[time-nuts] Antenna question about RHCP/LHCP I'm sure a time-nutcan answer

[Jim Lux]

On 6/4/12 10:44 PM, Chris Albertson wrote:
> On Mon, Jun 4, 2012 at 10:18 PM, Hal Murray<hmurray at megapathdsl.net>  wrote:
>>
>> What is the significance of the pointy tops of the long skinny antennas?
>>
>
> Guessing.   Terminates the end of the conductor to prevent a discontinuity
> and reflection

more likely it's for structural reasons. A lot of helical antennas are a 
wire or tape on a cruciform cross section core (rather than on a tube). 
You don't want that corner sticking out.
On the tube, it's easier to make a cone end, then a flat pillbox, and 
you don't have the diaphragm vibration mode on the flat end.

>
> How about the collars at the base of them?
>>
>
> Another guess: They kill multi path reflections from supporting structure

or provide a better match. Or both..

An awful lot of antenna designs out there work about the same, 
particularly for endfire helicals, so if you have a design that works in 
one application and you want to change it, you might just scale for 
size, rather than trying to come up with a completely new design.

it's like the HeliBowl antennas.. Turns out they're totally non critical 
and they all work just about the same.  Plastic party cup and cheap 
mixing bowl, and you're in business.


Broadside short helices, particularly quad helices are a bit trickier, 
especially if you're using the "one pair a bit long and the other pair a 
bit short" to get the 90 degree phasing.