[amsat-bb] Re: full duplex w/ 2 HT's

[Greg D]

So, all the on-line information seems to point to this solution - 
putting a diplexer (filter) on the 70cm side to tap off and absorb the 
incoming 2m signal. Isn't there also a design where one puts a filter on 
the 2m side, to tap off and absorb the 70 cm (3rd harmonic of 2m) signal?

I would think that any (reasonable) 70cm receiver would have enough 
rejection for a signal so far out of band, but the in-band 3rd harmonic 
can only be taken care of at its source. At least, I think that's the logic.

Is there anything worthy about the other design, or is the most common 
problem that our HTs can't possibly deal with a strong 2m signal on the 
70cm side because of their "DC to Daylight" receivers?

I've been on the mode-J birds without much of a problem with this sort 
of desense, though I did have a lot of trouble on AO-40 with reflected 
RF when aiming through a big oak tree behind the house. The 70cm uplink 
was interfering with the 13cm downlink, and I was pretty sure at the 
time that it was a 3rd harmonic sort of problem.

Just curious,

Greg KO6TH


Ronald Nutter wrote:
> Robert:
>
> I have used a duplexer on the UHF side of my satellite ops for several 
> years. I find that some radios have more of a desense problem than 
> others. Here is a link to my website where I briefly talk about what I 
> do on portable ops and using what is referred to as a Mode J filter - 
> http://www.ka4kyi.com/satellite_gear/misc_equipment/
>
> Ron Nutter
> KA4KYI
> AMSAT Area Coordinator for Missouri
>
> On 6/2/12 11:33 AM, Robert Coppock wrote:
>>
>>
>>
>> hola, i am trying to determine the best way to work full duplex on 
>> the birds with 2 HT's or radios. i cannot find any info so far on the 
>> web. my concern is to keep the transmit power out of the receive 
>> radio, i guess basically the difference between a diplexer and 
>> duplexer. since i am efficient (lazy) i thought i would just look for 
>> suggestions here while i am checking google also. what do i need, 
>> di/du/ plexer, and what is the correct wiring scheme ? i will be 
>> using the arrow antenna without the built in duplexer, from the 2 
>> feeds (vhf/uhf) and 2 HT's for now. my picture of it so far, is to 
>> wire the vhf radio direct to the vhf feed port on the arrow, and have 
>> the uhf port on the arrow come into a duplexer, with the duplexer uhf 
>> port going to the uhf radio, and the duplexer vhf port terminated 
>> into a 50 ohm dummy load. also, any suggestions of specific duplexers 
>> would be appreciated. i think i started on this project a few years 
>> ago, but never got around to get!
> ti!
>> ng it all together. grazie mille, Robert k f 0 g cn81 in the 
>> woodwork, literally, shasta/klamath national forests.
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