Topband: Spiral Top Hat 160M Vertical

[PaulKB8N at aol.com]

Gang,
 
I just finished constructing and "hanging" my spiral top vertical antenna  
on a high tree limb.  Here are some of the specifics:
 
- 16 turns of #14 dull gray coated wire on a 4'x4' tophat, turns spacing is 
 about 2"
- Top Hat details:  Constructed of 3/4" gray weather-resistant  electrical 
PVC conduit, the spiral wire is fed through holes drilled in the  conduit, 
the whole assembly and vertical wire are suspended from a high tree  branch, 
out about 12' from the tree trunk
- Height: approximately 30'
- Initial tests:  My Autek RF-1, admittedly a crude instrument, shows  
resonance around 1650 kHz, with very sharp bandwidth, tested against a  ground 
system of 8 short (30-50' radials)
 
Early conclusions:  I am quite thrilled to see how effectively the  spiral 
provides top loading.  My earlier, smaller spiral showed resonance  around 
2000 kHz, a few more turns made quite a difference. The narrow  bandwidth 
would seem to indicate very high Q, which is a good thing.  My  Autek gives me 
a rough idea of what type of components I will need to tune this  to 160M, 
probably with some series capacitance.
 
I had hoped to come up with an easier and hopefully better way to build a  
short vertical of the K6MM dimensions, without having to wind the long  
vertical distributed L PVC portion.  With this top hat, I can drop a  straight 
wire down to the ground without having to provide any additional  inductance. 
 Threading the 16 turns through the PVC spreaders was not easy,  but 
nonetheless was accomplished in about a half-hour.
 
This has turned out to be a very stealthy antenna.  The top hat  sits among 
several tree branches and is difficult to spot from almost any  direction.  
The gray vertical wire parallels several tree trunks and is  also difficult 
to spot. 
 
Decisions to be made:  I can either physically shorten the vertical to  
bring it to resonance, remove some turns off the spiral, or use a tuner to 
bring  the existing antenna to resonance.  I'm tempted to try the last option  
first.  This is not even a 3/8 wavelength electrically, it should exhibit  
the characteristics of a quarter wave vertical, albeit a very short one.
 
It will be a few days before I get the antenna on the air.   I also want to 
add some more radials, which may bring it closer to resonance on  160M.  I 
will keep you posted.
 
Paul, K5AF