Hi to everyone,
i need to replace my 1:1 balun on my yagi tribander 14-21-28 Mhz. I
have in my stuff a toroid FT 240-61 and some pieces of rg-142 coaxial
cable. Anyone can help me to know how many winding needs on toroid to
achieve good reactance on 14 to 28 Mhz ?
Thank you ,
Daniel.

On Sun, 8 Jun 2008 11:37:50 -0700 (PDT), wrote:

Hi to everyone,
i need to replace my 1:1 balun on my yagi tribander 14-21-28 Mhz. I
have in my stuff a toroid FT 240-61 and some pieces of rg-142 coaxial
cable. Anyone can help me to know how many winding needs on toroid to
achieve good reactance on 14 to 28 Mhz ?
Thank you ,
Daniel.

Here's a very good paper on balun and chokes.

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

Thank you Danny for the very interesting document on balun.

Here's a very good paper on balun and chokes.

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

Daniel wrote:
Hi to everyone,
i need to replace my 1:1 balun on my yagi tribander 14-21-28 Mhz. I
have in my stuff a toroid FT 240-61 and some pieces of rg-142 coaxial
cable. Anyone can help me to know how many winding needs on toroid to
achieve good reactance on 14 to 28 Mhz ?
Thank you ,
Daniel.

I used a dozen turns of RG58 on the same core, and achieved XL ~= 200 ohms
at 14MHz for a Cushcraft A3. RG142 is about the same OD, so you should be
able to fit that many turns on the toroid. If you want more XL, stack two
(or more) toroids.

Bryan WA7PRC

Dan and Bryan,

200 ohms seems too low. I just wound 10 turns on a FT-240-61 core,
single layer occupying about 70% of the core and made the following
measurements of choking impedance on an HP8753 network analyzer in a
calibrated test fixture. The choke was mounted clear of any metal.

14MHz: 220 + j2850
21MHz: 20K
28MHz: 895 - j3.9K

The 10 turns were parallel resonant at 20.9MHz and the calculations for
10 turns would be ~18uH at low frequencies.

The above choke would be very very good from 20 thru 10m.

73,
Larry, W0QE



Bryan wrote:
Daniel wrote:
Hi to everyone,
i need to replace my 1:1 balun on my yagi tribander 14-21-28 Mhz. I
have in my stuff a toroid FT 240-61 and some pieces of rg-142 coaxial
cable. Anyone can help me to know how many winding needs on toroid to
achieve good reactance on 14 to 28 Mhz ?
Thank you ,
Daniel.

I used a dozen turns of RG58 on the same core, and achieved XL ~= 200 ohms
at 14MHz for a Cushcraft A3. RG142 is about the same OD, so you should be
able to fit that many turns on the toroid. If you want more XL, stack two
(or more) toroids.

Bryan WA7PRC

wrote in news:92aeb7fd-91ff-4720-b516-dcb66c0aa796@
79g2000hsk.googlegroups.com:

Hi to everyone,
i need to replace my 1:1 balun on my yagi tribander 14-21-28 Mhz. I
have in my stuff a toroid FT 240-61 and some pieces of rg-142 coaxial
cable. Anyone can help me to know how many winding needs on toroid to
achieve good reactance on 14 to 28 Mhz ?
Thank you ,
Daniel.


Be wary of putting too many turns on, especially for a lower loss mix
such as 61.

As you put more turns on, the series R and X of the ideal choke increase,
but the distributed capacitance also increases. Self resonance isn't so
bad for lossy mixes, because the impedance is still fairly high where the
total ZX passes through zero, but with a lower loss mix, you may find
choking impedance at frequencies a bit above resonance become
unacceptably low.

It is really hard to assess this with spot frequency measurements.

To start of with, I would wind as many turns as you can fit with a inter-
turn spacing equal to the coax diameter... in other words, half the turns
that you could get by a completely filled single layer, then make sure
the turns are evenly spaced.

There is a cross over winding configuration used which its supporters
argue increases the self resonance frequency. I haven't seen that happen
reliably in my experiments, rather the reverse happens more often (ie
that it reduces the self resonant frequency a little).

Owen