RadioBanter - View Single Post - Newbie qrp homebrew transmitter; another question

November 15th 03, 12:02 AM

(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