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Author Topic: Two Dual Band Radios / Duplexer / Single Antenna  (Read 936 times)
KD5NVR
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« on: August 29, 2004, 08:47:57 PM »

I have two dual band radios. One for APRS and another for Echolink Simplex.  I know I can connect both radios to a common antenna through a duplexer, but can both radios be on 2 meters or both on 70 cm simultaneously.

KD5NVR
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K1CJS
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« Reply #1 on: August 29, 2004, 09:09:38 PM »

I wouldn't chance it, not operating on the same band.  Too much chance of overloading one or the other receiver in the set not doing the transmitting.  Besides, setting up a second antenna might cost the same or less than springing for the duplexer you'd need.  As far as mounting them, either separate them sufficiently horizontally, vertically, or both, and you'll have no problems.
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K1CJS
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« Reply #2 on: August 29, 2004, 09:09:39 PM »

I wouldn't chance it, not operating on the same band.  Too much chance of overloading one or the other receiver in the set not doing the transmitting.  Besides, setting up a second antenna might cost the same or less than springing for the duplexer you'd need.  As far as mounting them, either separate them sufficiently horizontally, vertically, or both, and you'll have no problems.
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KZ1X
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« Reply #3 on: August 29, 2004, 09:56:11 PM »

Actually, the device you're talking about is a diplexer, not a duplexer.  A diplexer allows devices that operate on different frequency ranges to share one antenna, and so forth. An example is a dual-ported Kenwood TW-4100 or Drake UV-3 connected to a diplexer and then to a dual-band mobile antenna such as an Austin 500C, or to a dual-band base station antenna such as a Hustler G6-270.  Diplexers are frequently sold by the Japanese (Comet, Diamond) in the ~$70 range.

A duplexer is the high-isolation device used to separate two devices on the SAME frequency range.  This is the device used on a full-duplex repeater, where a powerful transmitter and sensitive receiver share one common, same-band antenna.  A duplexer is a finely-tuned, two-frequency device, large in size, and costs several hundreds of dollars (frequently over $1000).

By the way, that is a long way of answering your question, which is, no.  Even with 50+ dB port isolation, you'd be well past the acceptable limit for your output filters and T/R switching diodes ... even if you DIDN'T make a mistake with tuning your VFO to the many verboten frequencies you'd be able to.
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N6AJR
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« Reply #4 on: August 29, 2004, 10:03:49 PM »

buy a switch..  buxcommco.com
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K5LXP
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« Reply #5 on: August 29, 2004, 11:29:25 PM »

Two separate antennas are the most practical solution for this application.  Desense within the same band is likely but the RF levels won't damage the other radio.

Mark K5LXP
Albuquerque, NM
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W8JI
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« Reply #6 on: August 30, 2004, 08:55:23 AM »

KZ1X is correct, it is a DIplexer you are talking about, although many manufacturers or retailers  incorrectly call them DUplexers.

DUplexers allow simultaneous transmission and reception on one band, while DIplexers allow multiple "things" to use a common wire or antenna on multiple bands.

Although it is easily possible to build a band seperation system that allows simultaneous transmitting and receiving on different bands, you are probably taking a chance with the cheap stuff being imported today. I wouldn't chance it without knowing the specs of the device.

I'd use antennas with a reasonable amount of distance between them, maybe 15-25 feet horizontal or 5-10 feet vertical seperation if you run low power.

73 Tom
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WA4MJF
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« Reply #7 on: August 30, 2004, 11:12:52 AM »

Well, I think he means duplexer as
he is talking about using the same band.

Yes, it should work. Make sure the
duplexer is tuned for the correct
frequencies and use good cable like a
minimum of RG-214 for the jumpers.


73 de Ronnie
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W7DJM
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« Reply #8 on: August 30, 2004, 11:44:10 AM »

""both radios be on 2 meters or both on 70 cm simultaneously.""


I am inferring that you have two dual band radios and expect to operate EITHER or BOTH radios, EITHER OR BOTH on 2 or 440.  

Nope.  Can't do that.

A DIPLEXER,  and it is called a duplexer by some manufacturers, might better be called something like a "dual band coupler."   These are small and inexpensive, and couple one dual band device, either an antenna or radio, to TWO single band devices, here one on 2m, the other on 70cm, again, either two antennas or two radios.

IN THIS CASE you can only operate two radios on separate bands, or you can operate a dual band, single port radio into two antennas.

NOW--if you had, say, 2- 2meter radios, and ALWAYS ALWAYS operated both on the SAME EXACT frequencies--say one on a repeater, and one on packet--you could use a good 2 meter duplexer (big and expensive)  to use the same antenna.

Good duplexers that will operate down to the 600khz split used for repeaters on 2 are VERY expensive.  Unless you were on some sort of crowded tower site, and weren't allowed two antennas, it would always be cheaper to install two antennas and two feedlines.
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W8JI
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« Reply #9 on: August 30, 2004, 09:06:22 PM »

That's right, I see now.

If you want to TX and RX on the same band....be prepaired to buy a duplexer. It would cost about $1000 for two meters and about $500 for 432MHz, and the split would have to be wide and the frequency constant.

Antenna separation would have to be very wide if you opted to use two different antennas. Maybe 50-100ft of vertical separation or a few hundred feet of horizontal separation.

The expense and complexity really depends on how good the radios are, and most dual band radios are not very good. A good radio requires less external filtering or less antenna separation.
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