On Jan 24, 12:32 pm, Michael Coslo wrote:
I'm putting together a homebrew mobile antenna based more or less on
"20 dollar mobile antenna" that appeared in QST some years ago.

For those not familiar, the antenna consists of a tapped coil at the
bottom, a spiral wound lower section, a tapped loading coil, and an
upper stinger.

My question involves that spiral wound lower section. What would this
be thought of? As antenna below the loading coil, as an inductor in
addition to the other two, or some sort of hybrid of antenna/inductor?

How would this be modeled in EZNEC?

Apologies in advance if this was an incredibly stupid thing to ask! 8^)

- 73 de Mike KB3EIA -

Basically the same as my "plastic bugcatchers" I make out of old CB and
ham
fiberglass antennas. The spiral just adds a bit more inductance. How
tight, and
how many turns would determine the overall loading. IE: just a few
spiral turns
won't do much for 80m. Your bigger lumped coil is going to be doing
most of
the dirty work. The spiral will just add a bit. But on the higher bands
like 10m,
that spiral could have quite a bit of effect on tuning.
Myself, I don't use the lower spiral section. Those smaller coil turns
are more
lossy than using the bigger lumped coil. It's really best to just use a
straight
lower section, and have all the loading at the center lumped coil.
That coil is higher Q, and more efficient. You don't want to degrade
things
by using the lower Q lower spiral windings, when you have a big coil
above
those.
My present best antenna is a 6 ft 20 meter hamstick that I butchered
up.
I did away with all the thin coil windings, but kept enough wire so I
could have a
fairly straight section at the bottom. I'll wrap maybe a turn or two
just to keep
it in place better, but I don't consider it part of the coil. I wanted
max performance.
So..Instead of placing the coil at the center of the fiberglass whip, I
mounted it
nearly at the top. The top one foot of the six foot whip is where the
coil is.
Then I have a 5 ft stinger whip attached to the top of the glass whip.
That makes it a true center fed, with 5 ft under and above the coil.
The tapped coil at the bottom is the matching device. That can vary to
the
install.. The higher you can mount the loading coil, the better,
unless
maybe the stinger is too short. I don't have that problem with a 5 ft
stinger.
The antenna is a stout talker. Even on 80m. And didn't cost me a cent.
All was
junk I aquired... BTW, for added effect, I often extend the base below
the
coil with a 3 ft hustler mast. In that case, I have 8 ft below the
coil, and
5 above. In that "parked" mode, it really kicks butt. Mine tunes 80-17
with the 5 ft stinger. On 15-12-10, I use shorter fixed length stingers
to tune, with
the coil bypassed. Naturally, the usual low band 11 ft antenna is a bit
tall for the
highest bands.. :/. I actually don't work the higher bands much. I'm on
40 in the
day, 80 at night most all the time. But I can work all the others if I
want to.
MK