On 4m I
am currently running 160W out to a 6 ele DK7ZB
The rig is an Elecraft
driving an OZ2M transverter which in
turn drives a homebrew PA
using the SD2931-10 device.
What's wrong with FM?
On 6m I use the K3 either barefoot or through an Acom 1000. The antenna is a 5 ele
This is a good design, definitely recommended, see full details here
. There is a kit for
the basic transverter developing about 100mW and also for a 25-30W PA
using a Mitsubishi module. Alternatively you could buy the module in UK
from GH Engineering
and buy/build the
rest from information on Bo's website (as I did).
Here's a view of the completed transverter
and on the right an inside view. Click on the images to see
them full size.
Here's the 25w PA before mounting the module
See this page
for antenna changeover info.
found the temperature stability of the crystal oscillator was not so
good and I added a Kuhne
crystal heater type QH40A.
This has improved the stability a
great deal and the drift of the whole system with my K3 is now less
There is a concern that transverters using a 42MHz local oscillator can
produce spurious emissions just below the 70MHz band. This arises from
3 x 42 =126MHz less 2 x 28 = 70. To test this, I set the
transceiver to 28.001, and looked at the spectrum from 69.997 to
70.002. Any spurious should appear at 69.998.
The spectrum is
shown below - 70.001 is rendered as 6000Hz. There is no
sign of any signal on 3000Hz - a suppression of more than 80dB.
This was my second 4m PA project of recent years - the first used a dual
The SD2931-10 design was taken from (a) the GI0GDP
on the 4m website for a 160W PA, and (b) the article by EI9GQ in RadCom (Aug 2008 p22)
for a 2m PA using the same device. These devices or near
equivalents are available on eBay from time to time at reasonable
prices. (I always felt very nervous about paying a lot for a device
which could expire in a microsecond!)
(For more power see the info from ON5VW
on a 600W PA for 6m using two SD2933.)
shows the SD2931-10 typically produces150W output
firstname.lastname@example.orgA dc input at 65% efficiency. It has about 0.5dB
compression at 170W output. I built mine without making a
pcb, but using double-sided board as shown.
Output spectrum is shown here
Points to note are the use of
mica compression trimmers on the output - there's a lot of rf current
flowing in this circuit! I started testing with a 12v supply
once tuned up I moved on to a 50v supply.
set the quiescent
current Idq = 0.25A. Trimmers now peak in a different place and with
1.4W rf input the PA gives 160W output for 4.64A or
and an efficiency of 69%. However, after testing Idq had
0.35A – so there is a need to temperature compensate the bias supply.
I incorporated a circuit to allow thermistor control of gate
voltage, see the circuit diagram
. Idq is
then set at 0.25A and drops slightly to 0.24A when hot.
The circuit was amended on 3 Feb 2011 by the addition of decoupling
around the 5V regulator which supplies the fan circuit. This suppresses
noise which otherwise can be heard in the 4m receiver!
point to note when testing PAs like this one is that typical digital
multimeters are badly affected by RF. For a reliable indication I had
to go back to a moving coil meter! I have also added a
circuit for thermostatic control of fans, which are set to
on at about 40C.
the antenna change-over relay I used a SDS Relais type S2.
is a small and quiet pc board sealed relay, chosen for its speed (6ms),
good hf characteristics and low price. It's available from Barend
The measured harmonic output is -36dB at 140MHz
and <-48dB at 210MHz and above. It is respectable but
needs a harmonic filter - the one I built is the coaxial stub filter by
G4SWX and is described on the G(M)3SEK website
(Look for "Filters:
Harmonic notch filters" under "The Best of In Practice"). I
measured its performance as follows:-
insertion loss <0.1dB
The PA and filter combination then provides at least 75dB rejection at
Further observations shortly.....
Operating cross band - a 6m bandstop filter
trying to operate cross band I found an excessive amount of noise
entering the 6m receiver from the 4m transmitter. The noise comes
from both the transverter and the PA. To overcome
this I designed a bandstop filter to go in the 4m transmitter
feedline. The filter was designed using Elsie
, a very useful
filter design program. The design is a Chebycheff
bandstop filter with series input. Parameters are 10MHz bandwidth,
order 3, 0.01dB ripple, centre frequency 50.2MHz. The design is
chosen for minimum loss at 70MHz as it needs to be put in the high
power output line.
For 19.7nH, use 10mm diam, 15mm long, 2t of 18g
For 837nH, use 10mm diam, 22mm long, 15t of 26g
For 510pF, I used 270pF || 220pF || 20pF ceramic
For 12pF, I used a 30pF trimmer adjusted for null at 50.15MHz
Predicted loss at 70MHz is 0.1dB, actual not measurable
Predicted loss at 50-50.5MHz is >40dB, actual ~30dB