Wideband FM QSOs
using
Gunn transceivers
January 22, 2005
Clint, KA7OEI, with the WFM 10 gig
setup. Click on picture for a larger version.
Most activity on the 10 GHz band is undertaken using Gunn
transceivers.
These units are self-contained transceivers, capable of both transmit
and
receive at the same time by virtue of the fact that the transmit
oscillator
is also the receiver's local oscillator.
It should go without saying that one cannot receive on the same
frequency
as one is transmitting, so a standard IF (Intermediate Frequency)
of 30 MHz has been chosen. While this frequency could be
practically
anything in the HF or low VHF range, 30 MHz was chosen because it is
still
in the HF range, but low enough that the mixer diode's output is still
good. An additional advantage is that there are not usually any
strong
signals present at or near 30 MHz. In the past, initial activity
was carried out on the bottom end of the FM broadcast band, at about
87-88
MHz but nowadays, every available slot on the FM band is used.
On 10 GHz, two types of Gunn transceivers are commonly used:
The so-called "Gunnplexers." This is a brand name, but it
generally
refers to the fancier gunn transceivers that not only have a mixer
diode,
but can be electronically tuned using a built-in varactor. This
electronic
tuning allows 50-100 MHz of frequency adjustment allowing tuning and
selection
of either of the two standard wideband FM frequencies.
Microwave "door openers." These typically have a mixer
diode, but
do not have electronic tuning. Fortunately, varying the Gunn
voltage
(typically from 7 to 9 volts for 10 GHz Gunn sources) will also vary
the
frequency by 5-15 MHz, the precise amount depending on a number of
factors.
While this is enough to allow modulation of the transmitted signal, it
does not allow tuning between the two standard wideband FM frequencies
so, unless test equipment is at hand, the mechanical tuning screw
cannot
be accurately adjusted to another frequency.
There are also some types of door openers that have appeared
that do not
have a mixer diode: Ron, K7RJ was looking at one of these, trying
to figure out how they could possibly work, until he realized that the
circuit extracted the mix frequency from the Gunn diode itself:
Apparently,
the return signal (or a received signal) actually superimposes itself
on
the Gunn voltage supply line. Preliminary tests indicate that
while
this scheme works, it does not result in as good as sensitivity as
using
a mixer diode, but it is possible that further refinements (such as
parallel
L/C networks in series with the power supply lead, better receiver
impedance
matching, amplification, etc.) may be able to improve performance.
Top and Middle: The
vantage point
of Ron, K7RJ, above the valley fog. Bottom: Ron, working N7MLD in DN31 from the traverse
range above Draper. Click on picture for a larger version.
Using the
transverter
(as
test equipment) during the contest:
At the time that we did this, my Gunn transceivers were of the "door
opener" type, being Solfan
units
manufactured in the late 1970's and used, until the mid 80's, in the
Dee's
Hamburger restaurant in Bountiful. Not having a varactor diode,
I'd
tuned the one that I take with me in the field to 10250 MHz, relying on
the fact that everyone else was either on 10280.
During the contest, the transverter proved to be extremely
useful.
Owing to the cold temperatures, my Gunn transceiver drifted up several
MHz, putting the intended calling frequency of 10250 MHz (paired with
10280
MHz) outside its tuning range. Fortunately, I'd brought along a
service
monitor.
With the transverter's wide range IF capability, I was able to
generate
a signal at 344 MHz that, when put into the transverter, caused it to
emit
a signal at 10280 - the receive frequency when using a 30 MHz IF (while
transmitting at 10250.) Using this scheme, I was able to retune
the
Gunn transceiver at will to verify that I was actually on the intended
frequency - something that came in very handy,
especially
when I worked others whose Gunn oscillators had also drifted off
frequency.
From my location (along Utah highway 111, in line with 6200 south) I
was able to work most of the others that were on 10 GHz that day using
Wideband
FM
on
the Gunn Transceiver:
Dale, WJ7L, near-ish Joe's Rock on Wasatch Blvd (a distance of
approximately
13.25 miles.)
Ron, K7RJ, on the traverse range between Utah and Salt Lake
counties (a
distance of approximately 15.5 miles.)
Charlie, N7MLD, who was above the Utah State capitol (approx.
13.75
miles)
and later moved to the International Center, near the airport (approx.
7 miles.)
I was also able to hear John, K7JL, (approx. 12 miles) but he was
unable
to hear me. It was clear, from the sound of the signal, that
there
was ground obstruction at my end. Later, I moved about 100 feet
to
the east to clear this obstruction, but some unknown problem seemed to
stop his Gunn transceiver from working. (I could work Ron from
that
location, but I neither Ron or I could hear John at that point.)
After the majority of the activity had subsided, I went to Ron's
location
above Draper to see his setup. From there Ron continued to work
Charlie,
N7MLD from various grid squares in Davis county (including DN41 and
DN31)
on 10 GHz, with the longest distance being approximately 45
miles.
Even at this distance, good signals were noted "horn-to-horn" (e.g. no
dish on either end.)
