Balanced Line - Lengths to Avoid

[KD6NRP]

I plan to build a horizontal doublet, feed it with 300-ohm window line, and use it with a balanced tuner. The objective is to create a single antenna with multi-band capability.

My understanding is there are certain balanced line lengths I should avoid. Here are a few questions:

1. For each band I want to operate on, I should avoid balanced line lengths equal 1/4 wavelength or multiple of 1/4 wavelength. Correct?

2. My transmission line will be 300-ohm window line with a velocity factor of 0.91. What formula should I use to take the VF into account and get the exact length? Is it (234/f) x 0.91 = length in feet, (234/f) x 1.11 = length in feet, or something else?

Regards,

Brian, KD6NRP

[KQ7W]

Brian - thanks for the Q's on 30, 160 etc.

Im sure ill get roasted here for this, but just run a ladderline to the shack leaving some slack and see how it goes, chances are it will tune just fine.

If it does not tune on some band you can always nip the line in 1 foot increments, I did that for only one of my 3 ladderline antennas and works FB on all bands.. the other 2 antennas worked 100% from the start using an arbitrary length.

Cu on the bands

Matt

[K7AAT]

  It seems to me that the critical lengths of your balanced feed line would depend on the length of your wire antenna ...  which you did not specify.

   Ed   K7AAT

[WB2WIK]

"Multi band" is a broad term.  It's impossible to achieve what you want to do with any particular length of line; you'd have to change the line length per band.

For example, if you make an 80m doublet fed with 300 Ohm line (or any impedance line!) and want to use it on 80m, it will match most easily when you make the line 1/2-wavelength, or a multiple of 1/2-wavelength, long.  1/2-wavelength with Vf = 0.91 is 128 feet at 3.5 MHz.  That will provide a low impedance (probably in the 50-70 Ohm range) at the "shack" end of the line.

That same length of cable will provide a very high impedance (in the thousands of Ohms) when you use the same doublet at 7 MHz.  This might work, but can be stressful on the tuner and if the Z is too high, you might not even find a match point.

Making the line a non-resonant length on the bands that have harmonic relationships like 80-40-20-15-10 meters means you've still "missed it" on bands that don't, like 60-30-17-12m.

I'd just make the line a convenient length that reaches where it needs to go and use a good tuner, and measure results.  If there are some bands that are very tricky to tune or simply won't tune, change the line length several feet and try again.

[AA4PB]

Well, a 1/4 wavelengh (or odd multiple) will convert a low impedance antenna feed to high impedance at the other end of the line and high to low (unless that antenna impedance matches the feedline). The problem is that with a doublet that is 1/2 wavelengh on the lowest frequency, the antenna impedance will vary from about 70 Ohms to maybe 1000 Ohms, depending on the frequency. In addition, the electrical length (wavelengths) of the feed line will vary with frequency. For a multiband antenna like this it is probably best to start with whatever length fits and then trim or add a little if your tuner can't match it on some bands. There is no "special length" that fits every case.

[WB6BYU]

It isn't as simple as "certain line lengths to avoid".

There are two considerations.  The one that lead to tables of line lengths in the Handbook was
based on the opportunity for common mode currents.  What most people forgot was that this
table was based on a particular set of assumptions that most stations did NOT meet.  A great
theoretical exercise, but one with little practical application unless you have a perfect RF ground
right at the rig.  The lengths will be different in practice, depending on many factors.  But in the
real world those lengths don't work any better or worse than anything else.

The other is the impedance seen by the tuner.  In this case you need to know the impedance
of the antenna on each band, the length of the coax, and the matching range of the tuner
you are using.  If your tuner works best with low impedance loads (for example, a coax tuner
followed by a 1 : 1 balun) the line lengths will be different than with something like a Johnson
Matchbox, which is better in the 600 - 1000 ohm range.  And in both cases you would want
different lengths depending on your antenna - a large loop will have present different impedances
to the feedline than a doublet.

So the best you can do is to look at the impedance of your antenna on each band (either based
on some reasonable guestimates or a computer model) and, for each band, run it through a
transmission line calculator (like VK1OD's here:  http://www.vk1od.net/calc/tl/tllc.php) to see
what the impedance is at the tuner end of the line.  If that impedance is in the range that
your tuner can match efficiently, you're good on that band.

Instead of all this analysis, most hams just set up the antenna and a convenient length of
twinlead to reach the tuner and see how it works.  If they have trouble matching on one or
more bands they add some extra feedline and try again.

But you can't make any general statements about what length will work without knowing the
details of both the antenna and the tuner you plan to use with it.

