John Sandin wrote:
It's me again. To refresh your memory, I'm building the "Dourg DeMaw
classic" transmitter as shown in the June 1967 QST. Details are at:
http://www.kcnet.com/~oyster/transmi...ansmitter.html
Things are working much better. I'm nearly ready to go back on the
air with it for real. I did a test, separating transmitter and
receiver by several miles, with identical dipoles at each end, and my
helper said that she received a nice strong signal at the fundamental
(7125 kHz). The problem is, she also heard a signal about half as
strong at a subharmonic (3562 kHz). Does anybody out there know what
might typically cause this to happen with a transmitter like mine? It
didn't occur to me to try a random wire at the receiving end as a
comparison. My receiver is an Icom R71A.
My homebrew lowpass filter is working. It's in its own box with
SO-232 connectors. I've verified that it prevents TVI, and also
silences any emissions at 2x, 3x, and 4x the fundamental. Without
it, I get a lot of TVI on a TV at home.
But I want to get rid of that 3562 kHz signal, which is right in the
80 meter band. I've had success so far by just experimenting with
rerouting ground connections and making the layout more efficient, so
I feel that the answer might be in that direction. Any advice would
be welcome. Please have a look at my website. Thanks!
-John Sandin KC0QWE
Remove the "T" to respond by e-mail
First off, beautiful job!
Can you put a dummy load across L2, leaving the amp out
of the circuit, and still hear the 3562? If you don't,
but can hear the 7125, the trouble would be in the amp.
I should think you can try that using the R71A and won't
need the miles of separation.
But - I think the problem may be RF getting into the power
supply. If you can power it from a 12 volt battery and
it doesn't create that 3562, that proves it.
Either way, you do need to change the design. The old
classic ran off a battery. You're using a power supply,
and there is a direct path for the RF to enter the supply.
Add a choke between pin 1 of the power supply connector and
the junction of the key and the tune/operate switch. That way,
RF can't flow into the supply. A 1000uH choke would have
2*pi*3.562*1000 (~223K) ohms reactance at 3.562. If there
is not enough room in the rig for a 1000uH of sufficient
current, you could still get over 70 K ohms reactance from
a 330 uH at about half the physical size. You could also
add a .1 uf to gnd across the junction of the choke and
pin 1. Those values aren't critical - you could use a
larger choke for even greater isolation, and a .01 if you
don't have a .1 You do want a choke that will carry the
current. A 500 mA choke would be more than enough for that.
Your transmitter probably draws around 300 mA. You can get
a 1000uH, 800 mA choke from Mouser for 85 cents or a smaller
330 uH 500 mA for 80 cents.