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Dummy asks about building first RF project...

D

Dave

Jan 1, 1970
0
Some time back I located the schematic (and a few of the previous builder's
notes) for an MFJ 1020A antenna tuner/RF amplifier. So I went out and
bought all the parts, and am considering spending some time actually trying
to build this puppy. Only I know nothing about working with RF, so I am
asking about the sorts of things I should watch out for and try to avoid. I
expect proper shielding is paramount, and keeping the length of component
leads short is next. What else is there? I would really like for this
thing to work as well as possible, and I don't mind spending time in that
endeavor. I am planning on hooking it up to a piece of coax connected to a
random-wire antenna for shortwave DXing. I am using coax to shield against
the noise from an AC compressor midway between the radio and the antenna.
What else would be good to do, or watch out for?

Apologies if any of this sounds really dumb, but computers and associated
peripherals is what they used to pay me to work on.

Many thanks,

Dave
[email protected]
 
M

Mantra

Jan 1, 1970
0
Dave said:
Some time back I located the schematic (and a few of the previous builder's
notes) for an MFJ 1020A antenna tuner/RF amplifier. So I went out and
bought all the parts, and am considering spending some time actually trying
to build this puppy. Only I know nothing about working with RF, so I am
asking about the sorts of things I should watch out for and try to avoid. I
expect proper shielding is paramount, and keeping the length of component
leads short is next. What else is there?

This is probably good enough to start. Most important points are: 1)
only by building stuff do you really learn anything, 2) you can learn
as much from what doesn't work as from what does, and 3) all things
that matter require practice and and persistence.
I would really like for this
thing to work as well as possible, and I don't mind spending time in that
endeavor. I am planning on hooking it up to a piece of coax connected to a
random-wire antenna for shortwave DXing. I am using coax to shield against
the noise from an AC compressor midway between the radio and the antenna.
What else would be good to do, or watch out for?

You can never properly amplify signal that isn't above the noise floor
in the first place. One good antenna is worth 10 good amplifiers.
Also noise floor is related to signal path bandwith - narrowing the
bandwidth lowers the noise floor and let's you pick-up weaker signals.
This is why many RF amplifiers are tuned rather than
wideband/broadband.
Apologies if any of this sounds really dumb, but computers and associated
peripherals is what they used to pay me to work on.

Not asking when you have a question is the only thing dumber - ignore
the trolls on this list that will say otherwise - we have more idiots
here now than we had 15-20 years ago.

Be aware that computers are massive RFI sources especially in the SW
bands. Eventually some test equipment may be useful to invest in if
you analog/RF hobby takes off - having the right instrument at the
right time can save lots of hours and frustration.

MM
 
F

Fred Stevens

Jan 1, 1970
0
Dave said:
Some time back I located the schematic (and a few of the previous builder's
notes) for an MFJ 1020A antenna tuner/RF amplifier. So I went out and
bought all the parts, and am considering spending some time actually trying
to build this puppy. Only I know nothing about working with RF, so I am
asking about the sorts of things I should watch out for and try to avoid. I
expect proper shielding is paramount, and keeping the length of component
leads short is next. What else is there? I would really like for this
thing to work as well as possible, and I don't mind spending time in that
endeavor. I am planning on hooking it up to a piece of coax connected to a
random-wire antenna for shortwave DXing. I am using coax to shield against
the noise from an AC compressor midway between the radio and the antenna.
What else would be good to do, or watch out for?

Apologies if any of this sounds really dumb, but computers and associated
peripherals is what they used to pay me to work on.

Many thanks,

Dave
[email protected]

Hi Dave,

I recommend that you read the Linear Technologies Application note
number 47 by Jim Williams - its a "must read" for RF beginners.

You can find it here:

http://www.linear.com/pub/document.html?pub_type=app&document=50

Regards,
Fred.
 
D

Dave

Jan 1, 1970
0
D

Dave

Jan 1, 1970
0
Mantra said:
This is probably good enough to start. Most important points are: 1)
only by building stuff do you really learn anything, 2) you can learn
as much from what doesn't work as from what does, and 3) all things
that matter require practice and and persistence.


You can never properly amplify signal that isn't above the noise floor
in the first place. One good antenna is worth 10 good amplifiers.
Also noise floor is related to signal path bandwith - narrowing the
bandwidth lowers the noise floor and let's you pick-up weaker signals.
This is why many RF amplifiers are tuned rather than
wideband/broadband.


Not asking when you have a question is the only thing dumber - ignore
the trolls on this list that will say otherwise - we have more idiots
here now than we had 15-20 years ago.

Be aware that computers are massive RFI sources especially in the SW
bands. Eventually some test equipment may be useful to invest in if
you analog/RF hobby takes off - having the right instrument at the
right time can save lots of hours and frustration.

MM

Thanks for the encouragement and feedback. I have an o-scope, a freq
counter and a DMM that also measures capacitance up to 220 uF. Any ideas on
what else I might evenatually need? I finally got my electronics shop set
up again after 20 years just this spring. I am thinking about buying a RLC
meter, to help with setting up the antenna for my shortwave radio. (I am
currently using a 50' random wire that is just thrown up on the roof.)

Oh, and I learned to ignore trolls a long time ago. Never wrestle with a
pig - you both get dirty, but the pig likes it.

Take it easy...

Dave
[email protected]
 
M

Matt J. McCullar

Jan 1, 1970
0
Good for you, Dave! Here's a man who thinks for himself.

For many years I always avoided RF hobby circuits whenever possible because
I was afraid I'd mistune something or not wind a coil properly and the
circuit would never work properly. But when I finally bit the bullet and
tried a few simple RF circuits, I became more comfortable with low-frequency
and high-frequency circuits (never could bring myself to the frustration
level of microwave circuitry, though).

One thing I learned that I never saw mentioned anywhere was this: Your RF
circuit must be *mechanically* stable. If anything can physically move or
flop around, you will notice this in your RF signal. I once built a QRP rig
(low-power Morse code transmitter) and wound the coil around a plastic film
can. But I did not glue down the plastic film can... it could move around a
little on the circuit board. Therefore, every time I moved the circuit
board, my output frequency would noticably drift. If I tapped it with my
finger, I could hear a distinct "poink" in the output signal on a radio
receiver. After I hot-glued the thing, that problem went away. Best if
your coil forms are much, much stiffer than that, too.
 
D

Dave

Jan 1, 1970
0
Matt J. McCullar said:
Good for you, Dave! Here's a man who thinks for himself.

For many years I always avoided RF hobby circuits whenever possible because
I was afraid I'd mistune something or not wind a coil properly and the
circuit would never work properly. But when I finally bit the bullet and
tried a few simple RF circuits, I became more comfortable with low-frequency
and high-frequency circuits (never could bring myself to the frustration
level of microwave circuitry, though).

One thing I learned that I never saw mentioned anywhere was this: Your RF
circuit must be *mechanically* stable. If anything can physically move or
flop around, you will notice this in your RF signal. I once built a QRP rig
(low-power Morse code transmitter) and wound the coil around a plastic film
can. But I did not glue down the plastic film can... it could move around a
little on the circuit board. Therefore, every time I moved the circuit
board, my output frequency would noticably drift. If I tapped it with my
finger, I could hear a distinct "poink" in the output signal on a radio
receiver. After I hot-glued the thing, that problem went away. Best if
your coil forms are much, much stiffer than that, too.

Thank you for reminding me of this particular pitfall. I have seen this in
action, but forgotten it. Much appreciated.

Dave
[email protected]
 
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