Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length of
wire). I connect a receive antenna to a spectum analyser and the antenna to
be tuned to the signal generators 50ohm output. I set the signal generator
to 400MHz and adjusted the length of the antenna to get a maximum peak on
the spectrum analyser. My question is when i have tuned the antenna for
maximum output at 400MHz is the antenna then tuned to 400MHz or is the
antenna's impeadance tuned to 50ohm or both?

Marcus,
I think the 'right' answer would be "both", sort of, since what you
are doing is looking for the maximum signal strength. Since the input
impedance of a 1/4 wave radiator isn't 50 ohms to start with, you are
also doing some impedance matching by length of radiator adjustment.
You seem to be getting the most benefit (signal strength) from using
'different' measuring devices than 'normal', but it's working, right?
And when you think about it, that's what most people do when they
'tune' an antenna, just different measuring devices. 'Resonance' and
impedance matching trade off, sort of. Is that the 'best' possible
way to do it? Mmmm, no. Is it a practical way to do it? Mmmm, yes,
sort of, if you've got the required devices.
- 'Doc

(Lots of 'wiggle-room' in that answer, not very satisfying huh? Oh
well...)

On Sun, 10 Feb 2008 13:20:21 -0000, "Marcus Tait"
wrote:

Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length of
wire). I connect a receive antenna to a spectum analyser and the antenna to
be tuned to the signal generators 50ohm output. I set the signal generator
to 400MHz and adjusted the length of the antenna to get a maximum peak on
the spectrum analyser. My question is when i have tuned the antenna for
maximum output at 400MHz is the antenna then tuned to 400MHz or is the
antenna's impeadance tuned to 50ohm or both?

Well, it could be, both, most times for practical use it's ok.

But antennas also have directional senitivity, and you may just
have optimized the direction diagrams for your experimental setup,
and not for DX wide distance connections. There is impedance match,
resonance and directional characteristics which we have to care for,
all three parameters!
The antenna may be in resonance, but Z not necessarily at 50 Ohms,
or it may have 50 ohms but be out of resonance and thus loose
efficiency and/or it may be radiating down to the earthworms.

Everything in the vicinity influences the impedance and resonance
parameters. Therefore normally we mount the antenna at it's proper
place, on the roof, on the mast or where we want it to use.
Then we measure either at the antenna footpoint or the bottom cable
end. I have seen people mounting an SWR meter at the antenna
in 10 meter height and then take readings with a telescope
from down under.


To optimize transmitter to antenna coupling we need a directional
coupler for 50 Ohm
http://en.wikipedia.org/wiki/Power_d...ional_couplers
and connect the antenna to the output port, the signal generator to
the input port, and the analyzer to the coupled port.
The isolated port must be terminated with a 50 Ohm resistor.
Then the antenna is optimally tuned for transmission when the
reflected signal gets to a minimum, that is, nothing is reflected back
to the analyzer.
The easier way is to use a SWR meter
http://en.wikipedia.org/wiki/SWR_meter where the signal generator is
connected to the input, the antenna to the output, and the reflected
signal is show on the meter instead of an analyzer.
This way we make sure the transmitter feeds all the power to the
transmitting antenna.

But where does the energy go?
Worst case, straight up to heaven.
We need a remote antenna to make sure we did everything right.

w.

On Sun, 10 Feb 2008 13:20:21 -0000, "Marcus Tait"
wrote:

Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length of
wire). I connect a receive antenna to a spectum analyser and the antenna to
be tuned to the signal generators 50ohm output. I set the signal generator
to 400MHz and adjusted the length of the antenna to get a maximum peak on
the spectrum analyser. My question is when i have tuned the antenna for
maximum output at 400MHz is the antenna then tuned to 400MHz or is the
antenna's impeadance tuned to 50ohm or both?


Hi Marcus,

You have tuned a system with a variable stub. To replicate its use in
a practical application, you would replace the generator with an equal
length line terminated in 50 Ohms shielded in a box the size of the
generator (with a dummy line going to the AC wiring to complete the
ground path). You would then have to replace the spectrum analyzer
with your transmitter/receiver in a similar sized box at the end of a
similar length of transmission line. Also during operation, you would
have to inhabit the same vicinity/volume of space as you did when you
clipped the wire. However, precision and resolution may not be that
demanding as you imply by asking for two places: 50 ±j0 Ohms.

