dipole antenna efficiency

[IK4YNG]

 

Hi,
I found another good link for the argument:
http://www.w5sf.com/article9.html

73,
IK4YNG Paolo

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[N2EY]

In a message dated 1/5/08 11:40:39 PM Eastern Standard Time, [hidden email]
writes:

> We simply do not have conductors that will handle RF with anything like the
> efficiency they will handle DC or low frequency AC. That's because all the
> RF current 'crowds' onto the very surface of a conductor.
>
> As we make an antenna physically smaller, the impedance drops. As the
> impedance drops, the RF currents and resistive losses go up. Even silver or
> gold - the best electrical conductors known - are not nearly good enough for
> the sorts of currents we see in small antennas.
>
> As an antenna is made shorter the resistive losses far exceed any other
> losses in the system. Of course those resistive losses occur in the matching
> network too. It's not just the antenna itself that converts RF into heat
> better than it makes electromagnetic waves.

This is so well said that it deserves repeating.

In *theory*, a dipole 1.3 feet long can be made to radiate 80 meter RF almost
the same as one 130 feet long. But in *practice*, the resistive losses of
real-world practical antenna systems of those sizes are very different.  

And what we're looking for are practical, real-world antennas that we can put
up in the limited space we have available.

---

Another factor to remember is that under good conditions we can do pretty
incredible stuff with very low power.100 watts into an antenna system that has 1%
efficiency will radiate the same amount of RF as 1 watt into an antenna
system with 100% efficiency. So even a poor antenna will sometimes let you make a
QSO.

---

It seems to me that one of the biggest stumbling blocks we amateurs have with
antennas is seeing and understanding the entire antenna *system* - which
includes the antenna itself, its surroundings, the feedline, matching networks,
etc. We have to consider all of them together. And like the links of a chain,
the weakest one will be the problem. A great antenna in poor surroundings will
work poorly, etc.

73 de Jim, N2EY


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[John Magliacane]

Ron D'Eau Claire wrote:

> We simply do not have conductors that will handle RF with
> anything like the efficiency they will handle DC or low frequency
> AC. That's because all the RF current 'crowds' onto the very
> surface of a conductor.

When Skin Effect losses become an issue in RF conductors and
inductors, Litz wire is typically used.  I see no reason why a
similar approach cannot be used (one of using multiple conductors
tied in parallel with each insulated from one another).

An "El Cheapo" approach might be to use multi-conductor cable
instead of a single, fat conductor to minimize Skin Effect losses.

Cookie, K5EWJ wrote:

> The capacitive reactance will require 397 microhenrys
> to cancel out which in turn will require a coil 4
> inches in diameter and about 24 inches long with 160
> turns (about 168 feet of wire).  Then you would need
> an 8333/1 balun transformer with its associated wire
> resistance.

Capacitance hats can be used to reduce the amount of
inductive loading required to establish resonance.

I agree that the impedance transformations appear astronomical,
but if performed in manageable steps (instead of a single
transformation), efficiency might be improved, and the process
might not seem so formidable or lossy.

David Woolley wrote:

> For antennas in practical locations, at least one other thing
> will happen:
>
> d) The antenna will convert part of the AC power into (near field)
> electromagnetic energy which will induce currents in the ground,
> building structure, wiring, water and gas pipes, etc.  Much of that
> energy will be converted to heat after it has lost, although some
> will be re-radiated (I believe, in extreme cases, if the current
> is induced in something large enough and resonant, the re-radiator
> can become the actual antenna and the antenna act as a feed device,
> but, normally, for low, and indoor antennas, this is where most of
> the energy turns to heat).

I fully agree that the surrounding environment is part of the antenna
SYSTEM, and should be taken into consideration regardless of whether
the antenna is half-wave or an electrically shortened version of a
half-wave antenna.

> This reflection abstraction causes a lot of confusion.  It is
> possibly easier to see it as simply a bad match between the
> transmitter source impedance (which is usually rather different
> from the optimum load impedance) and the antenna impedance,
> causing most of the DC input to the transmitter to end up as
> heat in the output devices.

Provided measures to re-reflect the energy back to the antenna in
phase with the incident power aren't made.  (And the device that
does this very nicely is the antenna tuner.)

