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Tuners and spurious responses

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Vic Rosenthal

Tuners and spurious responses

I just replaced my single-ended T-network tuner plus balun with a
massive old Johnson Matchbox. It is very selective, unlike the T
network. It is as if there is a sharp bandpass filter between the
antenna and the rig.

This may be totally imaginary, and there's no easy way to A/B test it,
but it seems as though the K3 sounds "cleaner" in some sense.
Could it be that since the mixer sees a much narrower spectrum, there
are fewer spurious responses?

Do those of you who use bandpass filters for SO2R or multi-transmitter
contesting notice such an effect?

It also seems that the better balance (my antenna system is a dipole fed
with balanced line) has reduced RF in the shack and possibly local noise
pickup.

--
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
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Don Wilhelm

Re: Tuners and spurious responses

Vic,

I have long been an advocate of the parallel tuned circuit matching
network, and an oldie but goodie of that is the Johnson Matchbox. It is
a bandswitching version of the old plug-in coil tuners. The matching
range is not as great as that with the plug-in coils with movable taps
on the coil, but it is quite useful even with the more limited matching
range.

Yes, it provides a bandpass response which keeps out 'crud' from bands
other than the one to which it is tuned. The high pass filter of the
"T" network tuner or the low pass filter of a "PI" network cannot
provide that same filtering.

Our local club has used fixed bandpass filters for the last 3 years with
a great improvement in station to station interference. The first year,
the 40 meter bandpass filter did not work as planned, and my Johnson
Matchbox was used in its place and did a fantastic job.

I admit, I am enslaved by the convenience of the Elecraft "L" network
ATUs, and I have not evaluated the comparison of receive noise with a
bandpass type ATU or filter. I am fortunate to have a relatively low
noise location for my station.

I cannot comment on the "RF-in-the-shack" aspects, but the balanced
Johnson Matchbox likely presents a better balanced load to your antenna
with parallel transmission line than any unbalanced tuner with a balun.

73,
Don W3FPR

On 12/22/2018 12:03 AM, Victor Rosenthal 4X6GP wrote:

> I just replaced my single-ended T-network tuner plus balun with a
> massive old Johnson Matchbox. It is very selective, unlike the T
> network. It is as if there is a sharp bandpass filter between the
> antenna and the rig.
>
> This may be totally imaginary, and there's no easy way to A/B test it,
> but it seems as though the K3 sounds "cleaner" in some sense.
> Could it be that since the mixer sees a much narrower spectrum, there
> are fewer spurious responses?
>
> Do those of you who use bandpass filters for SO2R or multi-transmitter
> contesting notice such an effect?
>
> It also seems that the better balance (my antenna system is a dipole fed
> with balanced line) has reduced RF in the shack and possibly local noise
> pickup.
>

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Barry LaZar

Re: Tuners and spurious responses

In reply to this post by Vic Rosenthal

Victor,
Somewhere in the collection of radio stuff I own, there is a Johnson
Matchbox which I haven't used in 20+ years, but I do remember a few
things about it. From what you say, I infer that you are using the KW
version, mine is the 275 Watt unit. I believe they used similar designs.
One of the things I sort of recall is that it was really a balanced feed
antenna tuner that could be used for unbalanced load tuning. And yes, it
was high Q meaning it was useful, sort of, for some filtering. However,
the final amplifier output circuit in the Johnson gear of the time was a
pi network which is useful as a low pass filter. And in those days low
pass filtering was important and the antenna tuner was important for its
intended design, matching only. Tuner efficiency was important as there
could be a lot of heat as AM, full carrier, was the mode of the day,
unless you ran CW. I did own a Johnson Viking II in my early days.

