[R-390] Antenna sweep
Bob Camp
ham at cq.nu
Sat Oct 27 18:57:57 EDT 2007
Hi
What does your antenna look like?
The only thing I can think of that would be 220 ohms at 3.75 MHz and
22 ohms at 1.65 MHz is some kind of loop ...
Bob
On Oct 27, 2007, at 4:55 PM, Tony Casorso wrote:
> This is a sweep of my antenna from 1MHz to 5MHz. I made it bigger so
> that the details can be seen.
>
> The underside of the curve (the cutout) is the antenna response. The
> flat(ish) top is the output of the sweep generator. This is put
> through a 2200 Ohm resistor to the antenna. The antenna resonse is
> with the probe on the antenna side of the 2200Ohm resistor, and the
> flat top is on the generator side of the resistor. The horizontal
> scale is 500KHz per division with 0KHz on the left edge of the
> screen and 5MHz on the right edge of the screen. The vertical scale
> is 10db per division.
>
> The antenna response peaks at about 3.75 MHz with another peak at
> 2MHz. The peak is 20db down from the generator output. This means
> the antenna looks like about 220 ohms at 3.75MHz. There are valleys
> at about 1.4MHz and 2.5MHz that are about 40db down from the
> generator output which means the antenna impedance is about 22 Ohms
> there.You can see a lone peak sticking above the generator output at
> about 1.65 MHz. Thats my friendly problem station.
>
> http://i25.photobucket.com/albums/c90/tonysradios/IMG_0828.jpg
>
> Tony
>
> ----- Original Message ----- From: "Tony Casorso" <canthony15 at msn.com>
> To: "Bob Camp" <ham at cq.nu>
> Cc: <r-390 at mailman.qth.net>
> Sent: Saturday, October 27, 2007 11:10 AM
> Subject: Re: [R-390] Re: Spurious Resonances
>
>
>> Hi Bob,
>>
>> OK. I get this. But I was not trying to show any distortion in the
>> pictures.
>>
>> All I was trying to show is that the Q of the preselector degrades
>> when the source impedance is low.
>> Thats it.
>>
>> If it's no surprise then so be it. The consequence of this is that,
>> with the receiver tuned to 3 MHz, a 1.5Mhz signal at the antenna
>> terminals is going to be 10 to 15db stronger at the grid of the RF
>> stage if the antenna impedance is low . This stronger signal is
>> more likely to cause overload and harmonic generation in the RF
>> stage. The downstream filtering in the plate of the RF stage and
>> the plate of the mixer stage will kill the 1.5MHz signal so the AGC
>> does not respond to it. So the RF stage has a strong signal at it's
>> grid (1.5MHz) and no AGC to compensate for it. So it distorts and
>> produces harmonics in the plate circuit which is tuned to 3MHz. The
>> harmonics are amplified and fed through the mixers and onward right
>> through the receiver.
>>
>> I don't think the loading of a 10Meg probe is going to kill the Q
>> that much at 3MHz (my "sharp" picture is made with the probe
>> connected after all). The capacitance will shift the resonant peak
>> slightly at 3MHz. Even if I did reduce the Q with my scope probe by
>> some amount, the low impedance source reduces it by a much greater
>> amount. If I had a 100x probe I might see a 40db attenuation at
>> 1.5MHz relative to the peak. But I would still see the 15 db
>> attenuation with the low impedance source.
>>
>> I found an antenna impedance calculation program on the web for
>> inverted Ls where you can specify the length of the horizontal part
>> and the vertical part as well as the length and gauge of the ground
>> lead and the gauge of the antenna wire. I tried it for my antenna
>> dimensions using zero for the length of the vertical segment. Not
>> that I believe it, but it came out 18 ohms at 1.65MHz.
>>
>> I wonder if I could sweep it with the antenna connected?? Probably
>> too much BCB stuff to see what is going on down there.
