Re: receiver input impedance?

On 7/9/09 20:43 , m II wrote:
D Peter Maus wrote:
On 7/9/09 19:52 , m II wrote:
I've been wondering how feeding two receivers from the same antenna
would affect performance.

If the input impedance is as high as I think, there shouldn't be much of
a drop.

I'm trying to compare two receivers on the same signals without the use
of an antenna pre-amp or some sort of active splitter.



mike

Depends on the impedance. Most coaxial inputs are 50 ohms. Single
wires can be much higher....over 300 ohms.

For simplicity let's consider 50 ohm coaxial input. Single wire high
impedance inputs are more complex to isolate.

Connecting two receivers to the same antenna, if the antenna is simply
split without isolation between the two inputs can cause the two
receivers to interact. Tuning one can affect reception in the other.
Birdies can be more numerous. One receiver can block the other.

Not to mention the antenna, as well as each receiver sees a
significant impedance drop.

So, you'll need some kind of isolation. A passive splitter can offer
enough isolation between receivers, but not in all cases. A passive also
halves the input to each receiver, which, in most cases, is not
significant as most decent receivers have more than enough gain to
compensate.

An active splitter will have better isolation, but not the same
gain/insertion loss port to port. As much as 3 db in the cases of some
of the good ones. A 3 db difference port to port can affect readability
of some weak signals, distorting your results.

Passive splitters are less expensive. Actives can run $200 or more
depending on in/out connectors and the number of ports. You can save
money by rolling your own. Mini-Circuits have components and
applications notes that make the project more accessible.

If a passive splitter will work for you, go for it. You can determine
the amount of interaction between receivers by setting them side by
side, and connecting them directly to the same antenna. If interaction
is low, a passive splitter may work well enough to get you where you
need to go.

If not, and your budget will have it, consider an active splitter. ICE
makes some really good ones. Stridsberg is another. You can also roll
your own. I have both ICE and Stridsberg. Little real world difference
between the two in service. You'll also need inline pads to offset any
variations port to port. You can build one. You can buy them ready made.
Connect one of the receivers with AVC off to one port of the active on a
medium level signal. Note the S meter. Connect the same receiver with
the same settings and same signal to the next port note the S-meter. Not
high precision, but your not exactly RNW Labs, here, either. Use
sufficient padding to balance the two ports.

You're ready to make your tests.


In your experience, do the active splitters add any noise of their own
to the signals? Is there any possibility of them amplifying any stray
signals from household sources?




Any active device will add noise. It's the nature of the beast. But any added thermal noise will be so overwhelmed by atmospheric noise you may, in practical terms, disregard it. ICE ands Stridsberg devices are quiet enough you can ignore them.

If you want to verify the implact of thermal noise from the active components, terminate the inputs to the active splitter, and connect it to your receiver. Compare that noise to the native noise of the receiver with a terminated input and you have your splitter noise.

You might also connect the splitter, terminate the input, measure the noise, then remove power to the splitter. Not as precise, but an indicator.

If your inputs are not open wires...if you're using coaxial inputs, and your system is properly grounded, your only path for household noise sources will be on the power lead to the active splitter. This can be a significant source of interference, hum and noise. So ferrite cores at each end of the power lead will be necessary. A grounded power supply will also help. But this usually isn't provided.



.

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  • Re: receiver input impedance?
    ... of an antenna pre-amp or some sort of active splitter. ... One receiver can block the other. ... do the active splitters add any noise of their own ...
    (rec.radio.shortwave)