; that they reduce the Q of the antenna, and therefore broaden the bandwidth. If we define bandwidth in terms of 3:1 SWR(50), a capacitive hat tends to decrease the bandwidth since it reduces the feedpoint impedance. For instance, the 3:1 SWR(50) bandwidth of a 1/4WL vertical is about 1.2 MHz while the 3:1 SWR(50) bandwidth of a 1/8WL vertical with horizontal top hat is about 0.5 MHz according to EZNEC. What would be the 3:1 SWR(50) bandwidth of the same 1/8WL vertical, with no horizontal top hat, under the same conditions? Actually I guess there would be two sets of conditions: - No base loading coil or matching network (in which case I'd guess it's outside the 3:1 range anyhow)? - A theoretical (but handy) zero-loss matching network at the base, giving it a 50-ohm feedpoint impedance at the desired center frequency? I think the latter is what was being referred to by the original poster. |
#2
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The Texas Bugcatcher and capacity hats
On Friday, February 14, 2014 7:25:29 PM UTC-6, David Platt wrote:
I think the latter is what was being referred to by the original poster. Sorry, I was looking at it from another angle - starting with a full-sized resonant antenna and adding a top hat for the purpose of decreasing its height. -- 73, Cecil, w5dxp.com |
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