Someone on another board I read posted a description of how to
construct a self-resonant, helically-wound vertical as an emergency
replacement for a 1/4-wave monopole for a MW broadcast station. His
conclusion was that its radiation resistance would be somewhere around
40 ohms, and could be driven at least at reduced power by a tx
expecting a 50 ohm load.

I responded as below, and thought it might be a topic for discussion
on rraa.
________

Such a configuration may be self-resonant, but that does not mean that
it
has the same radiation resistance as that of a linear monopole at its
1st
self-resonance (1/4-lambda).

According to John Kraus in "Antennas," 3rd edition, Chapter 8-22 --
the
radiation resistance of such a helical antenna is approximately the
same as
that of a linear monopole of the same overall height, and not to the
length
of the wire used to wind the helix.

The radiation resistance of a short monopole is approximately (h^2)/
312,
where h = height in electrical degrees. A 20-ft monopole is about
12-1/2
degrees at 1700 kHz, so the highest radiation resistance possible for
your
configuration for MW broadcast use is about half an ohm.

Kraus says in this chapter "The advantage of the helix over a straight
wire
or stub is that its inductance can resonate the antenna."

This means that a separate "loading coil" is not needed, which can
reduce
the fixed resistive losses in the antenna system. But it doesn't mean
that
a self-resonant, normal-mode helix has the radiation resistance and
radiation system efficiency of a linear, 1/4-wave monopole (other
things
equal).

RF

Richard Fry wrote:
But it doesn't mean that
a self-resonant, normal-mode helix has the radiation resistance and
radiation system efficiency of a linear, 1/4-wave monopole (other
things equal).

Here's a data point. Awhile back, using EZNEC to simulate
the delay through a 75m bugcatcher loading coil, I used
the coil pretty close to its self-resonant frequency of
~7 MHz. It had a 4 inch bottom section and a 4 inch
stinger so the antenna is about 14 inches long. The
resonant feedpoint impedance is 2 ohms (not including
ground losses) and the maximum gain is -14.8 dB.

Assuming a typical 1/4WL monopole has a maximum gain of
0 dB, the 1/4WL monopole is 30x the efficiency of this
short "helical" antenna on 6.85 MHz. So it seems that
the radiation resistance is ~1/30 that of a 1/4WL
monopole using mininec ground simulation.

Incidentally, a resonant helical monopole obviously
has a 90 degree delay from feedpoint to tip. The reason
that virtually zero phase shift occurs through the coil
is that one is sampling standing-wave current phase which
doesn't change (much) whether it is through a coil or
through a 1/4WL wire. Quoting my web page at:

http://www.w5dxp.com/current2.htm

"Standing wave current cannot be used to directly measure
either a valid amplitude change or a valid phase shift through
a loading coil. All of the reported conclusions based on
loading coil measurements using standing-wave current on
standing-wave antennas are conceptually flawed."
--
73, Cecil http://www.w5dxp.com

"Richard Fry" wrote in message
...
Someone on another board I read posted a description of how to
construct a self-resonant, helically-wound vertical as an emergency
replacement for a 1/4-wave monopole for a MW broadcast station. His
conclusion was that its radiation resistance would be somewhere around
40 ohms, and could be driven at least at reduced power by a tx
expecting a 50 ohm load.

I responded as below, and thought it might be a topic for discussion
on rraa.
________

Such a configuration may be self-resonant, but that does not mean that
it
has the same radiation resistance as that of a linear monopole at its
1st
self-resonance (1/4-lambda).

According to John Kraus in "Antennas," 3rd edition, Chapter 8-22 --
the
radiation resistance of such a helical antenna is approximately the
same as
that of a linear monopole of the same overall height, and not to the
length
of the wire used to wind the helix.

The radiation resistance of a short monopole is approximately (h^2)/
312,
where h = height in electrical degrees. A 20-ft monopole is about
12-1/2
degrees at 1700 kHz, so the highest radiation resistance possible for
your
configuration for MW broadcast use is about half an ohm.

Kraus says in this chapter "The advantage of the helix over a straight
wire
or stub is that its inductance can resonate the antenna."

This means that a separate "loading coil" is not needed, which can
reduce
the fixed resistive losses in the antenna system. But it doesn't mean
that
a self-resonant, normal-mode helix has the radiation resistance and
radiation system efficiency of a linear, 1/4-wave monopole (other
things
equal).

RF

Sounds right to me. The 'loading coil' is distributed throughout the
radiating element and, consequently, throughout the current distribution so
it might be possible to reduce the I^2R loss using the normal-mode helix.
It follows that terminal impedance involves an inductor with a non-uniform
current distribution throughout its length, so the magnitude of the reactive
part involves an integral.

Chris

christofire wrote:
The 'loading coil' is distributed throughout the
radiating element and, consequently, throughout the current distribution so
it might be possible to reduce the I^2R loss using the normal-mode helix.

For a fixed length of loaded monopole, a high-Q
center-loading coil (or top-loading coil) will
result in greater efficiency. Helical elements
have been measured in numerous 75m shootouts to
be relatively low-Q and therefore lossier than,
e.g. Texas Bugcatcher Coils.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
...
christofire wrote:
The 'loading coil' is distributed throughout the radiating element and,
consequently, throughout the current distribution so it might be possible
to reduce the I^2R loss using the normal-mode helix.

