chuck wrote in news:1174139919_4745
@sp6iad.superfeed.net:
Hmmm. You're thinking about the capacitive reactance of a short
antenna,
which can be high, and I'm thinking of the radiation resistance, the
real part of the antenna's impedance. When the short antenna is
matched,
the capacitive reactance is cancelled by the tuner and you are left
with
a radiation resistance that can be as small as a fraction of an ohm.
by
comparison, the radiation resistance of a quarterwave vertical antenna
over seawater is about 35 ohms. The base of the antenna is always a
current node on a short vertical antenna: current is a maximum there.
The high voltages (and low currents!) occur at the tops of these
antennas; not at their bases. Where would the current flow to at the
top?
When a tuner matches a short antenna's reactance and high impedance to,
say, 50 ohm coax, the current to the antenna is very low, in comparison
to the current in the coax at 50 ohms. Most of the current in this
circuit is confined to the series inductance inside the tuner, which
isn't radiated as H-field.
A 1/4 wave vertical over a good ground, like seawater, is closer to 12 to
20 ohms of resistive impedance, if the ground is at the feedpoint where
it should be, not 20' of strap away going down to the bilge. 1/4 wave
verticals have very low impedance, indeed. This makes a very heavy base
current at the feedpoint, if we're lucky, resulting in an impressive H-
field expanding away from the feedpoint at the base, dropping as you go
up towards the open end. A shortened antenna, like an 18' whip on 4 Mhz,
has almost no current at its high impedance feedpoint the tuner must deal
with. H-field suffers awful which causes the E-field to collapse to
match it. It radiates "poorly in all directions", a typical HF mobile
installation.
Many things, some reasonable, some not, can be done to "draw" the current
lobe up the antenna. Some are not practical in a marine environment and
certainly not "boat pretty", so won't be tolerated on yachts. The best
solution is to move the tuning inductor UP the antenna to its middle,
"center loaded". A center-loaded short antenna can be made to exhibit a
very low impedance at its base feedpoint, no where near 12 ohms of a full
1/4 wave vertical, but in the range of 20-30 ohms if the loading coil is
of sufficient size to be efficient, with low capacitance between windings
and low winding resistance, which wastes power turning it into heat. The
antenna appears a little inductive, which is easily balanced by shunt
capacitance at the base. Here's my center loading coil from 80-10
Meters:
http://www.texasbugcatcher.com/cata/tbcspec.htm#6inch
Look at coil #680, which is 6" in diameter on a Lexan form. The white
center insulator is very heavy Teflon. This coil is 4' up a 15' whip
from my feedpoint. I use a large clamp and braided copper strap
connected to the bottom of the coil to tune it by shorting the bottom
turns. Even that creates immense base current at resonance....(c;
On the same webpage, the #480XL coil is inserted in series with the #680,
and the #680 tuning short is used to tune the 15' beast below 3.5 Mhz.
At 1.8 Mhz, this is a VERY short antenna and VERY inductive.
The feedpoint at the car's trailer hitch has two different RF
autotransformers. The HF autotransformer is 10 turns of #10 over a very
heavy ferrite core tapped at every turn. Best impedance match 40-10M
occurs with the antenna tapped 4 turns from ground fed at 6-8 turns, the
coax input tap. 6T at 10M, 8T at 40M. On 80/75M, input is across all
10T, tapped at 5T, an impedance ratio of 4:1, works best. There isn't
enough natural inductance to tune below 3 Mhz, so another ferrite toroid
has 30T of #12, fixed tapped at 22T to operate on 1.8-2.0 Mhz with two
loading coils in series.
The antenna's capacitor hat is 8 stainless steel, about #12 wiresized,
welding rods turned round on the end into a loop to reduce corona, welded
to two stainless washers to hold them tight to a 10-24 whip screwed into
the 3' mast above the one (or two) loading coils. The capacitor hat is
approximately 4' across, and adds capacitive reactance at the top, where
it aids pulling the current lobe up the tuned antenna...instead of at the
base, where it radiates nothing. The capacitor hat and 3' mast above the
coils is removed (1/4 turn quick whip connectors) for operating above 20
meters 14-30 Mhz. Atop the capacitor hat is a cut down stainless CB whip
that creates a resonant antenna on 14.250 Mhz when you short out the
whole 6" coil. The antenna's length and only the capacitor hat resonate
15' to 14.250 by design as that's my favorite ham band. On 20M, VE8RCS
(the northern most amateur station in the world at a Canadian CG base
above the Arctic Circle) reports my mobile in Charleston to be as loud as
any legal ham station they can hear. I used to work them on 20M Packet
quite regularly on 14.105 "Network 105". Their QSL is a prized
posession. Packet, RTTY and the other digital modes are great fun when
traveling with a group of hams to a hamfest...(c;
If you deliver 100 watts to a short whip, it will radiate as well as a
quarter-wave vertical, assuming the same ground system, etc. There will
be MORE current at the base of the shorter antenna because power = I
squared x radiation resistance. Radiation resistance of a short antenna
is smaller than that of a quarter-wave antenna, so to keep power at 100
watts, I must INCREASE!
Nonsense! If it did, every broadcaster on the planet would be buying
50' of Rohn 25 and loading it up at the base....instead of spending
millions on full-sized 1/4 wave radiators like:
http://hawkins.pair.com/wsm/wsmtower.jpg
http://hawkins.pair.com/wcbs_wfan/cbsfan_twr17.jpg
http://hawkins.pair.com/wcbs_wfan/cbsfan_twr14.jpg
(100KW from TWO AM blowtorches is across that insulator)
All you Marine Radio guys need to see this webpage at NSS on Jim
Hawkins' website!
http://hawkins.pair.com/nss.shtml
Take the tour and see why their signal sounds like it does....(c;
Larry
--
Roll up to the long checkout line....
Yell, "ICE RAID!"
It's your turn to load the grocery belt...(c;