[Hammarlund] SP-600 JX-26

[James A. (Andy) Moorer]

----- Original Message ----- 

> Dan Cotsirilos wrote:
>
> "...what the heck kind of ant connecter that is on here supposed to be? it 
> is nuts!"
>

It is just sitting there, begging to be replaced by an SO-239. You just 
ground one side of it to the chassis and all will be well.

Sometimes Fair Radio or Leeds will have a cable-end connector for the Twinax 
on the SP-600. If they do, it will not be in their catalog - you will have 
to email them.

>
> By the way, the antenna input impedance of the SP-600 is 100 ohms, not our 
> "modern" standard 50 ohms. If you're a purist and want to match your 
> receiver to a 50 ohm resonant antenna for a specific frequency, you can 
> build an impedance transformer or, better yet, use an antenna tuner.  It's 
> too bad an antenna trimmer control wasn't built into the set.
>
> This brings me to a question for the gurus. The RF alignment instructions 
> for the SP-600 call for the signal generator output to be fed to the 
> antenna terminal through a 100 ohm resistor. If a 50 ohm resistor is 
> substituted, would the front end then be able to be tweaked to a 50 ohm 
> input?
>

I actually measured the impedance of the input to the SP-600 (using an HP 
250B Rx-meter if you must know). I got a bit startled. First off, you get 
the same reading whether the radio is powered up or not. I guess this 
shouldn't have been a surprise, but it was. Next is that the actual 
impedance at the antenna input is not 100 ohms - it bounces all over the 
place and is different on different bands. The lowest impedance is on the 
low two bands, and is around 70 ohms. By the time you get to 14 MHz, it is 
300-400 ohms. It is different at different parts of the same band, and the 
behavior is not consistent from one band to the next. One of these days, 
I'll post a graph on my web site.

To answer the question, the alignment of the front end will not be affected 
by any resistors you put in the line. If there was any capacitive or 
inductive component, it might affect the alignment, but no resistive 
component can make any difference to the resonant frequency (except in the 
extreme).

And no, aligning it with a 50-ohm resistor will not make it work any better 
with a 50-ohm antenna. To match it properly, you really need an antenna 
tuner. Or, you can do like the rest of us, just plug it into whatever 
antenna you have and enjoy twirling that big, brass weight.

James A. (Andy) Moorer
www.jamminpower.com