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 Post subject: What Wire for AM Loop Antenna?
PostPosted: May Fri 22, 2020 6:26 pm 
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I am preparing to make a simple adjustable AM loop antenna to improve reception on my vintage tube radio, and have done a fair amount of internet research on the topic. Basically, just planning to make 26 turns of wire around a 12" square box or "X" (wood or plastic), and connect to a 365pf capacitor so I can modulate from 550-1500khz, similar to this:

https://a4.pbase.com/o9/41/524341/1/153 ... G_4412.jpg

Although most examples of these antennae are generally the same, I've noticed that the type of wire is often quite different, and was thinking about using #35 enameled copper wire. Also considering a similar-gauged Litz wire, but I am not too familiar with it, and unsure if it is really that much better. I obviously want the unit to work as optimal as possible, so just curious what you experts recommend. Appreciate any suggestions/advice, and thanks for your time and attention!


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Fri 22, 2020 8:29 pm 
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Use #18 solid magnet wire.

Litz is for flexibility, the benefit gained from litz will be slight. Easily over shadowed by the losses in the frame. Most will be served in pink polyester yarn.

RCA used solid wire for the loops in the Radiola 26, Superhetrodyne and the AR-812. The Radiolas, 24, 25, 28 all had a fine strand covered in brown rayon braid. Simply because it was visible...

Add an extra turn or two, it can be removed and keep it spaced or it won't reach 1600 and get down to 540.

YMMV

Chas

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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Fri 22, 2020 8:57 pm 
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All you want to know about loop antennas. Litz was preferred by the old timers and still used by serious AM loop builders.

http://makearadio.com/loops/index.php

http://kr1s.kearman.com/html/hooploop.html

18 AWG is serious overkill and no benefit as far as Q or losses.

Rich


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Fri 22, 2020 9:21 pm 
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Thanks for the great advice and the links. That huge loop on the knitting rings was pretty amazing. This had me thinking (oh no) about some related questions:

-I believe the loop antenna is most useful for radios with an internal Ferrite bar. However, can I expect to see similar performance with radios that just have an external wire antenna?

-Is there an optimal gap that should be maintained between each wire turn? I've seen 1/4" and 1/8", but no idea if one is really better than the other.

-What is the relative increase in performance/benefit in using a thicker wire, such as 18-gauge?


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Fri 22, 2020 10:27 pm 
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Rich, W3HWJ wrote:
All you want to know about loop antennas. Litz was preferred by the old timers and still used by serious AM loop builders...18 AWG is serious overkill and no benefit as far as Q or losses.
Rich
The RCA engineers didn't think so, I take them as very intelligent folks.

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Last edited by Chas on May Fri 22, 2020 11:42 pm, edited 1 time in total.

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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Fri 22, 2020 10:55 pm 
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Litz is about 4x the cost of normal wire. RCA wanted to save money. You can make a good loop with stranded wire and a pretty good loop with solid wire. A really good loop will use Litz.

http://theradioboard.com/rb/viewtopic.p ... 743#p92743

You can couple to a radio with its own internal antenna or connect the loop to replace a wire antenna. Easy if you have a radio with terminals for an external antenna. I built my loops with a coupling coil. a dozen or more turns (depending on diameter and freq. range) for the main coil connected to a variable cap. Then a separate set of 2 or 3 turns connected to the antenna input of the radio.

Image From MakeARadio

Rich


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Sat 23, 2020 12:08 am 
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Here's a thread I did on the tunable loop I built;
viewtopic.php?f=12&t=241661&p=2024254#p2024254

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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Sat 23, 2020 12:31 am 
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Impressive and thanks for sharing. Still not sure if such an antenna is useful with radios lacking an internal Ferrite rod, but I figure it can't hurt. After even more research, I found this handy table, which indicates that I should have 20 wire turns (not 26 as I mentioned above) for a 12' square antenna http://www.hard-core-dx.com/nordicdx/an ... mloop.html

I decided to go with the 18 AWG solid magnet wire, mostly because of cost and shipping time. I'm not yet at the level where I'd notice the subtle benefits of Litz wire, and this is my first crack at building an antenna anyway. Still trying to determine what spacing is appropriate between each wire, but might try 1/8" unless you all have a better recommendation.


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Sat 23, 2020 1:12 am 
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As you've not bought materials yet, might I suggest you consider low loss material such as polyurethane, polyethylene, teflon, etc. Wood and PVC are lossy. If a material gets warm or hot in a microwave for 30 seconds, it's lossy.

I've experienced this a different way. My Tesla Coil's output increased noticeably when I went from a PVC form to polyethylene.

If you must use wood, dry it and seal it well so it can't retain moisture. AND design so the wire has minimal contact and proximity to bulky pieces of wood. This will help offset the losses by not using Litz.

