[AMPS] Re: 2m legal limit amps

w8jitom@postoffice.worldnet.att.net w8jitom@postoffice.worldnet.att.net
Wed, 23 Jul 1997 22:17:37 +0000


> Date:          Wed, 23 Jul 97 22:40:54 +0000

> Ian and I have agreed on the screen supply since our first chat a year
> ago. 
> The problem with the cathode resistor Tom is that it does not adapt very
> well at VHF and above. 

OK, that would be understandable because of phase shift due to stray 
C.

> I have a pair of TS940's and a nice 51J4 with all filters. Is this
> adequate?  Or would I be better to use another TS940....I have 4 of them,
> all with IRCI filters? Phase noise issue?

The IMD will be well above the phase noise level. The main problem is 
shielding and having an accurate attenuator. 

> I also have a Ballantine 300 RMS AC Voltmeter and a HP 3400A version to
> eliminate S Meter error; simple to plug into the audio line.  Ancient but
> damn accurate. 

You probably need the attenuator. The dynamic range would have to be 
over 40 dB with the AGC disabled. I am not sure, but I doubt the 
hum and noise at the audio port would allow that dynamic range.
   
> I am not against spending a few Pesos to build a jig that would yield
> real results. NO...not for 10KW tubes but a few of the popular 400-1000W
> (each) tubes. I think you know Tom that my primary interests these days
> are VHF and above at serious power. Maybe in November I can get warmed up
> to 160M again. 

Use some doubled sided PC board, and build a shielded housing for 
the test receiver with line bypasses and a feedthrough connector to 
the attenuator, and put the attenuator in the box. I used a box like 
this with a screen door for access to the knobs, and a push button 
precision attenuator (available from JFW and other companies, or 
stolen from a scrap HP analyzer ).

> I have noticed that my  51J4 hears absolutely nothing with the antenna
> port terminated. The TS-940 is good but not like the old Collins.

Then maybe you don't need the box.
 
> A few guys have suggested the HP141T system but I have heard all horror
> stories about them. Any comments? 

They overload easy, and so are a bit easy to misuse. If no one dumps 
ten watts into the input port they usually last a long time.
   
> I have a nice HP8568B available at times but that is pure overkill for my
> needs. I dont mind spending a few $$ for something that will give me a
> few years of trouble free use. From what I hear, the HP141T is unreliable
> and parts are no longer available. 

Old is the key word.
 
> What about some of these off-shore  DC-1300MHz Spectrum Analyzers? Any
> experience or comments?

I borrowed a few as evaluation units and they were poor, they had 
poor scale tracking and spurious responses. I'd just use a receiver, 
it'll work better AND be cheaper. Just find a good attenuator pad.

A spectrurm analyzer is good for looking at broadband signals or 
searching for unknown frequencies. If you know where to look, as with 
IMD or harmonics, a receiver is just as good or better.

73, Tom W8JI 

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