On 3/17/2015 5:19 PM, Brian Reay wrote:
On 17/03/15 08:12, Jeff wrote:
It is all here in the rest of the thread but here goes. I have only
three antenna mounting points available on my home for antennas to
serve:
a dedicated two meter transceiver for a digital node,
a dedicated Seventy Centimeter transceiver for a D-STAR hot spot.
a separate 120 CM transceiver
the six meter band of multiband transceiver
a dual band 146/446 transceiver

That is five radios that need to be able to use Six Separate VHF/UHF
frequencies with as many as four transceivers

operating at the same time. Two of those transceivers would be
operating as automatic stations so as many as three of

those transmitters may be transmitting at the same time. Two of the
three may even be on the same band given that their

will be an automatic station on both Two Meters and Seventy Centimeters
and I may be operating as a manual control operator

on either of those bands at any given time.

I initially asked for advise on which dual band antenna to use to
serve a 144 Mhz and a 440 MHz digital stations.

Someone then suggested that I use two tri-band antennas in order to
provide all band capability for FM operations on 6, 2, 1.25,

and 0.7 Meter bands while still operating the two automatic stations on
144 & 440. I would use vertical separation to reduce the

de-sensing on the common band stations.

I then asked for advise on any foreseeable problems with using the DCI
filters to effectively separate the signals at the radios

and yet allow the transceivers to share two multi-band antennas to cover
the needed bands with only two mounting points.

I'm saving the third mounting point for stacked Yagi-Uda beams to
provide additional range on 146/440 for the repeaters at the

extreme ends of the service area of the Forecast Office that I am
serving as Assistant Coordinator for Net Management

in their Skywarn Program. I am hoping that between the beams and two
amplifiers with receive pre amps I will be able to take reports when
appropriate from some of the more distant sub nets. I have done this
previously with temporary masts from this location so I am very hopeful
that it will work using more permanently mounted beams. All of that is
so you will know why the third mounting point is not available for the
needed omnidirectional vertical antennas.

I hope that makes the expanding questions clear.

Tom

The first thing that you have to consider is that none of the DFCI
filters will give you any protection when 2 radios are operating in the
same band, and paralleling things up with tee pieces is generally not a
good idea.


The issue with using Tee pieces is inadvertently inserting 'stubs' etc.
If care is taken to avoid this, eg by keeping leads very very short,
then at the frequencies being used, things should be OK. After all, a
duplexer is just a set of filters in one box (ie the connections are
very very very short). In fact, as the DCI filters are bandpass (based
on Tom's post, I've not looked at the specs), they are probably better
in some ways than some duplexers which just use LPFs and HPFs.
(Triplexers tend to use at least one BPF but some only use an
additional HPF.)


They do not have to be short - but as long as you have a multiple of 1/2
wavelength between the two diplexer inputs, each input will see the very
high impedance of the other input. Rather easy, since you are operating
on the third harmonic.

This could also be a good application for a hybrid ring duplexer. It's
a bit more complicated to build, but should provide better isolation.

In either case, the trick is going to be getting the lengths of the coax
correct. This is where a grid dip oscillator would be advantageous -
actually measure the coax, instead of depending on the listed velocity
factor.

And BTW - duplexers can also be bandpass. Can duplexers are typically
used on VHF and UHF repeaters. They can have very high Q (and therefore
very narrow passbands).

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