[WA3SKN]

I would just string up the dipole and feedline high and in the clear, and try using it.
Unless you hit the infamous "1/8 wavelength dipole with 1/8 wavelength feedline", you probably will have no problems at all!
73s.

-Mike.

[W4VR]

Depends on what the dipole impedance is at the feedpoint for the different bands.  About 10 years ago I put up a center-fed 178 foot horizontal dipole that I wanted to use on 160 through 10 as a temporary antenna.  I originally fed it with 120 feet of window line.  The Ameritron tuner handled it well on all bands, however on 40 meters I had RF in the shack.  I took off about 25 feet and the 40 meter problem went away.  It also loaded OK on all bands after I pruned the transmission line.  You did not say how long your dipole will be.  But, as others have said, "put it up with the feedline length you need and see how it works."  If you have a problem on one or more bands you can prune to optimize.

[W8JI]

I plan to build a horizontal doublet, feed it with 300-ohm window line, and use it with a balanced tuner. The objective is to create a single antenna with multi-band capability.

My understanding is there are certain balanced line lengths I should avoid. Here are a few questions:

1. For each band I want to operate on, I should avoid balanced line lengths equal 1/4 wavelength or multiple of 1/4 wavelength. Correct?

2. My transmission line will be 300-ohm window line with a velocity factor of 0.91. What formula should I use to take the VF into account and get the exact length? Is it (234/f) x 0.91 = length in feet, (234/f) x 1.11 = length in feet, or something else?

Regards,

Brian, KD6NRP

The bad or good length  of the feedline depends on the antenna element length.

[W9AC]

I agree with WB2WIK.  However, one way to look at it is to find a line length that presents a reasonable compromise if primary operation is on the harmonically-related HF bands. 

To Steve's point, an 80m dipole will have a Z below 100 ohms on that band.  But consider a 1/2 wave line feeding the 80m dipole at 7 MHz (66 feet, VF=1.0).  At 7 MHz, the feedpoint Z may be in the 4K-ohm area.  A 1/2 wave of line replicates the feedpoint Z exactly at the transmitter end of the line.  With this operating condition, the transmitter will see 4K-ohm.  Tuner losses and the ability to couple with a link tuner (if used) may become an issue although realistically, tuner losses are much more of a concern when the tuner sees a Z that goes very low -- say below 25 ohms.  You can look at QST product reviews to get a general idea as to tuner losses as a function of terminating Z for several commercially made unbalanced, and balanced tuners.

For harmonically-related bands, one compromise is to use line lengths in odd multiples of 1/8-wave at the lowest operating frequency. 

None of this may matter if the tuner remains reasonably efficient when the Z at the transmitter end of the line remains within its design specification especially if the tuner always sees between say...50-1200 ohms.  Unless a dipole antenna is much shorter than a half-wave, I don't think there's a feedpoint Z that goes deep in the low-Z area across the HF spectrum.

Another solution is to use the W5DXP method of line matching and dispense with the tuner.  One simply switches incremental lengths of balanced line.  Switching can be accomplished with knife switches or through remote-controlled relays.

Paul, W9AC

[N4JTE]

Google w5dxp, it's all there under ladderline  matching sections
bob

[W8JI]

Most people don't want to switch line lengths every time they change bands, and there really is no significant electrical performance advantage over a tuner.

What I do is pick a single line length that in combination with my antenna is not an extremely high impedance on any band. It's not that difficult to find a "sweet" length for multiple bands if the antenna is a reasonable length. With a half-wave antenna on the lowest band, avoiding a feedline electrical length that is near a multiple of 1/4 wavelength on the band lowest band is generally enough.

73 Tom

[WB2WIK]

Most people don't want to switch line lengths every time they change bands, and there really is no significant electrical performance advantage over a tuner.

What I do is pick a single line length that in combination with my antenna is not an extremely high impedance on any band. It's not that difficult to find a "sweet" length for multiple bands if the antenna is a reasonable length. With a half-wave antenna on the lowest band, avoiding a feedline electrical length that is near a multiple of 1/4 wavelength on the band lowest band is generally enough.

73 Tom

That's what I do, also.

The "sweet" length is one that allows me to run full power without arcing any tuner components!

That's about all that is really important.

[N7DM]

I use balanced lines on ALL my antennas...450 ohm twinlead, these days. Whatever feedline fits my installation.... is usually within the matching range of my Matching Gizmo. My 17 meter loop matches fine , 20 through 10...except for 15. I have a six foot chunk of twinlead that I clip on 'extra' when I work 15. Easy. 

73, dm

[W9OY]

There is one advantage to using a matrix of open wire line with relay switching and that is you can develop a system that automatically follows your radio and switches in the correct matching segments as you change bands, no "tuning" needed. 

73  W9OY