Actually, I mentioned reflected power to dispell the untrue but
widely held belief that reflected power "cancels" forward power
on a transmission line.  Forward and reflected power can peacefully
co-exist on a transmission line without interaction.  If this were
not true, repeater systems using duplexed antenna systems would
be impossible.

> As already pointed out, skin effect means that this is not true.
> People experimenting with small magnetic loops have to use large
> copper pipes to keep ohmic losses manageable.

The problem (in my mind) is that the copper pipe is really only a
single conductor, rather than the more efficient approach taken by
Litz wire where multiple conductors, each a skin depth in diameter,
are operated in parallel.

> For example, the KAT2 has a 10:1 SWR matching specification, but
> matching the antenna discussed here, at infinite height above the
> ground, needs a 250:1 range, or more.  They can also have power
> losses.

Agreed.  The impedance transformation has to (in my opinion) be
step-by-step process -- the same as a modern receiver design.
(In a receiver, we don't try to get all our gain and selectivity
in a single stage.  By the same token, we shouldn't try to make
a large impedance transformation in a single "stage", either.)

(Just my opinion...)

> > The penalties for using physically shortened antennas are:
> >
> > (a) Decreased operating bandwidth
>
> I'm not sure that is inevitably true.  My reference for normal mode
> helices included them in the section on broadband antennas.

I believe the Normal Mode Helix, as is used in HI-VHF television
broadcasting, is a travelling wave antenna, rather than a
resonant standing wave antenna, such as a dipole.

The bandwidth decreases in shortened resonant antennas as a
result of the antenna's increased Q.  The Q rises because the
RF energy must oscillate back and forth between feedpoint and
endpoint of the dipole many more times than it does in a
full-sized antenna before all the energy applied to the antenna
is finally radiated into space.

In travelling wave (non-resonant) antennas (rhombics, Beverages),
the RF energy travels down the length of the antenna just once.
Therefore, they must be very long in terms of wavelength to radiate
all the energy into space on the first shot.

Incidentally, I'm not advocating that everyone should cut their
antennas to 1/40th of their natural resonant length and compensate
for the shortened antenna using LC networks.

What I AM saying is that there are approaches that can be taken to
minimize losses and increase the performance of shortened antennas,
and these approaches are seldom used or discussed in Amateur Radio
circles.

Take a look at some of the antenna systems used at LF and VLF, and
apply those techniques to 80-meters.  Even if your efficiency is
only 50%, that's only half an S-Unit loss -- hardly a cause for
concern.


73, de John, KD2BD


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[David Cutter]

I've used several strands of enamelled copper wire twisted together.  I
usually twist 3 sets of 3 of quite thin wire, whatever is on hand; 0.2 to
0.45mm dia is easy to use; after twisting I give the group a good pull the
stretch and stiffen it a bit but it still remains very flexible indeed and
does not have a memory effect.  I tie the wires to a fence post then fit the
other ends into an electric hand drill 25m, it's surprising how much
twisting is needed. You must not let the wires touch the ground.   It is
also very slippery and does not cling to other wires which I find a great
advantage when manipulating multi-wire structures.  I call it "multi twist".

David
G3UNA

> An "El Cheapo" approach might be to use multi-conductor cable
> instead of a single, fat conductor to minimize Skin Effect losses.
>
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[Bob Cunnings NW8L]

The capacitance between individual strands in Litz wire limits it's
usefulness above 1 MHz or so. W8JI has an interesting discussion of
skin effect at:

http://www.w8ji.com/skindepth.htm

Bob NW8L

On Jan 6, 2008 3:54 PM, David Cutter <[hidden email]> wrote:

> I've used several strands of enamelled copper wire twisted together.  I
> usually twist 3 sets of 3 of quite thin wire, whatever is on hand; 0.2 to
> 0.45mm dia is easy to use; after twisting I give the group a good pull the
> stretch and stiffen it a bit but it still remains very flexible indeed and
> does not have a memory effect.  I tie the wires to a fence post then fit the
> other ends into an electric hand drill 25m, it's surprising how much
> twisting is needed. You must not let the wires touch the ground.   It is
> also very slippery and does not cling to other wires which I find a great
> advantage when manipulating multi-wire structures.  I call it "multi twist".
>
> David
> G3UNA
>
> > An "El Cheapo" approach might be to use multi-conductor cable
> > instead of a single, fat conductor to minimize Skin Effect losses.
> >
>
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[David Cutter]

I made this system primarily for its mechanical properties.  I will make
some measurements one fine day....

