[TowerTalk] Mosley antenna problems

K4SB k4sb@mindspring.com
Thu, 20 Jan 2000 15:53:26 +0000

Al Williams wrote:
> why dont antenna mfrs publish gain vs freq in addition to
> vswr vs freq?

Because they without exception claim greater gain than possible
for their antennas.

>   Is there a correlation of gain & vswr to freq?

Yes, to a small extent. Increasing VSWR will result is less power
being radiated ( and I'm using figures like 3 or 4 to 1. ) The
greater correlation is gain vs frequency. A properly design yagi
will ALWAYS exhibit greater gain ABOVE it's
resonant frequency, but at the same time LESS F/B.  Conversely,
you may have enormous increases in F/B with a decrease in gain at
frequencies BELOW resonance. For instance, I have 5 element LONG
boom yagis here for 10, 15 and 20. When modeling them, I at one
point had a F/B ration of over 90 db. However,
even a slight shift in frequency could degrade that figure by
over 30 db. And, the bandwidth of these antennas is severely
narrow, for example on the 20, it's
just a little over 200 kc between the 2:1 points.

> perhaps more importantly, can gain fall off but vswr remain
> relatively flat?

Most certainly, a reactive system, or a poorly designed feed
system can do this, along with many other factors. In actuality,
your VSWR meter is not telling the
truth. The simple truth is that VSWR in practice, is constant
ONLY when the VSWR is 1:1. ( and remember, I said in
practice....in actuality, it will vary even under these
conditions ) A classic example of this is the myth that changing
the length of a feedline ( neglecting transformers, ect ) will
change your VSWR. That is absolutely false! What you have done is
move the meter to a position where outgoing and reflected power
are not so much out of phase. On the other hand, you may
introduce reactance by the insertion of a usually short piece of
dissimilar coax at a precise distance from the load which will
then act as an impedence transformer and will provide a more
precise match. The EXCEPTION to this rule is of course, the
quarter wave transformer made from 70 ohm line and connected to
the load. If the load is 100 ohms, you will achieve for all
practical purposes a perfect and TRUE reading of 1:1.

Again, in practice, don't worship the SWR god. Depending on the
frequency, the feedline, and the feedline length, a VSWR of 10:1
will result in so little power loss as to be insignificant. 

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