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Question on Antenna Voltage

Andy asks

When one uses the equations for a loop antenna to determine what
voltage will be developed at the antenna terminals due to a passing
signal,
is the calculated voltage with the antenna terminated in an open
circuit,
or a resistance equal to the characteristic impedance of the antenna,
or some other load ?

I haven't seen any of the texts state specifically whether the
voltage
is open circuit or otherwise....

Same question for any antenna ---- if I use Power Density times
Capture Area , it implies that the antenna is terminated in a
resistive
matched load....but I haven't seen it specifically stated. Perhaps
the
issue is too obvious to write down. But I would like to see it.

I would appreciate some small discussion on this. It is a very
simple
thing to clear up if anyone here has first hand knowledge of the
issue.

Thanks, W4OAH

Question on Antenna Voltage

"AndyS" wrote in message
...
Andy asks

When one uses the equations for a loop antenna to determine what
voltage will be developed at the antenna terminals due to a passing
signal,
is the calculated voltage with the antenna terminated in an open
circuit,
or a resistance equal to the characteristic impedance of the antenna,
or some other load ?

I haven't seen any of the texts state specifically whether the
voltage
is open circuit or otherwise....

Same question for any antenna ---- if I use Power Density times
Capture Area , it implies that the antenna is terminated in a
resistive
matched load....but I haven't seen it specifically stated. Perhaps
the
issue is too obvious to write down. But I would like to see it.

I would appreciate some small discussion on this. It is a very
simple
thing to clear up if anyone here has first hand knowledge of the
issue.

Thanks, W4OAH

The simplest case is to specify that the antenna is terminated in a
resistance equal to its own radiation resistance (e.g. 73 ohms for a
thin-wire dipole) then the sums are made straightforward and equal amounts
of power are developed in the termination and the antenna. If the antenna
is essentially lossless, the power 'dissipated' in the radiation resistance
is re-radiated.

Open-circuit voltage wouldn't be a good guide to its coupling factor for a
lot of common antennas (which are called on to produce current) but, of
course, it works fine for active monopoles.

Chris

Question on Antenna Voltage

What sort of loop are you asking about? If it's a small loop (that is,
small in terms of wavelength):

AndyS wrote:
Andy asks

When one uses the equations for a loop antenna to determine what
voltage will be developed at the antenna terminals due to a passing
signal,
is the calculated voltage with the antenna terminated in an open
circuit,
or a resistance equal to the characteristic impedance of the antenna,
or some other load ?

Most likely open circuit. If not, it would be terminated in the complex
conjugate of its impedance as a source, which (for a small loop) will be
a very large impedance consisting of a large amount of inductive
reactance in series with a very small resistance.

I haven't seen any of the texts state specifically whether the
voltage
is open circuit or otherwise....

Same question for any antenna ---- if I use Power Density times
Capture Area , it implies that the antenna is terminated in a
resistive
matched load....but I haven't seen it specifically stated. Perhaps
the
issue is too obvious to write down. But I would like to see it.

Here it would be terminated in the complex conjugate of its impedance,
and the result of your calculation -- power -- would be the power
dissipated in that very small resistance.

The phrase "resistive matched load" you used above is an oxymoron
because of the very large reactance required for a matched load.

I would appreciate some small discussion on this. It is a very
simple
thing to clear up if anyone here has first hand knowledge of the
issue.

Thanks, W4OAH

Roy Lewallen, W7EL

Question on Antenna Voltage

On 22 jul, 00:19, AndyS wrote:
Andy asks

* * When one uses the equations for a loop antenna to determine what
voltage will be developed at the antenna terminals due to a passing
signal,
is the calculated voltage with the antenna terminated in an open
circuit,
or a resistance equal to the characteristic impedance of the antenna,
or some other load ?

* * I haven't seen any of the texts state specifically whether the
voltage
is open circuit or otherwise....

* * *Same question for any antenna ---- *if I use Power Density times
Capture Area , it implies that the antenna is terminated in a
resistive
matched load....but I haven't seen it specifically stated. *Perhaps
the
issue is too obvious to write down. *But I would like to see it.

* * I would appreciate some small discussion on this. *It is a very
simple
thing to clear up if anyone here has first hand knowledge *of the
issue.

* * * * * * * *Thanks, * W4OAH

Hello Andy,

In some cases you can check it yourself by calculating the no-load
voltage, compare this with the published data.

As for a single turn coil:

EMF = 2*pi*f*Aloop*B (all rms values), Pdens = E*H,
Zo=377 and B = u*H

EMF = 0.41*A*f*sqrt(Pdens), f in MHz, A in m^2, single turn.

Wire length of loop 0.25 lambda. Pdens = plane wave Power Flux
Density (W/m^2).

Formula can also be applied on multi-turn loops, as long as the
operating frequency is far below the self-resonant frequency of the
multi-turn loop.

When: (reactance of coil) Rload, output voltage is almost equal to
the unloaded voltage (EMF). For narrow band operation, you can add a
capacitor in series to cancel the inductance. You will get the EMF
across an about 50 Ohms resistive load (as radiation resistance of
small loops is far less then 50 Ohms). I used this approach on a
single turn loop made out of strip material (the strip material is to
lower the self inductance of the loop).

Of course when using good matching, you can get more output power with
a given PFD.

Generally spoken, antennas for EMC measurement purposes (mostly not
well- matched for the HF and lower range) are mostly specified with a
given load (mostly 50 Ohms).

For antennas specified with capture area:
Pout = (capture area)*Pdens.
Capture area = gain*lamda^2/4pi. Gain NOT in dB.

The actual loaded voltage depends on the input impedance of the
antenna. The no load voltage is twice the loaded voltage, only when
the antenna is perfectly matched to a resistive load.

When the gain, antenna factor or capture area is specified under non-
perfectly matched condition or is specified for a perfectly matched
non-resistive load, the EMF is generally NOT twice the loaded output
voltage. As an example you can consider a non-tuned loop.

I hope this helps you a bit.

Best regards,

Wim
PA3DJS
www.tetech.nl
Remove the obvious three-letter combination in case of PM.

Question on Antenna Voltage

On Jul 21, 11:22 pm, Owen Duffy wrote:

The article athttp://www.vk1od.net/antenna/SmallUntunedSquareLoop/index.htmshould
answer your question.

Owen

Andy replies:

Exactly what I was looking for. Thanks, Owen.....

W4OAH

Question on Antenna Voltage

AndyS wrote in
:

On Jul 21, 11:22 pm, Owen Duffy wrote:

The article
athttp://www.vk1od.net/antenna/SmallUntunedSquareLoop/index.htmshould
answer your question.

Owen

Andy replies:

Exactly what I was looking for. Thanks, Owen.....

Thanks Andy, pleased you found it helpful.

73
Owen