Why doesn't maximum field strength and minimum SWR occur at the same frequency?
 

I have always heard that this is true, and that I should tune antennas
for maximum field strength and not for minimum SWR. Why wouldn't they
occur at the same time?

Nathan

nathan wrote:
I have always heard that this is true, and that I should tune antennas
for maximum field strength and not for minimum SWR. Why wouldn't they
occur at the same time?

I read somewhere that maximum power is transferred when the load is
the complex conjugate of the impedance presented to the load by the
transmission line. I think that was explained in "Transmission Lines"
by Chipman. Minimum SWR seems to occur when the load impedance is equal
to the characteristic impedance of the transmission line. These are not
necessarily the same points if Z0 has a reactive component.

Also, if the field strength measurement is not being made in the far
field, it may not be an accurate indication of far field radiation
which is what matters. Measured near field photons may simply recombine
with the antenna's free electrons and not contribute to far field
radiation.
--
73, Cecil http://www.qsl.net/w5dxp



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Your transmitter is designed to drive a load of 50 Ohms. This corresponds to
an SWR of 1:1 if you are using 50 Ohm coax. A particular transmitte may very
well put out more power into , say, 30 Ohms than 50, but it might not be
linear, or the output transistors could overheat.

Tam/WB2TT

"nathan" wrote in message
om...
I have always heard that this is true, and that I should tune antennas
for maximum field strength and not for minimum SWR. Why wouldn't they
occur at the same time?

Nathan

"nathan" wrote in message
om...
I have always heard that this is true, and that I should tune antennas
for maximum field strength and not for minimum SWR. Why wouldn't they
occur at the same time?

Nathan

Simply speaking, resonance, and the closest impedance match, are frequently
different points.

My CB antenna on the car is that way. 100 ohms resistive when resonant.
I leave it resonant.
I would have preferred nearer to 50, but that's not a huge thing.

Nathan wrote:
"Why wouldn`t they occur at the same time?"

Suppose you adjust the length of a 1/4-wave antenna to resonance. And,
suppose its impedance is 37 ohms. The antenna has no reactance and is
resonant but it does not match the coax in spite of zero reactance
because the surge impedance of the coax is not 37 ohms.

Best regards, Richard Harrison, KB5WZI

There are several reasons why maximum field strength and minimum SWR don't
occur at the same points, necessarily. The field strength can be considered
part of the "efficiency" of the overall system. SWR just relates to the match
between the feedline and the coax. Think of the dummy load example; it might
exhibit close to a 1:1 SWR but radiates little power.

nathan wrote:

I have always heard that this is true, and that I should tune antennas
for maximum field strength and not for minimum SWR. Why wouldn't they
occur at the same time?

Nathan

Maximum field strength, far field strength, occurs when the antenna is
radiating the MAXIMUM power delivered to it. [Irms^2*Rradiation]

Minimum VSWR occurs when the load impedance of the antenna matches the
characteristic impedance [Zo] of the transmission line.

At my previous location I had a very efficient antenna, 1/2 wavelength,
fed with 600 ohm transmission line with a VSWR of more than 20:1.

The difference is between the radiation resistance [Rradiation] of the
antenna and the feedpoint impedance of the antenna. They are NOT the
same thing!

Deacon Dave, W1MCE

Hi,

no question, minimum SWR results maximum power on air.

greeting Leo Baumann

Nonsense! I can give you any number of cases where your statement is false.

"Leo Baumann" wrote in message
...
Hi,

no question, minimum SWR results maximum power on air.

greeting Leo Baumann

"Leo Baumann" wrote in message
...
Hi,

no question, minimum SWR results maximum power on air.

Fits the definition of a dummy load.. Minimum SWR.
Not much radiation there, though.