A couple of questions:

1. I have some Type 43 beads that will fit over RG8X that I am putting
up. The OD is about a 1/2" inch as is the length. Will this work for
~3.8Mhz and how many of those beads will I need to use? I can get type
77 in the right size, would there be any advantage to using type 77? I
would like it to handle legal limit - and I would only run that if the
SWR was under 2:1.

2. Why do most 1:1 baluns use that section of RG-303 coax with the
small type 73 beads? Is that a teflon coax that can handle heat
better?

Thanks!
Scott, WU2X

On Mon, 13 Aug 2007 15:19:10 -0000, wrote:

A couple of questions:

1. I have some Type 43 beads that will fit over RG8X that I am putting
up. The OD is about a 1/2" inch as is the length. Will this work for
~3.8Mhz and how many of those beads will I need to use? I can get type
77 in the right size, would there be any advantage to using type 77? I
would like it to handle legal limit - and I would only run that if the
SWR was under 2:1.

2. Why do most 1:1 baluns use that section of RG-303 coax with the
small type 73 beads? Is that a teflon coax that can handle heat
better?

Scott, here's a link that you or anyone else contemplating making
baluns should read first.

http://audiosystemsgroup.com/RFI-Ham.pdf

One of the best papers I've seen on the subject.

Danny, K6MHE

wrote:
A couple of questions:

1. I have some Type 43 beads that will fit over RG8X that I am putting
up. The OD is about a 1/2" inch as is the length. Will this work for
~3.8Mhz and how many of those beads will I need to use? I can get type
77 in the right size, would there be any advantage to using type 77? I
would like it to handle legal limit - and I would only run that if the
SWR was under 2:1.

See Jim Brown's choke compendium:
http://www.audiosystemsgroup.com/RFI-Ham.pdf

He talks about the tradeoffs between the various materials and design
criteria. You can look and see if your 43 will be good enough, or if
you should go out and buy something else. There's a new mix, #31,
specifically designed for this kind of thing.

#43 is optimized for VHF applications.

There's also the thing that materials that are good for transformers
might not be good for chokes (i.e. in a choke, resistance is good, in a
transformer, bad).

#43 material apparently also has a thermal runaway problem.. it gets
hot, the impedance drops, more heat is dissipated, the impedance drops
more, etc.




2. Why do most 1:1 baluns use that section of RG-303 coax with the
small type 73 beads? Is that a teflon coax that can handle heat
better?

yes
Although, one has to wonder about such an economy, since keeping the
coax from melting is only part of the story. Heating of the ferrite is
bad too. For #43, heating it to 60C drops the permeability by about
20-25% relative to 25C.



Thanks!
Scott, WU2X

Thanks for all the really good references. I was able to get some
Type 31 from Amidon that I am going to go with.

-Scott, WU2X