David
G3UNA


----- Original Message -----
From: "Bob Cunnings" <[hidden email]>
To: <[hidden email]>
Sent: Sunday, January 06, 2008 11:06 PM
Subject: Re: [Elecraft] dipole antenna efficiency

> The capacitance between individual strands in Litz wire limits it's
> usefulness above 1 MHz or so. W8JI has an interesting discussion of
> skin effect at:
>
> http://www.w8ji.com/skindepth.htm
>
> Bob NW8L
>
> On Jan 6, 2008 3:54 PM, David Cutter <[hidden email]> wrote:
>> I've used several strands of enamelled copper wire twisted together.  I
>> usually twist 3 sets of 3 of quite thin wire, whatever is on hand; 0.2 to
>> 0.45mm dia is easy to use; after twisting I give the group a good pull
>> the
>> stretch and stiffen it a bit but it still remains very flexible indeed
>> and
>> does not have a memory effect.  I tie the wires to a fence post then fit
>> the
>> other ends into an electric hand drill 25m, it's surprising how much
>> twisting is needed. You must not let the wires touch the ground.   It is
>> also very slippery and does not cling to other wires which I find a great
>> advantage when manipulating multi-wire structures.  I call it "multi
>> twist".
>>
>> David
>> G3UNA
>>
>> > An "El Cheapo" approach might be to use multi-conductor cable
>> > instead of a single, fat conductor to minimize Skin Effect losses.
>> >
>>
>> _______________________________________________
>> Elecraft mailing list
>> Post to: [hidden email]
>> You must be a subscriber to post to the list.
>> Subscriber Info (Addr. Change, sub, unsub etc.):
>>  http://mailman.qth.net/mailman/listinfo/elecraft
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[WILLIS COOKE]

Bill, let me first state that operating QRP makes a
good antenna much more a requirement than operating
QRO.  Of course, the antenna does not need to handle
as much power, so some of the components can be
lighter weight.  For someone to give you a really good
suggestion we need to know a few things:

1.  Do you plan to operate mobil in motion?

2.  How long do you stay in one place?

3.  Do you usually park in commercial RV Parks, or on
private property?

4.  What sort of restrictions on antenna installation
do you anticipate?

5.  Are you reasonably physically fit so that you can
erect masts, etc.?

6.  Is the roof of your RV strong enough for you to be
on top of the RV to erect the antenna?

7.  Anything else that might give us clues about
problems that you may encounter?

8.  Are you thinking a K1, 10 watt K2 or just what for
a rig?

9.  What kind of operating and bands do you consider
required and what do you consider desirable?

Cookie, K5EWJ

--- bill KE5KWE <[hidden email]> wrote:

>
> Let me bring this down to the practical world.  My
> QTH is a 42.5 ft. Travel
> Trailer which is insulated from the ground by 6
> Rubber Tires and Air.  It
> has Metal, I presume Aluminum, siding and a Rubber
> over Wood Roof which is
> approx. 10 ft. off the ground.  My K2 is almost
> built and I will be
> operating CW exclusively QRP.  Are there any
> suggestions for either a
> commercially built antenna or a homebrew.
> (Recognizing that I am not a EE
> and have no formal electrical background!)
>
> Thanks
>
> Bill Fogel, KE5KWE
> "On the Road in the USA"
>
>
> This thread started with a person trying to find an
> antenna that would work reasonably well that does
> not
> take up much space.
> --
> View this message in context:
>

http://www.nabble.com/dipole-antenna-efficiency-tp14609719p14650029.html

> Sent from the Elecraft mailing list archive at
> Nabble.com.
>
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[David Woolley (E.L)]

bill KE5KWE wrote:
> Let me bring this down to the practical world.  My QTH is a 42.5 ft. Travel
> Trailer which is insulated from the ground by 6 Rubber Tires and Air.  It

Is it connected to the mains electricity supply?  If it is, you will
probably need to consult someone familiar with your local electrical
codes as, at least in the UK, rules for such structures are much
stricter than for permanent buildings. At the very least, that will
affect if and how you provide an RF ground.

