[Antennas] Buried ladder line?

[Darryl J. Kelly]

>At 07:05 PM 10/13/2003, you Bob Lay wrote:
>>[snip]
>>
>>Meanwhile, if you have the determination for it, the best plan for a
>>balanced tuner is the one by Dick Measures, published in the Feb 1990 QST.
I
>>have personally seen one built exactly to those plans. The owner/builder was

>>kind enough to let me examine it and operate it. It is probably even better

>>and more versatile than the Johnson Matchbox.
>
>I have both and use both. Dick Measures design, built properly. will tune a

>much wider impedance (differential)  range than the Matchbox, however, it 

>does not handle high common mode conditions as well as the Johnson 
>Matchbox. If your antenna and transmission line are well balanced  it will

>work very well. But for antenna systems that are not well balanced - such 

>as an off center fed dipole -  Measure's tuner wouldn't necessarily work 
>much better addressing the common mode problem than a conventional T type 

>tuner with a balun. In other words Measure's design removes differential 
>impedance stress from the balun, but falls remove the common mode impedance

>stress. (Common mode current's path is to ground and that path, in 
>Measure's design, is through the balun)
>
>I feel Measure's design is a step up from the T-tuner/balun combo 
>(particularly for the QRO operator), but the link-type tuner, similar to 
>the Johnson Matchbox design, is still the best for matching balanced 
>transmission lines.
>
>
>>Meanwhile, I am hoping to find one of the smaller Johnson Matchboxes that

>>was designed for about 300 watts (AM service) - they are said to be able to

>>handle 1500 watts p.e.p. with no trouble.
>
>The small tuner is said to have better efficiency
>
>
>>To build the Dick Measures design you will need one big variable cap and two

>>identical rotary inductors whose shafts will be ganged together with a
>>toothed belt and driven by a front panel crank with counter. It's a serious

>>project. You may want to look at some of the other designs, as well, that

>>have appeared in the ARRL Antenna Book.
>
>What out for the big capacitor as it will also have a big minimum 
>capacitance that can be a killer on 10-meters. I had better luck using a 
>smaller capacitor and switching in addition parallel capacitors as needed 

>for the lower frequencies.
>
>Yes, you are quite right it does take some time to find parts and build.
>
>73,
>Danny, K6MHE
>
>
>
I have a balanced tuner built with two E. F. Johnson 229-202s, 15 microhenries
and I use a split stator 100-100 capacitor. I use jumpers on the cap and for
most bands I jump the capacitor in series, and for 80 and 30 meters jump it
in parallel. Antenna is 136' center fed zepp, inverted vee, feed with 300 ohm
twinlead. Hope this is of interest.
Darryl, KK5IB

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