Antenna field strength question
MCB
For an antenna operating in receive - What field strength present on the
antenna will result in an power output of 3W? The gain of the antenna is 1
(0dBi) and perfectly matched into 50ohms.
Richard Clark
Hi OM,
Looks like you should be consulting the work of Tesla. Be warned he
went mad and broke (don't know which order) trying to fill a load with
significant power transmitted cross town (Colorado Springs).
Now, I only start with this presumption because the wording of your
question lacks a significant amount of detail.
How tall the antenna?
What frequency of operation?
What separation of transmitter from receiver?
What power in transmitter?
Field strength is specified in volts/meter and you already know how
many volts you need for 50Ohms to render 3W, it remains to be seen if
the structures and frequency support that value.
One does not ordinarily speak of power OUTPUT from a receiving antenna
(true, that is what is available and sought, but typical antennas
rarely offer anything but milliWatts at very close range). If you
wanted 3W from a 3W transmitter, you would use direct connection (skip
antennas altogether). If you wanted 3W from a 6W transmitter, you
need a similar antenna within the space that one would knock down the
other if it fell. A direct connection is simpler still. If you
wanted 3W from a 60W transmitter, you will still need to be quite
close and it would be hard to not extend that direct connection
instead. Square-Law is going to kill you unless you are next door to
a half-megaWatt VOA site.
73's
Richard Clark, KB7QHC
MCB
More information: The antenna is an active dipole. There is a 3W RF limiter
on the output which connects directly to a low noise amplifier (LNA). So the
antenna can cope with up to 3W on its output before any damage to the
limiter or LNA. I need to know the field strength as I have a transmitter
within 10 metres transmitting 100W into another dipole at the same frequency
as the RX !! (the receiver is isolated).
So I need to know what the field strength in V/m will produce 3W on the
output of the antenna?....... Anyone know how to tackle this problem?
Thanks
"Richard Clark" <kb7...@attbi.com> wrote in message
news:kjbrfvkumht4me1h2...@4ax.com...
Richard Clark
>Thanks Richard
>
>More information: The antenna is an active dipole. There is a 3W RF limiter
>on the output which connects directly to a low noise amplifier (LNA). So the
>antenna can cope with up to 3W on its output before any damage to the
>limiter or LNA. I need to know the field strength as I have a transmitter
>within 10 metres transmitting 100W into another dipole at the same frequency
>as the RX !! (the receiver is isolated).
>
>So I need to know what the field strength in V/m will produce 3W on the
>output of the antenna?....... Anyone know how to tackle this problem?
>
>Thanks
>
Hi OM,
You still have a lot of unknowns.
What frequency?
Are the antennas in the same polarization (horiz/vert)?
What is the gain of the "active dipole" amplifier?
Is the LNA the active part of the "active dipole?"
(It doesn't seem to be so by your description.)
This may be academic because you say the output is limited to 3W.
What is the power input limit of the LNA?
If you are operating HF,
Why are you using an LNA?
If you are not operating HF, but rather VHF (2M) you are probably OK,
but in all reality that is an issue with the input power ability of
the "active dipole" amplifier, not its output.
MCB
=> The active dipole consists of a RF limiter and LNA all in one package.
The maximum input power to the LNA is 100mW before damage. The limiter
limits power levels between (50mW and 3W) to 50mW - which is fed into the
LNA. The RF limiter can withstand 3W before damage. To solve my problem all
you need to know is that more than 3W will damage the antenna.
So looking at it a different way I wish to know what is the maximum field
strength that can be applied to the antenna before damage? (with the TX and
RX antennas copolar (vertically polarised) at 2m).
How do I calculate the field strength, knowing the gain of the antenna
(0dBi), the output power from the dipole (3W), if I assume that the
impedance is 50ohms? Are there any other quantities that I am not taking
into account? Does the impedance of free space (377ohms) come into it?
"Richard Clark" <kb7...@attbi.com> wrote in message
news:aq0sfvc9bki896do7...@4ax.com...
Dave Shrader
the second which you have not yet asked!
Where do I get a calibrated [accurate] field intensity meter? BTW, they
are moderately expensive.
Deacon Dave, W1MCE
MCB
I have access to a calibration dipole and spectrum analyser to make a
incident power measurement. I can then calculate the field strength from
this no problem. (Just having trouble solving my orginal problem).
"Dave Shrader" <david....@attbi.com> wrote in message
news:3EFEC7E0...@attbi.com...
Richard Clark
>Yes you are right they are expensive...
