[Elecraft] OT a bit: K3 Ant Tuner

Fri Jul 5 15:51:57 EDT 2019

As commonly used, bandpass filters have one main use - to protect receivers. That may be the local receiver, protecting it from sting out of band signals, or a neighbor receiver, gain protecting it from strong out of band signals.
This quite well describes SO2R stations, where we are very concerned about receiver damage. The BPF is commonly placed between the transceiver and antenna, which for a multiple of reasons needs to be 50 ohms impedance.
One of the major jobs given to the BPF is to suppress harmonic content of the transmitted signal, for which the antenna load will most certainly NOT present a 50 ohm load to the BPF. There are many references for this, I would refer to W2VJN’s publication “Managing Interstaion Interference” and the excellent articles by K9YC (who will most likely join into the discussion shortly). Jim has done a lot of testing with BPFs, and has some very interesting articles comparing and discussing their use, as well as other articles about how to avoid damaging receivers in SO2R stations with their use, as well as the use of stubs for harmonic suppression.

The point is, BPFs are designed for both scenarios, to pass signals in-band, where the load impedance is 50 ohms, and reject out of band signals, where the impedance is almost never50 ohms. Note that most BPFs do not do much for in-band signal rejection (by design). Using them to protect a receiver in the same band is a recipe for disaster.

There is another point being missed. The K3 design, like most transceivers, contains a Low Pass filter (LPF) after the PA (used for both the low power output and the KPA3 if installed), followed by a directional coupler. The directional coupler needs to see 50 ohms at both the input and output for it to measure signals accurately. It can pretty well be assumed that when the internal ATU is in bypass, the K3 is close to 50 ohms at the antenna port. Thus, in bypass, the BPF should see the load it needs at its output. 

Again, the purpose of the BPF is to protect receivers, so the load we generally discuss is the K3 receiver, which will be 50 ohms due to the directional coupler input, But we also need for it to see an in-band 50 ohm load at its other end so that it can properly do its job. Note that BPFs are bidirectional - it really doesn’t matter how you connect them they do the same job both ways. They will generally see higher signal levels when in the active TX path (and thus dissipate more heat), but they are active in both directions, and for passing signals with very little loss, need to see a proper load.

73!
Jack, W6FB

> On Jul 5, 2019, at 12:27 PM, Joe Subich, W4TV <lists at subich.com> wrote:
> 
> 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 <marklgoldberg at gmail.com> 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 <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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