You're offering a scenario different from the original. As I read it,
originally a tuner was used to flatten the load that terminates the BPF output.
Your analysis assumes a matched driver and a mismatch on the output of the BPF.
Then the OP mentioned a second tuner between the TX output and the input of the
BPF, which can be assumed to match the BPF input. The BPF is now matched at
both ends.
To answer the OP's question, the answer is, IMHO of course, yes.
Wes N7WS
On 7/5/2019 10:18 AM, Mark Goldberg wrote:
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 <[email protected]> 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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