Hi Robert

Good to see you on again! Yes quite a lot of endless possibilities!

It all pretty well boils down to how much loss you can "stand". Any
unswitched configuration will lose at least half the signal in a 2 way
split. This probably isnt a big deal at HF (since received noise is
always much greater than the receiver noise floor), but may be "just
enough" to cause problems at VHF+. Physical switches also introduce loss
that varies with quality and operating frequency. One day too you'll
want to have two receivers going concurrently and if you have a switched
config will curse having it that way!

My suggestion? Check that there isnt any DC present on the antenna
terminal of all your radios (that might be used for external preamps
etc) and just hard connect the two scanners to the one antenna. Do some
signal checks on a few known stations and see if you are losing enough
to be troubled.

Do a few diferent evolutions to check the HF radio/antenna in tandam
with the VHF/UHF system and standalone. I would expect problems here as
some VHF/UHF antennas will look like dead shorts at HF.

If you have a signal generator set that up as a beacon in your yard. You
may also be able to hear harmonic content to check up higher. (I often
use the 5th harmonic of my HF radio to freq check my VHF radio, after
having calibrated my HF radio to WWV)

Combiners are often transformers on toroids of varying sizes per
frequency range. VHF+UHF combiners can be made of etched PCB's. You'll
find quite a few designs in Ap Notes by the likes of Motorola if you
look around. This may of course not be an option for you. Single
frequency (+ odd multiple) splitters are much easier to build. All you
need is coax!

In the end I would hope you can get away with low pass filter on the HF
antenna (an old CB TX filter will do) and a high pass filter on the
Scantenna (maybe a TVI filter meant for 75 ohm TV installs), a broadband
combiner/splitter and 4 outputs, one to each RX.. You may even be able
to use a CATV splitter. They supposedly work down to 5MHz nowadays (for
cable internet upload) Thats about the cheapest I can think of, but of
course there will be some loss. I dont think CATV splitters go past
about 1GHz for example.

As to your off vs on Q2. In a theroetical world your splitter device
would have a specfic Z and for max power transfer your receivers would
have the same. It never happens that way in real life though. In your
case too you are receiving only so the problems become a lot simpler!
Yes you would lsoe a proportion of the signal. Only you can determine if
that is significant.

And to "meaningful" loss? Try it! Changing your antenna height may even
negate all the loss you are concerned about. Thicker coax may be an
option too if you are interested in a lot of distance UHF work. Then
again if you are that interested a preamp may also be in order!

Apologies for not being specific!

Cheers Bob W5/VK2YQA


Robert11 wrote:
Hi,

Just an elderly sw listener from the old days.
Not very sharp with antenna theory, frankly.