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Author Topic: 2 rigs, 1 TriBand antenna. Ideas?  (Read 2146 times)
N6YFM
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Posts: 516




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« on: February 09, 2017, 01:09:35 PM »

I use a single tri-band Comet CX-333 base antenna on the roof (2m, 222Mhz, 440Mhz).
For rigs, I have a 222Mhz base radio, and then a dual-band 2m/440 base radio.

I am getting tired of working the antenna switch all the time.

What do people find is the best practical solution?   Do you purchase;

QTY 1  Triplexor like Comet CS-324
and then add back to back a
QTY1   2m/440  Duplexor like a Comet CF-416A

Would putting those back-2-back be practical for sending 2m and 440 over the
same wire to the dual-band rig, and then sending just the 222Mhz to that rig?
Again, 2 rigs, 1 antenna.

Or is it fairly easy to make your own combo unit?

Thanks,

Neal
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ONAIR
Member

Posts: 3536




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« Reply #1 on: February 09, 2017, 04:40:34 PM »

I use a single tri-band Comet CX-333 base antenna on the roof (2m, 222Mhz, 440Mhz).
For rigs, I have a 222Mhz base radio, and then a dual-band 2m/440 base radio.

I am getting tired of working the antenna switch all the time.

What do people find is the best practical solution?   Do you purchase;

QTY 1  Triplexor like Comet CS-324
and then add back to back a
QTY1   2m/440  Duplexor like a Comet CF-416A

Would putting those back-2-back be practical for sending 2m and 440 over the
same wire to the dual-band rig, and then sending just the 222Mhz to that rig?
Again, 2 rigs, 1 antenna.

Or is it fairly easy to make your own combo unit?

Thanks,

Neal
   I had a similar issue.  Was easier for me to just put up a separate antenna.
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W9IQ
Member

Posts: 1708




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« Reply #2 on: February 09, 2017, 06:23:13 PM »

Neal,

You need to consider the isolation between the two radios. This is not a problem with a dual band radio as the designer has taken this into consideration. But if the isolation from the 222 MHz port to either of the other ports is not sufficient, you could damage your dual band radio when transmitting on 222 MHz and vice versa.

As an example, if your 222 MHz transmitter puts out 40 watts (46 dBm) and you want to limit the power of this signal to that of a VHF S9+60 level (-33 dBm) on the dual band radio, you will require about 80 dB of isolation. That is hard to achieve with the commercially available diplexers and triplexers.

The triplexer you referenced claims 55 dB of isolation. 40 watts out from your 222 MHz rig would put 0.125 mW (-9 dBm) into your dual band radio. Can it take that power level? Unfortunately, there is probably only one way to find out for certain - it is equivalent to testing the match to see if it will light when needed...

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
Member

Posts: 1708




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« Reply #3 on: February 09, 2017, 10:36:30 PM »

Neal,

I was giving my earlier post a little bit more thought and realized that I could give you an interesting datapoint based on the Friis equation.

If two radios operating on 2 meters are connected to Diamond X50A antennas (4.5 dBi) spaced 850 feet apart, the path loss between the two antennas is 55 dB. This is the same attenuation claimed by the triplexer.

If the transmitter was putting out 40 watts, the other receiver would experience a VHF S9+83dB signal or 0.12 milliwatts.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
K4JJL
Member

Posts: 833




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« Reply #4 on: February 10, 2017, 09:51:00 AM »

If the transmitter was putting out 40 watts, the other receiver would experience a VHF S9+83dB signal or 0.12 milliwatts.

-9dBm isn't that bad of a figure.  I put 0dBm into repeater receivers all the time to adjust audio levels in voters.

The preselector filters might attenuate it even more keeping the signal from overloading the front-end, granted your radio has a preselector.  I've seen the crap designed now days.  Wouldn't surprise me one bit if the preselection was minimal.
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W9IQ
Member

Posts: 1708




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« Reply #5 on: February 11, 2017, 04:51:56 PM »

Neal,

One other point to keep in mind if you go with the triplexer/diplexer is to keep a radio or dummy load attached to each port or the isolation will not be effective.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
K6BRN
Member

Posts: 490




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« Reply #6 on: February 11, 2017, 07:43:06 PM »

Neal:

In days past, dual band 2M/70CM FM radios had two outputs, one for each band.  So to add 1.25M from a second mono-band radio onto a single coax feed with 2M and 70CM just required a decent lumped-element triplexer (small, inexpensive and not terribly efficient or sharp) like the Comet CFX-422, to combine the separate 2M, 1.25M and 70CM feeds.  And this worked just fine with no damage to any connected radio (as proven by experience with a number of radios).

Combining signals from a stand-alone 1.25M radio and those from a newer dual band 2M/70CM radio (with a built-in 2M/70CM duplexer and single combined output) to feed a single tri-band antenna, like a comet CX-333 can seem a little tricky, because the 1.25M band has to be slotted in between the 2M and 70CM bands, which are already combined.  I'm not aware of any commercial duplexer (triplexer?) that can do this job "out of the box" (maybe someone here does??? Please jump in).  But a simple and proven solution does exist that costs just a little more money and adds a tad more signal loss.

I am using a Comet FF142 (2M/70CM duplexer) back to back with a Comet CFX-422 (2M/1.25M/70CM triplexer) to combine the feeds from an Alinco DR235 MKIII 1.25M FM radio (25W) with a Kenwood TM-G707 2M/70CM FM radio (50W).  In this arrangement, the FF142 is used backwards, as a diplexer/splitter, to separate the combined 2M/70CM output from the Kenwood into two feeds, one for 2M and the other for 70CM, which are then fed into the CFX-422 triplexer, that combines the now completely separate 2M, 1.25M and 70CM feeds.  Surprisingly, this silly arrangement has only about 1db loss (measured on the frequencies I use) on any band and provides enough isolation, when combined with the radios built-in RX band-pass filters to prevent damage.  In fact, there seems to be no interference between the two radios that I can detect, and it is easy to RX on one radio while TX'ing on the other, even at full power.  FM can cover up a host of interference issues, so this may be why the arrangement works so well, in part.

What's the catch?  The cost of an extra duplexer (~$60) and a small amount of added signal loss.  But no more antenna switching (and no switch cost, ~$60).  And if you are only using FM and repeaters, this small loss will make no significant difference.  I'd hesitate to extend this approach for use with higher power amplifiers - but with these radios, it works just fine.

Best Regards,

Brian - K6BRN

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