In article ,
Gary Schafer wrote:

The coil in the center of a marine antenna that is designed to be used
on 2 mhz is indeed a choke at anything above 7 mhz or so. It is not
intended to be such but that is what it is when the antenna is used
above its intended range. All of the antenna above the coil is
electrically disconnected from the lower part of the antenna. You
could physically remove that upper portion and notice little if any
difference at higher frequencies.

That is the reason that type of antenna is not used on any system that
operates above 4 mhz. A straight whip (no coil involved) is the only
thing that will work satisfactorily in a multi band system. (trap
antenna being the exception)

Those old antennas with the loading coil in them perform much better
on 2 mhz than the straight whip antenna of the same physical length
but they are very poor on the higher bands as part of the antenna is
not there electrically. It is then a very short antenna at the higher
frequencies.

Regards
Gary



On Thu, 20 Nov 2003 21:37:05 -0500 (EST), (Ron
Thornton) wrote:

The coil in the center of an antenna is not a choke it is just an
inductor that electrically lengthens the antenna. This method is used
well above 2-4 mhz in mobile radio. It is not used much above VHF
because a full wave length is already relatively small.

Ron


Just a few comments here on this thread.......

Gary is right on in his assesment of the operation of the end feed
loaded antenna. The coil does indeed act as a choke on frequencies
higher than 4XResonant. In the "Old Days" we used Morad 2800 and 3600
loaded whips with 15 to 35 ft of wire below them as antennas in the
MF Frequency Range. They were just great below 5 Mhz, and not half
bad above that as the coil only choked off the upper 102" whip and
the coil itself, and left the bottom 10ft of antenna and 35 Ft of
wire as an antenna for the HF Ranges above 5 Mhz. This 45 Ft of
antenna did just fine as an antenna in those ranges as it would be
tuned (Fixed tuned antenna tuner era) as 3/4 Wavelength at these higher
frequencies. today's autotuners have a similar tuning firmware routine
that adds output Capacitance untill it finds the 3/4 Wavelength point.
They don't do as good of job as a Fixed tuned tuner due to the step size
and the sense circuits, but they will get close, and be fairly effecent.
The trick to make this type of antenna work is to get some wire under
the antenna, so that you have an effective antenna length once the coil
chokes off, as the frequency rises. Most of the North Pacific Fishing
Fleet uses this type of antenna system for MF, and HF, comms.

Bruce in alaska
--
add a 2 before @

In article ,
(Ron Thornton) wrote:

Well I guess if I tried to push 30 mhz thru a 2 mhz transmitter you
could call the transmitter a choke too. But in reality it ain't and
the coil in the middle of an antenna ain't either. What you describe as
a choke is a tuning inductor, the choking is an inconsequential behavior
of the tuning at a frequency the antenna was never designed to operate
at.

Ron


Ron, Ron,

Loaded Whips, are just that, wires with a big coil in the middle, that
effectivly lengthen the antenna. They are designed to be 1/4 Wavelength
at the Designed Frequency. ( Morad 2800 & 3400 ect) these can be
resonated as 3/4 Wavelenth at higher HF Frequencies very easily by
adding output shunt capacitance in the tuner. The coil does decouple
the antenna above it at something near 4X Designed Resonance. These
type of antennas have been around in Marine Radio for 50 years, and
Eddie Zanbergen RIP (Chief Engineer @ Morad) designed, built, and sold
them throughout the world, and the properties of Impedance vs Frequency
are well documented for them over the years. After one tunes the old
fixed tuned Antenna Tuners, common in the Marine Radio Service for 20
or 30 years, you get a feel for how they tune and how they work. They
DO tune up very well, and ARE as effecent as any other Marine Antenna
System. Been there, Done that, on hundreds of vessels from 20' to 1200'.


Bruce in alaska
--
add a 2 before @

Wires with a big coil in the middle is what I'm talking about. I guess
this is a semantics thing. I can't get used to calling it a choke cause
in fixed station if we wanted to change freq that much we went out and
threw up another 3 curtain full wave Rhombic. You mobile guys are
strange, you want one antenna to do it all.

