Box Loop Update

[crainbebo]

I am in the middle of constructing a 3 1/2 foot box loop that will be used for AM DX on the Tecsun PL-660 and Grundig G5.

Currently, the body is constructed, but we need to get some super glue to mount it in place completely. This antenna is around 31 inches x 45 inches. This antenna is not wooden like most DXers use, this is a PVC loop. I have bought a half-lb of wire which will be plenty for the loop. The antenna can fit 11 turns of wire, which may mean a gigantic difference in AM DX.

After the wire is placed on, I am planning to mount a piece of wood to the top, for the tuning knob and variable capacitor. This will help (hopefully!) get rid of bleed from KIXI, KKNW and KXPA, since they are all 2 1/2 miles south of me in the Mercer Slough.

I'll have more updates as this project goes along.

-crainbebo

 

[Schroedingers Cat]

What capacitance would it take to tune it?

Are the windings inside or outside the PVC pipe? Inside would be tricky, but I've heard of it being done, feeding each turn through successive straight pipe and elbows, and then assembling. But once it's assembled, you're stuck unless the inductance is about right for your capacitor options. I guess you could just get a tight fit without cementing/acetone.

 

[crainbebo]

The capacitance is 365 pf.

The windings will be outside the pipe. Most wooden box loops I've seen have the turns of wire outside it...

Do you think this option is OK, or would the DX be worse than let's say, a wooden box loop? Let me know.

-crainbebo

 

[stacker]

PVC should be fine for MW frequencies. As frequency goes up it starts to conduct.

 

[crainbebo]

So would there be a huge difference on stations like CBK 540, KPQ 560, etc. compared to upper-band KFBK/KGA etc.? Or not?

-crainbebo

 

[gar hi]

After I got my little Terk loop from the store a couple years ago, I considered building a much bigger loop using a pizza box but I didn't because I had no clue as to where to get a tuning capacitor for a loop.

Where can you find one?

In my experience with my Terk loop anyway, there's a noticeable difference in signal strength with daytime DXing but I haven't noticed any difference at night with skywaves.

My goal with a large loop antenna would be to maybe pull in one of the New York stations daytime with an actual ID on the east coast of Florida and also hear some stations here on the Gulf coast from Texas that I haven't yet been able to get.

 

[cd637299]

^ I have an issue similar to gar's. Once I tried to build one of these things, but without the capacitor, it's a waste of time.

I think I'll just wait a few years until the number of AM stations starts to whittle down....then and only then would I be able to hear long distances!

I'm happy with my Select-a-Tenna....but could I build one even better? Even the YouTube videos on AM loops don't seem to explain much.

How about a "Building an AM Loop for Dummies" book?

cd

 

[crainbebo]

I got mine from this site
http://www.angelfire.com/electronic2/index1/

13.95 for the capacitor + $1.80 for the tuning knob

-crainbebo

 

[Schroedingers Cat]

Mail order and online is about the only place to find variable capacitors these days. Usually you want a 365 pF, but it would seem that if you had more turns, you might want less capacitance. You could try the 365 pF, but if you can only peak the bottom of the band at the unmeshed end, you might want to try a smaller capacitance.

 

[rbrucecarter5]

I built a few of these things:

http://earmark.net/gesr/loop/

Those pages are getting a bit out of date, I really need to update them.

First of all - you can buy a cheap radio in a flea market, garage sale, thrift shop, or on eBay. Probably, it goes against your grain to scrap a perfectly good radio to get a tuning capacitor, but I made the psychological adjustment years ago. I've bought a lot of wireless mice to get the fabulous 455 kHz ceramic filter (for AM) as well as cheap radios for tuning capacitors. There are usually some other useful parts in there as well.

PVC pipe is heavier than wood, which is the reason I don't use it. Gluing it, though, is easy. The primer and glue is in the same department as the PVC pipe at the hardware store. They used to sell a one step adhesive, but the glue sniffing morons ruined that the same as they ruin all other sources of really good glue.

Most tuning caps you salvage out of newer radios - the little plastic ones - are more like 230 pF than 365 pF, which means more turns of wire. The good news is - for some reason it is easier to cover the whole AM band with one of those caps than it is with a 365 pF cap. Probably the reason they went to that value in the first place, to help cover up to 1700 kHz.

A lot of loop makers have to have switches, either on the number of turns, or adding a fixed cap onto a variable, etc. to cover the whole band. A bit of experimentation gets it right, but don't be surprised if you have trouble covering the band. I can usually get very close, but that is after years of experimentation.

I've done a bit of backtracking on one point - I no longer make the turns flush with each other. On all but the smallest loops - this makes inter-turn capacitance so large that the loop won't go to high frequencies. I've stopped making anything larger than 3 feet with ribbon cable for that reason. Separate the turns as much as you can, it will be easier to tune the whole band.

I've never done the pizza box thing - I am too concerned about attracting roaches to the residue - they like cardboard as it is. But I've heard of pizza box loops and beer can tuning capacitors - a bit kludgy, but they should work.

 

[boombox]

So would there be a huge difference on stations like CBK 540, KPQ 560, etc. compared to upper-band KFBK/KGA etc.? Or not?

-crainbebo

The plastic shouldn't interfere with your reception on MW -- the Select-A-Tenna and Radio Shack loops I use (I read somewhere that the RS Loop was made by Terk) are encased in plastic, and they work fine throughout the MW spectrum.

 

[gar hi]

I got mine from this site
http://www.angelfire.com/electronic2/index1/

13.95 for the capacitor + $1.80 for the tuning knob

-crainbebo

Thanks!

I've wondered, though.

I've assumed bigger is better when it comes to loops but does anyone know if that's actually the case?

Haven't been able to find out through looking myself.