73,
Barry
K3NDM

------ Original Message ------
From: "Victor Rosenthal 4X6GP" <[hidden email]>
To: "Elecraft Reflector" <[hidden email]>
Sent: 12/22/2018 12:03:05 AM
Subject: [Elecraft] Tuners and spurious responses

>I just replaced my single-ended T-network tuner plus balun with a massive old Johnson Matchbox. It is very selective, unlike the T network. It is as if there is a sharp bandpass filter between the antenna and the rig.
>
>This may be totally imaginary, and there's no easy way to A/B test it, but it seems as though the K3 sounds "cleaner" in some sense.
>Could it be that since the mixer sees a much narrower spectrum, there are fewer spurious responses?
>
>Do those of you who use bandpass filters for SO2R or multi-transmitter contesting notice such an effect?
>
>It also seems that the better balance (my antenna system is a dipole fed with balanced line) has reduced RF in the shack and possibly local noise pickup.
>
>-- 73,
>Victor, 4X6GP
>Rehovot, Israel
>Formerly K2VCO
>CWops no. 5
>http://www.qsl.net/k2vco/
>______________________________________________________________
>Elecraft mailing list
>Home: http://mailman.qth.net/mailman/listinfo/elecraft
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:[hidden email]
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html
>Message delivered to [hidden email]

alorona

Re: Tuners and spurious responses

In reply to this post by Vic Rosenthal

Vic, it may well be your imagination (!) but you may also be hearing the rejection that your tuner gives you, particularly to strong AM stations in the broadcast band. My measurements, as well as circuit simulation, show about a 40 dB rejection of AM stations when the tuner is tuned to 40 meters.

Circuit simulation of the Johnson circuit shows not all that high a Q, but it certainly is starting to look like a broad bandpass response. (It's actually more high-pass than band-pass.) That's why it's effective against the broadcast band. It's yet another argument to use a tuner like that one.

Interestingly, I just moved from a link-coupled tuner back to an unbalanced tuner with balun because my measurements of common-mode current on the transmission line show that the balun is more effective at suppressing it. The link-coupled tuner acts more like a voltage balun which would be okay if the antenna were inherently balanced, but in many cases the current balun suppresses common-mode better when the antenna is in an environment that makes it not well-balanced. When power lines or houses or cars or other things are in the antenna's near field it tends to make the antenna present an unbalanced load to the transmission line. That's when equal currents (not voltages) work better. But I'm repeating what has long been known.

If you can measure this stuff, like with an RF current meter, it becomes much clearer.

Enjoy your new, cleaner reception, thanks to that tuner!

Al W6LX

>>> This may be totally imaginary,
>>> --
>>> 73,
>>> Victor, 4X6GP

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Vic Rosenthal

Re: Tuners and spurious responses

I have a 50 kW BC station in line of sight with my antenna. When I used a vertical antenna without a tuner, it was enough to overpower the bias in the K3’s T/R switch and generate spurs all over 40 and 30 meters. I fixed it with a highpass filter before changing to a horizontal antenna .
When I rotate my dipole, there is a point where one end gets close to a structure. The tuning changes, so I know it has an effect, which must unbalance the system. Maybe coincidentally and maybe not, local noise increases at that point. I think I will try isolating the tuner from ground and feeding it through a balun.

Victor 4X6GP

> On 22 Dec 2018, at 9:05, Al Lorona <[hidden email]> wrote:
>
> Vic, it may well be your imagination (!) but you may also be hearing the rejection that your tuner gives you, particularly to strong AM stations in the broadcast band. My measurements, as well as circuit simulation, show about a 40 dB rejection of AM stations when the tuner is tuned to 40 meters.
>
> Circuit simulation of the Johnson circuit shows not all that high a Q, but it certainly is starting to look like a broad bandpass response. (It's actually more high-pass than band-pass.) That's why it's effective against the broadcast band. It's yet another argument to use a tuner like that one.
>
> Interestingly, I just moved from a link-coupled tuner back to an unbalanced tuner with balun because my measurements of common-mode current on the transmission line show that the balun is more effective at suppressing it. The link-coupled tuner acts more like a voltage balun which would be okay if the antenna were inherently balanced, but in many cases the current balun suppresses common-mode better when the antenna is in an environment that makes it not well-balanced. When power lines or houses or cars or other things are in the antenna's near field it tends to make the antenna present an unbalanced load to the transmission line. That's when equal currents (not voltages) work better. But I'm repeating what has long been known.
>
> If you can measure this stuff, like with an RF current meter, it becomes much clearer.
>
> Enjoy your new, cleaner reception, thanks to that tuner!
>
> Al W6LX
>
>
>>>> This may be totally imaginary,
>>>> --
>>>> 73,
>>>> Victor, 4X6GP
>

Bob McGraw - K4TAX

Re: Tuners and spurious responses

In reply to this post by Vic Rosenthal

The Johnson Matchbox configuration is indeed a banpass filter meaning it
attenuates both above and below the frequency to which it is tuned.    I
use mine at Field Day to provide attenuation to stations operating both
above and below the band being used. The amount of attenuation does
vary as it is not symmetrical in nature.

