[Antennas] Feedlines, Tuners, and myths

2002年5月30日 18:42:59 -0700

I started to relook at the antenna system as a whole. I looked at it with 
the reminants of my physics training. The antenna system should be viewed 
with the aspect of waves rather than elctron travel. I worked on this and 
ended up with a wave things thing. I have asked one person to review 
it. Then I started to read an article that was sent to me a while ago. i 
didnot read it then as I had recieved many comments on that it was not good 
as ARRL no longer publishes it. It is "ANOTHER LOOK AT REFLECTIONS" by 
Maxwell. Now you are suggesting a book by him.
I have bought many books on antennas, including books that have chapters 
thereon . Too many to cite. So when you cite Relfictions II, I am 
intreged. What cost and what info.
Chris opr VE7HCB
At 01:02 PM 2002年03月11日 -0800, Pat W wrote:
> I am writing this only to try to dispel some myths out
>there in Ham Radio land about feedlines and tuners.
>Unfortunately some wrong beliefs continue to get
>propagated and it seems necessary to examine what
>happens when we connect our transmitter(and receiver)
>to our feedline.
>>I am not an engineer but have managed over time to
>finally grasp the concepts necessary to build effient
>antenna systems. Please don't flame me for my simple
>analysis. These ideas finally gelled after reading
>Reflections II" by Walt Maxwell. I recommend this book
>for others who have been confused in the past.
>>Here are some of my ideas:
>> All feedlines are transformers. Any feedline must
>transform the load impedance to an input impedance,
>depending on the line length. If the line length
>happens to be exactly 1/4 wavelength, the
>load is transformed by the equation Zi=Z0**2/ZL, where
>Zi is the input impedance, ZL is the load impedance,
>and Z0 is the feedline impedance.
> From the equation, note that when the feedline
>impedance equals the load impedance, then the
>transformation = 1, or Zi=ZL=Z0. That is why
>we can use a 50 ohm coax on a resonant antenna(assuming
>50 ohms of load).
>Taking advantage of this equation, we can feed an off
>resonant antenna by choosing the correct feedline
>length.
>Example, an 80 meter dipole on 40 meters with a ZL of
>5000 ohms, and feedline Z0=500 ohms will provide about
>a 50 ohm input impedance! A perfect match.
>>If the line length is exactly 1/2 wavelength long, then
>the Zi = ZL regardless of the feedline impedance Z0.
>(this is only true when the load is not reactive, ie,
>ZL=R + j0).
>>If the line length is neither of the above then all
>bets are off and the Zi is transformed by the
>"Transmission Line Equation". See the ARRL handbook for
>the definition but do not expect to understand it.
>>Note that the equations apply to all elements making up
>the feedline, including anything grounded between the
>transmitter and the antenna!!!
>>2). The SWR on the feedline is constant and cannot
>change by changing the feedline length. If the SWR
>does change, then there is a problem with the feedline
>caused by what are called 'common mode currents'.
>In other words, the feedline has become part of the
>antenna and changing the feedline length is changing
>the antenna length. When this situation occurs, a
>balun is mandatory to prevent the common mode
>currents. However, changing the feeline length may
>allow a tuner to find a better match but that does not
>mean that the SWR has changed, only that the
>transformation that occurs is creating a different r +j
>for the tuner.
>>3). Tuners do not "fool" the transmitter into seeing a
>different load.
> A tuner provides what is called a 'Conjugate Match'
>between the transmitter and the antenna. The conjugate
>match, in a simplified way, takes reflected power and
>adds it to the outgoing forward power to be delivered
>to the load. For example, if 1/3 of the power is
>reflected at the load, then 2/3 of the power is
>absorbed by the load. The tuner takes the reflected 1/3
>power and adds it to the outgoing forward power to be
>delivered to the load. A 100 watt transmitter in this
>case, would have 150 watts outgoing, 50 watts
>reflected, and 100 watts(or 2/3 of 150) delivered to
>the antenna.
>Tuners can have losses and are not meant to match
>anything we can think of. The reflected power adds to
>the loss in the line, which can be quite high when
>using coax. Open wire lines are much less lossy which
>allows high SWR on the line with minor signal
>degradation.
>>4). "A low SWR tells me what the feedline is doing,not
>what the antenna is doing"(From LB Cebik). We seem to
>be obsessed over SWR and forget that is not the primary
>motive here. A lossy coax feedline dampens the
>SWR curve as the load SWR increases which can fool us
>into thinking our antenna is efficient, when is it is
>not.
>The G5RV antenna is NOT an all band antenna. The longer
>the coax feed, the higher the losses and corresponding
>dampened SWR curve. A G5RV with 66' of RG58 can have
>SWR over 100:1 at the antenna, but will show 15:1 at
>the transmitter and have 10 dB of loss! A very nice
>dummy load. The best G5RV is one with no coax!!.
> The typical 33' of ladder line is meant to provide 1/2
>wavelength of feedline on 20 meters to make the load
>impedance transparent thru the feedline.
>On other bands, it's different of course.
>> Pat W0OPW
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