Re: PL-380 - tuning knob broke again :( (also a TP question)


Jim has lucidly explained several good reasons for using a low inductance loop and transformer (tuning range and flexible wire). The same reasons govern the historical use of low-inductance loops with HiFi receivers. Generally, high-impedance signal inputs like the Si4734 uses are great when the receiver is near the antenna. But lower impedance signals are needed when the antenna is remote from the receiver.

I agree with Jim's reasoning and would like to add two more reasons to use a transformer.

1) The transformer provides some galvanic isolation to sensitive components in the radio. The Si4734 AM input is very robust to ESD discharges _for_a_tiny_chip_, but if you bring that input to wires outside the case, more robust ESD protection circuits are highly recommended. If you look inside your radio, you'll see that the FM whip has protection diodes on the PCB to prevent you from zapping the IC inputs.

2) The transformer provides a balanced-to-unbalanced (balun) signal conversion. Without this, the loop sorta looks like two antennas at once: an ideal loop plus a big hunk of wire hanging on a tuned, high-impedance node. This has a tendency to pick up noise and ruin the loop's nulling capabilities. Even the PL-380's internal loopstick is affected a little by this issue.

I once connected a high-Q inductor to the AM input of the Si4734 without the loopstick. I was mainly experimenting with the chip's tuning function, but I noticed that with just the tuned inductor, the radio could pick up several stations. So I connected the high-impedance side of the tank to the FM whip antenna. The resulting signals were huge -- reading over 90 dBuV (PL-300WT)! A loop is not the only way to pick up AM signals. Whips can be excellent too, but don't provide good nulling (with a single whip), and need a good ground and balun to avoid picking up man-made noise.


--- In ultralightdx@..., "jim_kr1s" <jkearman@...> wrote:

--- In ultralightdx@..., "farmerik" <farmerik@> wrote:

Would using 300 ohm or 450 ohm ladder line allow a more turns or a
larger loop to match the input on the PL-360? It would have to go
through the 1/8th inch plug, but most of the lead could be higher, is it

I have no experience with the PL-360, but my intuition is that the
antenna is 240 uH or thereabouts. That means it appears right across the
varactor. Two wires side-by-side have capacitance. I forget the
capacitance-per-foot of 300-ohm twinlead but you could look it up. But
-- that capacitance will appear in parallel with the tuned circuit, and
will increase the minimum tuning capacitance. That will likely prevent
the antenna from tuning to the top of the band.

Now, if you used a plug-in transformer and matching winding, yeah I
guess you could use twinlead with a higher inductance antenna. But why?
When you can use lightweight, flexible, cheap, and readily available
speaker wire, why use that heavy, clumsy stuff? Twinlead was invented
for use on VHF television. It's a waste on MW. I have 300 feet of it,
but I never considered using it for this purpose.

There is no reason to use a higher-inductance antenna. The idea for
this project was sparked by the SiLabs datasheet. It mentions using a 5
: 1 turns ratio transformer to accommodate those little AM loop antennas
supplied with AM/FM tuners. It turned out that my antenna had a slightly
different inductance, so I needed a different turns ratio (3.8 : 1), but
the principle is the same.

I find hanging my loops from the ceiling, over head, is much easier
than making 'legs' and finding a spot WITHOUT a radio or speaker in my
upstairs room!

Look at the Hoop Loop <> .
The hoop is solidly attached to the base with screws. You could hang it
any way you like, without the base, as long as you used non-conductive
material like monofilament or mason's string. You'll find stranded
speaker wire much friendlier than stiff twinlead if you're rotating a
hanging antenna, BTW.


Jim, KR1S <>

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