[Icom] Summary of 60 Meter Performance of IC-765 (long)

[George, W5YR]

  I have completed several  tests on my 765 and present the following
results for those who might be interested in using a 765 on 60 meters. The
765 is typical in many respects of equipment of the late 80's and early
90's. All measurements were made from computer-generated transmit audio
signals introduced via Pin 4 of ACC1, thereby avoiding any effects of the
microphone preamp, etc.

As most 765 owners know, snipping a single diode opens up the transmitter to
the entire tunable range.

My unit, Serial No. 2437, purchased in August 1990 provides the following
performance:

PSK31
-------

Using the suggested dial frequency of 5346.5 kHz USB, 1500 Hz below the
channel center frequency of 5348 kHz, the 765 produces 40 watts PEP output
from a PSK31 IDLE two-tone signal with a visually  undistorted r-f envelope
waveform as viewed on a scope.

The corresponding average power is 20 watts which confirms the lack of
distortion. More power can be obtained but I would refrain from operating at
more than 60 watts PEP. I have not measured IMD at either of these levels,
but experience with the scope and wattmeter suggest that the IMD will be in
the -35 dB range or better.

SSB
----

The following occupied bandwidth (power level vs frequency) table was
obtained on 20 meters by feeding sinewave signals of the indicated frequency
into the ACC1 transmit audio input (Pin 4). Power output was set to be 100
watts at 1000 Hz as a reference. All output power readings were taken with a
Bird 43 wattmeter using slugs previously calibrated against an HP 436A
precision power meter.

Frequency        Power Output            Reference
      Hz                     watts                        dB

1000                        100                            0

  420                          50                           -3

  365                           25                          -6

  273                             5                         -13

  205                              0.25                   -26

1000                            100                          0

2800                              50                         -3

2820                               25                        -6

2860                                 5                      -13

2910                                 0.25                  -26

Note that the commonly used "half-power bandwidth" - measured at the -3dB
levels - is 420 Hz to 2800 Hz or 2380 Hz. However, the FCC defines "occupied
bandwidth" as the range between the frequencies where the signal level
is -26 dBc, the reference being the level in the passband.

For the 765 the "FCC" occupied bandwidth, on my unit at least, runs from 205
Hz to 2910 Hz for a total bandwidth of 2705 Hz.

We must be very careful in interpreting these results. We have just stated
that the actual measured occupied bandwidth is only 2705 Hz while we are
allowed a 2800 Hz channel bandwidth. It would appear that we are legal if we
do, in fact, drive the 765 with audio signals ranging from 205 Hz to 2910
Hz.

However, that 2705 Hz bandwidth does not simply lie within the channel
"somewhere." Very specifically, it extends from 205 Hz above our suppressed
or dial frequency of 5346.5 Hz, or  at 5346.705 Hz on the low end, to 5346.5
Hz plus 2910 Hz or 5349.41 Hz, on the high end.

So, taking my 765 as an example,  we find that if we do use modulation
frequencies from 205 to 2910 Hz, our transmitted signal will extend from
5346.705 kHz to 5349.41 kHz. Is this legal?

The FCC defines the total channel width as 2800 Hz centered at 5348 kHz.
This means that the channel boundaries are 5348 - 1.4 = 5346.6 Hz on the low
end and 5348 + 1.4 =  5349.4 kHz on the high end.

And so the answer is - not quite! Our lower transmitted bandwidth limit is
105 Hz higher than the low boundary but our upper limit is actually 10 Hz
above the channel boundary.

Now, that is close enough to call attention to the accuracy of my
measurements since I could be off only a few Hz on the high end - where it
counts - and the upper limit would just fit within the channel. But, let's
work with what we have, with the knowledge that measurement tolerances are
involved.

So, we are left in an individual decision situation. Each of us should
either measure the occupied bandwidth of their transmitter as I did and/or
just play it safe and do whatever it takes to restrict both the low and high
end of the modulation frequency range to buy some headroom and ensure that
we do not crowd the upper channel boundary.

Remember that with a rig like the 765 and most like its generation, we have
no external control over the transmit bandwidth, as we do in the PRO and
PRO2 for example. So, our only recourse to be on the safe side is to limit
the audio frequency range to gain a few hundred Hz safety margin.

(The 765 transmit bandwidth is determined by the properties of the FL-30
filter operating at about 9 MHz and the FL-96 at 455 kHz.)

There is a "fix" to this problem, but one which I fear will cause even more
problems than it might cure. That is, simply set the dial lower in frequency
than 1500 Hz below the channel center frequency by a sufficient amount to
squeeze the actual transmitted bandwidth into the channel limits.

If we do that we can drop the dial down to 5346.45 kHz. This will place the
lower -26 dB point at 5346.655 kHz while the upper -26 dB point will reside
2705 Hz higher or at 5349.36 kHz.

Result? Our 2705 Hz bandwidth is now safely placed within the allotted 2800
Hz channel with 55 Hz to spare on the low end and 40 Hz headroom on the high
end. Close, but legal - if everything is completely accurate. Not much room
here for error . . .

So, what is the problem? Well, if I set up my 765 like that and you set up
your Brand X rig to the dial frequency  recommended for that channel,
5346.5 kHz, then my audio will differ from yours, and vice versa, by the
difference in dial frequencies, assuming complete accuracy of calibration.
That means that I will sound 50 Hz "off frequency" to you.

So, either you will change your dial out of habit or ask me to change mine.
Either action could easily put one or both of us "out of band." And in this
case, I suspect that our channel neighbors are going to be looking very hard
for the slightest departure from our strict frequency assignments.

The only real solution to all this that I can see is for everyone to agree
to set their dials 1500 Hz lower than the channel center frequency, as
recommended by ARRL and as practiced in England, and hope that all our dials
are fairly closely calibrated.

Then run our transmit audio as narrow as we can and still retain
intelligibility. For a rig like my 765 - again assuming that my measurements
are reasonably accurate - I am right on the edge of being legal if I use the
full 205 to 2910 Hz audio range.

I think that we will all have to make a concerted effort to NOT twiddle the
dial to bring in a QSO partner "on frequency" if he/she sounds a little high
or low. The FCC didn't leave us much wiggle room here and a 50 Hz shift up
or down in return for a more natural sound could well put part of our
transmitted signal out of the assigned channel.

 This has been an overly long posting , as is usual when I get wound up on
something, but I wanted to be sure to emphasize the challenge that we have
been given to operate in these narrow channels with amateur equipment that
was never designed to be used in such a fashion, although obviously it can
be.

Between now and July 3 midnight local time, let's all spend a little time
measuring our rig's transmit bandwidth and do whatever it takes at the audio
end to ensure that we do not inadvertently slip outside the channel.

If I have made any errors here or if something is unclear, please let me
know. Privately if you feel that your comments are not of general interest
and to the list if they are. I hope that this information will encourage
those with appropriate equipment to make use of our new 60 meter band. I
also hope that it will make everyone aware of the importance of proper dial
frequency selection and audio band limits.

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr@att.net>