RadioBanter - View Single Post - An antenna question--43 ft vertical

[email protected] · July 8th 15, 08:20 PM posted to rec.radio.amateur.antenna

Brian Reay wrote:
On 08/07/15 19:36, wrote:
Brian Reay wrote:
wrote:
Brian Reay wrote:
On 06/07/15 01:21, wrote:
John S wrote:
On 7/5/2015 5:24 PM, wrote:
Roger Hayter wrote:
wrote:

The output impedance of an amateur transmitter IS approximately 50 Ohms
as is trivially shown by reading the specifications for the transmitter
which was designed and manufactured to match a 50 Ohm load.

Do you think all those manuals are lies?

You are starting with a false premise which makes everything after that
false.

A quick google demonstrates dozens of specification sheets that say the
transmitter is designed for a 50 ohm load, and none that mention its
output impedance.

If the source impedance were other than 50 Ohms, the SWR with 50 Ohm
coax and a 50 Ohm antenna would be high. It is not.

Where is the source impedance found on a Smith chart? Also, if you have
EZNEC, you will not find a place to specify source impedance but it will
show the SWR.

A Smith chart is normalized to 1.

That is true but is doesn't address the point. There should still be
somewhere to represent the source impedance, albeit normalised.

The purpose of a Smith chart it to match a SOURCE to a LOAD.

EZNEC allows you to set the impedance to anything you want and assumes
the transmission line matches the transmitter.

Likewise, that is a sweeping statement which evades the point.

The main purpose of EZNEC is to design an antenna for amateur radio use
and all commercial amateur radio transmitters have an output impedance
of 50 Ohms.

Neither of those responses address the points.

It is clear you cannot support your assertions.

EZNEC can set the source impedance to any value one desires, but the
default is 50 Ohms as most people are interested in 50 Ohm systems
as the majority of transmitters are designed for 50 Ohm loads.

EZNEC calculates the SWR for an antenna presented to the SOURCE
in the model.

For most simulations, the SOURCE is placed at the antenna terminals,
which represents what an attached transmission line will see.

The transmission line most commonly used is 50 Ohm coax.

EZNEC will also allow you to put a SOURCE at one end of a transmission
line with the other end of the line at the antenna terminals.

In this simulation, EZNEC simulates what the transmitter would see if
it were connected to the transmission line/antenna system.

SWR is defined in terms of SOURCE impedance and LOAD impedance.

SWR = (1 + |r|)/(1 - |r|)

Where r = reflection coefficient and

r = (Zl - Zo)/(Zl + Zo)

Where Zl = complex load impedance, Zo = complex source impedance.

The matter at hand isn't SWR it is the output impedance of PAs.

Nope, the matter at hand is the definition of SWR.

That seems to be you sticking point. You are assuming the PA is a
transmission line, rather than an active source.

Nope, the matter at hand is the definition of SWR, which is defined
in terms of SOURCE impedance and LOAD impedance.

It does not matter in the slightest if the SOURCE impedance is the
output of a transmitter or the end of a transmission line.

You confusion isn't helped by the habit of some manufacturers including
SWR in the PA spec.

Manufacturers specify the LOAD impedance for the transmitter, and
sometimes the SWR range that the transmitter will handle.

Modern PAs are designed to drive a load of 50 ohms, they don't have a
source impedance of 50 ohms. If they are driven into the wrong load,
they can operated outside there safe area of operation. If the power
isn't reduced, they can be damaged.

Once again, SWR is defined in terms of SOURCE impedance and LOAD
impedance. The normal LOAD for a transmitter is one end of a piece
of coax with an antenna on the other end.

The SWR at the near end of a piece of coax may or may not be the
same as the SWR at the far end of the coax.

--
Jim Pennino