>>
>> Tony
>>
>> ----- Original Message ----- From: "Bob Camp" <ham at cq.nu>
>> To: "Tony Casorso" <canthony15 at msn.com>
>> Cc: <r-390 at mailman.qth.net>
>> Sent: Saturday, October 27, 2007 10:36 AM
>> Subject: Re: [R-390] Re: Spurious Resonances
>>
>>
>>> Hi
>>>
>>> Any filter is going to respond to it's termination impedances.
>>> Both the input and output impedance are significant.
>>>
>>> The grid of the RF stage looks like > 100 meg ohms resistive at
>>> RF. It's got a bit of capacitance, but not a whole lot.
>>>
>>> If you look at the preselector like a single tuned circuit, it's
>>> going to have Q. The Q is going to be determined by:
>>>
>>> 1) The resistive portion of the source resistance driving the
>>> preselector, relative to the reactances.
>>> 2) The Q of the coils themselves
>>> 3) The resistive portion of the load resistance, relative to the
>>> reactances.
>>>
>>> The "relative to the reactances" part gets a bit crazy because you
>>> normally have part of the resistances in series form and the
>>> other part of the resistances in parallel form. There is a great
>>> thing called the "Q formula" that lets you convert them all back
>>> and forth. It was a staple in EE texts up through about 1954.
>>>
>>> Vacuum tubes have really low Gm compared to just about anything
>>> else. To get a stage to have gain, you need very high impedances.
>>> The normal way to make stuff work was to run impedances way
>>> higher than we do today. You can easily fake yourself out going
>>> from solid state to tube. Loading is a much bigger deal when
>>> checking tube circuits.
>>>
>>> A measurement you can make on a solid state amp with very little
>>> interaction, is much more difficult to make on a tube circuit. An
>>> easy way to see this is using a DVM on an R390. The numbers don't
>>> come out right. The input impedance of the DVM is not as high as
>>> that of a VTVM. The DVM is fine on solid state, it's not up to
>>> the task with tubes.
>>>
>>> By going to the plate for your measurement, the load resistance is
>>> no longer interacting with the preselector at all. The source
>>> resistance is still there, but the antenna has source resistance
>>> as well. Once you have the load impedance out of the way you can
>>> be reasonably sure that what you are measuring is "real".
>>>
>>> Since you are after second order distortion, every db you knock
>>> down the input counts as two db at the output. If you were
>>> talking about third order (intermod) distortion, each db of
>>> attenuation would count three db. A 30 db selectivity in the
>>> preselector will take second order distortion down 60 db and
>>> third order down 90 db. At low levels, a tube amp should have
>>> second order distortion down > 30 db with no preselection at all.
>>>
>>> If the response you see is at exactly twice the input frequency,
>>> it's second order distortion. The conversion scheme in a 390 does
>>> not produce other spurs that follow a 2:1 ratio like that. You
>>> can write a book about all the possible spurs that can occur (and
>>> people have). The math is pretty simple. It's been mentioned
>>> before in this thread. Take the two sine waves, add them or
>>> subtract them. Take that and raise it to an integer power. Powers
>>> of 0,1,2,3,4, and 5 are commonly used. If you have a mixer you
>>> have three signals rather than just two. The same "raise it to a
>>> power" is used. You can include am, fm, and pm in the signal
>>> equations. AM, and FM modulation transfer differently through
>>> each spur generating process. AM is transfered by all of them to
>>> the extent that the amplitude of the output will follow he
>>> amplitude of the input through the db/db ratio for that response.
>>>
>>> So much fun .....
>>>
>>> Bob
>>>
>>>
>>>
>>>
>>>
>>> On Oct 27, 2007, at 11:39 AM, Tony Casorso wrote:
>>>
>>>> Hi Bob,
>>>>
>>>> I'm not sure what you are getting at. I used a 10x scope probe
>>>> connected to the Grid of the RF stage. I want to measure the
>>>> response of the preselector. I do not want to swamp anything.
