ANTENNA SHARING
 

Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30 -
512 MHz. They would both be tuned into seperate frequencies and need
to share a common broadband, omni directional, vertical monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation and
harmonics would be considered? Would these factors not mean that the
input impedance to the set tuned into the different frequency would
remain high thus not effect matching?

ANTENNA SHARING
 

I'll let others delve into the real depths here...

It is possible (some would say likely) that if you dont separate the
radios by some antenna line filtering (eg diplexer) one transmitter
could damage the others receiver. This will happen even if they are on
differing frequencies because it is likely the first amplifying device
in line will be made to work across a broad range of frequencies. There
is a good chance that you may not notice the damage if the signals you
normally listen to are strong, only to find that the blown front device
is needed when they get weak.

If you wanted to consider a diplexer you have another big problem in
that your operating frequencies overlap and are very wide. Most of these
filters tend to work on a fairly narrow band or widely separated wider
bands. If you segment the transceivers to specific operating frequencies
you can bet that one day you'll want to use the "wrong" transceiver...

You also want to retain a good antenna/transceiver line match. Adding a
T piece and more equipment tends to upset that somewhat. The (extra)
coax length itself will be a load that best case will reduce the
effective power output.

The input impedance of the receiver wont vary as a fucntion of the
receiver operating frequency. Unless of course there is some front end
filtering which is switched in and out. (Thats unusual nowadays - not
having filters keeps the cost down)

I'd suggest that a cheaper path is to use 2 antennas as a
diplexer/filter system will be quite expensive. The other way to do it
might be via coaxial relays that switched the unused receiver antenna to
an attenuator or "open" whilst the other radio is transmitting. This
would take some figuring because you want the relays to changeover
before the radio starts transmitting.

Hope this is of some help.

Cheers Bob


Big Nose wrote:

Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30 -
512 MHz. They would both be tuned into seperate frequencies and need
to share a common broadband, omni directional, vertical monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation and
harmonics would be considered? Would these factors not mean that the
input impedance to the set tuned into the different frequency would
remain high thus not effect matching?

ANTENNA SHARING
 

Big Nose wrote:

Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30 -
512 MHz. They would both be tuned into seperate frequencies and need
to share a common broadband, omni directional, vertical monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation and
harmonics would be considered? Would these factors not mean that the
input impedance to the set tuned into the different frequency would
remain high thus not effect matching?


Simple...

Get a two port circulator. That'll solve your problem ( but at a price. Try
E-Bay ). At your low frequencies, try build one yourself.

Regarding the impedance, that is not that problematic. You can basically
transform any impedance to your taste. You will incur signal loss tho'.

Also, as you unfortunately are looking at the lower end of the RF spectra,
all these parts will be bulky but it's still possible.


Cheers


Dan / M0DFI

ANTENNA SHARING
 

Dan Andersson wrote in
:

Big Nose wrote:

Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30
- 512 MHz. They would both be tuned into seperate frequencies and
need to share a common broadband, omni directional, vertical
monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation
and harmonics would be considered? Would these factors not mean that
the input impedance to the set tuned into the different frequency
would remain high thus not effect matching?


Simple...

Get a two port circulator. That'll solve your problem ( but at a
price. Try E-Bay ). At your low frequencies, try build one yourself.

Regarding the impedance, that is not that problematic. You can
basically transform any impedance to your taste. You will incur signal
loss tho'.

Also, as you unfortunately are looking at the lower end of the RF
spectra, all these parts will be bulky but it's still possible.

Possible, but a bit tricky to set up, since you'll need quite a few db of
isolation between the ports for this to work even reasonably. 60db would
be enough for separations in the range of 10mhz or more, but below that,
you're starting to need 75 or 80db in order to hear something on the
other receiver.

If it's only necessary to use one radio at a time, then maybe 25 or 30db
would do it OK, since all you really need to do then is prevent receiver
damage. But if it's desired to use both radios simultaneously, then the
more isolation, the better.


