Design of a Yagi is a comprimise of SWR, gain, and F/B as they are
interdependent. If you optimize for one, the others will change. For low-band
beam, F/B is usually most important, but at the expense of gain, SWR or both.
If you were to optimize a beam for lowest SWR, you would comprimise gain and/or
F/B.
I don't think it would be a valid statement "that because a Yagi has high SWR
it has poor F/B". I think it would be fair to say that if you had LOW SWR, you
are lacking in the gain or F/B department.
-John, N9RF
----- Original Message -----
From: Gary Smith
To: amps@contesting.com
Sent: Friday, April 24, 2009 11:02 AM
Subject: Re: [Amps] High SWR
A comment: I have been told that a high swr on a yagi antenna reduces the
front to back ratio. Any truth (facts?) to support this info?
73,
Gary...wa6fgi
----- Original Message -----
From: Gary Schafer
To: 'Scott McGrath' ; amps@contesting.com
Sent: Thursday, April 23, 2009 9:26 AM
Subject: Re: [Amps] High SWR
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Scott McGrath
> In reality and especially with a tube amplifier and a Pi output tank the
> antenna system can be brought into resonance at a wide range of
> impedances and resonance is what we are looking for and it really is
> Antenna System Goodness in MOST cases. As the reflected power is
> definitely real but it is best expressed as the reactive power in a
> system which effectively is lost. With ladder line if long enough will
> re-radiate some of this 'lost' power.
Reflected power is not best expressed as reactive power. It is real power
and it is not lost power just because it is reflected. when reflected power
sees the matching network in the final amp or antenna tuner it then gets
re-reflected back toward the antenna to be ultimately radiated along with
the rest of the power. The only "lost power" is that due to the feed line
loss on both the forward and reflected power.
With ladder line the loss is normally very low and very little forward power
is lost and very little reflected power is lost. There can be very high swr
(high reflected power) on the line and most all of the reflected power will
end up getting back to the antenna to be radiated by the antenna.
Ladder line will not re_radiate reflected power any more than it will
radiate forward power. If the line is well balanced it will not radiate
either forward or reflected power that may be on it.
73
Gary K4FMX
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