TopBand: Vertical Matching

Tue, 4 Nov 1997 22:43:27 +0000

> From:          "Mike Michon" <michon@eatel.net>

Hi Mike,

> So, is it possible to match this arrangement so we can eliminate the
> tuner?? 

Yes,why not use surplus CATV cable?

>If so will we be able to have a broad enough match to accomodate
> how spread out the band gets during the contest??

Tune it for the middle of the contest area, a 1/4 marconi (even 
one made from wire) is TWICE as broad as a half wave antenna. That 
because the L/D ratio is twice as good! 

If you could use two wires (even if they are thinner) spaced a foot 
apart, the bandwidth would be even better because the antenna would 
look thicker!

> If we continue to use the tuner, can we do anything to make the match
> closer to what the tuner can handle??

Most tuners need a lot more C in the network than they have. The low 
C causes the circulating currents and voltages to be needlessly high, 
and that increases loss and heating. A Tee network tuner will 
actually handle MORE power with higher efficiency if the impedance is 
high, than if it is low. The optimum impedance for a "T" tuner using 
250 pF caps is around 2000 ohms! That's where it has the lowest loss, 
and the widest bandwidth.

> Would a balun at the vertical do any good as far as matching.  The feedline
> is usually about 6 inches apart so I am guessing about 500-600 ohms for it,
> but we can space it closer if necessary or beneficial.

Not using a balun virtually guaranties feedline radiation, and signal 
ingress into the system. You are feeding an UNbalanced antenna with 
a balanced line, so common mode currents will be induced in the line. 
Some line length and tuner configurations will minimize the problems, 
but NONE will eliminate them.

Unbalanced and balanced junctions should always be properly treated 
(that requires a balun of some form), unless you want feedline 
radiation.

73, Tom W8JI

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