I've seen the demonstrations on YouTube but the radio the pass the loop over is kind of small.

The smaller the built in AM antenna in the radio is, the bigger difference the loop makes.

My Sangean PR-D5 has a 200 mm built in ferrite antenna, about as big as they are in portables, and my Terk loop still makes a big difference on many weak stations in the daytime.

 

[kenglish]

Here's a cool loop calculator:

http://www.angelfire.com/mb/amandx/loop.html

For parts, especially the variable caps, try "The Crystal Set Society", here:
http://www.midnightscience.com/catalog5.html

 

[rbrucecarter5]

I've wondered, though.

I've assumed bigger is better when it comes to loops but does anyone know if that's actually the case?

Haven't been able to find out through looking myself.

I've seen the demonstrations on YouTube but the radio the pass the loop over is kind of small.

The smaller the built in AM antenna in the radio is, the bigger difference the loop makes.

My Sangean PR-D5 has a 200 mm built in ferrite antenna, about as big as they are in portables, and my Terk loop still makes a big difference on many weak stations in the daytime.

Yes - the bigger the loop the better. Read my pages ---

www.earmark.net/gesr/loop

The more cross sectional area the better. After that, lower the resistance of the wire by using as large a gauge as possible.

 

[crainbebo]

rbrucecarter5, I heard that you used a four foot box loop, and you had to take it out of Dallas due to overloading and bleed from local AM stations. Does that mean I have to go up in the Cascades to do some night DX, or will the capacitor work even 2 miles away from a huge 50,000 watt AM (KIXI 880)?

-crainbebo

 

[gar hi]

I've wondered, though.

I've assumed bigger is better when it comes to loops but does anyone know if that's actually the case?

Haven't been able to find out through looking myself.

I've seen the demonstrations on YouTube but the radio the pass the loop over is kind of small.

The smaller the built in AM antenna in the radio is, the bigger difference the loop makes.

My Sangean PR-D5 has a 200 mm built in ferrite antenna, about as big as they are in portables, and my Terk loop still makes a big difference on many weak stations in the daytime.

Yes - the bigger the loop the better. Read my pages ---

www.earmark.net/gesr/loop

The more cross sectional area the better. After that, lower the resistance of the wire by using as large a gauge as possible.

OK, I see. Thanks. Didn't get to check it out when you first posted it.

I'm thinking a pizza box may be too light and easily be blown down in the wind if I'm at the beach.

Imagine how the New York stations would come in at Daytona Beach daytime with a good loop!

 

[stacker]

So would there be a huge difference on stations like CBK 540, KPQ 560, etc. compared to upper-band KFBK/KGA etc.? Or not?

-crainbebo

No difference. PVC is good throughout the MW band. It not until you get to VHF and UHF when it begins to act less like an insulator.

 

[rbrucecarter5]

rbrucecarter5, I heard that you used a four foot box loop, and you had to take it out of Dallas due to overloading and bleed from local AM stations. Does that mean I have to go up in the Cascades to do some night DX, or will the capacitor work even 2 miles away from a huge 50,000 watt AM (KIXI 880)?

-crainbebo

I would never use a loop in the Continental US at night - there are so many stations on the air that it would saturate any AM radio that you can name. It is true that the 4 foot loop did not work in Dallas. You would think Dallas would be a better location for the Chicago stations, WSB, WSM, etc. - but there was not a trace of them in the daytime. But in Lubbock, the story was completely different. There is one other factor - the ground conductivity - which is amazing in Lubbock.

If you use a loop two miles from a 50 kW station, you might be able to use it as a source of power! That is a bit tongue in cheek, but the last time I put a loop near a transmitter I was making for a downhole RF tool - I was able to get output in the tens of volts.

 

[audioguy]

I use a box loop all the time in the Chicago area. I made my small box loop using a copier paper box (you could also use a "bankers box" just as well). I believe I wound about 20 some odd turns of thermoplastic insulated stranded hookup wire around it and tuned it with a 365 mmf variable cap that I just mounted to the cardboard box with a couple of washers to keep the screws from breaking through. I used clear packing tape to secure the turns.

This is not very durable but it is not very expensive either and you can build it in about 10 minutes. You may have to experiment with the number of turns (I don't have it here with me at the moment).

With this setup I can easily pull in WLW at high noon with a $20 Walkman headphone radio here in the north Chicago suburbs.

It is fairly sharp and won't overload your radio unless you are pretty close to a tower and have it tuned to that frequency.

The improvement in signal strength with this loop appears to be in the neighborhood of 20 dB.

 

[boombox]

rbrucecarter5, I heard that you used a four foot box loop, and you had to take it out of Dallas due to overloading and bleed from local AM stations. Does that mean I have to go up in the Cascades to do some night DX, or will the capacitor work even 2 miles away from a huge 50,000 watt AM (KIXI 880)?

-crainbebo

You shouldn't have much of a problem over most of the MW spectrum. You may have some swamping near 880 -- just place the radio farther away from the loop and retune, and the overall setup gets more selective... With my S-A-T and Radio Shack Loops, I have to do that with KIRO, although I'm farther away from it than you are from KIXI, so you may have more of an issue with KIXI....

In my case, even if KIRO's swamping 700 or 720, if there is a station on one of those two frequencies, the loop will pull the station in through the swamping during fade-ups. Depending on the radio, I may have to 'null' KIRO, or place the loop a bit further away from the radio to hear the station on 700.

The thing to do is build the loop and work with it, see what it will do. You shouldn't have to go to North Bend to use it, though.....

I have a 4 foot spiral loop (still in storage) that I frequently used for DXing in the mid to late 1980's with a boombox (which is a good one, with a decent loopstick), and don't recall any problems using the loop, and the local AM band isn't much more crowded now than it was back then.