[R-390] Re: Image Problem

Sun Oct 21 22:19:20 EDT 2007

HI

I do believe that the transmitter output on a broadcast AM setup must  
have all harmonics at -60 dbc. Most gear that I have seen has the  
harmonics at -100 or so. It's sort of a "matter of pride" thing to  
keep your signal far more clean than the regs require.

After the transmitter you have what ever is being done to match and  
phase the antennas. Your guess is as good as mine here. Let's say  
another 20 to 40 db.

Once you hit the antenna you are often feeding a sub 1/4 wave at the  
BC band. You might or might not have a 1/4 wave at the second  
harmonic. My *guess* is that the antenna isn't going to be very "hot"  
on the second harmonic. For guesses say -3 db, but it might be +3 or  
-20 ...

More or less the radiated signal should have the second harmonic  
below -80 and likely way below. Normally you will get far more  
harmonics and crud from rusty fence wire, or loose down spouts on  
your house than directly from a broadcast transmitter. The main  
exception to that rule is stuff like remote repeaters that runs very  
low power with nobody paying much attention at all to it. You rarely  
see them on AM, but they do exist on FM.

Bob

On Oct 21, 2007, at 9:32 PM, Jim M. wrote:

> How do we know that the broadcasters aren't actually radiating a  
> harmonic?
> What are the FCC requirements for harmonics on the BCST band?
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net [mailto:r-390- 
> bounces at mailman.qth.net]
> On Behalf Of Bob Camp
> Sent: Sunday, October 21, 2007 7:32 PM
> To: Tony Casorso
> Cc: R-390 List
> Subject: Re: [R-390] Re: Image Problem
>
> Hi
>
> Roughly speaking, assuming that 1 micro volt is 0 db:
>
> 100 mv is 10^5 micro volts = +100 db
>
> If your carrier meter is calibrated for 0 = 1 uv then a +40 reading
> would be 60 db second order rejection.
>
> That's actually pretty good overload performance for that kind of  
> input.
>
> One mili volt would be 40 db rejection which is not quite as good.
> You do have a *lot* of signal there .....
>
> You have to be very careful how you measure levels inside an R390.
> The impedances are very high. Unless you have a real good FET probe,
> it's very hard to measure anything without de-tuning or loading the
> circuits. That said a 5:1 reduction from the front end tuning doesn't
> sound like a real good performance from the front end.
>
> Bob
>
>
> On Oct 21, 2007, at 5:11 PM, Tony Casorso wrote:
>
>> I'm losing track now :). I'm no longer positive where the sig gen
>> was. I tried it so many ways. I have checked the AGC before against
>> the curve in the manual and it is pretty close. You could pick off
>> -4.2 volts on the curve and have some idea what I was applying at
>> that time.
>>
>> Tony
>>
>> ----- Original Message ----- From: <SHELLY199 at aol.com>
>> To: <r-390 at mailman.qth.net>
>> Sent: Sunday, October 21, 2007 2:58 PM
>> Subject: [R-390] Re: Image Problem
>>
>>
>>>
>>> Tony,
>>>
>>> When you measured the  voltages below what was the gen input  
>>> voltage?
>>>
>>> Voltages are
>>>
>>> With  no AGC applied:
>>> Plate 201v
>>> screen 178v
>>> cathode 2.1v
>>> grid  -.5v
>>>
>>> With AGC applied:
>>> Plate 215v
>>> screen 201v
>>> cathode  .22v
>>> grid -4.2v
>>>
>>>
>>> The agc, with the Langford mod, should be  about:
>>>
>>> 10 Uv rf input = -4.36 Vdc
>>> 100Uv rf input = 6.66
>>> 1000 Uv rf  input = 8.53
>>>
>>> I don't know if this will help, but you are talking some  very big
>>> rf input
>>> voltages (20mv).
>>>
>>> I also had a problem with the Ist rf  amp not responding to the
>>> AGC voltage
>>> once.   Tube tested good and the  agc voltage was there.  Was
>>> fixed with
>>> another rf amp tube.  That took  a lot of work.
>>>
>>> By the way, if you use In914 or In4148 diodes in that  Langford
>>> mod I've
>>> found that one of the diodes will fail.  I use  1n5060.  I think
>>> it's breakdown
>>> when the AGC switch is turned from slow to  medium. This has
>>> happened in at
>>> least 5 IF decks.
>>>
>>> Rich WD2Q
>>>
>>>
>>>
>>>
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