Parts for compact loop antenna - toroid help

[Daniel Reynolds-2]

I'm thinking about building a non-tunable compact loop for 40m (7040) using
RG-8. If I have good success, I'll be making other loops for 20m and 30m
(mono-band / tuned to the CW-QRP frequency). I need some help figuring out
which toroid to buy to build a portable compact loop like is shown here:

http://www.qsl.net/aa5tb/loop.html

It looks like I can buy the cable, toroid, and even the banana plug/bnc adapter
at Ocean State Electronics. My only question is - which toroid core should I
buy for this application? I've seen comments on the web about using a T-50-2
for the Rock Loop, and I've seen other vendors selling a T-50-6 which
supposedly covers the entire HF band. But I cannot find either one at Ocean
State Electronics. For 40m, should I buy a T-50-61? What about 30m? T-50-61 or
-67?

Thanks for your help,
Daniel / AA0NI
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[N8LP]

I haven't built a loop like this, but here is some general info about
ferrite vs. powdered iron...

If  the toroid will be used only as a transformer and not as a coil,
then you can use either ferrite or powdered iron. For low-band only work
you could use FT-50-43 or -61. For all bands, I would use -61. FT-50
should be good up to about 50W. The -61 core will require more turns on
gthe secondary. If you use powdered iron, T-50-2 is a far better choice
than -6, especially at lower frequencies.

If you need a coil instead of a xfmr, forget ferrite... it saturates too
easily.

73,
Larry N8LP


Daniel Reynolds wrote:

>I'm thinking about building a non-tunable compact loop for 40m (7040) using
>RG-8. If I have good success, I'll be making other loops for 20m and 30m
>(mono-band / tuned to the CW-QRP frequency). I need some help figuring out
>which toroid to buy to build a portable compact loop like is shown here:
>
>http://www.qsl.net/aa5tb/loop.html
>
>It looks like I can buy the cable, toroid, and even the banana plug/bnc adapter
>at Ocean State Electronics. My only question is - which toroid core should I
>buy for this application? I've seen comments on the web about using a T-50-2
>for the Rock Loop, and I've seen other vendors selling a T-50-6 which
>supposedly covers the entire HF band. But I cannot find either one at Ocean
>State Electronics. For 40m, should I buy a T-50-61? What about 30m? T-50-61 or
>-67?
>
>Thanks for your help,
>Daniel / AA0NI
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>
>
>  
>

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[Stuart Rohre]

Daniel,
You could forgo the core toroid entirely and use a small loop of smaller
coax than that of the antenna loop coax.
73,
Stuart
K5KVH
Google on "small loops" for a variety of feed versions.


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[Charles Greene]

Daniel,

The small powered iron toroids like the T-50-2 and T-50-6 are used for
tuned circuits, as their loss is less.  The -2 is used for VHF and the high
end of the HF band; the -6 is used for the mid and low end of the HF
band.  The ferrite cores, which begin with F do not work well in tuned
circuits.  They begin with an F, like F-50-61 or F-50-43.  The latter are
used for baluns, RFI chokes and other than tuned circuits.  Most any
electronics supply house carries  full line of powered iron
toroids.  Amidon which sometimes advertises in QST is a source.

Chas, W1CG

At 02:18 PM 10/6/2004, Daniel Reynolds wrote:

>I'm thinking about building a non-tunable compact loop for 40m (7040) using
>RG-8. If I have good success, I'll be making other loops for 20m and 30m
>(mono-band / tuned to the CW-QRP frequency). I need some help figuring out
>which toroid to buy to build a portable compact loop like is shown here:
>
>http://www.qsl.net/aa5tb/loop.html
>
>It looks like I can buy the cable, toroid, and even the banana plug/bnc
>adapter
>at Ocean State Electronics. My only question is - which toroid core should I
>buy for this application? I've seen comments on the web about using a T-50-2
>for the Rock Loop, and I've seen other vendors selling a T-50-6 which
>supposedly covers the entire HF band. But I cannot find either one at Ocean
>State Electronics. For 40m, should I buy a T-50-61? What about 30m? T-50-61 or
>-67?
>
>Thanks for your help,
>Daniel / AA0NI

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[G3VVT]

To confuse the matter of toroids further, the catalogs I have show the iron  
dust toroid T50-2 made by Micrometals as working in the mid to low HF, 0.25 -  
10MHz range and identified by a red color.
 
