[Elecraft] Balun for K-1

Sun Jul 21 08:00:01 2002

Hi,

You have to consider the application.  While the premise that an air core=20
balun has adequate bandwidth for a K1 4 bander is correct, this only holds=
=20
for working into an unbalanced load of 50 ohms or somewhere near 50 ohms=20
which is fine for a beam.  However, an open wire transmission line feeding=
=20
a dipole on multiple bands can easily have an impedance well over 500 ohms=
=20
on some frequencies.  You have to calculate the impedance of the balun at=20
the frequency in use; the rule-of-thumb is at least 5 times the Zl, or 2500=
=20
ohms for a load of 500 ohms.  That may take a lot of turns of air wound=20
core, which decreases the highest frequency response.  The reference Tom=20
quoted lists the loss of the ferrite bead balun as around 5% which is=20
insignificant at 5 watts, being around .25 DB.  On the other hand, an air=20
wound balun is a good way to get started as Jim said, and it won't hurt=20
anything.  But it just may not have the desired result of producing=20
balanced currents in an open wire transmission line at all frequencies.

Here's some numbers which may be helpful for the small ferrite core.  15=20
turns of RG-174 on an F114-77 core has an inductance of 370 uhy, which has=
=20
an impedance of 16,265 ohms at 7 MHz.  A little high, and I didn't measure=
=20
the frequency response or efficiency as I use this one for an EMI choke on=
=20
an audio line.  13 turns of RG 174 on a F114-43 has an inductance of 106=20
uhy, which has an impedance of 4660 ohms at 7 MHz which sounds like a good=
=20
number to start with.  I would be interested in what your G-R R.F. Bridge=20
says about this one, Sandy, if you happen to try it.  The F114-43 is a good=
=20
core to start with as they are $1 ea. at Sams Small Parts.

At 10:26 PM 7/20/2002 -0500, Sandy, W5TVW wrote:
>I would imagine (haven't tried it yet, by the way!) that a choke balun
>wound on a very high permability ferrite toroid (a small one say 1"" in=20
>diameter)
>with RG-174 coaxial cable would do a fine job from 7-21 Mhz. for the
>4 band card K1.  I plan on trying this and making some measurements
>to see what happens with a G-R R.F. Bridge.  Will report on the
>findings soonest.
>73,
>Sandy W5TVW
>----- Original Message -----
>From: "Tom Hammond N=D8SS" <[email protected]>
>To: "James R. Duffey" <[email protected]>
>Cc: <[email protected]>
>Sent: Saturday, July 20, 2002 4:07 PM
>Subject: Re: [Elecraft] Balun for K-1
>
>
>| Hi Jim:
>|
>| >I think it is unecessarily complicated for the task at hand. For the use
>| >over the short frequency span of interest to a K-1 4 band module user,=
=20
>7 MHz
>| >to 21 MHz,  a simple coiled coax choke balun, 6 or 7 turns should=
 provide
>| >adequate choking reactance. It is simple, can be made in minutes, and is
>| >effective. With it you can be on the air in minutes. The numbers you=
 quote
>| >indicate the effectiveness of the simple coiled balun, the balun in=
 column
>| >5, 8 turns sceamble wound into a 6 5/8 in coil, has a peak choking=20
>reactance
>| >of 8530 ohms and a 200 Ohm choking reactance bandwidth (recommended by
>| >deMaw) from 2 MHz to 16 MHz (an 8:1 bandwidth. There are those who think=
 a
>| >200 Ohm reactance bandwidth is inadequate, so the 500 Ohm choking=
 reactance
>| >bandwidth is from 3.5 MHz to 10.5 MHz, still a respectable 3:1 ratio.
>| >Winding a coiled coax choke balun as I suggested should be adequate for=
 the
>| >4 band K-1, which was the application I was addressing. Other=
 applications
>| >may require more care and a better balun. When improvements are made to=
 the
>| >antenna system at a later time, the balun can be upgraded. But to get=20
>on the
>| >air quickly with a minimum of fuss and acceptable performance, the=
 coiled
>| >coax balun is hard to beat.
>|
>| I agree completely with your response!
>|
>| I was mainly posting the article for the benefit of all of us on the
>| reflector, rather than directing it at anyone is particular.
>|
>| Of course, MANY (thousands, I'm sure) scramble-wound baluns have been=
 made
>| over the years (my tribander sported one for probably 15-18 years=
 itself).
>| And we never knew the difference between the way it worked and the way it
>| might have performed had it been wound in a solenoidal fashion. Heck...=
 we
>| STILL don't now... <G>
>|
>| But, thanks to the article, I now know that if I want to obtain PEAK
>| performance from my coaxial-cable baluns, I should probably wind them in=
 a
>| single-layer coil, rather than in the manner I'd been using for many=
 years.
>| As you state, in many instances a single-layer coil is not required to
>| obtain more than adequate performance.
>|
>| >The posting was in keeping with my QRP KISS (Keep it simple stupid)=
 policy.
>| >I find the KISS policy keeps me out of trouble and on the air more than
>| >pursuing optimum (and oftem more complicated) solutions. Don't get=20
>offended,
>| >the stupid in KISS is not meant to apply to anybody but me.
>|
>| I'm not offended in the least... and I do appreciate your further
>| discussion (above) of the subject. Thanks for taking the time to respond.
>| And, don't worry, MOST CERTAINLY applies to me very well...!!
>|
>| 73,
>|
>| Tom Hammond   N0SS
>|
>|
>| _______________________________________________
>| Elecraft mailing list: [email protected]
>| You must be a list member to post to the list.
>| Postings must be plain text (no HTML or attachments).
>| See: http://mailman.qth.net/mailman/listinfo/elecraft
>| Elecraft Web Page: http://www.elecraft.com
>|
>|
>
>_______________________________________________
>Elecraft mailing list: [email protected]
>You must be a list member to post to the list.
>Postings must be plain text (no HTML or attachments).
>See: http://mailman.qth.net/mailman/listinfo/elecraft
>Elecraft Web Page: http://www.elecraft.com

73, Chas, W1CG
K2 #462