Probably you were using a less than optimum balun which had little
common mode rejection or poor balance.   The best way to check the two
configurations is to measure the current in each leg of the balanced
feed line. Many baluns do not do a good job or making a "balanced"
feed.    The work of DJ0IP {see his website} has a lot of information
from real field measurements on baluns, good ones and bad ones.

73

Bob, K4TAX

On 12/21/2018 11:03 PM, Victor Rosenthal 4X6GP wrote:

> I just replaced my single-ended T-network tuner plus balun with a
> massive old Johnson Matchbox. It is very selective, unlike the T
> network. It is as if there is a sharp bandpass filter between the
> antenna and the rig.
>
> This may be totally imaginary, and there's no easy way to A/B test it,
> but it seems as though the K3 sounds "cleaner" in some sense.
> Could it be that since the mixer sees a much narrower spectrum, there
> are fewer spurious responses?
>
> Do those of you who use bandpass filters for SO2R or multi-transmitter
> contesting notice such an effect?
>
> It also seems that the better balance (my antenna system is a dipole
> fed with balanced line) has reduced RF in the shack and possibly local
> noise pickup.
>

K9MA

Re: Tuners and spurious responses

While I seriously doubt any rejection of nearby frequencies by a tuner
is likely to have any effect on the K3 receiver,
I'd point out that any tuner configuration other than an L network can
provide a match over a wide range of Q. One generally tries to tune them
for minimum Q to minimize losses. If tuned to a high Q, however, both
the T and pi networks generally will provide some rejection of adjacent
frequencies. Just how much is impossible to predict, unless you know
just how the antenna impedance varies with frequency. At far removed
frequencies, of course, a T does act like a high pass, and a pi like a
low pass, but in neither case do they match the antenna to 50 Ohms,
unless it happens to actually be 50 Ohms at some frequency.

That said, is suppose it IS possible that a tuner/antenna combination
just happened to have a deep null right on the frequency of a nearby
broadcast station which was causing intermodulation. If that were the
case, a more reliable solution would be a trap or stub.

73,
Scott K9MA

On 12/22/2018 08:52, Bob McGraw K4TAX wrote:

> The Johnson Matchbox configuration is indeed a banpass filter meaning
> it attenuates both above and below the frequency to which it is
> tuned.    I use mine at Field Day to provide attenuation to stations
> operating both above and below the band being used. The amount of
> attenuation does vary as it is not symmetrical in nature.
>
> Probably you were using a less than optimum balun which had little
> common mode rejection or poor balance.   The best way to check the two
> configurations is to measure the current in each leg of the balanced
> feed line. Many baluns do not do a good job or making a "balanced"
> feed.    The work of DJ0IP {see his website} has a lot of information
> from real field measurements on baluns, good ones and bad ones.
>
> 73
>
> Bob, K4TAX
>
> On 12/21/2018 11:03 PM, Victor Rosenthal 4X6GP wrote:
>> I just replaced my single-ended T-network tuner plus balun with a
>> massive old Johnson Matchbox. It is very selective, unlike the T
>> network. It is as if there is a sharp bandpass filter between the
>> antenna and the rig.
>>
>> This may be totally imaginary, and there's no easy way to A/B test
>> it, but it seems as though the K3 sounds "cleaner" in some sense.
>> Could it be that since the mixer sees a much narrower spectrum, there
>> are fewer spurious responses?
>>
>> Do those of you who use bandpass filters for SO2R or
>> multi-transmitter contesting notice such an effect?
>>
>> It also seems that the better balance (my antenna system is a dipole
>> fed with balanced line) has reduced RF in the shack and possibly
>> local noise pickup.
>>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

--
Scott K9MA

[hidden email]