The T50-6 seems the best for all round HF use with a listing of 2 - 30Mhz  
and identified by a yellow color. Have used these in home  constructed HF radios
in the past with good success.
 
For VHF there are several listed: T50-12 (green/white), T50-17  (blue/yellow)
and the -22 version (green/orange) seems to be available in the  smaller
sizes. All cover 20 - 200MHz with varying temperature stability.
 
Bob, G3VVT
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[Daniel Reynolds-2]

I'm just posting this so that someone can tell me ... NO - DON'T DO THAT - or -
DO THIS.

I know I could use a loop inside of a compact loop, but a toroid is more
appealing for me because of the size and appearance. It looks like the
consensus is that I need an Iron Core Toroid ... something along the lines of a
T-50-6 (or larger size diameter T-114-6). This will be used as a 'transformer'
between a turn (or a few turns) of wire run between the center conductor and
shield of my coax, and a single turn of a compact loop. I plan to use this for
80-20m.

When I inquired with the original author of the webpage (aa5tb) - he thought he
used a ferrite core ... FT-114-43 maybe - he didn't know for certain. It sounds
like from some of you - that a ferrite core has significantly less coupling
than an iron core.

Can I safely assume that if a core is of a low density (reduced coupling), that
I can make up for it with more turns on the primary/coax side - however, if it
is a high density core - it would be very hard to reduce coupling since the
secondary/compact loop side is already limited to one turn?

Finally - many of you were able to tell me about the cores - BUT - what core
would you use for this application (coupling your transmission line to a
compact transmitting loop)?

Thanks again - and sorry about the Off Topic post (albeit - it is for a
QRP/Elecraft radio).

- Daniel / AA0NI

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[Masleid, Michael A.]

Hi Daniel

>I'm just posting this so that someone can tell me ... NO - DON'T DO THAT - or -
>DO THIS.

OK, use FT 43 material.

You want to make a broadband transformer.

You want tight coupling, but you need the right turns ratio.

If the small loop has a radiation plus loss resistance around 1 ohm,
you need a 1:7 turn ratio to match 50 ohms.  You want the shunt reactance
of this 7 turn coil to be much more than 50 ohms.  FT 43 can do that.

If you run 8 watts, you need 20 volts on 7 turns.  An FT-50-43 core
will run about 138 gauss at 3.5 MHz to do this.  Use a bigger core to
reduce loss, handle more power, and have enough room for the windings.

73, de Michael, AB9GV
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[Charles Greene]

Well Dan,

A F-114-43 should work ok for a transformer.  I don't know of any design
data.  When I said the iron core was for tuned circuits, I meant a coil and
condenser type.  The ferrite cores are better for broadband transformers.

73,  Chas

At 10:24 AM 10/7/2004, Daniel Reynolds wrote:

>I'm just posting this so that someone can tell me ... NO - DON'T DO THAT -
>or -
>DO THIS.
>
>I know I could use a loop inside of a compact loop, but a toroid is more
>appealing for me because of the size and appearance. It looks like the
>consensus is that I need an Iron Core Toroid ... something along the lines
>of a
>T-50-6 (or larger size diameter T-114-6). This will be used as a 'transformer'
>between a turn (or a few turns) of wire run between the center conductor and
>shield of my coax, and a single turn of a compact loop. I plan to use this for
>80-20m.
>
>When I inquired with the original author of the webpage (aa5tb) - he
>thought he
>used a ferrite core ... FT-114-43 maybe - he didn't know for certain. It
>sounds
>like from some of you - that a ferrite core has significantly less coupling

Probably the other way around.  The ferrite core has a lot more
permeability, which means you can get more inductance with fewer turns.

>than an iron core.
>
>Can I safely assume that if a core is of a low density (reduced coupling),
>that
>I can make up for it with more turns on the primary/coax side - however, if it
>is a high density core - it would be very hard to reduce coupling since the
>secondary/compact loop side is already limited to one turn?
>
>Finally - many of you were able to tell me about the cores - BUT - what core
>would you use for this application (coupling your transmission line to a
>compact transmitting loop)?

To tell the truth, I wouldn't use any core.  I'd use a coax or copper
tubing coupling loop.


>Thanks again - and sorry about the Off Topic post (albeit - it is for a
>QRP/Elecraft radio).
>
>- Daniel / AA0NI


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