In other words, your work would suffer compounding issues of proximity
and parasitic relations - lack of isolation. As a quarterwave antenna
does not present 50 Ohms without extenuating factors, achieving that
figure away from the bench would be more luck than engineering.

73's
Richard Clark, KB7QHC

On 10 feb, 14:20, "Marcus Tait" wrote:
Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length of
wire). I connect a receive antenna to a spectum analyser and the antenna to
be tuned to the signal generators 50ohm output. I set the signal generator
to 400MHz and adjusted the length of the antenna to get a maximum peak on
the spectrum analyser. My question is when i have tuned the antenna for
maximum output at 400MHz is the antenna then tuned to 400MHz or is the
antenna's impeadance tuned to 50ohm or both?

Hello Marcus,

You method does not guarantee optimum match (hence maximum radiated
power). Matching involves reactive component = 0, resistive component
= 50 Ohm (for a 50 Ohms generator).

When your quarter wave wire (connected to RF generator) has a real
impedance (Im(Z) = 0). The impedance can be anything between about 18
Ohm and 50 Ohms. It depends on the termination of the coax shield at
the quarter wave wire connection. So resonance (generally spoken, no
reactive components), does not guarantee maximum power transfer.

Only when you can prove by other means that your quarter wave antenna
is 50+j0 Ohms when in resonance, maximum reading on the analyzer
corresponds to optimum match and maximum radiated power (assuming no
ohmic losses in the antenna). As a first approximation, you can see
the quarter wave antenna as a series resonant circuit with a series
resistor that represents the "losses" due to electro-magnetic field
radiation.

Best regards,

Wim
PA3DJS
www.tetech.nl

In article ,
"Marcus Tait" wrote:

Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length of
wire). I connect a receive antenna to a spectum analyser and the antenna to
be tuned to the signal generators 50ohm output. I set the signal generator
to 400MHz and adjusted the length of the antenna to get a maximum peak on
the spectrum analyser. My question is when i have tuned the antenna for
maximum output at 400MHz is the antenna then tuned to 400MHz or is the
antenna's impeadance tuned to 50ohm or both?

The way the "Big Boys" do this is with an Impedance Bridge connected
to the Spectrum Analyzer, Tracking Generator, Antenna, and 50 Ohm
Termination Load. Then prune the antenna for the biggest NULL, at the
desired Operating Frequency. Been using this method for years on
MF/HF/VHF Marine Coast Station Antennas. sure takes the guesswork out
of the system.

--
Bruce in alaska
add path after fast to reply

"Marcus Tait" wrote in
:

Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length
of wire). I connect a receive antenna to a spectum analyser and the
antenna to be tuned to the signal generators 50ohm output. I set the
signal generator to 400MHz and adjusted the length of the antenna to
get a maximum peak on the spectrum analyser. My question is when i
have tuned the antenna for maximum output at 400MHz is the antenna
then tuned to 400MHz or is the antenna's impeadance tuned to 50ohm or
both?

My view is that you have adjusted the antenna to accept the maximum power
from your source which should in this case have an equivalent source
impedance of 50+j0.

Contrary to popular belief, radio transmitters do not necessarily or even
usually have an equivalent source impedance close to 50+j0.

You should not be surprised if you replaced the generator with a
transmitter and performed the same procedure, that the optimum length
might be a little different, and that if changed the feed line length a
little, the optimum length might be a little different again.

You would more usually want to adjust the antenna to be within a given
VSWR tolerance, and to know the final VSWR, you will need some form of
directional coupler on the signal source to determine rho etc.

Owen

"Owen Duffy" wrote in message
...
"Marcus Tait" wrote in
:

Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length
of wire). I connect a receive antenna to a spectum analyser and the
antenna to be tuned to the signal generators 50ohm output. I set the
signal generator to 400MHz and adjusted the length of the antenna to
get a maximum peak on the spectrum analyser. My question is when i
have tuned the antenna for maximum output at 400MHz is the antenna
then tuned to 400MHz or is the antenna's impeadance tuned to 50ohm or
both?

My view is that you have adjusted the antenna to accept the maximum power
from your source which should in this case have an equivalent source
impedance of 50+j0.

Contrary to popular belief, radio transmitters do not necessarily or even
usually have an equivalent source impedance close to 50+j0.

