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D. Peter Maus · July 12th 09, 02:20 AM posted to rec.radio.shortwave

On 07/11/09 19:50, wrote:
On Jul 10, 8:50 am, "D. Peter
wrote:
On 07/10/09 10:17, wrote:

On Jul 9, 7:22 pm, D Peter wrote:
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.
The atmospheric noise will dominate for HF, but if you get a
multicoupler for UHF, you probably do want to worry about the noise
figure. If you do it right, the performance can be better with the
multicoupler, i.e. if the noise figure of the preamp is better than
the radio.
Yeah, UHF, well anything above HF, is a different animal
entirely. And there, a active multicoupler can nearly always improve
things. And an active will definitely introduce additional noise.
So, care in selection and installation of an active device is crucial.

Working VHF/UHF its even more important to take care when working
multiple receivers off an single antenna. LO leakage is often more
difficult to contain, and receiver interaction is almost a given. It
can be done...hell, it's done every day...but care must be taken.
And it's not always easy.

Appliances around the house can be the source of really annoying
interfering signals up that high. Especially, if you have an HDTV in
the house.

If you wait long enough, the MiniCircuits splitter you want will show
up. I've bought them new as well. Pricey, but the quality is very
good.

An active multicoupler has the potential to improve the SNR IF (big
if) the noise figure of the multicoupler( amplifier less the splitter
loss) is better than that of the radio itself. Remember, the game is
input referred input noise.

I've got a custom preamp made by Angle Linear for the mil air band
that uses a dual stage PHEMPT amp. The gain is high enough to that
even after a 4 way split, the noise figure is probably better than the
radios I am using.

An interesting thing about this preamp is it has no limiting. Normally
you wouldn't think of a preamp needing a limiting, but this thing with
dual gain stages and lots of beef could fry a radio input if someone
keyed up right next to it. With the splitter attached (Minicuit), the
power is reduced enough that this isn't an issue.

Bear in mind that a preamp followed by a splitter is not the same
as a multicoupler, and may not give you the isolation you seek to
prevent receiver interaction while you do your test.

A active multicoupler, such as the models by ICE, have discreet
active devices at each output. This severely attenuates backward
leakage from one receiver to the input of another. It's also why
there is a variation in insertion loss/gain port to port...each
active device has its own characteristics. Its own gain profile. Its
own noise profile, too.

A splitter offers only a few db of isolation port to port. You
may need 20 db or more of isolation port to port to prevent
interaction.

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