Ron

Just can't figure out how to get a Rhombic on a sailboat. We don't expect
one antenna to do it all. I have separate antennas for HF, VHF, TV, GPS,
WX SAT, DGPS, FM, HAM VHF, HAM UHF. If only a single antenna
could do it all.

Doug, k3qt
s/v Callista

"Ron Thornton" wrote in message
...
Wires with a big coil in the middle is what I'm talking about. I guess
this is a semantics thing. I can't get used to calling it a choke cause
in fixed station if we wanted to change freq that much we went out and
threw up another 3 curtain full wave Rhombic. You mobile guys are
strange, you want one antenna to do it all.

Ron

In article ,
(Ron Thornton) wrote:

Wires with a big coil in the middle is what I'm talking about. I guess
this is a semantics thing. I can't get used to calling it a choke cause
in fixed station if we wanted to change freq that much we went out and
threw up another 3 curtain full wave Rhombic. You mobile guys are
strange, you want one antenna to do it all.

Ron


Ron, on a vessel, you ostly only have room for one antenna.....


Bruce in alaska
--
add a 2 before @

I have five on my 26 foot sail boat. Marine VHF on the stern, Amateur on
the port spreader, TV on the starboard spreader, A scanner and a GPS on the
mast head.
One Ground in a square foot of copper clad on the keel. (Salt water sailor
only)



"Bruce in Alaska" wrote
(Ron Thornton) wrote:
Wires with a big coil in the middle is what I'm talking about. I guess
this is a semantics thing. I can't get used to calling it a choke cause
in fixed station if we wanted to change freq that much we went out and
threw up another 3 curtain full wave Rhombic. You mobile guys are
strange, you want one antenna to do it all.
Ron

Ron, on a vessel, you ostly only have room for one antenna.....

"Roger Gt" wrote in message
. com...
I have five on my 26 foot sail boat. Marine VHF on the stern, Amateur on
the port spreader, TV on the starboard spreader, A scanner and a GPS on the
mast head.
One Ground in a square foot of copper clad on the keel. (Salt water sailor
only)



"Bruce in Alaska" wrote
(Ron Thornton) wrote:
Wires with a big coil in the middle is what I'm talking about. I guess
this is a semantics thing. I can't get used to calling it a choke cause
in fixed station if we wanted to change freq that much we went out and
threw up another 3 curtain full wave Rhombic. You mobile guys are
strange, you want one antenna to do it all.
Ron

Ron, on a vessel, you ostly only have room for one antenna.....

"Roger Gt" wrote in message
. com...
I have five on my 26 foot sail boat. Marine VHF on the stern, Amateur on
the port spreader, TV on the starboard spreader, A scanner and a GPS on the
mast head.

Why would one mount a gps antenna on the masthead? Wouldn't the normal roll
of the boat cause that antenna to travel several feet and cause the position to
change
on each cycle? I would think rail or arch mounted would be more stable. Ours is
mounted
in the overhead of the pilot house in the opening for the sliding hatch, which
is lexan.

Leanne
S/V Fundy

Sorry I miss-spoke.
GPS on the cabin roof.
Scanner on the Mast head.

"Leanne" wrote in message
...

"Roger Gt" wrote in message
. com...
I have five on my 26 foot sail boat. Marine VHF on the stern, Amateur
on
the port spreader, TV on the starboard spreader, A scanner and a GPS on
the
mast head.

Why would one mount a gps antenna on the masthead? Wouldn't the normal
roll
of the boat cause that antenna to travel several feet and cause the
position to
change
on each cycle? I would think rail or arch mounted would be more stable.
Ours is
mounted
in the overhead of the pilot house in the opening for the sliding hatch,
which
is lexan.

Leanne
S/V Fundy

Roger Gt wrote:

One Ground in a square foot of copper clad on the keel. (Salt water sailor
only)

I guess the big steel/iron ballast keel (couple of thousand
pounds AIUI) on bottom of my Westerly would make a 'good' RF
ground? Connect to SS keel bolt/s accessible in bilge. Taking
other precautions for elctrolysis and/or 'ground' currents from
shore power?
Terry.