[R-390] Antenna sweep

Tony Casorso canthony15 at msn.com
Sat Oct 27 16:55:39 EDT 2007 

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
>>>>> _____________________________________________________________
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>>>
>>
>
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