RadioBanter - View Single Post - Balun 1:1 with FT 240-61 toroid

Owen Duffy · June 10th 08, 05:05 AM posted to rec.radio.amateur.antenna

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