开云体育

Al,
A folded dipole is around 300 ohms. This explains the use of 300 ohm twin
lead
on many TV antenna on old. If you really want to delve deeply into the topic
the
link below will give you more than needed with some programs that will crunch
the numbers.
_
()

Enjoy,

Eric

Is there a nominal impedance or approximate impedance for a loop
antenna? For example, a dipole is considered 72-ohm, a bevarage 450-
ohm. What is a nominal loops Z?

I have a 4-1/2 foot diameter loop antenna with 12 turns paralled by a
capacitor for the antenna and one additional turn for the pickup coil.
The loop is for the 200 - 500kHz band. About how much is the impedance
of such an antenna? Thanks.

Al

Hi Al,
Using very approximate input values to Reg Edward's "rjeloop3" program, gives 6000 ohms for the tuned loop, at 200 kHz.

73,
Steve

Hi Al,
By the way, all mail direct to you has been bouncing as of last evening. Your ISP rejects anything I try to send to you.
Steve

Hello,

1)A loop is an inductor so impedance (complex number) equals j by
omega by L ??? (the dominant factor of the total impedance since an
actual loop has ohmic resistance and capacitance). The nominal
impedance you are referring is something a little different (see (3)).

2)After all you actually _tune_ i.e. resonate the system with a
variable capacitor (I guess you are talking about a tuned loop
antenna). So in practice you don't need that value.

3)We always forget that the characteristic impedance of an antenna is
actually the _radiation resistance_. What most people don't know is
when we match the radiation resistance of areceiving antenna with the rest
system, the antenna *transmits* half of the received energy. Check
(near
the bottom of the page, it has
math but makes clear in plaintext many, many, many misconceptions... I
remember a
series of posts here or at Shortwave-SWL-Antenna Group about
receiving antenna impedance. The link above answers many questions....)

(ducks to avoid flames)

Manolis

In a message dated 11/03/2007 07:11:50 GMT Standard Time,
mpetrakis@... writes:

)We always forget that the characteristic impedance of an antenna is
actually the _radiation resistance_. What most people don't know is
when we match the radiation resistance of areceiving antenna with the rest
system, the antenna *transmits* half of the received energy. Check
_
() (near
the bottom of the page, it has
--------------------------------------------

That sounds a bit like pseudo science based on the maximum power transfer
theorem, whereby it can be shown that the maximum power transfer from any
source occurs when the impedance of the source matches the impedance of the
"receiver".
For a conjugate match the inductive and capacitive reactances, equal and
opposite, cancel and the resulting impedances are purely resistive and also
equal.
I've seen debate before on what happens to the "extra" energy but in
practice half is dissipated in the source resistance and half in the receiver.
It matters not whether you consider it to be retransmitted or just not
available in the first place, conservation of energy dictates there's no more at
the end of the process than at the beginning.

regards

Nigel
GM8PZR

At 08:43 PM 3/10/2007 +0000, you wrote:

Is there a nominal impedance or approximate impedance for a loop
antenna? For example, a dipole is considered 72-ohm, a bevarage 450-
ohm. What is a nominal loops Z?

I have a 4-1/2 foot diameter loop antenna with 12 turns paralled by a
capacitor for the antenna and one additional turn for the pickup coil.
The loop is for the 200 - 500kHz band. About how much is the impedance
of such an antenna? Thanks.

Al

Yes, and no. The answer is yes if you compare a naturally resonant dipole, 1/2 wavelength .... approx. 72 ohms, to a naturally resonant loop, 1 wavelength .... something over 100 ohms. Once you start to reduce the size of the loop and resonant it with an external capacitance the answer is no (eg a nominal impedance.)

I am not prepared (perhaps able as well) to quote theory or mathematical modeling of the situation, but I can give some insights from practical experience. In my experience if the LC circuit is resonant with relatively large inductance compared to capacitance the impedance seems to be relatively lower in value. If you have the opposite situation eg with a relatively small inductance and larger capacitance then the impedance seems to be much higher.

The reason I make the statement of higher or lower in impedance is that I find that to couple to a resonant circuit of the first kind (high inductance and low capacitance) is that it is less susceptible to 'loading' when I attempt to couple to it. On the other hand the high C versus L tuned circuit is very touchy to coupling and requires a very light (high impedance) load. The way you wish to go will depend on the objective in your design.

If you design a tuned loop for reception in the MW band I find that the most effective combination is the one with the higher C vs L combination. However, tuning and loading are touchy but performance ... eg narrowness of null and amount of antenna radiation are maximum. I usually couple to a receiver through mutual inductance .... and often best performance is found with a separation of 2 to 3 feet and more. Thus, I find a pickup loop counter productive unless it is designed for critical adjustment.

This is one of the challenges for the xtal receiver group who try to maximize signal pickup with signal separation. Best performance always requires high c low L inductors and high impedance audio load earphones or transformers.

In your second question, if you know the inductance of the of the 12 turn loop and capacitance to resonant at the particular frequency of interest ... it will be possible to calculate an approximate impedance value. If you move off resonant frequency it will be different hi hi .... There is no nominal impedance.

Jim

Thanks to all for the replies to my questions. Steve I forgot about
rjeloop3 and all the info it provides. I'll look into it a bit more.
I don't understand why you are having problems e-mailing to me. I am
getting mail from other sources. please send me a test message and
let's see what happens.

The main reason I was asking about the Z of my loop (it is a tuned
loop by the way, I just replaced the variable cap with varactors and
it is working great) is that I am interested in building a balun
(matching transformer) for it. In order to do this I need some value
to match to. The receiver side is nominal 50-ohms and if the loop is
450-ohms then I know I need a Z match of 9:1. I can't build a
matching transformer if I don't know what I am matching to. That is
why I wanted some nominal Z value for the loop.

Manolis, are you saying that if I tune (peak) my loop, it is already
at the best Z match, and I therefore can conclude that I do not need
a matching transformer?

The antenna is basically working very well and I am pleased with it,
but one must continually tinker and strive to do better. Right?

Al KA5JGV
San Antonio, TX