The reason you want to minimize losses its to obtain as high Q as you can. High Q results in sharper tuning, which results in concentrating weak signal into a smaller bandwidth, so it sounds louder.


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Sat 23, 2020 1:16 am 
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You can employ a tunable loop with a set that doesn't have an internal air or ferrite loop, by connecting a lead to the antenna terminal, and wrapping it once or twice around the loop, to act as a pickup coil.

The number of turns the loop will need in order to cover 540-1600 Kc might have to be experimented with a bit, as I had to do for mine, because it's dependent on the size of the loop area.

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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Sat 23, 2020 1:23 am 
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Thanks, fifties. So...if I already have a wire antenna coming out of the back of my radio, can I just attach it to the external AM loop?


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Sat 23, 2020 6:24 am 
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Think, "inductive coupling". If a receiver has a built in antenna, and the loop is placed near it, the RF gain from the loop inductively couples to the built in antenna.

With a set that's designed for a long wire antenna, you attach a length of wire to it's antenna terminal, and then wrap it once or twice around the tunable loop, for inductive coupling. The loop is not directly connected.

There are tunable loops you can buy that do have a lead to directly connect to a set's antenna terminal. I have a Grundig AN-200 sitting on top of a console TRF, connected to it's antenna terminal, and it works quite well.

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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Sat 23, 2020 3:33 pm 
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OK, great...that's pretty much what I was thinking. There's actually an alligator clip at the end of my radio's long wire antenna (I got it that way), so was also thinking about clipping it somewhere on the loop as well. However, I just don't know where it would be most appropriate to do that. I'm sure wrapping it a few times as you mentioned is probably just as good.


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Sat 23, 2020 8:49 pm 
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flyer92 wrote:
-Is there an optimal gap that should be maintained between each wire turn? I've seen 1/4" and 1/8", but no idea if one is really better than the other.

-What is the relative increase in performance/benefit in using a thicker wire, such as 18-gauge?

The spacing between turns affects the inductance significantly. If there is too much spacing then you'll need more turns. Since you want to connect it to a 365pF variable capacitor, you'll need to wind the loop to have about 240uH inductance. Some people may direct you to use an online calculator called Professor Coyle. Unfortunately, it's very inaccurate for these loops. Instead, you can use the following spreadsheet to calculate the inductance of various shapes of loop antennas. It's very accurate:
http://electronbunker.ca/Extras/Downloa ... alLoop.ods
It should also be mentioned that there is no benefit to winding the loop right down to the center of the frame. Any windings in the inner area contribute almost nothing to performance, and is generally a waste of wire. A good rule of thumb is to make the depth of the winding no more than 1/3 of the mean radius.

The benefit of thicker wire is mainly strength. I agree with the others that what little benefit you might get by using Litz, isn't going to be enough to justify its high cost.


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Mon 25, 2020 4:10 am 
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I have Litz, but it is the new stuff with the pink polyester over-wrap, and I think it would look strange in a restoration or home-brew. Can the polyester be successfully dyed? If so, will the dye liquid soaking between the wire strands adversely affect its characteristics? I am thinking that a green dye might counter the pink and result in a sort of brown (better than pink IMHO). Exploring options here. Your thoughts?
R/ John


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Mon 25, 2020 5:48 am 
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Liquid dye will not affect RF signals.

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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Mon 25, 2020 7:35 pm 
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Google [How to dye polyester yarn]

https://www.ritdye.com/instructions/how ... ic-fibers/

GL

Chas

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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Mon 25, 2020 11:37 pm 
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So....after borrowing a friend's Tecsun AN200 loop antenna, I tried to inductively couple it with my old tube radio's wire antenna. Nothing happened, which makes me believe I would have similar (or worse) results with a homebrew version. As such, I am about ready to scrap this whole loop antenna project, unless there's an optimal place to physically connect the radio's antenna. Mine came with a handy alligator clip at the end, so not sure if it would be best to clip it to one end of the loop, on one of the air capacitor's 4 leads, or onto the chassis of the capacitor itself. Maybe this will be just as futile as inductive coupling, but thought it might be worth a shot if you think it merits the effort.


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Mon 25, 2020 11:43 pm 
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You can use this with an older radio that uses a wire antenna, but you need to add a one turn pickup loop to the loop antenna. Then connect one end of the pickup loop to the radio's antenna input, and the other end to the radio's ground.


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 Post subject: Re: What Wire for AM Loop Antenna?
PostPosted: May Tue 26, 2020 1:39 am 
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What is the make and model of the radio?

Knowing that, the schematic can be found, the type of antenna input can be determined?

Randomly connecting to the radios internal components can create a shock hazard at worst or potentially damage "tuning" coils..

Chas

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