[Top] [All Lists]

Re: [TenTec] OT SWR VS Power Loss

To: Alfred Lorona <w6wqc@dslextreme.com>, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] OT SWR VS Power Loss
From: Ken Brown <ken.d.brown@hawaiiantel.net>
Reply-to: ken.d.brown@hawaiiantel.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sun, 06 Jul 2008 12:48:18 -1000
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>

There are few ways to tune an antenna. One method is to alter the 
antenna's dimensions, which in the simplest antennas means to adjust the 
length. Another method is to adjust the distance from ground or other 
large conductive objects. Either of these two methods can change the 
feed point impedance and the resonance of the antenna, so these methods 
can really be called "tuning the antenna".

Whether adjusting the inductor on a base loaded vertical is "tuning the 
antenna" depends on whether you consider the inductor to be part of the 

There are many ways to tune an antenna system, without tuning the 
antenna. Most "antenna tuners" do just this. They do not tune the 
antenna, they tune the antenna system. Kurt N. Sterba has written some 
very good explanations of how an "antenna tuner" "transmatch" or other 
impedance transforming device can be used to tune the antenna system,  
so that the source (transmitter) sees a load it can work most 
effectively with. Unfortunately Kurt always concludes that the antenna 
has therefore been tuned. Sorry Kurt, the antenna still has the same 
impedance at it's feed point, the tuner has tuned the system as a whole, 
without tuning the antenna.

Does it matter where the impedance matching is done? Depends on a lot of 
things. How long the feed line is, and what kind of feed line, how much 
power you are putting into the feed line are some of the first 
considerations. In the vast majority of amateur applications, at the 200 
watt level or less, the expense and maintenance of a remote tuner at the 
antenna feed point will not be justified compared to the convenience and 
simplicity of having the matching device at the shack end of the feed 
line. Only at higher frequencies where the feed line loss becomes 
significant, and at higher powers where you could have excessive heating 
or dielectric breakdown due to high currents and voltages along the feed 
line when the SWR is high, is it likely to matter much.

>> it may be argued that the tuner at the antenna actually
>> resonates the antenna and so improves its radiation efficiency at some
>> cost from losses in the tuner.
> As Jerry points out, there are some amongst us who believe that a tuner at 
> the antenna feed point  tunes the antenna wire to resonance. It does not. 
> Consider a wire longer than a halfwave length with, say, a feedpoint 
> impedance of 120 ohms instead of the 70 ohms exhibited by a resonant dipole. 
> The tuner does not change the impedance of 120 ohms to 70 ohms which it 
> would have to do in order bring the wire to resonance. What the tuner does 
> do is transform the 120 ohm antenna impedance to the characteristic 
> impedance of the feedline. The antenna wire remains at 120 ohms. If the 
> feedline is 50 ohm coax,  the tuner transforms 120 ohms to 50 ohms. The 
> feedline is them terminated in 50 ohms which makes it a flat line with no 
> loss due to a finite swr. There is a gain in radiation but it is due to the 
> elimination of power lost in the feedline that was due to a finite swr and 
> not because the antenna wire is brought to resonance.
> The feedline looks into 50 ohms at the input of the tuner. The antenna looks 
> into 120 ohms at the output of the tuner. The two are said to be impedance 
> matched. Both parties are happy and maximum power transfer occurs between 
> them. Incidentally, an impedance match between the end of the feedline and 
> the antenna occurs whether the tuner is at the junction of the two or at the 
> rig end.
> 73, AL 
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec

TenTec mailing list

<Prev in Thread] Current Thread [Next in Thread>