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Re: [TowerTalk] Multi-band dipoles

Subject: Re: [TowerTalk] Multi-band dipoles
From: Jim Brown <>
Date: Thu, 21 Apr 2011 09:30:18 -0700
List-post: <">>
On 4/20/2011 10:19 PM, Grant Saviers wrote:
> Bottom line, even with the article's promise of a "no tweaks" design,
> you will likely need to prune to get what you want,

I've built, measured, and used a LOT of dipoles in the last seven years, 
and I'd call anyone who advertises a "no-tweak design" either a huckster 
or a fool!  There are simply too many variables -- the dielectric 
constant of the wire that you use, surrounding objects, ground 
conductivity, and the height above ground among them.

Another important point -- the feedpoint impedance at resonance of a 
half wave dipole varies a lot depending on its height above ground.  Low 
dipoles (a half wave or less) tend to be closer to 50 ohms and high 
dipoles tend to be closer to 75 ohms. This simple difference will cause 
a shift of 1% or so in the minimum SWR. Further, if you use 75 ohm coax 
and measure the SWR with a 50 ohm SWR bridge, you will get wrong answers 
for the SWR!

Think about this -- two things about SWR matter. First, the match 
between the antenna and the transmission line determines the loss in the 
line. Second, the feedline impedance where it connects to the 
transmitter output needs to be close to 50 ohms to get max output and 
keep the transmitter happy. #2 is the problem that antenna tuners fix.  
Further, power amps with a tuned output stage (most of those using 
tubes) are able to be happy over a moderately wide range of load 
impedance. That is, the tuned output stage acts as an antenna tuner.

Getting back to the original question in this thread about antennas for 
160/80 for a DX trip one thing I would not leave home without is a good, 
high power antenna tuner!

73, Jim Brown K9YC

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