On 2019-07-05 1:55 PM, Jack Brindle via Elecraft wrote:
The external ATU takes care of the antenna matching, and should
> present a 50 ohm load to the BPF.
This only hold true for the *single frequency* on which the external
antenna tuner is "tuned". Typically (depending on the tuner Q and
losses), the SWR seen by the BPF will be *HIGHER* away from the one
"matched" frequency (e.g., the other end of the band) than it would
be without the external tuner in place.
As such, the tuner in/tuner out will increase losses and heating in
the BPF. If the BPF is a marginal design, the added loss/heat could
be fatal to the BPF.
The proper way to handle a rig with built-in tuner is to bypass the
internal tuner (or tune it into a 50 Ohm load in the middle of the
band) and do *all tuning with the external tuner* which assures the
BPF always sees a 50 OHM load.
73,
... Joe, W4TV
On 2019-07-05 1:55 PM, Jack Brindle via Elecraft wrote:
Lets step back a bit and look at the system here. The K3, with ATU, drives into
the BPF, when then drives into the external ATU and finally the antenna.
The external ATU takes care of the antenna matching, and should present a 50
ohm load to the BPF. The BPF, because of its design, should present a 50 ohm
load to the ATU as well, so everything is matched there.
Before anyone jumps on this, remember the signals go both ways, outbound for
transmit, inbound for receive. Also, we have a fundamental principal thrown at
every EE student, that for best transmission of signal, the source and load
impedances should match.
OK, so let’s look at the K3 side. The K3 antenna port connects directly to the
BPF’s radio port. Again, we are presuming the BPF is designed for 50 ohms
resistive source/load. The K3 is designed to transmit into a 50 ohm load, but
it may not itself be a 50 ohm source. And, the receiver input may not be 50
ohms as well. Adding a tuned ATU does bring this to 50 ohms, providing a proper
match into the BPF, so that optimum signal flows both ways. So it could
actually be beneficial for the ATU to be in-line and properly tuned. The best
way to perform the tune would be to tune the ATU into a 50 ohm dummy load, but
tuning into the BPF at low power should work also. The exception to this would
be if the BPF changes impedance when power is applied, but then if this happens
the best place for that BPF is the trash can.
Now having said this, the ATU can also be bypassed in the K3, and that circuit
may provide a proper 50 ohm source/load for the BPF. I would probably take this
approach myself (and in fact do so in my station). The reason I take this
approach is to avoid the small loss that the internal ATU presents. In
contesting, every db matters.
Note that we are talking nits at this point. Unless the ATU was tuned for a
load quite a bit off 50 ohms resistive, (quite possible), then retuning won’t
make much difference. Perhaps the op had this situation, and thought it better
to return the ATU than to bypass it, or he didn’t even think to bypass it. The
point is, both methods will work, and I doubt he did anything wrong it
performing the tune.
73!
Jack, W6FB
On Jul 5, 2019, at 10:18 AM, Mark Goldberg <[email protected]> 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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