Re: New antenna, new loggings

Tony Germanotta

Jim, I hope you feel better soon. At least with your new no-knobs antenna situation you can DX from the sickbed.

Have you thought about building one with more turns and feeding it with 300-ohm TV antenna wire? Would you be able to eliminate the internal transformer that way?  You could use a shunt to ground plug and be able to use both internal and external antennas that way. 

And farmerik, one really simple way to get the bigger loop is to use a hula hoop as a form. Folks have been doing that for years. It's especially easy with just a couple turns of wire inside and no resonating capacitor. You can often buy them for a couple bucks at toy stores these days. Some of the really cheap ones even have wooden or plastic plugs to hold them together. You remove the staples holding it together, pull it apart, run your wire through a couple times and then push it back together with the two ends on opposite sides of the plug. 

On Mar 20, 2010, at 10:44 PM, jim_kr1s wrote:


--- In, "farmerik" wrote:
> Jim - I really don't understand the theory well enough to follow exactly what you are doing. I had hoped a unit which plugged into the stock model PL-360 could be made and sold by some enterprising member for fumble fingered folks like me. I guess the PL-360 lacks too many important features, like the adjustable selectivity to interest anyone. It might be nice to try one on a TERK or the TECSUN small loop though. Now if I can only stop buying larger radios on eBay, maybe I will order more ULR's! - FARMERIK

Good luck cornering the market on radios! My system is not complicated. The existing antenna is replaced by a toroid, which is less susceptible to picking up signals than a bar. Now the radio is happy, as it sees a tunable LC network across the MW band. When you match impedances, you obtain the greatest energy transfer from one circuit to the next. Because my hoop loop has only 3 turns of wire, it has a lower impedance than the 60-turn resonator winding on the toroid. I calculated that a 15-turn winding, wound over the 60-turn winding, would be about right. It worked well, but it's always worth adding or removing turns to get the best results across the band. As it happened, on mine, 13 turns was the lucky number. I was quite surprised at the difference removing only two turns made, and the increase was uniform across the band.

Why use a 3-turn antenna? The low impedance allows use of low-impedance balanced feed line, in this case small-gauge speaker wire! Also, the quilt hoops are less than an inch wide. It would take about 15 turns to make a coil that resonated with the standard 365-pF variable cap (or the varactor in the radio). That many turns so close together would make it hard to tune the loop to frequencies higher in the band, because of the inherent "self capacitance" of the closely spaced windings. I know Tecsun is getting away with it on their small loops; you can see the windings. I commend them! My experience with high-inductance loop antennas using close spacing hasn't been good.

I don't think the PL-360 is DXer friendly. You could plug my system into the PL-360 in place of its antenna, but you'd need a small box to contain the matching transformer, and it would eliminate the one advantage of the -360, its small size.

The PL-360 apparently duplicates the tuning arrangement of the PL-300/wt, PL--310 and PL-380. An on-chip voltage-variable capacitor (VVC) automatically tunes the antenna coil on LW and MW. (As built, tuning on LW is not really happening, as the coil inductance is too low.) The Terk, Tecsun and SAT antennas contain their own tuning capacitors, which resonate their antenna coils just like the on-chip VVC in the Tecsun radios. So if you plugged one of those antennas directly into the radio, you'd have the on-chip VVC in parallel with the capacitor in the external antenna. I don't think it would work as intended on MW. I've been fighting an invasion of space-alien body snatchers. Once I can stay on my feet for more than a few minutes I'll try connecting the Terk directly to my modified PL-380 just to see.


Jim, KR1S 

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