[AMRadio] antenna tuners


Gary Schafer garyschafer at comcast.net
Fri Apr 21 11:18:09 EDT 2006


Hi Dave,

What the MFJ will show you at the end of your feed line is the result of the
feed line and antenna as you noted. The only way to know where the antenna
itself is resonant is to measure it right at the antenna. Or you can measure
it through a 1/2 wave length of feed line which will reflect what is at the
antenna to the other end. However that is only good at one frequency, where
the feed line is exactly 1/2 wave length.

A coax length of anything other than a 1/2 wave length is going to transform
the impedance seen at the antenna (if it is not 50 ohms) to something else
at the other end of it.

By having the meter in the shack showing a low swr or finding the frequency
where the swr dips does not mean that is where the antenna is resonant. It
only means that is the frequency where the impedance is transformed to best
match the transmitter. It is not necessarily the resonant frequency of the
antenna.

If the antenna resistance at resonance is not 50 ohms, changing its length
(or frequency) will introduce reactance which adds or subtracts from the
resistance until it presents 50 ohms at a particular frequency.
Note that it will present 50 ohms to the feed line but it will no longer be
resonant.

73
Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of david knepper
> Sent: Friday, April 21, 2006 5:22 AM
> To: garyschafer at comcast.net; Discussion of AM Radio
> Subject: Re: [AMRadio] antenna tuners
> 
> Could you please explain, using an MFJ antenna analyzer, what is the meter
> showing on the instrument?
> 
> I would think that the total system, that is feedline and flat top or
> antenna if you prefer,  is resonating at the point shown on the meter? Or
> am
> I wrong?  In any case, I love that device for checking out the "resonant"
> point of the antenna system - note that I did not say just antenna.
> 
> 
> Thank you
> 
> Dave, W3ST
> Publisher of the Collins Journal
> Secretary to the Collins Radio Association
> www.collinsra.com - the CRA Website
> Now with PayPal
> CRA Nets: 3805 Khz every Monday at 8 PM EST
> and 14255 every Saturday at 12 Noon EST
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer at comcast.net>
> To: "'Discussion of AM Radio'" <amradio at mailman.qth.net>
> Sent: Thursday, April 20, 2006 10:12 PM
> Subject: RE: [AMRadio] antenna tuners
> 
> 
> > This is another tidbit to keep in mind for those that still may think
> that
> > an antenna has to be resonant to give 1:1 swr.
> >
> > A dipole antenna rarely is 50 ohms at resonance. It is very dependent on
> > height above ground as to what impedance it presents at the feed point.
> It
> > can range anywhere from below 30 ohms to above 70 ohms.
> >
> > So if you cut your antenna so that you have 1:1 swr at the transmitter
> end
> > of the coax, the antenna is probably not tuned to resonance! You have
> > detuned the antenna to change its impedance that the coax sees.
> >
> > Only rarely does a resonant antenna turn out to be 50 ohms.
> >
> > 73
> > Gary  K4FMX
> >
> >
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