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[TowerTalk] Re: Force12

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Subject: [TowerTalk] Re: Force12
From: (Tom Rauch)
Date: Fri, 12 Oct 2001 12:14:14 -0400
Hi Yuri,

We always can learn useful things when people disagree. Rather 
than call you names and insult you in return, let me point out 
something useful to people using stubs. 

> Nice try to dance out of it :-)   Who is talking about coaxial
> dipoles? So it worked for some 50 years, now you "calculate" it
> doesn't?

While I certainly respect your and K7GCO's technical abilities and 
clear logical thinking, it appears you may be far off base in this.

Looking at a high dipole on EZnec, a #12 wire dipole has an 
impedance of 67 j0 at resonance on 3.7 Mhz

A RG-8 shorted stub resonant on 3.7 MHz has an input impedance 
of 2700 ohms j0.

The parallel combination is 65 ohms, and the stub only adds a few 
percent of loss. Not too bad at resonance.

On 3.5 MHz the antenna Z is 60 -93j (SWR 4.63).
The parallel stub is 124.8 +560j

Now let's look at how each part of stub impedance affects the 

The combined impedance of the stub and antenna is 64-28j and the 
SWR is 1.75:1.  That looks good, and if we just leap to a 
conclusion it looks like the stub helped cancel reactance. 

Now let's use a lossless stub:
With lossless stub across the feedpoint SWR is 4.44

Woops! The reactance only changes the SWR from 4.63:1 down to 
4.44 : 1  So much for the misplaced notion reactance of the stub 
offsets or cancels reactance of the antenna!

Now let's just add a parallel 125 ohm resistor, instead of a 125 
+561J stub. This would be pure loss resistance equal to the 
resistance of the stub:

SWR is now 1.88:1 

Very clearly, the SWR decrease from 4.63 down to 1.75 from the 
stub is almost entirely due to the added loss resistance of the 

Indeed the primary way a parallel stub adds bandwidth is through 
loss resistance. Bandwidth will get better if we use an RG-58 stub, 
because loss is higher with RG-58. The better the cable is, the 
less sense it makes to add it. As a matter of fact there is very little 
disadvantage to just using a resistor over a stub, if the goal is 

Now this applies to the Coaxial dipole, or to stub systems for 
nulling out adjacent bands, and stubs used to load antennas. 
Stubs have to used carefully, or we can add more loss than we 
might expect.

It isn't that I just don't trust level-headed technical advice and  
direct to-the-point concise answers from you and GCO, it's just 
that I have looked at stubs many times over the years.
73, Tom W8JI 

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