You haven't actually specified what the constraints on your antenna are.
  Are you constrained to not having an external antenna?  Are  you
allowed one but it has to fit within the area of the trailer.   How high
above the trailer can you go?  Are you allowed to install ground rods
(electrical codes apart)?

Without either an external antenna or a ground connection, you are
probably lost.

--
David Woolley
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RFC1855 says there should be an address here, but, in a world of spam,
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[David Woolley (E.L)]

John Magliacane wrote:
>
> When Skin Effect losses become an issue in RF conductors and
> inductors, Litz wire is typically used.  I see no reason why a

One has to ask why this isn't used in all the commercial "miracle" antennas.

>
> I believe the Normal Mode Helix, as is used in HI-VHF television
> broadcasting, is a travelling wave antenna, rather than a
> resonant standing wave antenna, such as a dipole.

You are thinking of axial mode helices.  Normal mode helices are the
most common antennas on VHF hand-helds, and are often called "rubber ducks".



--
David Woolley
Emails are not formal business letters, whatever businesses may want.
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that is no longer good advice, as archive address hiding may not work.
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[Eric Swartz - WA6HHQ]

(I apologize for posting this to the list - Lisa is having trouble
contacting Thom.)

Thom, Lisa has been emailing you since last Wednesday to confirm your K3
order, which should be shipping this week or early next. She has not
received any replies to her emails. Her email includes an attached .pdf
of your invoice for confirmation. (Its possible your anti-spam filter
may be catching this.)

Please email Lisa ([hidden email] and/or [hidden email]) or call
her at 831-662-8345. We'd like to get this out to you as soon as
possible. :-)

73, Eric  WA6HHQ
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[John Magliacane]

> John Magliacane wrote:
>
> >
> > I believe the Normal Mode Helix, as is used in HI-VHF television
> > broadcasting, is a travelling wave antenna, rather than a
> > resonant standing wave antenna, such as a dipole.

David Woolley wrote:

> You are thinking of axial mode helices.  Normal mode helices are
> the most common antennas on VHF hand-helds, and are often called
> "rubber ducks".

I suppose various antennas go by the same name, even though their
operation is considerably different.

I was referring to a Normal Mode Helix also known as a "Side Fire
Helix".  It's described in the Antenna Engineering Handbook
(Johnson), and used in horizontally and circularly polarized
omni-directional applications (mostly VHF and UHF-TV broadcast).
It's a traveling wave antenna where the helix is wound around a
supporting vertical mast that serves as a reflector.

Axial mode helices are a different beast.  They are typically used to
provide broadband, UNI-DIRECTIONAL, circular polarization (OSCAR
satellite communications).  Invented by John Kraus, W8JK.

I always considered a "rubber duck" to be nothing more than a
continuously loaded monopole, and is considerably different in
operation from the other two helical antennas described above.

> > When Skin Effect losses become an issue in RF conductors and
> > inductors, Litz wire is typically used.  I see no reason why a
>
> One has to ask why this isn't used in all the commercial
> "miracle" antennas.

Look at some of the older antennas used for FM broadcast, such as the
Collins or RCA ring dipole arrays.  These are physically short,
capacitively loaded, gamma matched dipoles.  Some implementations of
these antennas place a number of identical dipole rings in parallel
with one another to reduce ohmic losses and improve efficiency.

These antennas may not be made from Litz wire, but the underlying
concept of distributing RF current among closely placed parallel
conductors in an effort reduce resistive losses is still the same.


73, de John, KD2BD


Visit John on the Web at:

        http://kd2bd.ham.org/
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[Thom LaCosta]

On Mon, 7 Jan 2008, Eric Swartz - WA6HHQ, Elecraft wrote:

> (I apologize for posting this to the list - Lisa is having trouble contacting
> Thom.)

She would have no problems if she sent it to the correct address.

I have answered her from the correct address telling her not to use the
incorrect address, and submitting a change in the order based on the email you
sent me relative to the balanced tuner.

She can also send the information to this address.

I trust this post with the copies sent to various addresses and all the ships at
sea will correct the problem.