>>>> The two pictures show a sharp response from the preselector in
>>>> one case, and a not so sharp response in the other case. The
>>>> only difference is the impedance of the signal source connected
>>>> to the antenna.
>>>>
>>>> Tony
>>>> ----- Original Message ----- From: "Bob Camp" <ham at cq.nu>
>>>> To: "Tony Casorso" <canthony15 at msn.com>
>>>> Cc: <r-390 at mailman.qth.net>
>>>> Sent: Saturday, October 27, 2007 9:04 AM
>>>> Subject: Re: [R-390] Re: Spurious Resonances
>>>>
>>>>
>>>>> Hi
>>>>>
>>>>> What are you picking off the output with?
>>>>>
>>>>> Picking off at the plate would be a *much* better idea if you
>>>>> are trying to measure the input circuit. You will have far less
>>>>> impact from the probe that way. If you want to suppress the
>>>>> effect of the output tank, swamp it with a low probe impedance.
>>>>>
>>>>> Bob
>>>>>
>>>>>
>>>>> On Oct 27, 2007, at 10:35 AM, Tony Casorso wrote:
>>>>>
>>>>>> How lucky can I get? I posted a set of images this morning
>>>>>> that had a "spurious response" in it. It turns out it was
>>>>>> cause by the settings on my sweeper (DC Offset was cranked
>>>>>> over to one side for some reason). The mailer wus unable to
>>>>>> post it for some reason so I get to try again with good
>>>>>> pictures.
>>>>>>
>>>>>> Here are links to couple of pictures. I am sweeping the
>>>>>> preselector in my R-390a. The left edge of the sweep is
>>>>>> 1.5MHz. The right edge is 4.5MHz. The center is approximately
>>>>>> 3MHz. The receiver is tuned to 3.00MHz and the power is off. I
>>>>>> am measuring the response at the grid of the RF stage.
>>>>>>
>>>>>> The broader response picture is with the 25 Ohm (gen with 50
>>>>>> Ohm rterminator) sweep gen directly connected to the balanced
>>>>>> input. The sharper response is with a 2.2K resistor in series
>>>>>> with one side of the balanced input.
>>>>>>
>>>>>> It looks like I was right that low source impedance broadens
>>>>>> the frontend response.
>>>>>>
>>>>>> http://i25.photobucket.com/albums/c90/tonysradios/IMG_0825.jpg
>>>>>>
>>>>>> http://i25.photobucket.com/albums/c90/tonysradios/IMG_0826.jpg
>>>>>>
>>>>>> Tony
>>>>>>
>>>>>> ----- Original Message ----- From: "Tony Casorso" <canthony15 at msn.com
>>>>>> >
>>>>>> To: <r-390 at mailman.qth.net>
>>>>>> Sent: Saturday, October 27, 2007 8:09 AM
>>>>>> Subject: [R-390] Re: Spurious Resonances
>>>>>>
>>>>>>
>>>>>> I should havethought about it for a few more minutes.
>>>>>>
>>>>>> Maybe the sweeper has a strong second harmonic that is visible
>>>>>> near 1.5.
>>>>>>
>>>>>> Tony
>>>>>> _____________________________________________________________
>>>>>> R-390 mailing list
>>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>>>>> Help: http://mailman.qth.net/faq.htm
>>>>>> Post: mailto:R-390 at mailman.qth.net
>>>>>> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>>>>>> _____________________________________________________________
>>>>>> R-390 mailing list
>>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>>>>> Help: http://mailman.qth.net/faq.htm
>>>>>> Post: mailto:R-390 at mailman.qth.net
>>>>>> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>>>>>>
>>>>
>>>
>>
>> _____________________________________________________________
>> R-390 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>> Help: http://mailman.qth.net/faq.htm
>> Post: mailto:R-390 at mailman.qth.net
>> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>
> _____________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>
More information about the R-390
mailing list