RE: [AMRadio] BC-610 terminating impedance

[John Coleman ARS WA5BXO]

        I may have missed something here amongst all the messages and I
really am not too familiar with the BC610 but I thought it had variable
loading with an adjustable 3-4 turn link.  If so, it should be able to
load into a fairly wide range of loads.  On my variable link rig I can
go down to about 10 ohms load by pulling the link out to maintain the
proper plate current, but with the link in all the way the proper
loading (indicated by plate current)is reached when the load is about
100 ohms.  Of course if the load is 10 ohms and the link pulled out for
proper plate current I need to be very careful not to pour the coals on
with the plate voltage or the link will over heat.  I have been able to
load to proper plate current with a 300 ohm load but I had to parallel
tune my link instead of series tune.  

        As for SWR bridges, I think many people get the properties of
loads and sources mixed up.  All the SWR meters I have measure the match
between the line and the load and they don't give a hoot about the
source as long as there is no reactance in the line from the source.

        SWR is defined as the ratio of transmission line to load.  Not
source to line.  

        Given the scenario of a XMITER (say 100 watts) that has been
adjusted to match a 50 ohm dummy load on a 1/2 wave length of coax cable
and the SWR meter is at the load end of the cable.  If the load
resistance is increase to 100 ohms the SWR meter should show 2:1 and the
transmitter should have 1/2 output RF current, Hence 1/2 power.  If the
transmitters load is adjusted to compensate and bring the RF current and
plate current back up to normal it will be putting out 100 watts again.
But the SWR meter should still show 2:1.  The 100 ohm load will get all
must a hot as the 50 ohm load and in many cases you will not be able to
tell the difference in temperature.  If there is a slight difference it
will be caused by the slight increase in loss of the transmission line
due to the SWR of 2:1.  This is generally not of any consequence on 80
or 40 meters.  For the most part the load is still getting the 100 watts
even though it is 100 ohms instead of 50 ohms.    

        By the same token many SWR meters that are not at the load end
of the cable may not give the correct reading and especially if the
source is not a resistive output as most transmitters are not unless
they or pre adjusted with the desired load attached with a very short
cable.

        The reason I said a 1/2 wave length of coax in the above
scenario, is because that with a 1/2 wave length (must take in effect of
slow velocity of coax) the value of load resistance is reflected at the
source.  At a 1/4 wave length is either multiplied of divided by the SWR
depending on whether the load is greater that the line characteristic
impedance or less than it.

        Of course if the SWR is 1:1 then it doesn't matter how long the
line is or where you measure it.

        The only need I have found for an SWR meter is, as an indicator
of the tuning of an antenna tuner for random length wire or balanced
line doublets.  Also many solid state radios have no tuning built in and
can only work right into a 50 ohm non reactive load.  In this case a PI
type tuner for coax fed antennas is very useful for fine tuning to the
proper load. (Basically, it takes the place of an internal tune and load
procedure, as older tube type radios had.)  The SWR meter is a good
indicator to use when it is placed at the input of the tuner and the
transmitter has been pre-tuned into a 50 ohm dummy load.  When the
antenna tuner is adjusted correctly, as indicated by the SWR meter, the
transmitter will be loaded just as it was with the dummy load and will
not have to be retuned.

SEE http://www.qsl.net/wa5bxo/swr/swr.htm

John,
WA5BXO