In article ,
Greg Doughty wrote:
Hello to all. I continue to return here as antenna's are the hardest thing to
figure out. We can end the cw argument by testing antenna knowledge hi hi!
Anyways, I have a rock mite 20m qrp that I built and am giving it a go with a
random wire hooked to an mfj 901b tuner. I am using 22 gauge stranded at about
16 feet. The tuner has a ground wire as well. Here is the trick . . . I have
a buzz in the headphones only at certain times. I can move the headphone wire
around and it changes the tone and amplitude of the buzz. In fact it sometimes
goes away all together. However, when I put my hand on the final in the
altoids can, it also lessons it as well as when I put my hand on the can. It
sounds like a ground problem but not sure how to fix it. I tried different
grounds but to no avail.
Direct-conversion receivers are rather notorious for being subject to
a phenomenon called "tunable hum".
According to what I've read in "Experimental Methods in RF Design",
the root cause of this problem is local-oscillator radiation and
modulation. The D-C receiver has an oscillator running at all times,
which is being fed into the mixer. Mixers have a finite amount of
isolation between the L/O port and the RF-in port, and some amount of
the L/O power is coupled back out through the RF-in port. In a simple
D-C receiver, this L/O power can be coupled right out to the antenna,
and radiated. This can cause two effects:
- It's audible on other receivers in the area, as a "birdie" at the
receiver's LO frequency.
- It can be coupled into nearby power wiring, and mixed (in the RF
sense) with the 60-cycle powerline waveform and the waveform's
harmonics by nonlinear devices hooked to the power line - i.e.
power supply rectifiers. This mixing (which is in effect a
modulation) can create RF sidebands, located 60 and 120 and 180 and
240 etc. Hz on either side of the local-oscillator frequency.
These sidebands radiate out through the power wiring, are picked up
by the antenna, and are demodulated by the D-C receiver's mixer.
In sum, then, you end up hearing powerline hum and harmonics in
your 'phones, even if the D-C receiver is powered by batteries and
has no connection at all to the power mains.
The best fix for this is to include an RF preamp or buffer between the
receiver's antenna and mixer. The benefit doesn't come from the
preamp gain - it comes from the additional isolation that the preamp
adds to the circuit, thus blocking the L/O leakage from the mixer
before it can reach the antenna and be radiated. As best as I can
recall (I don't have the schematic handy) the Rockmite does not
include such a preamp.
If your Rockmite isn't battery powered, try that. Also, try using a
balanced antenna, located further away from your listening area (and
from power wiring, etc.) - this may reduce the degree to which the L/O
leakage couples into the power lines and creates sidebands.
I do hear some small amount of buzz/hum on my Rockmite, even when
battery powered and even when hooked to an outdoor inverted-V dipole
located some distance from the building and from any power wires.
Simple D-C receivers can also be subject to strong-signal overload and
intermodulation. It's not uncommon to hear AM broadcast-band signals
on a Rockmite or Pixey or similar, due to strong-signal overload of
the mixer. There's a simple modification available for the Rockmite
which greatly reduces this problem... if I recall correctly it's a
single resistor, placed in parallel with the diodes located just
before the mixer. It's possible that your hum/buzz is coming from
intermodulation from a strong local RF source of some sort - if so,
this mod might help matters.
Another problem I am having is the swr meter I have
will not do less than 5 watts(RS) so I am trying to wing it with the tuner and
try to get the highest noise level. Once again though, if I turn the
inductance past d/e I get the hum again. Also if I turn up the antenna
adjustment, the buzz gets worse. I know this is a long one, but I am hoping
someone can help me out.
Unfortunately, the increase in buzz may indicate that you're
establishing a better match with the antenna... you'll transmit more
effectively, receive more effectively, but also leak the
local-oscillator power more effectively.
If you want a tuner better suited to QRP operation, I can recommend
the Norcal QRP (now AMQRP) BLT kit. It's a simple Z-match tuner, good
for up to about 5 watts, capable of feeding balanced antennas or (with
one additional jack and a switch) coax-fed antennas, and including a
simple resistive-bridge SWR meter which provides a useful tuning
indication with only a few milliwatts of power. It's a nice match for
the Rockmite.
http://www.amqrp.org/kits/blt/ for the kit. If you've got a
well-stocked junk box you might already have everything you need to
build a similar design. Least-common component is probably the little
plastic varicaps - you could certainly substitute broadcast-band air
variable caps if you have a couple handy.
--
Dave Platt AE6EO
Hosting the Jade Warrior home page:
http://www.radagast.org/jade-warrior
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