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 -

On Jan 24, 12:32*pm, Michael Coslo wrote
* * * * 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 -

Hi Mike, I built one similar to this myself. I always assumed the
spiral wound lower section added some distributed inductance that would
reduce the amount needed for the upper loading coil. The inductance
added by the spiral section could be modeled by adding a couple of
inductive loads on the lower section. The effect of the loads will be
to change the current distribution slightly on the lower section of the
antenna. The spiral bottom does radiate.

The goal on this type of antenna is to adjust the center loading coil
such that the antenna looks capacitive, and adjust the bottom loading
coil to cancel Xc. Hopefully your impedance now looks like 50 ohms
resistive. The spiral and center loading attempt to move the current
distribution up the antenna so that more radiation occurs on the upper
parts.

When I modeled mine, I only had a center inductive load, and the
results were satisfactory. My goal in modeling was to design matching
networks, I wasn't too concerned about current distribution. Also had
a top hat, modeled as crossed wires. Later discarded top hat, not
aerodynamic.

Homebrew mobile antennas IMO need center loading, no top hat, can
accept power (loads up), and will stay attached to your vehicle at
80mph in a driving rainstorm.

Gary N4AST

wrote in
ups.com:

Hi Mike, I built one similar to this myself. I always assumed the
spiral wound lower section added some distributed inductance that
would reduce the amount needed for the upper loading coil. The
inductance added by the spiral section could be modeled by adding a
couple of inductive loads on the lower section. The effect of the
loads will be to change the current distribution slightly on the lower
section of the antenna. The spiral bottom does radiate.

Sounds sensible. Is there any future in trying to get this thing to work
on 40 meters?

The goal on this type of antenna is to adjust the center loading
coil
such that the antenna looks capacitive, and adjust the bottom loading
coil to cancel Xc. Hopefully your impedance now looks like 50 ohms
resistive. The spiral and center loading attempt to move the current
distribution up the antenna so that more radiation occurs on the upper
parts.

When I modeled mine, I only had a center inductive load, and the
results were satisfactory. My goal in modeling was to design matching
networks, I wasn't too concerned about current distribution. Also had
a top hat, modeled as crossed wires. Later discarded top hat, not
aerodynamic.

Homebrew mobile antennas IMO need center loading, no top hat, can
accept power (loads up), and will stay attached to your vehicle at
80mph in a driving rainstorm.


Thanks much, Gary. When the weather breaks, It should be ready to
roll.

- 73 de Mike KB3EIA -

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 -

Hi Mike,

Anything other than a straight conductor would add inductive loading, with
less (if any) contribution to radiation. Close spacing of the inductor turns
will add distributed capacitance. The closer to the feedpoint, the greater
the I^2*R loss and the larger conductor should be, to keep the loss as low
as possible. So, electrically, the best place for loading is at/toward the
top end.

My mobile antenna setup consists of Hustler/Newtronics parts... but not
quite out-of-the-box. For 20m, I use their RM15 (15m) loading coil + a 49"
whip with about 6" cut off of it. The whip came from Larsen (pn W490).
Though this effectively moves the loading coil toward the high-current end,
it adds considerable radiating surface area. When the band was in, it was
good enough to work the UK (using 100W) from my QTH near Seattle! I haven't
(yet) tried an RM12 loading coil with an even longer W640 (64") whip. My
setups for the other HF bands are similarly taller than the Hustler setup...
the 75m antenna is quite tall! :-))

Bryan WA7PRC

Mike Coslo wrote in
36:

wrote in
ups.com:

Hi Mike, I built one similar to this myself. I always assumed the
spiral wound lower section added some distributed inductance that
would reduce the amount needed for the upper loading coil. The
inductance added by the spiral section could be modeled by adding a
couple of inductive loads on the lower section. The effect of the
loads will be to change the current distribution slightly on the
lower section of the antenna. The spiral bottom does radiate.

Sounds sensible. Is there any future in trying to get this thing to
work on 40 meters?

This is actually not that different from the commercial "hamstick" mobile
antennas. I have a set of them on my balcony that I use. 40m is very
noisy here, but I can regularly be heard down the coast in Oregon and
California and have worked Japan in the wee small hours (from just east
of Vancouver, BC).

They're not very efficient on 80, a bit better on 40, and OK on 30, 20,
15 and 10 (those are the bands I have antennas for).


--
Dave Oldridge+
ICQ 1800667

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