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alorona

Re: Tuners and spurious responses

No, not a null, but a rolloff. When I quoted -40 dB I didn't mean a notch at one frequency, but the stopband level reached by the time you get well into the broadcast band. So that would be -40 dB on all AM stations below a certain frequency.
The K3 is a good receiver, but every receiver has its limits and certainly the K3 will suffer once an interfering signal gets above a certain level.
A trap or stub would work, but my point was that it would be unnecessary if using a link-coupled tuner. Attenuating a 50 kW station by 40 dB makes it sound like a 5 W station.
Al W6LX

>>> That said, is suppose it IS possible that a tuner/antenna combination
>>> just happened to have a deep null right on the frequency of a nearby
>>> broadcast station

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K9MA

Re: Tuners and spurious responses

I haven't analyzed them thoroughly, but I can see how a link coupled
tuner could provide more far out rejection both above and below the
operating frequency than an L, T, or Pi.

Link coupled tuners can also, of course, be inherently balanced. I've
never been a fan of using baluns with single-ended tuners to feed
unmatched balanced antennas, because it's just about impossible to build
a practical balun that can cover the whole range of possible impedances.
Open wire can operate with a 10:1 SWR with modest losses, but the
impedance at the tuner could be anywhere from 45 to 4500 Ohms. The
problem with link coupled tuners is that tapped coils are cumbersome and
somewhat dangerous. The differential capacitor in the Johnson Matchbox
was a way around that, but it was expensive and limited the range of the
tuner.

73,
Scott K9MA

On 12/22/2018 21:20, Al Lorona wrote:

> No, not a null, but a rolloff. When I quoted -40 dB I didn't mean a
> notch at one frequency, but the stopband level reached by the time you
> get well into the broadcast band. So that would be -40 dB on all AM
> stations below a certain frequency.
>
> The K3 is a good receiver, but every receiver has its limits and
> certainly the K3 will suffer once an interfering signal gets above a
> certain level.
>
> A trap or stub would work, but my point was that it would be
> unnecessary if using a link-coupled tuner. Attenuating a 50 kW station
> by 40 dB makes it sound like a 5 W station.
>
> Al W6LX
>
>
>
>
> >>> That said, is suppose it IS possible that a tuner/antenna combination
> >>> just happened to have a deep null right on the frequency of a nearby
> >>> broadcast station
>
>
>

Wes Stewart-2

Re: Tuners and spurious responses

In reply to this post by K9MA

Generally speaking, with components of the same Q, an L network will be the
lowest loss solution to any matching problem. The network Q is set only by the
two impedances to be matched. (I did once contrived a situation where matching
the very low R, high C gate impedance of an FET could be done with lower loss
using a series pair of L networks, but this is unusual.)

In correspondence with Dean Straw 20+ years ago when we were prepping my article
on ladderline, I pointed out to him that the problem with T networks with three
variables was that there are an infinite number of solutions, with only one
giving the lowest loss and some giving huge losses, with the operator being
clueless. I believe the same would be true with the Match Box. (By what I'm
sure must be coincidence, it was shortly after that an ARRL favored author wrote
article on tuner losses.)

Any rejection due to an antenna matching system is just serendipity and
depending on it for that purpose is foolhardy, IMHO, of course.

To pick one nit with Scott, a pi-network can be high pass and a tee-network can
be low pass.

Wes N7WS

On 12/22/2018 7:21 PM, K9MA wrote:

> While I seriously doubt any rejection of nearby frequencies by a tuner is
> likely to have any effect on the K3 receiver,
> I'd point out that any tuner configuration other than an L network can provide
> a match over a wide range of Q. One generally tries to tune them for minimum Q
> to minimize losses. If tuned to a high Q, however, both the T and pi networks
> generally will provide some rejection of adjacent frequencies. Just how much
> is impossible to predict, unless you know just how the antenna impedance
> varies with frequency. At far removed frequencies, of course, a T does act
> like a high pass, and a pi like a low pass, but in neither case do they match
> the antenna to 50 Ohms, unless it happens to actually be 50 Ohms at some
> frequency.
>
> That said, is suppose it IS possible that a tuner/antenna combination just
> happened to have a deep null right on the frequency of a nearby broadcast
> station which was causing intermodulation. If that were the case, a more
> reliable solution would be a trap or stub.
>
> 73,
> Scott K9MA
>
>
> On 12/22/2018 08:52, Bob McGraw K4TAX wrote:
>> The Johnson Matchbox configuration is indeed a banpass filter meaning it
>> attenuates both above and below the frequency to which it is tuned.    I use
>> mine at Field Day to provide attenuation to stations operating both above and
>> below the band being used. The amount of attenuation does vary as it is not
>> symmetrical in nature.
>>
>> Probably you were using a less than optimum balun which had little common
>> mode rejection or poor balance.   The best way to check the two
>> configurations is to measure the current in each leg of the balanced feed
>> line. Many baluns do not do a good job or making a "balanced" feed.    The
>> work of DJ0IP {see his website} has a lot of information from real field
>> measurements on baluns, good ones and bad ones.
>>
>> 73
>>
>> Bob, K4TAX
>>
>> On 12/21/2018 11:03 PM, Victor Rosenthal 4X6GP wrote:
>>> I just replaced my single-ended T-network tuner plus balun with a massive
>>> old Johnson Matchbox. It is very selective, unlike the T network. It is as
>>> if there is a sharp bandpass filter between the antenna and the rig.
>>>
>>> This may be totally imaginary, and there's no easy way to A/B test it, but
>>> it seems as though the K3 sounds "cleaner" in some sense.
>>> Could it be that since the mixer sees a much narrower spectrum, there are
>>> fewer spurious responses?
>>>
>>> Do those of you who use bandpass filters for SO2R or multi-transmitter
>>> contesting notice such an effect?
>>>
>>> It also seems that the better balance (my antenna system is a dipole fed
>>> with balanced line) has reduced RF in the shack and possibly local noise
>>> pickup.
>>>

Bob Nielsen-4

Re: Tuners and spurious responses

In reply to this post by alorona

DLW Associates makes a filter which has 40 dB rejection across the
entire broadcast band. It is available from DX Engineering.

On 12/22/18 7:20 PM, Al Lorona wrote:

> No, not a null, but a rolloff. When I quoted -40 dB I didn't mean a notch at one frequency, but the stopband level reached by the time you get well into the broadcast band. So that would be -40 dB on all AM stations below a certain frequency.
> The K3 is a good receiver, but every receiver has its limits and certainly the K3 will suffer once an interfering signal gets above a certain level.
> A trap or stub would work, but my point was that it would be unnecessary if using a link-coupled tuner. Attenuating a 50 kW station by 40 dB makes it sound like a 5 W station.
> Al W6LX
>
>
>
>>>> That said, is suppose it IS possible that a tuner/antenna combination
>>>> just happened to have a deep null right on the frequency of a nearby
>>>> broadcast station
>
>
>
> ______________________________________________________________
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W2xj

Re: Tuners and spurious responses

You can easily build a highpass filter.

Sent from my iPad

> On Dec 22, 2018, at 10:55 PM, Bob Nielsen, N7XY <[hidden email]> wrote:
>
> DLW Associates makes a filter which has 40 dB rejection across the entire broadcast band. It is available from DX Engineering.
>
>> On 12/22/18 7:20 PM, Al Lorona wrote:
>> No, not a null, but a rolloff. When I quoted -40 dB I didn't mean a notch at one frequency, but the stopband level reached by the time you get well into the broadcast band. So that would be -40 dB on all AM stations below a certain frequency.
>> The K3 is a good receiver, but every receiver has its limits and certainly the K3 will suffer once an interfering signal gets above a certain level.
>> A trap or stub would work, but my point was that it would be unnecessary if using a link-coupled tuner. Attenuating a 50 kW station by 40 dB makes it sound like a 5 W station.
>> Al W6LX
>>
>>
>>
>>>>> That said, is suppose it IS possible that a tuner/antenna combination
>>>>> just happened to have a deep null right on the frequency of a nearby
>>>>> broadcast station
>>
>>
>> ______________________________________________________________
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alorona

Re: Tuners and spurious responses

In reply to this post by Wes Stewart-2

One man’s serendipity is another man’s looking at the available antenna tuner topologies and choosing one with a very useful secondary function.

Al W6LX

> Any rejection due to an antenna matching system is just serendipity and depending on it for that purpose is foolhardy, IMHO, of course.
>
> Wes N7WS

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