[Elecraft] KAT3A Question

[Mark Goldberg]

On Sat, Apr 11, 2020 at 6:09 PM Bob McGraw K4TAX <rmcgraw at blomand.net>
wrote:

> My 6M yagi shows a 5.3:1 SWR as indicated on my K3S on 28.3MHz with the
> ATU in Bypass mode.   When the KAT3A ATU  in the radio resolves a match,
> the indicated SWR indicated is 1.1:1 on the radio.
>
> Using the same antenna but using my KAT500 ATU the best SWR resolved is
> 1.6:1 as indicated on the radio.
>
> Now my antenna analyzer on 28.3 MHz with my 6M yagi connected shows and
> SWR of 7.5:1, a Z value of 16.6 ohms, and R value of 7.3 ohms and the X
> value of 14.7.
>
> That same antenna on the analyzer at 50.2MHz shows the SWR at 1.2:1, Z
> at 39.9, R at 40.0 and Z at 0.0
>
> My conclusion:  although the SWR values may be indicating acceptable
> values, the 6 meter Yagi operated on 10 meters would make a very lousy
> antenna.   You'd be much better off to use an 80 meter dipole or better
> yet, just a 10 meter dipole.
>

In addition, a Yagi operated at a non resonant frequency, especially below
it's design frequency is likely to not act like a Yagi at all. The pattern
depends on the elements being close to resonance. More wire / aluminum is
usually better. A physically short antenna may be tunable but it won't be
efficient. Much of the power will be dissipated in the tuner or balun or
feedline as losses. A physically short antenna + tuner + high power often
results in smoke somewhere! I use a 300 foot horizontal look and it can be
tuned from 80 to 6 meters. Who knows what the pattern is at high
frequencies but I can make contacts!

So, a tuner is a good idea, but it won't solve every problem. Because it is
easy to understand, Hams use SWR, but in actuality the same number for SWR
results from an infinite number of impedances ( R and X), not all of which
can be handled by the tuner the same way. ARRL has done some comparison
testing with tuners with various impedances, I believe. I don't know if the
losses have been measured, but you can take two different impedances that
result in an SWR of 4:1 and only some can be tuned, depending on the tuner.

73,

Mark
W7MLG