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. 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.

73
73
Greg

I have a rock mite 20m qrp. 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.
Hi Greg, First of all I have never used a rock mite, but I will just give
some thoughts. If the receiver is a direct conversion type, they are bad about
picking up 60 hz hum. Does the buzz sound like 60 or 120 hz hum? The finger
touching the final increasing the hum should be a clue.
The antenna may or not be a problem. QRP swr and watt meters are not common,
you may have to look at building one.
Consider using a 20m coax fed dipole, only 33' feet long, and will work ok
20' high. This is balanced, to a degree, and will not require a tuner
Get an antenna analyzer (MFJ 259B ect.) and make sure the impedance at the
tuner input is 50 ohms, assuming that is what the rock mite wants to see.


73 Gary N4AST

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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave gave some good suggestions, but let me add one more. You might be
hearing detected video from a strong TV station. Years ago when I was
testing audio amplifiers for the little "Optimized" transceiver, I was
driven nearly to distraction by that until I finally figured out what it
was. In my case, the headphone wire was the main antenna for the video,
and a good bypass with very short leads right at the headphone jack
cured it. But in your case it could be getting in via the antenna or
power supply leads as well. You can bypass the power supply leads, but
it takes a little knowledge and care to let HF through the antenna lead
while effectively stopping VHF or UHF TV.

Winding the power supply and headphone leads a few turns on ferrite
toroids close to the rig can also be very effective. A good type of
ferrite to use is type 43, which is common, although most other kinds
will work fairly well. You'll probably want to use separate toroids for
power supply and headphone, for convenience. Keep the two conductors
together as you pass them through the core.

Here's how to identify detected video. Hold perfectly still, and see if
the buzz changes. The sound of video will change with the transmitted
picture. If you can correlate the changing sound with the picture from a
local channel, you have the smoking gun. But even if you can't, a buzz
that changes rougness and loudness while you hold still is pretty good
evidence. If you have a 'scope, you can identify video by using line
triggering. Video will look like a pattern that slowly scrolls by,
taking about 20 seconds to drift a full "cycle".

Roy Lewallen, W7EL

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. 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.

73
73
Greg