>
>I have access to a calibration dipole and spectrum analyser to make a
>incident power measurement. I can then calculate the field strength from
>this no problem. (Just having trouble solving my orginal problem).
>
Hi OM,
Barring quite a number of questions left unanswered, have you
considered the potential of blowing out the front end of the spectrum
analyser to make this measurement?
Do you know the instrument's maximum power tolerance?
MCB
the cal dipole and spectrum analyser (as a safe guard), everything was
OK.......... Hence I know the field strength where the active antenna is
going to be placed, just don't know what field strength will damage it....
"Richard Clark" <kb7...@attbi.com> wrote in message
news:iocufv898f58n6sr4...@4ax.com...
Richard Clark
>Already made some preliminary measurements, used a 20dB attenuator between
>the cal dipole and spectrum analyser (as a safe guard), everything was
>OK.......... Hence I know the field strength where the active antenna is
>going to be placed, just don't know what field strength will damage it....
>
Hi OM,
I don't understand.
Why do you need the computation when you already know the answer?
Roy Lewallen
wavelength. Manipulating some equations from King, Mimno, and Wing's
_Transmission Lines, Antennas, and Waveguides_, the field necessary to
generate a particular power P in a load connected to a receiving antenna is
Erms = (1/WL) * sqrt((240*pi^2*P)/G)
where Erms is the RMS field in V/m, WL is the wavelength in meters, and
G is a complex function calculated and graphed in the text. (It's f in
the text; I used G to avoid confusion with frequency.) This is for an
infinitely thin antenna parallel to the impinging field, terminated with
the complex conjugate of its feepoint impedance. For a half wave dipole,
G is about 0.82. But a half wave dipole has a gain of 1.64 relative to
an antenna with 0 dBi gain. So a field sqrt(1.64) = 1.28 times greater
would be required for the 0 dBi antenna. Plugging in the numbers,
Erms = 119.2/WL volts per meter.
It should be possible to confirm this with NEC-2 by modeling a
terminated dipole with an applied plane wave source. (EZNEC doesn't have
the capability to model the plane wave source.)
Roy Lewallen, W7EL
at...@general2.asu.edu
A little simpler equation than the one Roy gave if you know the Gain
(here it is not in dB, so for 0dB the Gain is 1) is from reciprocity:
Matched Cross Section = Gain*lamdba^2/(4*pi)
Assuming the wave matches the polarization of the antenna, the matched
received power is the incident Power/Area times the cross section.
P = Gain*lambda^2/(4*pi)*(Incident Power/area)
A plane wave with rms electric field of E_rms has an incident
Power/Area in SI units of E_rms^2/(mu_0 c), where c is the speed of
light and mu_0 is the permeability of vacuum 4*pi*10^(-7) Newtons/Amperes^2.
Plugging in, using a numerical factor of 3*10^8 m/s for c, mu_0*c has
a numerical value of 120*pi, and solving for E_rms gives
Erms = (1/lambda)*sqrt(480*pi^2*P/Gain)
Comparing with the equation Roy posted, we see that the G factor in
that equation is just Gain/2 for a thin wire, and he verifies that for
the half wave dipole where Gain=1.64 and G is .82.
73 Kevin w9cf
In rec.radio.amateur.antenna Roy Lewallen <w7...@eznec.com> wrote:
: The one critical piece of missing information is the frequency or
: Roy Lewallen, W7EL
:>
:>
Tom Bruhns
> Thanks Richard
>
> More information: The antenna is an active dipole. There is a 3W RF limiter
> on the output which connects directly to a low noise amplifier (LNA). So the
> antenna can cope with up to 3W on its output before any damage to the
> limiter or LNA. I need to know the field strength as I have a transmitter
> within 10 metres transmitting 100W into another dipole at the same frequency
> as the RX !! (the receiver is isolated).
>
> So I need to know what the field strength in V/m will produce 3W on the
> output of the antenna?....... Anyone know how to tackle this problem?
(Aside: Thanks to Roy and Kevin for their postings on this subject!
I'm especially glad to see that Kevin still checks in here
occasionally.)
As an experimentalist, I'd suggest simply commandeering the antenna
into which you're feeding the 100 watts for long enough to do a
network measurement between the antennas. Since you really only care
at one frequency, simply reduce the transmitted power to 1W, for
example, for long enough to find the power at the appropriate point in
the active antenna chain. It's a linear system, right? So 100 times
the input power will result in 100 times the "output" power. All that
seems much easier than going through the intermediate step of
measuring field strength...and since it sounds like you may be within
the near field of the transmitting antenna, you may not be accurately
characterizing the field strength in any event.
Cheers,
Tom