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Author Topic: Duplexer for dualband crossed yagi  (Read 415 times)
W5MIT
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Posts: 22




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« on: October 26, 2002, 10:35:02 PM »

Must I use a duplexer if I wish to only use one radio (half-duplex) on a dual-band crossed yagi such as the Arrow II Satellite antenna?

Essentially, I'm wondering if I will experience high SWR or reduced efficiency if I transfer RF power to both gamma matches simultaneously-- even though I only am transmitting on one band at a time.
Thanks!
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AC5UP
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Posts: 3825




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« Reply #1 on: October 26, 2002, 11:28:42 PM »

All I know about this is from my experience with a Kenwood TM-733 2m/70cm rig...

The two pigtails are connected to an outboard duplexer that feeds one coax line to the antenna. It crossband repeats very nicely and, although I rarely use the feature, have noticed no particular reduction in RX sensitivity or other ill effects. There are a few frequency combinations where the isolation is less than perfect, but those are few.

If you run two coax lines to the antenna you won't need a duplexer.

- AC5UP
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AB8IG
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Posts: 131




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« Reply #2 on: October 27, 2002, 09:02:47 AM »

I'm not sure I understand your query, but I'll give my best shot.

Most commercial VHF/UHF multiband antennas are designed to work with a single feedline. Any necessary signal separation is either built into the design or included with the antenna (chokes, etc.). If any ancillary equipment is required it should be specified in their manual. Don't worry about loading a mulitband antenna from  a single band radio. Be careful not to exceed the wattage rating of the antenna.

If your rig has more than one antenna connection and you want to use a single feed line for multiple frequencies you will need a diplexer or triplexer as required.

Jim
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N6AJR
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Posts: 9891




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« Reply #3 on: October 27, 2002, 02:12:37 PM »

Rf is lazy.  If you feed a 10-15-20 meter fan dipole with a 10 meter signal, it sees the 10 meter resonant antenna as the perfect 50 ohm load so most of its power goes there, the 15 and 20 meter antenns have an impeadence in the hundreds to thousands of ohns at this frequency and RF being lazy won't go there. So even on a uhf/vhf antenna, if you feed it with one or the other it will ignore the band it doesn't like.  The duplexer lets you either feed a multiband antenna with 1 cable from 2 rigs or uses a multiband rig with a signel output to go to more than one antenna.  For your stated purpose, you don't need a duplexer.. hope this helps.  tom N6AJR
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KG4UDX
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« Reply #4 on: October 27, 2002, 09:31:26 PM »

The arrow satellite antenna has seperate coax for the 2m and 70cm elements.  You can get a duplexer with it so you can feed both into the same radio, or use them seperatly.
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