Litz wire for AM ferrite Rod Antenna?

[YD]

Late at night, by candle light, John Larkin

Yes, but you have to be careful that nothing overloads in the
front-end, or in the first IF stage, from that 50 KW monster. A decent
antanna Q helps some. Fets are really good for avoiding nonlinearity.

So, low-gain jfets or mosfets in the front end and maybe the first IF,
and pile up selectivity and gain in later IF stages. Manual stage gain
pots, rather than AGC, would be fun.

John

Ack, I have a 200 kW 600 kHz transmitter just over a kilometer away.
Great for RFI immunity testing.

- YD.

 

[John Larkin]

Late at night, by candle light, John Larkin


Ack, I have a 200 kW 600 kHz transmitter just over a kilometer away.
Great for RFI immunity testing.

- YD.

We look up at this beast from our back window. Something like 22
megawatts of AM, FM, TV, and HDTV...

http://en.wikipedia.org/wiki/Sutro_Tower


and we're in a wooden building... no shielding at all. All the scope
traces are fuzzy.

John

 

[john jardine]

We look up at this beast from our back window. Something like 22
megawatts of AM, FM, TV, and HDTV...

http://en.wikipedia.org/wiki/Sutro_Tower


and we're in a wooden building... no shielding at all. All the scope
traces are fuzzy.

John

Ugh!.
Thought I had it bad with an interfering 10uV of line scan noise from the PC
monitor a few feet away. Even considered seeing if the LCD types were any
better. Clearly I'm living on easy street.

How on earth do you cope when designing low level stuff stuff, a screened
room any worth?.

 

[John Larkin]

Ugh!.
Thought I had it bad with an interfering 10uV of line scan noise from the PC
monitor a few feet away. Even considered seeing if the LCD types were any
better. Clearly I'm living on easy street.

How on earth do you cope when designing low level stuff stuff, a screened
room any worth?.

We design it to be EMI hard, which you may as well do anyhow.
Multilayer boards, solid ground planes, signal bypass caps and ferrite
beads, and no low-level opamp type things directly exposed to the
outside world. The RF is terrible here, but less intense than a CE
suceptability test or a cell phone nearby. And an EMI hard design
tends to have better ESD resistance and, actually, just be more
reliable.

We do occasionally use an old analog, even tube-type scope, to see
low-level stuff. The digital scopes fuzz up seriously and there's not
much you can do about it. Interestingly, the 12 and 20 GHz sampling
scopes are very clean, probably because they are all 50 ohm coax and
SMA connectors.

We do have an old unused bathroom we may convert to a screen room so
we can do crude radiated emissions tests. That would be an interesting
project.

John

 

[Bill Bowden]

Late at night, by candle light, John Larkin
<[email protected]> penned this immortal
opus:







Ack, I have a 200 kW 600 kHz transmitter just over a kilometer away.
Great for RFI immunity testing.

Is that KOGO clear channel (600Khz) in San Diego?
Up here in LA area, we have clear channel KFI on 640KHz.
They both carry Coast to Coast with Art Bell, George Noory,
etc. Some of the stuff is pretty good, but I stay away from the remote
viewing, flying saucers, "out of body experiences" and shadow people
stuff. Some of the callers ghost stories are very original. I like
good story tellers, even if it's BS.

-Bill

 

[YD]

Late at night, by candle light, "Bill Bowden" <[email protected]>
penned this immortal opus:

Is that KOGO clear channel (600Khz) in San Diego?
Up here in LA area, we have clear channel KFI on 640KHz.
They both carry Coast to Coast with Art Bell, George Noory,
etc. Some of the stuff is pretty good, but I stay away from the remote
viewing, flying saucers, "out of body experiences" and shadow people
stuff. Some of the callers ghost stories are very original. I like
good story tellers, even if it's BS.

