Newbie qrp homebrew transmitter; another question
 

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

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

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

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

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.

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.

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

Farhan, I took your advice about tuning out the subharmonic, although
I used a dummy load instead of a 12" wire. I did find a tuning point
at which the subharmonic went away. The article says to tune it so
that there is a minimum of illumination at the PA indicator lamp, and
a maximum of illumination at the RF indicator lamp. I found a point
where there is NO illumination at the PA indicator lamp, while the RF
lamp glows brightly. Prior to this test, I always tried to have at
least some illumination at the PA indicator lamp. Perhaps no
illumination is best for this particular incarnation of this rig,
given that it still has some problems. I have no scope and no
wavemeter (yet) and not a lot of knowledge, so I rely a lot on trial
and error, probably too much.

The tuning point described above is also the point where any
self-oscillation stops, the subharmonic is silent, and the output is
the greatest as measured via a voltmeter across the dummy load,
rectified with a diode and capacitor as suggested by KC6WDK. I'm now
getting 12 v there.

Now I'll continue to try to find the source of the subharmonic, and
also the chirp, which is still a little troublesome. I am also going
to try some suggestions made by others who responded.

I realize that I'm rather handicapped, having a 1-watt rig that uses
crystals, and having only a "Technician with Morse Code" license. I
missed passing the General exam by one question. :(

Thanks to all.


On 14 Nov 2003 16:02:58 -0800, (Ashhar Farhan)
wrote:

(John Sandin) wrote in message ...

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.


if she has received the signal across the town, then you are putting
out a pretty strong sub-harmonic. a couple of things have to be
checked up:

1) are u sure that the signal is actually due to the crystal? it might
be that the oscillator is freely running on its own. to check that,
test that the signal is stable, if u move ur hand around the coils or
touch them, the signal should not shift its frequency. if it does
that, then the crystal is not really a part of the oscillation.

2) if you have figured out that the crystal is oscillating properly,
next, you might have mistuned your PA to 3.5MHz instead of 7MHz. Try
retuning the PA, disconnect the antenna from the receiver attach a
foot of wire to the output of the tx and tune the tx for maximum
output on 7MHz. Then switch the rx to 3.5MHz and see if you can 'tune
out' the 3.5MHz trace.

these are quick fixes. the larger problem that you are faced with
really is that you don't exactly know what is going on. the above two
solutions are based on diagnosing the problem emperically.

what you really need is a wavemeter. If you have another variable cap
lying around, it might be a good idea to hook one up. It will serve
you for years. It takes about 4 components to wire it up. You can
caliberate it easily using a regular TX for 3.5 and 7. Now you can
couple the wavemeter to each of your TX coils and tune it to the
correct frequency.

if we are to assume that the crystal is actually 7MHz, i see no way
that it can give sub-harmonics in a proper design. the only culprit
could be the tuned circuits.

- farhan


-John Sandin KC0QWE

Remove the "T" to respond by e-mail

Farhan, I took your advice about tuning out the subharmonic, although
I used a dummy load instead of a 12" wire. I did find a tuning point
at which the subharmonic went away. The article says to tune it so
that there is a minimum of illumination at the PA indicator lamp, and
a maximum of illumination at the RF indicator lamp. I found a point
where there is NO illumination at the PA indicator lamp, while the RF
lamp glows brightly. Prior to this test, I always tried to have at
least some illumination at the PA indicator lamp. Perhaps no
illumination is best for this particular incarnation of this rig,
given that it still has some problems. I have no scope and no
wavemeter (yet) and not a lot of knowledge, so I rely a lot on trial
and error, probably too much.

The tuning point described above is also the point where any
self-oscillation stops, the subharmonic is silent, and the output is
the greatest as measured via a voltmeter across the dummy load,
rectified with a diode and capacitor as suggested by KC6WDK. I'm now
getting 12 v there.

Now I'll continue to try to find the source of the subharmonic, and
also the chirp, which is still a little troublesome. I am also going
to try some suggestions made by others who responded.

I realize that I'm rather handicapped, having a 1-watt rig that uses
crystals, and having only a "Technician with Morse Code" license. I
missed passing the General exam by one question. :(

Thanks to all.


On 14 Nov 2003 16:02:58 -0800, (Ashhar Farhan)
wrote:

(John Sandin) wrote in message ...

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.


if she has received the signal across the town, then you are putting
out a pretty strong sub-harmonic. a couple of things have to be
checked up:

1) are u sure that the signal is actually due to the crystal? it might
be that the oscillator is freely running on its own. to check that,
test that the signal is stable, if u move ur hand around the coils or
touch them, the signal should not shift its frequency. if it does
that, then the crystal is not really a part of the oscillation.

2) if you have figured out that the crystal is oscillating properly,
next, you might have mistuned your PA to 3.5MHz instead of 7MHz. Try
retuning the PA, disconnect the antenna from the receiver attach a
foot of wire to the output of the tx and tune the tx for maximum
output on 7MHz. Then switch the rx to 3.5MHz and see if you can 'tune
out' the 3.5MHz trace.

these are quick fixes. the larger problem that you are faced with
really is that you don't exactly know what is going on. the above two
solutions are based on diagnosing the problem emperically.

what you really need is a wavemeter. If you have another variable cap
lying around, it might be a good idea to hook one up. It will serve
you for years. It takes about 4 components to wire it up. You can
caliberate it easily using a regular TX for 3.5 and 7. Now you can
couple the wavemeter to each of your TX coils and tune it to the
correct frequency.

if we are to assume that the crystal is actually 7MHz, i see no way
that it can give sub-harmonics in a proper design. the only culprit
could be the tuned circuits.

- farhan


-John Sandin KC0QWE

Remove the "T" to respond by e-mail