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In a message dated 6/18/2007 11:00:49 PM Eastern Standard Time,
davidgriffin@... writes:
Although, Brian indicates 25% as the max circumference rather than 33%.
Does anyone know why either of these sizes is considered max for a small
transmitting loop?
Also, what is the LoopCalc program comparing to when it states that a
certain configuration is , e.g., 51% efficient. Compared to a standard
dipole?
Thanks, Dave


Hi Dave,
Once the circumference of a small transmitting loop is much more than
about 30-33% of a wavelength the loop becomes self-resonant or actually
resonant below the operating frequency, so there is no way to tune the loop.
I think all antenna efficiency programs use a resonant dipole in free
space as the standard for antenna efficiency.
73 Todd WD4NGG



*** See what's free at .

I've been playing with KI6GD's LoopCalc program for small transmitting
loops. And I have a couple of questions for those of you who have used
it. First, I notice that when the circumference of the loop exceeds
0.33 wavelength the programs says such length is too large for a small
transmitting loop. That same sentiment is voiced in W2BRI's faq at

Although, Brian indicates 25% as the max circumference rather than 33%.
Does anyone know why either of these sizes is considered max for a small
transmitting loop?
Also, what is the LoopCalc program comparing to when it states that a
certain configuration is , e.g., 51% efficient. Compared to a standard
dipole?
Thanks, Dave

--- In loopantennas@..., DavidGriffin <davidgriffin@...>
wrote:

Also, what is the LoopCalc program comparing to when it states that
a
certain configuration is , e.g., 51% efficient. Compared to a standard
dipole?

Hi David,

Loop efficiency relates to resistive losses in the loop. If there
were no losses the efficiency would be 100%. Small loops have very
low resistance due to radiation, on the order of tenths of an ohm.
Compare this to 50 ohms for a dipole up a half wavelength. At such
low loop resistance, losses in the conductor out of which the loop is
constructed become significant. That is why small loops are made out
of pipe rather than wire in order to increased surface area and why
copper is preferred over aluminum, as copper has better conductivity
(lower resistance).

Dave WA6YSO

self resonance is the problem above a particular diameter. you can actually
see this as you watch the capacitor needed for a given frequency as you
increase the size.

efficiency is the ratio of the skin effect impedence over the sum of the
skin effect plus free space impedence (which is pretty small.) note that the
efficiency doesn't take into account any joints which will reduce the
efficiency even further.

73

jim ab3cv

Although the cost is high I recommend the use of Cool-Amp
_ ()
for use on small loops and RF ground busses. We used this
product at the shop on all the copper buss connections for the
platers to lower the resistance. It leaves a thin silver coating on
copper and brass. I have used it on PC boards as well as RF
buss. A little bit goes a long way. Anything that can be done to
reduce the skin resistance has a good pay off in the end.

Eric

Thanks Jim, Todd and Dave. Good points. Helps my understanding a lot.
Dave G.
N2chi

--- In loopantennas@..., "Jim Miller" <jim@...> wrote:

self resonance is the problem above a particular diameter. you can

actually

see this as you watch the capacitor needed for a given frequency as you
increase the size.

efficiency is the ratio of the skin effect impedence over the sum of

the

skin effect plus free space impedence (which is pretty small.) note

that the

efficiency doesn't take into account any joints which will reduce the
efficiency even further.

73

jim ab3cv

seems i screwed up my efficiency calculation. but the thought was right...

i'll post corrected version in a few minutes.

jtm

Rf = free space impedence (small for small loops)

Rs = skin effect resistance (depends on bulk resistivity, usable skin area,
frequency)

Efficiency = Rf / (Rf + Rs)

You can see as Rs approaches zero efficiency approaches 1 and as Rs equals
Rf efficiency is halved.

Note that in addition to skin effect other resistances could be in an actual
implementation such as the resistance of joints to construct the loop,
connection to the capacitor and the implementation of the capacitor itself.

All those would add to Rs and further degrade the efficiency.

73

jim ab3cv

Also, what is the LoopCalc program comparing to when it states that a
certain configuration is , e.g., 51% efficient. Compared to a standard
dipole?

I think all antenna efficiency programs use a resonant dipole in free
space as the standard for antenna efficiency.

Actually, as far as efficiency is concerned, the reference is ideal or no
loss. Efficiency, unlike gain, is not compared to a dipole or anything
else. 100% efficiency just means that all the power you feed into the
antenna, gets turned into radio waves and none is lost in the form of heat
(resistance losses). 50% means half your input power is lost as heat and
never gets out.

After dealing with the power lost to heat, then what's left over is affected
by the antenna's gain ... which IS compared to a reference antenna (dipole
or isotropic), and is a function of direction (since antenna gain is all
about concentrating signals in some directions while sacrificing signal in
other directions).

Andy

Hi Andy,
Yes, I agree. It makes sense that that efficiency would be a power in -
power out ratio. I was wondering how to compare it, though. E.g.,
what would be the efficiency of a plain old dipole up 30 feet in my
backyard? If the efficiency for that arrangement were, say, twice that
of a proposed rigid loop as calculated via the KI6GD program, I would
have second thoughts about spending the money to build such a loop (or I
would increase circumference up to 33 % or increase pipe diameter up to
the limit of my wallet.)
As I play with the program, using "reasonable" copper diameters and side
lengths (reasonable to me), I come up with efficiencies in the 25 to 40
% range.
Thanks,
Dave

--- In loopantennas@..., Andy <Andy.groups@...> wrote:

Also, what is the LoopCalc program comparing to when it states that a
certain configuration is , e.g., 51% efficient. Compared to a

standard

dipole?

I think all antenna efficiency programs use a resonant dipole

in free

space as the standard for antenna efficiency.

Actually, as far as efficiency is concerned, the reference is ideal or

no

loss. Efficiency, unlike gain, is not compared to a dipole or anything
else. 100% efficiency just means that all the power you feed into the
antenna, gets turned into radio waves and none is lost in the form of

heat

(resistance losses). 50% means half your input power is lost as heat

and

never gets out.

After dealing with the power lost to heat, then what's left over is

affected

by the antenna's gain ... which IS compared to a reference antenna

(dipole

or isotropic), and is a function of direction (since antenna gain is

all

about concentrating signals in some directions while sacrificing signal

in

other directions).

Andy