You should not be surprised if you replaced the generator with a
transmitter and performed the same procedure, that the optimum length
might be a little different, and that if changed the feed line length a
little, the optimum length might be a little different again.

You would more usually want to adjust the antenna to be within a given
VSWR tolerance, and to know the final VSWR, you will need some form of
directional coupler on the signal source to determine rho etc.

Owen

The corollary to what you are saying is that a particular transmitter might
put out more power when presented with a load other than 50 Ohms. Assuming
the output is voltage limited, as to 13.8 V at the drain, more than likely a
hot ( high gain ) transistor will put out more power into a lower impedance.
This would also increase the temperature rise in the transistor.

Tam/WB2TT

You've gotten several answers, but let me add one more, from perhaps a
little different point of view.

The question of whether the antenna is "tuned to 400 MHz" isn't an easy
one to answer, because it doesn't have any real single meaning. As you
adjust an antenna's length in the vicinity of a quarter wavelength,
there's no magic length where the antenna is "tuned" to work better than
for another length. You can change the length a fair amount in this
region without having any appreciable effect on the pattern or gain. So
your adjusting isn't impacting the pattern or gain of the antenna to any
appreciable degree.

What you are doing is changing the impedance at the base of the antenna.
That impedance is transformed through whatever length of transmission
line you have connected to it, and that transformed impedance is what
the signal generator sees. The signal generator, having a fixed output
impedance of 50 ohms, will deliver the most power when it sees 50 ohms
resistive, causing the spectrum analyzer to show the maximum received
signal. So what you've done is to adjust the antenna feedpoint impedance
so that, when transformed through the line, it provides the best match
to the signal generator output impedance of 50 + j0 ohms. You could say
that the antenna itself is "tuned to 400 MHz" when its feedpoint is
purely resistive (that is, the antenna is resonant). If you have a
perfect 50 ohm match at the signal generator, then you know that the
antenna impedance is 50 + j0 ohms (assuming you're using 50 ohm
transmission line between the generator and antenna). But if you don't
have a perfect match at the generator, the antenna might or might not be
resonant at the point where the match at the generator is best. In
practice, for a 1/4 wave antenna, it'll be very close. But as I said in
the beginning, that's of really no importance in determining the antenna
performance.

If you were to put an adjustable matching network between the signal
generator and antenna so you could assure that the signal generator
always sees 50 + j0 ohms, you'd find that the antenna length adjustment
(over a moderate range) has virtually no effect on the received signal
strength as long as you adjust the matching network for maximum received
signal after each length adjustment. But most VHF transmitters and
receivers are designed to work with a 50 ohm load and don't have any
built in adjustable matching capability, so the adjustment you've done
will result in the best overall performance when connected to such a
transmitter or receiver.

Roy Lewallen, W7EL

Marcus Tait wrote:
Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length of
wire). I connect a receive antenna to a spectum analyser and the antenna to
be tuned to the signal generators 50ohm output. I set the signal generator
to 400MHz and adjusted the length of the antenna to get a maximum peak on
the spectrum analyser. My question is when i have tuned the antenna for
maximum output at 400MHz is the antenna then tuned to 400MHz or is the
antenna's impeadance tuned to 50ohm or both?

Bruce in alaska wrote:
In article ,
"Marcus Tait" wrote:


Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length of
wire). I connect a receive antenna to a spectum analyser and the antenna to
be tuned to the signal generators 50ohm output. I set the signal generator
to 400MHz and adjusted the length of the antenna to get a maximum peak on
the spectrum analyser. My question is when i have tuned the antenna for
maximum output at 400MHz is the antenna then tuned to 400MHz or is the
antenna's impeadance tuned to 50ohm or both?


The way the "Big Boys" do this is with an Impedance Bridge connected
to the Spectrum Analyzer, Tracking Generator, Antenna, and 50 Ohm
Termination Load.

Or with their handy dandy Vector Network Analyzer, which essentially
rolls all of the above stuff into one convenient package. There's a
fair number of mfrs selling VNAs targeted to this kind of market (i.e.
standing out in the wind on a tower, as opposed to in a nice quiet
anechoic chamber indoors). There's some nice stuff for antenna ranges
too (fiber optic connections to the remote measurement point, for instance).