Topband: Input Impedance of the FT1000/S-Meter Calibration issues

[Hardy Landskov]

I have measured essentially the same thing; not real good but they pass
signals. If you look at the schematic, the attenuators are really simple
voltage dividers without regard to maintaining any kind of decent return
loss as might be required by a phased Flag Array, or phased Beverage Array.
How much more would it have cost to put in DPDT relays and do it right? Not
much, maybe another $0.25 a relay.
It's really this kind of crap that I object to especially when one spends
this kind of money.  It's just cheap s__t design.
My opinion....
N7RT

At 02:35 PM 2/9/04 EST, K8LV1 at aol.com wrote:
>Recent comments on this topic finally started to focus on one of the weak 
>links in trying to establish meaningful S-meter calibration. That is, the 
>question of the true input Z of the RX. This is very important since
S-meters 
>essentially measure the voltage present at the RX input.
>
>In order to get ahandle on this I took some test equipment over to K8LX's FB 
>contest station (where FT1K's always abound) and Steve and I made accurate 
>measurements, which are summarized below. The figures show the actual
measured Z 
>in all positions of the input gain/attenuator switch. The figures for each 
>band report Z for the switch settings (respectively) NOR, 6db, 12db, and
18db. 
>Values did not change noticeably between IPO and NOR so we didn't bother 
>reporting them separately.
>
>1.8MHZ:  (37+j18)  (68+j15) (108+j63) 108-j80)
>
>3.5MHZ  (32-j27)  (49-j19)  (36-j62)  (36-j62)
>
>7.o MHZ  (60+j35)  (58+j33)  (37-j25)  (22-j27)
>
>14.0MHZ  (17+j24)  (16+j19)  (18+j21)  (10+j16)
>
>21 MHZ  (65+j11)  (52+j30)  (43+j52)  (16+j78)
>
>28MHZ  (38+j32)  (76+j0)  (94-j58)  (94-j58)
>
>We were particularly disturbed by the 20 meter readings, thinking that maybe 
>that radio was screwed up in the front end. So we measured another FT1K
and it 
>gave essentially the same readings!
>
>These readings illustrate the futility of trying to calibrate the S-meter in 
>absolute units, with  over 10-20 db of variation at the input terminals. 
>Another interesting note: I used an HP 4815A Vector Impedance meter which
'excites' 
>the DUT with a constant current of 12uA, which would produce 600uV across a 
>50 ohm load (about -50dbm). As we tested at various frequencies, when we
tuned 
>the RX precisely to the 4815 for maximum meter reading, the S-meter readings 
>went from S3 or S4  to well over S9, as would be expected with the input Z 
>jumping around as reported.
>
>73
>Eric von Valtier K8LV
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