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Re: [TowerTalk] step away from stepIR

To: "Rob Atkinson, K5UJ" <>,<>
Subject: Re: [TowerTalk] step away from stepIR
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Tue, 12 Jun 2007 12:02:21 -0400
List-post: <>
> It's safe to assume that the feedline in a typical 
> installation is at least
> 60 or 70 feet long, probably longer in most cases.   This 
> is usually a
> significant length in terms of wavelength above 30 m.   I 
> also assume that
> reflected power, or vswr, is being measured in the shack. 
> One thing that
> bothers me about this is that this doesn't guarantee that 
> the Z at the
> feedpoint is actually 50 j0.

It does guarantee it, at least within limits we care about.

It is impossible to come up with a 50 j0 impedance at the 
end of ANY length cable with modest losses unless the load 
is very near 50 j0.

>It's probably close enough on the lower bands
> but on 10 or 12 meters, where the feed is likely to be 
> mulitple lambda in
> length, I wonder if using the antenna as a tuner gets the 
> user into
> something like the "my feedline tunes my antenna!" type of 
> situation i.e.
> the meter in the shack may show true unity along the line, 
> or maybe you just
 > happen to have adjusted the antenna so that the length of 
feedline you are
> using and your frequency result in a nice looking match at 
> the shack but the
> feedpoint, if you could measure there, would tell a 
> different story?

Doesn't work like that. SWR except for small gradual changes 
by loss remians the same along the line length as long as 
the SWR meter and feedline are the same normalized 
impedance. Now if you had a 75 ohm meter with 50 ohm cable, 
or some other mismatched combination between the line and 
meter, SWR reading would vary with length (even though SWR 
really doesn't change along the line in that case either).

> The other question i have is, how do you know the element 
> length that gives
> you a match to 50 ohm feedline, is the antenna dimension 
> that gives you
> optimum gain and F/B?  Is the element length for 50 ohm Z 
> always the length
> for best performance?

No, but we assume they have that all worked out.

> I essentially question giving no reflected power the 
> highest priority which
> seems to be what happens with these antennas.  Or maybe 
> most of the time you
> get close enough to having your cake and eating it too so 
> that my concerns
> are minor issues?

I never worry about SWR at all if it is less than 1.5 or 
2:1. There are cases where it is a worry, but not in my 

73 Tom


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