[Elecraft] FW: The use of a doublet

[Don Wilhelm]

Phil,

Ron speaks truth!
The "magic" is that the feedline acts as an impedance transformer, and 
open wire line (or dry twinlead) will operate at high SWRs with 
negligible loss.

We have formulas for quarterwave (or odd multiple) transmission line 
sections, but not often mentioned is that the transformation for 'in 
between' transmission line lengths can be beneficial to provide a decent 
feedpoint impedance for the tuner.  The relationships for the ham bands 
will be such that often a compromise can be found in the transmission 
line length so it can be matched by commonly available tuners.

Ron's homebrew link coupled tuner can match most any impedance 
efficiently - and I will wager that it has plug-in coils to cover the 
various bands, but the same is not true of many commercially available 
tuners - particularly the popular T match.  The T match tuner can have 
settings that do nothing but 'heat the tuner' while indicating a low SWR 
at the input.

The net result is that certain antenna lengths give rise to specific 
antenna feedpoint impedances for each of the ham bands, and the 
transmission line electrical length will transform that inpedance to 
some other value - with a lot of judicious calculations, one can come up 
with a combination of lengths that will provide a range of (for example) 
25 ohms to 1000 ohms at the end of the transmission line, and most 
available tuners can handle that impedance range with low loss in the 
tuner itself.  The antenna and parallel feedline is inherently low loss 
(when the 300 ohm feeder is dry).

73,
Don W3FPR

Ron D'Eau Claire wrote:
> Tnx Phil. 
>
> Note that using the lengths do not improve the performance, only the
> impedance excursions involved and they can be handled by a wide range tuner
> quite nicely.
>
> That said, some tuners show excessive losses at extreme impedances even
> though they may show a 1:1 SWR on the coax link to the rig. I use a homebrew
> fully-balanced link-coupled tuner with my doublets that I have considerable
> confidence in having low losses across its range and the ability to match
> everything from a fraction of an ohm to many thousands of ohms. Using many
> "commercial" tuners I'd be much more concerned. 
>
> The ARRL Handbooks have a table showing the combinations of wire length and
> feeder length that produce the least extreme feed point impedances as well. 
>
> Ron AC7AC
>
> -----Original Message-----
>
> I was asked why 90 feet of 300 ohm ladder line by the group.  Ron of DX
> Engineering responds below.
>
>
> Philip LaMarche 
> LaMarche Enterprises, Inc.
> www.instantgourmetspices.com
>
>
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