[Antennas] Next dumb question

[Terry Conboy]

Indeed, a large enough capacity hat can reduce or totally eliminate the 
need for a loading coil.  Use of capacity hats can increase the 
bandwidth and radiation resistance significantly, which can improve 
efficiently for a given ground loss resistance.  One thing to be careful 
of is to not to mount any loading coil too close the the hat, or the 
added parallel capacitance across the coil can increase circulating 
currents in the coil and lower the effective coil Q (increase losses). 
Probably a good rule of thumb is to space it away from the hat by at 
least by the length of the coil.

Also, if the hat conductors slope down from horizontal, the fields can 
partially cancel the radiation from the vertical conductor which will 
slightly lower the radiation resistance.  Sometimes adding more hat 
wires of shorter length is a viable solution, if you are stuck with 
supports that give a similar droop angle. Obviously, there are tradeoffs 
for maximum efficiency between coil size and making drooping hat wires 
longer.

Generally, it's a good idea to keep the capacity hat conductors 
symmetrical around the vertical.  Any asymmetry can result in high angle 
horizontally polarized signals, which can be good or bad, depending on 
desired path lengths.  (The inverted-L can be thought of as a single 
wire hat, and it can produce significant NVIS signals.)

73, Terry N6RY

On 2012-11-23 7:28 PM, fred forbush wrote:
> Concerning the cap. hat the way I understand it is that the electrostatic
> field surrounding the hat interacts with the electromagnetic field of the
> loanding coil beneath it. I've found that using a cap. hat reduces the
> amount of coil needed when tuning to a frequency.
> 73, Fred
> k6kub
>
>
> On Fri, Nov 23, 2012 at 11:08 AM, Michael Josefsson <mj at isy.liu.se> wrote:
>
>> On 23 Nov 2012, at 18:39, Jim Miller wrote:
>>
>>> Is a capacity hat electrically connected (metal to metal) to the antenna
>>> itself or is it just sitting there insulated but "capacitively coupled"?
>>>
>> It is electrically connected.
>>
>>> The coil I bought was supposed to be for 15 to 80 meters.  Well.. It only
>>> tunes down (no coil) to about 4.1 and SWR is too high to use even at
>> 3.963
>>> MHz (and it won't go into the 15 meter band either, 20 is tops).  OK,
>>> shorter whip for 15.
>>>
>>> But, for 80 meters, thinking about stationary operation and adding a
>>> temporary top hat to bring me into the top end of the band.
>>>
>>> Better yet, also thinking about adding a few turns of extra coil at the
>> base
>>> to bring the whole thing down a bit.  How much would it take me to move
>> from
>>> 4.1 down to 3.9 or so using existing coil for adjustment and my 102 in
>> whip?
>>> Say #12 THHN close wound on a 2 inch form (6,8,?)
>>>
>> Everything depends on everything else... Trial and horror is your path :/
>>
>> Cheers anyway:)
>>
>> /Micke
>>