-Bill

Nope, it's Radio Gaucha in southern Brazil. When they installed it
about 15 years back you could hear it on the telephone! Great for
soccer fans but a pain for everybody else. Then they had some crews
running around placing filters and what-not all over town and it's
pretty much under control now. It still gets in if you're not careful
and crystal radios are a waste of time.

- YD.

 

[Rich Grise]

We look up at this beast from our back window. Something like 22
megawatts of AM, FM, TV, and HDTV...

http://en.wikipedia.org/wiki/Sutro_Tower

and we're in a wooden building... no shielding at all. All the scope
traces are fuzzy.

Doesn't it have a pattern something like this?
("side" view): http://www.marcspages.co.uk/tech/antgain2.gif

IOW, aren't you kind of "in the shadow"?

Cheers!
Rich

 

[John Popelish]

I measured the Qs of a couple ferrite rods, one with litz wire and the
other with solid wire of about the same AWG. Both Qs measure around 80
unloaded. I don't see any difference using litz wire.
I used a single loose turn of wire to couple a generator to the LC
circuit at 533KHz. The 3dB rolloff points occur at around 529KHz and
536KHz for a bandwith of about 7 Khz and Q of 533/7 = 76.

The rod with the litz wire is loaded by the gate of a jfet, so I
believe there is no load, the other rod had no external loads
connected.

Not very impressive results, but the bandwidth seems reasonable, and
the audio quality is good using a little 2.5 inch speaker.

Sounds pretty reasonable to me. With that Q, I suspect the
limit is more the ferrite than the wire type.

How much of the rod's length is covered with the coil?
Turns in the middle third produce higher Q than turns at the
ends.

Is the wire wrapped right on the rod, or over some sort of
insulating tube? Spacing the wire about a wire diameter or
so from the rod can improve the Q at the upper end of the band.

 

[Bill Bowden]

I think you are interpreting this correctly.  Keep in mind
that this is the Q of the coil, unloaded by any receiver
circuit.  If the circuit adds a significant load, the Q
differences for a tuned antenna would be a smaller ratio
different.  22 AWG is also pretty heavy wire for a typical
antenna coil.  With smaller wire, the Qs would be smaller
but closer.

I measured the Qs of a couple ferrite rods, one with litz wire and the
other with solid wire of about the same AWG. Both Qs measure around 80
unloaded. I don't see any difference using litz wire.
I used a single loose turn of wire to couple a generator to the LC
circuit at 533KHz. The 3dB rolloff points occur at around 529KHz and
536KHz for a bandwith of about 7 Khz and Q of 533/7 = 76.

The rod with the litz wire is loaded by the gate of a jfet, so I
believe there is no load, the other rod had no external loads
connected.

Not very impressive results, but the bandwidth seems reasonable, and
the audio quality is good using a little 2.5 inch speaker.

-Bill

 

[Bill Bowden]

Sounds pretty reasonable to me. With that Q, I suspect the
limit is more the ferrite than the wire type.

How much of the rod's length is covered with the coil?
Turns in the middle third produce higher Q than turns at the
ends.

Is the wire wrapped right on the rod, or over some sort of
insulating tube? Spacing the wire about a wire diameter or
so from the rod can improve the Q at the upper end of the band.

The rod with litz wire is a original unit from a 1972 Radio Shack AM/
FM modle. The rod is 4.75 inch long by 3/8 diameter with a winding
length of about 1 inch (on a paper form) situated about 1/3 the way
from one end to allow fine tuning adjustment by sliding the winding
slightly up an down the rod.

The other rod with solid wire, I wound myself. It's a little shorter
at 4 inches (same diameter) and the winding is similar 1 inch long
situated in the middle of the rod on a similar paper form.

I'll run another test at the upper end of the band to see what happens
there and report results.

I etched the circuit board with a solid ground plane on one side to
avoid possible oscillation problems, but I found I need to space the
antenna rod about 1/2 inch away or more to avoid losing signal. This
makes the box size bigger than I planned, but I can live with it.
Guess it's not a good idea to use a solid ground plane with little AM
radios?

-Bill