[Moon-Net] LNA Relay

Magnus Lindgren sm6xma at gmail.com
Mon Sep 30 02:17:18 CEST 2019


Hi!

The port-to-port isolation you mention is the inherent isolation of the 
horn itself. When installed in a dish this is considerably reduced since 
the dish reverses the sense of circular polarisation of a reflected 
wave. I.e., some of the energy emitted at the tx probe in the horn will 
be reflected back by the dish (in opposite polarization) to the rx port.

Please refer to my Master's Thesis which can be found at the following 
location:

"A 1296 MHz Earth–Moon–Earth Communication System – Theory and Practice"

http://publications.lib.chalmers.se/records/fulltext/217884/217884.pdf#pagemode=bookmarks

More specifically, point 4 in section "8.10 Antenna temperature 
estimation" is of interest. I estimated the resulting "in-dish" 
port-to-port isolation to only 18dB.

73's


Magnus, SM6XMA/DL6XMA

On 29/09/2019 03:39, Don Hawbaker via Moon-net wrote:
> Just off the top of my head, I think you can get only 20 to 30 db 
> isolation out of a septum feed.
> 
> If you are transmitting plus 53 db or more, you LNA is going to get hit 
> with 23 to 33 dbm.  As I recall, they don’t like more than about zero 
> dbm long term, so you would need an isolation relay.
> 
> But like I said, these are numbers just off the top of my head.  You can 
> Google and find the exact numbers.
> 
> 
> 
> 
>> On Sep 28, 2019 at 1:26 PM, <James C via Moon-net 
>> <mailto:moon-net at mailman.pe1itr.com>> wrote:
>>
>> FWIW,  Besides the additional protection an iso relay provides, it 
>> also ensures your LNA is always looking at a 50ohm impedance to deter 
>> self oscillation. Something to think about anyway,. GL, 73 .-James K7KQA
>>
>> On Sat, Sep 28, 2019 at 9:32 AM Stephen Hanselman via Moon-net 
>> <moon-net at mailman.pe1itr.com <mailto:moon-net at mailman.pe1itr.com>> wrote:
>>
>>     Vlada,
>>     Are you suggesting that an isolation relay is not needed on a
>>     septum feed?  That would make my 1296 system simpler, but I’m a
>>     bit worried about my LNA
>>
>>
>>     73's
>>
>>     __ __
>>
>>     Stephen Hanselman, KC4SW____
>>
>>
>>
>>     Sent from my iPad
>>
>>     On Sep 27, 2019, at 02:12, vlada via Moon-net
>>     <moon-net at mailman.pe1itr.com <mailto:moon-net at mailman.pe1itr.com>>
>>     wrote:
>>
>>>     Hi Voja,
>>>
>>>     as regards LNA input protection, the transfer relay behave
>>>     actually in the same way as a single port relay.
>>>
>>>     So, the level of protection just depends on the isolation of the
>>>     particular relay and proper function of the sequencer or similar
>>>     function circuitry.
>>>
>>>     Additional relay at LNA input is almost necessary in case of
>>>     separate but close RX/TX feeds (like septum feed) with limited
>>>     isolation in-between.
>>>
>>>     Optionally, if the relay isolation is insufficient or you are
>>>     looking for a secured protection, you could use an additional
>>>     protection directly on LNA input either outside or inside (like a
>>>     power limiter used e.g. in spectrum analyzers or an antiparalel
>>>     vhf Schottky diodes etc.).
>>>
>>>     However, al these units will obviously somehow (more or less)
>>>     decrease the noise performance of LNA.
>>>
>>>     73, Vlada
>>>
>>>     OK1KIR
>>>
>>>     ______________________________________________________________
>>>     > Od: "Vojislav Bandobranski via Moon-net"
>>>     <moon-net at mailman.pe1itr.com <mailto:moon-net at mailman.pe1itr.com>>
>>>     > Komu: "Moon-Net" <moon-net at mailman.pe1itr.com
>>>     <mailto:moon-net at mailman.pe1itr.com>>
>>>     > Datum: 26.09.2019 21:39
>>>     > Předmět: [Moon-Net] LNA Relay
>>>     >
>>>
>>>     Hi,
>>>     does anyone have experience with using a transfer rf switch (with
>>>     4 connectors) in LNA?
>>>     Is it necessary to add another relay in front of LNA input for
>>>     insulation?
>>>     I mean simple rx/tx transition.
>>>
>>>     -- 
>>>     73 de Voja/YU7C
>>>
>>>
>>>     ----------
>>>
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> 
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