BL2 w/Multi-Band Dipole

[David Dietrich]

Hello Everyone!

I am planning on building a multi-band dipole fed with 450 Ohm ladder line soon.  I have a K1 & K2, both with the ATU's installed, and the BL2 switchable balun.  I was wondering if I could connect the balun directly to the back of the K1/K2 when I use this antenna?  My other option would be to use one of my 18" foot RG8X jumpers between the rig and the balun.  I know someone here has done it, or at least knows whether this will work or not.  I am also posting this question to QRP-L, so I apologize if you see this message twice.

Thanks ES 73,

David
KC9EHQ
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[Jim Dunstan]

At 09:13 AM 4/2/2012 -0700, you wrote:

>Hello Everyone!
>
>I am planning on building a multi-band dipole fed with 450 Ohm ladder line
>soon.  I have a K1 & K2, both with the ATU's installed, and the BL2
>switchable balun.  I was wondering if I could connect the balun directly to
>the back of the K1/K2 when I use this antenna?  My other option would be to
>use one of my 18" foot RG8X jumpers between the rig and the balun.  I know
>someone here has done it, or at least knows whether this will work or not.
>I am also posting this question to QRP-L, so I apologize if you see this
>message twice.
>
>Thanks ES 73,
>
>David
>KC9EHQ

Hi David

When feeding a multi-band dipole with parallel feed line the first choice
is to bring the feed line all the way to the ATU or balun at the operating
position.  If the feed line is properly dressed (that is, care is taken to
maintain balance) there will very little RF radiation from the feed
line.  Since you are running QRP this strategy is further re-enforced ...
since you want maximum efficiency.  The farther you separate the balun from
the operating position with coax the greater will be the RF loss due to the
characteristic of coax cable.  The usual reason for this separation is the
difficulty that is often encountered in bringing the parallel line into the
shack.

One tip on easing the parallel feed line from outside house to the
operating position is to transition the 450 ohm feed line to 300 ohm twin
lead the short distance to the balun connected to the radio.  The 300 ohm
twin lead is not as efficient at the 450 ohm line but much more efficient
than making the transition with coax.

Jim, VE3CI


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[Nr4c]

Ron, I'm not reading what you're reading in this.  I read that he is
going to run the ladder-line from the antenna, to the radio.  He seems
to be asking if he can attach the BNC conector on the BL2 directly to
the back of the radio.  I am considering doing just that with my KX3
when it arrives.  This eleminates the coax jumper altogether.  I would
think it would be a very low loss way to go.  My only concern is that
the whole thing gets pretty long and bulky hanging off the KX3. Prob not
as much an issue with a K3/K2 radio.  What do you think?

...bill  nr4c

On Mon, 2 Apr 2012 09:13:16 -0700, Ron D'Eau Claire wrote:

> It sounds like you are planning to feed a single wire in the center
> with 450
> ohm ladder line so there will be standing waves on the feed line.
>
> In that case it's *always* more efficient to bring the parallel wire
> feed
> line all the way to the rig and avoid the use of 50-ohm coaxial line
> in the
> system. How much depends upon the SWR, which varies widely with
> frequency,
> and the length of the coax, but you can lose half of your power, or
> more, in
> a coax link with high SWR.
>
> 73, Ron AC7AC
>
> -----Original Message-----
>
>
> Hello Everyone!
>
> I am planning on building a multi-band dipole fed with 450 Ohm ladder
> line
> soon.  I have a K1 & K2, both with the ATU's installed, and the BL2
> switchable balun.  I was wondering if I could connect the balun
> directly to
> the back of the K1/K2 when I use this antenna?  My other option would
> be to
> use one of my 18" foot RG8X jumpers between the rig and the balun.  I
> know
> someone here has done it, or at least knows whether this will work or
> not. 
> I am also posting this question to QRP-L, so I apologize if you see
> this
> message twice.
>
> Thanks ES 73,
>
> David
> KC9EHQ
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

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[Mike K8CN]

Hi David,

An 18 inch jumper cable won't produce a dramatic shift in feedpoint
impedance over that presented by a direct connection (via a double male BNC)
to the K1/K2 ATU even on 10 meters.  If an adequate SWR can be obtained by
the ATU (VSWR less than 2.0) with the coax jumper in place on the bands of
interest (you may need to try the two different balun ratios), you can be
happy.

If you want to see the possible difference in the impedances presented to
the ATU for any given impedance at the BL-2 BNC connector, the TLW program,
written by Dean, N6BV and supplied with recent editions of the ARRL Antenna
Book, will calculate for you the transformed impedance at the end of an
arbitrary length of feedline.  It's not necessary in your practical
situation, but it is interesting and educational if you've not worked much
with feedlines.

The only concern with the direct connection is the case when there may be
repetitive movement of the 450 ohm window/ladder line; you'll want to have
some separate strain relief to prevent repeated flexure of the BNC female
connector.  The ATU connectors on my K2 are well-secured to the case, but
the connector bodies are plastic.  For an indoor shack environment you
should have no problem either way.

73,
Mike, K8CN

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[Mike K8CN]

Hi David,

An 18 inch jumper cable won't produce a dramatic shift in feedpoint impedance over that presented by a direct connection (via a double male BNC) to the K1/K2 ATU even on 10 meters.  If an adequate SWR can be obtained by the ATU (VSWR less than 2.0) with the coax jumper in place on the bands of interest (you may need to try the two different balun ratios), you can be happy.

If you want to see the possible difference in the impedances presented to the ATU for any given impedance at the BL2 BNC connector, the TLW program, written by Dean, N6BV and supplied with recent editions of the ARRL Antenna Book, will calculate for you the transformed impedance at the end of an arbitrary length of feedline.  It's not necessary in your practical situation, but it is interesting and educational if you've not worked much with feedlines.

The only concern with the direct connection is the case when there may be repetitive movement of the 450 ohm window/ladder line; you'll want to have some separate strain relief to prevent repeated flexure of the BNC female connector.  The ATU connectors on my K2 are well-secured to the case, but the connector bodies are plastic.  For an indoor shack environment you should have no problem either way.

73,
Mike, K8CN