--
Dave Oldridge+
ICQ 1800667

ANTENNA SHARING
 

Well, one thing for darn sure - those 2 radios sharing the antenna should be
able to hear each other really really good - (till they destroy each other's
front end.)
Given the parameters (sharing some of the same frequencies) - the only way
to share the antenna is by unplugging the coax from one radio and pluggin it
into the other. - Or using a good coax switch - (which could conceivably be
a relay)
But I may be wrong.
My 2¢
w4pmj
Hal


"Big Nose" wrote in message
oups.com...
Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30 -
512 MHz. They would both be tuned into seperate frequencies and need
to share a common broadband, omni directional, vertical monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation and
harmonics would be considered? Would these factors not mean that the
input impedance to the set tuned into the different frequency would
remain high thus not effect matching?

ANTENNA SHARING
 

Dave Oldridge wrote:

Dan Andersson wrote in
:

Big Nose wrote:

Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30
- 512 MHz. They would both be tuned into seperate frequencies and
need to share a common broadband, omni directional, vertical
monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation
and harmonics would be considered? Would these factors not mean that
the input impedance to the set tuned into the different frequency
would remain high thus not effect matching?


Simple...

Get a two port circulator. That'll solve your problem ( but at a
price. Try E-Bay ). At your low frequencies, try build one yourself.

Regarding the impedance, that is not that problematic. You can
basically transform any impedance to your taste. You will incur signal
loss tho'.

Also, as you unfortunately are looking at the lower end of the RF
spectra, all these parts will be bulky but it's still possible.

Possible, but a bit tricky to set up, since you'll need quite a few db of
isolation between the ports for this to work even reasonably. 60db would
be enough for separations in the range of 10mhz or more, but below that,
you're starting to need 75 or 80db in order to hear something on the
other receiver.

If it's only necessary to use one radio at a time, then maybe 25 or 30db
would do it OK, since all you really need to do then is prevent receiver
damage. But if it's desired to use both radios simultaneously, then the
more isolation, the better.




Well, it's actually not that tricky!

If you use a dual port circulator, you will not have any problems.

The only problem is to find a broadband dual port circulator.

Dan

ANTENNA SHARING
 

I don't think its quite that simple (circulator) and I'm not sure how the
circulator solves the problem particularly if it has only two ports. In
addition a circulator for 30-512 probably isn't in existence. Perhaps more
explanation is needed on this, Dan.

This depends on the actual frequencies.
A diplexer must have two non-overlapping frequency ranges to "split" in two.
Like one is 30 - 90 and the other is 100-512. Each can be quite wide.
What your post has is OVERLAPPING frequencies and that is a problem. If one
was 30-90 and the other 120-512 that would be easier. There are units that
are quite inexpensive that will do 2-150 and 200-512.

You seem to have the correct idea about the impedance "seen" at one, but at
the frequency of the other, but this applies when using some methods, not
all.

If the two radios will be operated at the same time, then the problem is
more strict in the requirements of the method used. When one transmitter is
transmitting, it puts considerable power into the system at the transmit
frequency, and harmonics as you well know, however, a transmitter also puts
out considerable NOISE all over the place. There is typically more noise
nearer to the transmit frequency. This noise must be reduced to prevent it
from desensing the other receiver and this may require additional filtering.

I know this makes things more difficult, but from your post, it looks that
way.

73, Steve, K,9.D;C'I



"Dan Andersson" wrote in message
...
Big Nose wrote:

Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30 -
512 MHz. They would both be tuned into seperate frequencies and need
to share a common broadband, omni directional, vertical monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation and
harmonics would be considered? Would these factors not mean that the
input impedance to the set tuned into the different frequency would
remain high thus not effect matching?


Simple...

Get a two port circulator. That'll solve your problem ( but at a price.
Try
E-Bay ). At your low frequencies, try build one yourself.

Regarding the impedance, that is not that problematic. You can basically
transform any impedance to your taste. You will incur signal loss tho'.

Also, as you unfortunately are looking at the lower end of the RF spectra,
all these parts will be bulky but it's still possible.


Cheers


Dan / M0DFI