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Re: AO-40 Portable, Duplexers, and Down Converters, and only 2 cables?

Hi Bill,

You certainly could do this!  The things to consider are:

1.) As you mentioned, will the duplexer pass DC?  Different ones may or 
may not.  Some may also short DC to ground.  It all depends on the 
design and structure.  Since you have the duplexers, you should be able 
to test if they pass DC or if they short it.  Also, if DC is injected 
on one port - does it show up on all of them (ie: is there DC 
continuity between all ports)?  That could cause a problem as many 
antennas (on the UHF side) are DC shorts and your rig could see DC 
power on the UHF antenna port.  You might need some DC blocks.

2.) In terms of the power you need some idea of the rejection specs on 
the duplexer.  Usually, duplexers are a Low Pass/High Pass filter 
configuration.  So, the tough parts are what are the specs?  If the 2m 
port of the duplexer has a rejection of say 30 to 40 dB of the UHF 
uplink signal, then consider that if you run 1 Watt (30 dBm), the AIDC 
would see 0 dBm or 1 mW with 30 dB rejection (ignoring cable loss).  Is 
1 mW enough to kill it?  Probably not but I'm not sure.  It follows 
then if you ran 10 W, the AIDC would see 10 mW, etc.  The two questions 
then are how much rejection does the duplexer have out of band and how 
much power the AIDC will see before melting down.  If your planned 
output power - cable loss - isolation is within the AIDC meltdown 
level, then there's no big deal.  One other thing you could do too is 
to add a 3 to 6 dB attenuator in the path to the AIDC.  You've got 
plenty of gain in the AIDC, and the extra attenuation will help prevent 
the d/c from being overloaded as well if the attenuator sees too much 
power, it will fail.

Perhaps you've thought of all this before.  But other than that, I see 
no reason why you couldn't do it.  If your duplexers do not pass DC, 
then you could add a bias tee to the line going to the AIDC after the 
second duplexer.  Yes, you'd have to run a line of DC cable, but it 
would save you a run of coax.



On Wednesday, Jun 4, 2003, at 11:37 America/Chicago, Bill Acito W1PA 

> With the thought of trying to minimize the amount of cabling I have to
> lug around, and assuming I use a dish with a AIDC downconverter, a
> safety bias-tee (not the power option from the FT-847), and already 
> have
> the duplexers handy, could I do the following:
> r|--2m--[Bias Tee]----|                  |-----AIDC-----Dish
> i|                    |Dup>----------<Dup|
> g|--70cm--------------|                  |--------------70cm Beam
> Assuming the duplexers provide a DC path for the AIDC.
> How much power could I run and not smoke the AIDC --- what would happen
> first, Leila siren, or downconverter smoke?  :-)
Jon Ogden
NA9D (ex: KE9NA)

Citizen of the People's Democratic Republik of Illinois

Life Member: ARRL, NRA
Member:  AMSAT, DXCC

Ham Radio Webpage: http://www.qsl.net/na9d   <- Updated on 1/12/03!!!
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"Man's mind, once stretched by a new idea, never regains its original 
                 - Oliver Wendell Holmes, Jr.

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