Well, there it is. My halo antenna -- assembled and tuned to
frequency: Spot on 50.200 Mcs.
Not clear yet? Here is a close up:
Just having a little fun with you here...
As we all know, a halo antenna
has to have a pair of ugly
pie pans mounted at the ends of the halo -- at the gap. Right?
Some men see a thing and ask "HUH?"
I see a thing and ask "HUH? ... There's gotta be
a better way!"
I didn't really want the 'traditional' 2-disk tuning capacitor at
the gap for my antenna. For one thing, it just adds extra cross
section and exacerbates wind load problems. Too, it was Just One
More Thing for rain, snow, sleet, & dead bugs to mess with and
screw up the tuning when mobile.
It's just a #$&@&$# capacitor! There's no law that says it
has to be on the
outside.
So... I designed a halo tuning capacitor to go on the
inside.
The 'traditional' tuning device is a single, 2-plate capacitor
across the two ends of the gap.
My internal 'solution' turns out to be two capacitors in series
across the gap.
I can't really believe that this idea is original to me.
But, I can not think of any place that I've ever seen this scheme
before.
(Gee! Maybe I've had an
Original Thought . Do you
suppose?)
I forgot to photograph
this piece of the action.
So, you're stuck with a drawing I made:
I got the Teflon cylinder from my dad's
Collection(s)-of-Stuff. He had 3 of them: 2" long, 1 3/8" O.D., and
with a 1/2" hole through the center. I had one of them turned down
as you see in the drawing above. The outside shoulders were turned
on a lathe such that it was A Very Snug Fit into the halo end(s).
The 3/4" center section guaranteed that the (re)assembly of the gap
was always consistent.
A 1/2" diameter, 5" long aluminum tube was slipped into, and
centered in, the Teflon cylinder and the gap was assembled.
There was enough 'spring' out at the end of the halo to permit one
end to pull off the Teflon cylinder and over the 1/2" tube without
undo stress or strain. Then I would let it swing back in -- either
over or under the Teflon cylinder -- while I futzed with the 1/2"
aluminum tube.
This 1/2"-tube-to-1/2"-hole fit was Good And Tight -- something I
wanted in order to defeat movement by road vibration. The first
measurement showed the resonant frequency to be in the mid-40 Mcs.
Using a pipe cutter, I whacked off about 1/2" of the aluminum tube
-- re-inserted
and re-centered it -- and re-assembled the
gap. The resonant freq. was now around 47 Mcs. I used this to
calculate the number of 1/16"-per-Mcs to do additional cuts on the
tube -- and then I cut even less than the amount that was
'calculated'. I continued this up to about 49.9 Mcs. Then, I
stopped cutting. Instead, I started sliding the tube to one side of
the Teflon cylinder, and zeroed in on my target freq. of 50.200
Mcs. The
MFJ-259 and the
IC-706 both claim the 2:1
SWR is over 400 Kcs.
One other item: As you can see from the pictures up above, I
installed heat shrink tubing over the gap. This affected the
resonant frequency by about 170 Kcs. -- down. What I did was slide
the (un-shrunk!) heat shrink tubing over one of the halo ends and
off to the side while fiddling with the Teflon cylinder and the
small aluminum tube. After I re-assembled the gap, I slipped the
heat shrink tube over the gap.
Then I did my
measurements (with the
MFJ-259.) The heat shrink tubing I used
was the stuff you can find in the electrical department of your
hardware store that is 1" un-shrunk and 4" long -- comes
2-to-a-package. The stuff is probably a skoosh over 1", and my lawn
chair tubing measured 31/32".
Once I had the thing tuned to 50.200, I ran a thin bead of hot melt
glue around each halo tube about 1" back from the end(s).
Thin -- because the heat shrink tube has to slide over it!
Then I shrunk the heat shrink tubing -- carefully using a propane
torch in my case. I think you can see the 'bumps' of the hot melt
glue under the heat shrink tubing up in picture number 2. I'm
hoping this will be a Good Seal against water. If water were to
wick-in, under the heat shrink tubing, it would wreak havoc with
the halo's resonance until it dried out.
One Last Comment: Tune it on the high side of your target frequency
as far a you can live with.
Everything hereafter will conspire to lower the resonant
freq.: rain, dust, mud, bugs, bird poop.
Nothing will raise it.
Taa Daa! It's all done!!
Well, I do have some final comments and miscellaneous reflections,
... and some more pictures:
6M Halo -
Comments - Reflections
W3DHJ 6M Halo - Front Page
W3DHJ
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