Re: Guy Atkin's Independent Testing of PL-380 7.5" Loopstick

Tony Germanotta

Yea, those component stereo antennas are worthless as antennas, but can be perfect to slide over a radio as an inductor to feed the result of other antennas into it. They're no better, really, than just winding a few loops of wire around the radio, but you can put the proper plug on the end and not have to worry about it.  I like to haunt thrift shops and flea markets and often find them for a buck or so. I have a couple here just waiting for the right project. 

I'm with you on the 2010 replacement with a monster internal antenna.

For an easily portable boost on my radios, I rely on something I found in an old Popular Electronics article from the 60s. An AM Booster that was just a box with a ferrite rod and tuning capacitor inside that you were to set beside a transistor radio. I eliminated the box to make it even more portable. 

I took a flat ferrite from an old scrap transistor radio (again one of those buck bin finds). If you are very careful, you can sometimes even remove the tuning capacitor without severing those fine connecting wires to the loop coil. If not, pay attention to which terminals they are attached to, these caps are usually multi-section mica or PVC models, especially on AM-FM radios. (I mark the right ones and their leads with a Sharpie, it doesn't matter which one goes to which terminal, though.) If they break off, just solder them back together when the parts are removed. 

After I do that, I hot glue the little cap onto the ferrite bar and cover the entire mess in electrical tape for protection and insulation. For a quickie, you can just tape it all together. My red tape model looks particularly fetching. Then you attach a strip of stick-on velcro on the bottom and find the sweet spot on each individual radio and put some of the other type of velcro there. (I usually use the soft felt on the radio and the rough teeth on the antenna, for comfort sake when I don't have them attached.) Now you just stick your antenna on top (or the side or the back, depending on where the internal loop is best positioned. You usually want the turns on the external loop to be right over those on the internal radio. The PL-310 with its separated turns has a wide sweet spot, other radios with tightly spaced loops are more finicky. And there are spots on some digital radios where it can induce or magnify noise, I have found. So it is best to do the sliding around before you put on the velcro.) Then you tune in your station on the radio, and tune the external antenna cap. (Often these things have a dial string pulley attached. Once you eliminate the string, I have found those large plastic pulleys make fine dial knobs. The plastic tends to be wide enough to eliminate hand capacitance effects.) It isn't nearly a DX machine like a 27" ferrite, but it often adds just enough gain to bring something out of the circuit noise. And it is very small and very portable. It also allows you to null on all angles, which sometimes is the difference especially at sundown when things are coming in on all sorts of angles.

You can use larger ferrites, but all the ones I have are round, so they don't stick on so well. And on some radios, putting the antenna right on top of the case spoils the q of the internal antenna. On those, you want to try using a spacer of some sort, like those level frames on the sliders. Most of the time, with these very small antennas you can't get that far away, though. One or two finger widths is about the max. So you can find or build a little plastic or cardboard box that meets the bill and just stick some of that Dollar Store velcro on both sides and go to town.  Again, don't expect PVC loop performance, or even the boost a Select-A-Tenna will give you. But it does answer one of my needs: another dial to tweak on these digital masterpieces. How many times can you step through bandwidths after all?  And you can stack these things side by side on the top of the radio. I'm not sure, but it seems to me that having two tuned 3 inch loops is better than one 6 inch loop with one cap. And it's jet another knob to fiddle with.

On Feb 12, 2010, at 12:23 AM, jim_kr1s wrote:


--- In, Tony Germanotta ...> wrote:
> When I come up for air in a month, I guess I could combine the hobby by building a regen amp for a box loop to feed into one of those non-tuned stereo set air-core AM loops I can couple to the UL by slipping it around the radio.

One of those things arrived in the mail today. It was smaller than I expected. You can tune one with the radio by connecting it through an impedance-matching transformer. Mine looks like about 11 uH, so a transformer with 5:1 turns ratio, as mentioned in the data sheet, should work. But the antenna is physically too small to seem very good, and is wound with plain stranded wire. I've been trying larger loops. You want to keep the inductance low to keep the feed line impedance low. It doesn't take much wire on a large frame to get to 10-15 uH, but this looks like the way to go. You get a large capture area and the radio tunes the antenna for you.

> And pretty soon this ultra light outfit will take up an entire room. And that's the charm of what John and you others discovered. The pure joy of using little radios to do amazing things. I find I keep going back to a station until I can conquer it without loops or any other assistance beyond a pair of headphones.

Any radio will do. I get a lot of pleasure from the regen; the PL-380 selectivity is good and I like the portability, but if Tecsun got it right and made an ICF-2010 sized radio with a decent internal antenna wound with real Litz wire, and that bloody soft mute defeated, I'd jump on it. Of course we'd all try to make it better, but it would make a good starting point.


Jim, KR1S 

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