[AMRadio] Cable splitters for receivers


james.liles at comcast.net james.liles at comcast.net
Sun Jan 12 15:55:12 EST 2014


Hi Geoff:

The difference in sensitivity when two radios live on the same line will 
vary markedly.  If you are listening to one radio on 20 meters and the other 
is peaked via pre-selector or frequency dial peaking on 80 meters, the 80 
meter radio presents a tuned circuit with very low impedance across the 
line ---- possibly a few ohms.  Even on the same band as you tune one to the 
top and other to the bottom, you cannot help but hear the difference, 
especially if the Q is high in the tuned circuits.

The SR-150 has a loaded Q of approximately 85 on 20 meters.  If you connect 
two SR-150's to the same antenna you would expect a 3 to 6 db loss but truth 
is you will have at least a 10+db loss if each is tuned to opposite ends of 
the band.

All bets are off if the radio is poorly aligned or has faulty parts in the 
RF pre-selector area.  One other note.  A radio can have an RF pre-selector 
that was designed by the bean counters; as an afterthought.  Band 4 on the 
Hal S20R, SX-24, SX-25 etc.  Those radios need a bunch of rework and some 
witchcraft to make them right.

Kindest regards Jim K9AXN

-----Original Message----- 
From: Geoff
Sent: Saturday, January 11, 2014 2:03 PM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] Cable splitters for receivers


On 01/12/2014 01:04 PM, Donald Chester wrote:
>
> I once used two receivers, a 75A-4 and a R-390, just by connecting the 
> same transmission line to both receivers using a T-connector. It didn't 
> seem to attenuate either receiver's signal level noticeably. The greatest 
> danger in doing that is that spurious products from one receiver will show 
> up in the other.
>

I did that same thing with a pair of Hallicrafter SX-73's (R-274-D) and
what I noticed was, that trying to peak the 'Antenna' tuning on one
receiver, detuned the other.

I'm going to ass/u/me that the input impedance on the Hallcrafter SX-73
is ~50ohms, simply for the mere fact that they are fed via SO-239's.

So, approx 25ohms at each receiver is pretty low - about half of what
they're looking for.

That's a fairly low impedance, but to have the tuning of one affect the
other was unexpected.  Perhaps a 2:1 transformer, and parallel each on
the secondary may have eliminated that phenomenon.


-Geoff/W5OMR

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