For a fixed length of loaded monopole, a high-Q
center-loading coil (or top-loading coil) will
result in greater efficiency. Helical elements
have been measured in numerous 75m shootouts to
be relatively low-Q and therefore lossier than,
e.g. Texas Bugcatcher Coils.
--
73, Cecil http://www.w5dxp.com


That's interesting. I imagine it would be difficult to support a helix with
a length of many metres whilst keeping the losses low, and of course there
may be a greater total length of conductor in the continuously-loaded case.
Otherwise, I can't think of a fundamental reason why a long normal-mode
helix should have lower Q than a monopole of the same length with a series
inductor somewhere in it.

The Tesla coil brigade appear to have sorted out suitable tube materials in
lengths up to a couple of metres or so with low loss at frequencies up to
the medium wave band or thereabouts.

Chris

On Feb 22, 3:01*pm, Cecil Moore wrote:
christofire wrote:
The 'loading coil' is distributed throughout the
radiating element and, consequently, throughout the current distribution so
it might be possible to reduce the I^2R loss using the normal-mode helix.

wrote:
Yep... The only way to get a helical wound stick to really
work well is to have a decent length stinger on it.

One thing stands out loud and clear from the 75m
mobile shootout results. The longer the straight
high current bottom section under the loading coil,
the greater the radiated signal. The best 75m
mobile antennas that I have ever seen were not
center-loaded. They were top loaded.

One was my entry which was a CB whip with a
horizontal top loading coil and capacity hat at
the top which equaled the top-rated bugcatchers
and screwdrivers and bested all the others.

The other was a 10 foot long, 2 inch diameter
copper pipe with a loading coil and top hat
on top out in El Paso. I forget who entered it.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
wrote:
Yep... The only way to get a helical wound stick to really
work well is to have a decent length stinger on it.

One thing stands out loud and clear from the 75m
mobile shootout results. The longer the straight
high current bottom section under the loading coil,
the greater the radiated signal. The best 75m
mobile antennas that I have ever seen were not
center-loaded. They were top loaded.

How well do they travel, Cecil? It's an experience with a capacitance
hat at the top, I have to imagine a loading coil up there would be
trying to turn the whole thing into a horizontal antenna.


One was my entry which was a CB whip with a
horizontal top loading coil and capacity hat at
the top which equaled the top-rated bugcatchers
and screwdrivers and bested all the others.

The other was a 10 foot long, 2 inch diameter
copper pipe with a loading coil and top hat
on top out in El Paso. I forget who entered it.

That kind of answers my question for one way to support the coil on top.
A big honkin' support. Did these folks travel with their antennas up? At
least here in the PA mountains, the trees and a lot of the bridges don't
put up with much antenna on top of the car.

My VHF/UHF antenna scores a few trees, and I ended up putting a string
and loop on my Bugcatcher to pull the stinger down, and it still hits
the occasional object.

Michael Coslo wrote:
How well do they travel, Cecil?

Here's an example of a pretty good 75m mobile antenna.

http://www.w5dxp.com/shootout.htm
--
73, Cecil http://www.w5dxp.com
"Government 'help' to business is just as disastrous as
government persecution..." Ayn Rand

On Feb 23, 6:48*am, Cecil Moore wrote:
wrote:
Yep... The only way to get a helical wound stick to really
work well is to have a decent length stinger on it.

One thing stands out loud and clear from the 75m
mobile shootout results. The longer the straight
high current bottom section under the loading coil,
the greater the radiated signal. The best 75m
mobile antennas that I have ever seen were not
center-loaded. They were top loaded.

One was my entry which was a CB whip with a
horizontal top loading coil and capacity hat at
the top which equaled the top-rated bugcatchers
and screwdrivers and bested all the others.

The other was a 10 foot long, 2 inch diameter
copper pipe with a loading coil and top hat
on top out in El Paso. I forget who entered it.
--
73, Cecil *http://www.w5dxp.com

That's why I carry an extra lower mast section.
Really helps the efficiency.
And the good part is adding more lower mast
below the coil does not change the resonant
frequency near as much as adding more stinger
above the coil. So in my case, I have my 10 ft
plastic bugcatcher, which is center loaded when
driving. But when I'm parked, I can add the extra
3 ft mast and get a pretty good boost.
And the resonant frequency hasn't changed too
much, and I can basically use the same coil taps,
and stinger lengths.
Or at least for the lower bands.
If you added three feet above the coil, the
resonant frequency would be way off from where
you started. Maybe not in the same band any more..

I wish I had one even taller.. I'd like to find a 5-6
foot lower mast. I think the Hustler bunch sells
one that tall.
Course.. Knowing me, I'd end up using both of them
coupled together if the antenna mount would stand
up to it. :/
Just the setup I have now with 8 ft under the coil
works pretty well. Some peoples whole antenna
is about that long.. :/
I use a 5 ft stinger whip on 80/40..
Various shorter ones for the higher bands when
the coil is bypassed.