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
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"160-meters is a band for men, not for sissies!" - SM5EDX
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