non-destructive coupling of hoop loop to PL-310


Tony Germanotta
 

Gang, I have been watching Jim's experiments with his hoop loop for months but was buried in a project and couldn't dig in. When I finished the project, I decided to give the hoop loop a try, but I wanted to keep my PL-310's internal loopstick in place, because I really enjoy barefoot dxing whenever possible. So I wondered why not use the existing antenna coil as the tuned section in a transformer and just wind on a takeoff coil that would tune and feed the hoop loop.

I opened up the PL-310 (six screws, don't forget the ones under the flip stand and in the battery compartment) and got my first look at the loopstick. What a miserable excuse for ferrite material that is. Mine was as rough as concrete, or one of those bunion files. I saw that there was enough room to thread a wire around the loop without taking it out. I didn't want to risk breaking the ferrite rod or one of the wires. I also noticed that on the far end, by the wrist strap, there was a foam rubber shock absorber that wasn't actually glued to the stick. That meant I could make a turn at the end rather than try to fish each one out, then press down the foam and slide it down the ferrite rod to its place. I cut off about a foot of magnet wire I had in a junk box, I didn't measure it, but it seemed about the same diameter as the loop already on the stick. I don't think it matters that much in this context. Then I started my first loop as close as I could to the end of the actual antenna coil. Leave yourself plenty of wire to go out of that wrist strap hole and later to connect to a plug outside the radio.

I didn't try to overwrap the existing coil, because there really wasn't enough space for that without taking the loopstick out. Using Jim's calculations, I put on 15 turns, pulling each one as close to the last as I could. (Steve's calculations show I probably should have made it no more than 12 turns for maximum efficiency.) Magnet wire really doesn't wrap that neatly this way, especially on a rough rod with sharp edges. You don't want to rub the enamel coating off because it will short the turns together and you won't know you are one or more turns short. But it doesn't have to be perfect, my experiment shows. All it needs to do is inductively couple the radio and the external antenna. When I finished the turns, squeezing each past the foam support, I tried to get them as close together as possible and then twisted the two ends together until they went out of the little wrist strap hole.

I put the radio back together again and didn't even wind up with any extra parts. Then I found a couple plastic 1/8" phono plug sockets I had in a parts box. These are square and are perfect for hot gluing on the hoop loop and the radio. Next I used a dremel sandpaper tool to scrape the enamel off the part of the wire that needed to be soldered to the boxes and hooked it all up.

So how does it work?
With no antenna plugged in, it's a stock PL-310. A fine radio with plenty of sensitivity and selectivity.

With the hoop loop, however, it becomes a monster. Here's a quick bandscan I did this afternoon, from 3:30 to 4:30 p.m. from my house in Chesapeake Virginia, that's at the far southeastern tip of the state. I'm about 20 miles from the Atlantic or from the North Carolina border.

I won't give the station IDs unless I am sure of them, because I really wasn't logging, just testing for signal strengths and quality. The first set of numbers is the reading using the hoop loop, the second is the PL-310 barefoot and oriented in the same direction as the loop was.

The noise floor at my house today seemed to be 15 dbu. The audio always was higher fidelity using the loop, even on powerful locals. The nulling is quite sharp, you can almost point the hoop's sides at a station and peak it, and if you slide it 5 or 10 degrees off, you get a dramatic drop in signal. That should bode well for nulling out pests. The orientation of the PL-310 itself never seemed to matter, the air core loop seems to totally overwhelm the internal antenna's directional pickup pattern. But the hoop loop never seemed to overload the PL-310. Even local monsters stayed on their assigned frequencies and the hoop allowed reception on nearby frequencies as well.

Also, it seemed that the soft mute effect of offtuning by 1 or 2 Khz was much lower with the hoop loop. I didn't have to turn the volume up the two notches I usually have to, probably because the signal, even on the relatively weak stations, remained within the AGC parameters while using the Hoop Loop or maybe it just fools the chip.

You do see more meter swinging with the loop, which is probably due to it being more sensitive to atmospheric wrinkles. Now I'm wondering how many turns I need to wrap around a spare 7" ferrite I have around here to make it a more portable option......



Frequency Signal S/N
530 (VB tourist info station) 19 11
15 01

540 26 11
15 02

560 38 25
19 22

660 WFAN NYC (350 miles or so) 20 03 (There fine)
15 01 (nothing really readable)

1020 WIBG OC NJ (about 300 miles) 24 00 (but clear copy, talking about Sixer's hiring Doug Collins with ID at 4:28 p.m.)
(1.9 KW daytime station) 15 00 (nothing but noise)

1050 (local monster) 67 25 (no sign of overloading, no bleeding to adjacent channels)
47 25

1240 32 18
18 08

1330 22 17
15 01

1370 (Baltimore), about 270 miles 28 20
15 05

1420 27 08 (readable signal, "Great Balls of Fire" by the Killer
15 00 (just noise)

1450 22 08
15 00

1530 25 13
15 02

1650 64 25
49 25

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