On Fri, 14 Nov 2003 04:26:08 GMT,
(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!

This tx of yours is fixed frequency, right? If so, why not just
knock-up a simple notch filter for 3562? Or even a high pass? I know
it's not establishing the cause of the sproggie, but at least it's a
quick expedient to stop you confusing the chaps on 80m!
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

I don't think filtering the spur is a good answer; it's sort of brute force.
Better to find the cause and eliminate it.

If the oscillator is on 40 meters it would be strange to find a subharmonic.
On the other hand, if it's on 80 and there is a doubler, that could explain
it. (I haven't looked at the diagram.)

Assuming the oscillator is on 40, the 80 meter spur might occur because the
oscillator is squegging, or cutting out, on alternate cycles. Or, if it's a
crystal, the crystal might be fractured.

If the oscillator is on 80, reduce its amplitude and investigate the Q of
the succeeding circuits.

An excellent approach would be to look at the waveforms with an oscilloscope
to see just what's going on there.

73, Bob K6DDX

On Fri, 14 Nov 2003 08:36:36 -0800, "Bob" wrote:

An excellent approach would be to look at the waveforms with an oscilloscope
to see just what's going on there.

What would a 7Mhz waveform look like with a 3.5 sub-harmonic say 6 db
down? Does it display as two seperate, distinct waveforms or one
distorted one?

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Fri, 14 Nov 2003 08:36:36 -0800, "Bob" wrote:

An excellent approach would be to look at the waveforms with an oscilloscope
to see just what's going on there.

What would a 7Mhz waveform look like with a 3.5 sub-harmonic say 6 db
down? Does it display as two seperate, distinct waveforms or one
distorted one?

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

I don't think filtering the spur is a good answer; it's sort of brute force.
Better to find the cause and eliminate it.

If the oscillator is on 40 meters it would be strange to find a subharmonic.
On the other hand, if it's on 80 and there is a doubler, that could explain
it. (I haven't looked at the diagram.)

Assuming the oscillator is on 40, the 80 meter spur might occur because the
oscillator is squegging, or cutting out, on alternate cycles. Or, if it's a
crystal, the crystal might be fractured.

If the oscillator is on 80, reduce its amplitude and investigate the Q of
the succeeding circuits.

An excellent approach would be to look at the waveforms with an oscilloscope
to see just what's going on there.

73, Bob K6DDX