[Elecraft] OT a bit: K3 Ant Tuner

[Mark Goldberg]

I could not let this go. I've done a lot of work with bandpass filters. The
mismatch will degrade the filter.

For an example, I used Elsie, which is a filter calculator. Using the
example 20 Meter bandpass filter, the passband loss is about .25 dB with
matched 50 ohms in and out. Changing the output impedance to 38+j12 (38
ohms plus 135 nH inductor at 14.15 MHz, about 1.5:1 SWR), the passband
losses increase to about 0.4 - .44 dB and vary more over the band.

Here are the schematics and plots, anyone is welcome to check my
calculations, as I do make mistakes!

https://drive.google.com/file/d/1kj31IL_px6nVyRadW4nOi_c6FLgyaRyk/view?usp=sharing

The loss in the filter will almost double. For 100W in, the loss goes from
about 6W to about 10W. So, it is not a good idea. A worse match will result
in even more losses, perhaps overheating and destroying the filter.

73,

Mark
W7MLG

On Fri, Jul 5, 2019 at 8:32 AM Don Wilhelm <donwilh at embarqmail.com> wrote:

> Rich,
>
> Yes and no.  With 2 antenna tuners in-line, there will be a bit more
> loss due to inductor winding resistance, but other than that, it should
> do harm.
>
> Several bandpass filters indicate that they should be between the rig
> and the tuner (so the bandpass filters are not run at a high SWR).
> If the power rating of the bandpass filter is marginal with respect to
> the rig power, then I would observe that caution. Refer to the bandpass
> filter specs.
>
> 73,
> Don W3FPR
>
> On 7/5/2019 10:57 AM, Rich wrote:
> > To minimize emails direct replies would be nice.  I searched the web and
> > could not find an answer.   I know there are a ton of smart folks on
> > this list so I thought I would ask.
> >
> > On Field Day at typical setup is :
> >
> > K3 (or any radio) - bandpass filter -  External Antenna Tuner  - Antenna
> >
> > So the antenna was tuned via the external tuner, but saw a guy then
> > using the K3 ant tuner to touch up the SWR between the radio and the
> > bandpass filter.  Is that an acceptable practice?
> >
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