[Antennas] Buried ladder line?

2003年10月13日 20:45:15 -0700

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