Tunning a Balanced SSB Antenna With a Long-wire Tuner

[Bint al Kham]

Any one have thoughts on tuning a balanced antenna for that "All Band" performance? I see that there are tuners in the ham world specifically for balanced (dipole, doublet, loop) antennas. I am curious:

1) How would one use a single-line end-feed tuner like the Icom marine backstay tuners (AT-130, AT-140) to tune a balanced antenna? Would one leg go the ground term and the other to the ant term? (Sounds good...but am I gonna fry something? MFJ suggests I might at the 160 m band, see third URL below)

2) Are there limitations in the tunability of a balanced antenna to cover the 1.6 - 28 MHz range?

3) Would you set up the balanced antenna for mid-range and tune to either side?

4) Or set it as long as possible and tune down the spectrum?

This is really all about avoiding the counterpoise, and tolerating a non-back stay antenna.

For the source of my pondering see:

The All-Band Doublet

All Band Tuner for Random Length Dipoles

http://www.mfjenterprises.com/man/pdf/MFJ-1777.pdf


Thanks in advance smart people!

Ken

[btrayfors]

Hi, Ken...

Well, some interesting pondering it is!

The German balanced antenna tuner is very interesting, indeed, but not for a boat I think. Could be great in a land setup, though, where you want to run more than about 150 watts PEP output....the limit of many automatic tuners. While the $700 price is a bit steep, the unit seems to be well designed and constructed, and it's considerably less than you'd pay for a good high-power end-fed auto tuner like the SG-235.

I think the limitations for boat use are of two types:

1. to get an all-band balanced dipole antenna, you'd need a pretty long one...probably 100 feet or more. On most small boats, there simply wouldn't be enough room; and

2. the radiation pattern would vary greatly by band, and likely would do so in an unpredictable way.

I don't know if you were thinking about a vertical or sloping orientation, or an inverted vee, or what. Either way, my guess is that the two considerations above would be seriously limiting aboard a small boat.

Could be great fun on a large ship, though, or a home QTH :-)

I've been trying for many years to come up with a multi-band solution better than the traditional backstay, and which doesn't require a counterpoise :-)

Keep up the pondering. Maybe you'll have a breakthrough.

Cheers,

Bill

[cooper]

propagation is not one of my areas, but has always facinated me. I am sure that this is going to come across as a bit simple but....The antena feed wire is shielded so it does not act as an antena in its own right. Would it be possible to have a sheild "sock" that could be moved up or down to change the effective length of a , for example , backstay antena. ? I realise that this would be crude but coupled with an antena tuner, would it help to come closer to the wave (or fraction) length ?

[S/V Illusion]

The tuners you are familiar with (AT 120, 130, 140) are intended specifically for end-fed antennas. If you want to build a balanced antenna, get a tuner designed for that application. Be careful what you wish for - a balanced antenna is much more difficult to match atenna impedance, will not radiate an effective signal on anything except it's fundamantal or harmonic frequency and requires a specific feedline twinlead which is not well suited for marine use. You're better off using a random length wire.

Cooper - think of the feedline as part of the antenna. A tuner is actually mis-named as it is not some magic device but simply a method to match the imedances of the antenna and radio output only. The only advantage coax has in this context is minimizing feedline radiation. Nothing you do to "move" the outer braid of the coax will change the electrical length of the feedline or impedance mis-match if one exists and nothing will change the radiation pattern except the antenna design.

[fairbank56]

Originally Posted by Bint al Kham

1) How would one use a single-line end-feed tuner like the Icom marine backstay tuners (AT-130, AT-140) to tune a balanced antenna? Would one leg go the ground term and the other to the ant term? (Sounds good...but am I gonna fry something? MFJ suggests I might at the 160 m band, see third URL below)

Many hams have used this method for tuning horizontal dipoles at their homes. I use this method at home using an SEA1612B auto-tuner but use a 4:1 balun on the tuner output and feed the dipole with ladder line. Works great. SEA used to sell the baluns as an accessory for this very reason. Here's what they say about it in their service manual:

"In order to support balanced antenna configurations, a special toroidal balun is added to the standard coupler output terminal. This balun transformer consists of a two wire transmission line of intermediate impedance wound with high voltage wire. It operates as a 4:1 transformer, which means that the impedance actually seen by the antenna coupler is 1/4 of the drive point impedance shown at the feedline terminus. To insure good efficiency in the system, extremely short (in terms of wavelength) antennas should be avoided."

Eric

[Bint al Kham]

Eric, that doesn't sound bad at all. Would the balun technique work with the Icom AT tuners?

I've read that the SEA1612B was a precursor to the widely acclaimed and accursed SGC 230.

Sounds like its time to chat with SGC and Icom about this balun option for feeding a balanced antenna with an end-feed tuner. Alas, that probably means the end of the dream. Asking funny (maybe even not so funny) questions of electronics vendors can often be a dead end.

Ken

[fairbank56]

Originally Posted by Bint al Kham

Eric, that doesn't sound bad at all. Would the balun technique work with the Icom AT tuners?

Yes, it should work fine with them. I just used an Elecraft BL2 balun kit and adapted it to the 1612. There's plenty of extra room inside the 1612 enclosure for it and I just added another output insulator for the ladder line. The Icom tuner's are very tight for space so you would have to mount it outside the enclosure, but certainly do-able.

Eric

[cooper]

sv illusion..I was not talking about the feed wire, but the antenna itself. effectively changing the standing length, or to put it another way shortening the antenna and lengthening the antenna...

[S/V Illusion]

Originally Posted by cooper

sv illusion..I was not talking about the feed wire, but the antenna itself. effectively changing the standing length, or to put it another way shortening the antenna and lengthening the antenna...

I misunderstood, sorry.
The only way to change the electrical length of an antenna is a capacitance link which could work as you described but only if the movable portion is located at the extremities of the antenna. The physical movement of an electrically conductive material in close proximity to the antenna can change the resonance frequency somewhat but not sufficiently to make the antenna resonant across a wide frequency range. A collateral unwanted effect of doing tthis is to also change the antenna's impedance not to mention the physical complexity of such a system.

Also, the shield on coax feed line is not intended to preclude it acting as part of an antenna. A mismatched antenna will create RF on the feedlline braid allowing it to radiate as will an unbalanced antenna such as when just the inner conductor of the coax is connected to a single wire antenna and the braid to ground.

Hope I didn't further confuse the issue...

[Bint al Kham]

Copper,
Your idea reminded me of a trapped dipole article I once read. This thing uses traps to electrically lengthen and shorten the dipole:

Coaxial-Cable Trap Dipole by dxzone.com

Ken