Thom,EIEIO
Email, Internet, Electronic Information Officer

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[David Woolley (E.L)]

John Magliacane wrote:

> It's a traveling wave antenna where the helix is wound around a
> supporting vertical mast that serves as a reflector.

None of the references I found (including university course notes), when
researching this at the weekend, described anything other than a
dielectric core.  They did suggest a spectrum of behaviour from
basically linear polarized, to circular polarized, depending on the pitch.

Also, travelling wave antennas normally radiate almost parallel to the
length of antenna.

> capacitively loaded, gamma matched dipoles.  Some implementations of
> these antennas place a number of identical dipole rings in parallel
> with one another to reduce ohmic losses and improve efficiency.

I think I can produce a fairly good qualitative argument that that might
not work for radiating elements, even if there is an advantage for
lumped inductors.

If the individual wires strongly interact, which will be the limiting
case as you bring them close together, they will behave like the
individual elements of the cross section of a single conductor, and
therefor suffer a skin effect which will exclude current from the inner
ones and the insides of outer ones.

If they are positioned so that they don't interact with each other, each
will act as an independent radiator and have the radiation resistance it
would have had in isolation.  It will also have the the same ohmic loss
resistance.  So, although you reduce the ohmic resistance by paralleling
them, you also reduce the radiation resistance, in proportion.

For a non-qualitative proof, you would need to demonstrate that there
isn't some peak in performance, between these two extremes
>
> These antennas may not be made from Litz wire, but the underlying
> concept of distributing RF current among closely placed parallel
> conductors in an effort reduce resistive losses is still the same.

Note that a fundamental part of the concept of Litz wire is that it is
intertwined, so that no one strand is always outside or always inside.


--
David Woolley
Emails are not formal business letters, whatever businesses may want.
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that is no longer good advice, as archive address hiding may not work.
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[John Magliacane]

> John Magliacane wrote:
>
> > It's a traveling wave antenna where the helix is wound around a
> > supporting vertical mast that serves as a reflector.

David Woolley <[hidden email]> wrote:

> None of the references I found (including university course
> notes), when researching this at the weekend, described anything
> other than a dielectric core.  They did suggest a spectrum of
> behaviour from basically linear polarized, to circular polarized,
> depending on the pitch.

Johnson's "Antenna Engineering Handbook" (McGraw-Hill) among others
describes the antenna design to which I am referring.  A preview of
chapter 28 where the antenna is described is available at:

http://books.google.com/books?id=xTSNJhVlHGgC&pg=PP1&sig=qdNrli_UaJsiZB_gq3cRqvjy4DY#PPT961,M1

Dielectric Communications (formerly RCA) manufactures such an antenna
(model TCL) for the purpose I originally described.  Their older
print catalog describes the antenna in more detail than what is
available on the web:

http://www.dielectric.com/broadcast/images/tcl1.jpg
http://www.dielectric.com/broadcast/vhf.asp

Jampro does a better job of describing and illustrating the antenna
on-line.  In particular, they state:

"The JTC Circularly Polarized Spiral Antenna consists of a supporting
pole around which are multiple stainless steel spirals wound at
specific pitch angles and spaced from the pipe by heavy-duty
fiberglass-reinforced low-loss insulators. The array acts as a
traveling wave antenna, with the main beam broadside to the support
pole and exceptionally low radiation along the axis of the pole. It
produces an omni-directional azimuth pattern with low VSWR."

Full description and photos are available here:

http://www.jampro.com/index.php?page=jtc-circularly-polarized-spiral-tv-antenna


73, de John, KD2BD


Visit John on the Web at:

        http://kd2bd.ham.org/
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[David Woolley (E.L)]

John Magliacane wrote:
>
> http://books.google.com/books?id=SNJhVlHGgC&pg=PP1&sig=qdNrli_UaJsiZB_gq3cRqvjy4DY#PPT961,M1
>
> http://www.jampro.com/index.php?page=c-circularly-polarized-spiral-tv-antenna

Interesting, although it doesn't seem to be a compact antenna (each turn
of the helix is several wavelengths) and the multiple helices appear to
be there to achieve the radiation pattern and polarization, not
performance (you don't use stainless steel for low losses).
--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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