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Re: 70 cm Mast mount Preamp



At 06:19 1999-09-16 -0400, you wrote:

Hi all,
I have used the SSB 70 cm preamps both at the club station
here at the Engineering College of Copenhagen and at home.

In both cases it were necessary to mount a series resonance
circuit on 145.800 MHz in parallel with the input to get rid of the 2 meter
signal in mode-J. The resonance circuit is placed "behind" the TR relay.
At the clubstation that was not enough - we had to place a 70 cm bandpass
filter at the input of the amplifier. That cured the coupling.

The only difference between my home QTH set-up and the club station is
a 90 cm parabolic dish between the 2 meter antenna and the 70 cm antenna.
The distance between the 2 meter antenna and the 70 cm antenna is
approximately 2 meters.

The one at my home QTH have had two major breakdowns. The first one 
an electrolytic capacitor in the timing circuit - the other blown out
protection diodes. The transistor was OK after that.

73 OZ1MY/Ib


>Pat;
>I, too, recommend the SSB units.  Have used home-brewed jobs, which work
>well, but the relay co$t can really get you.  The SSBs have helical
>filters in them, to cut down on the out of band junk; criticl at 2m,
>becoming that way at 70cm.  They also have adjustable gain, which means
>you can set it up to your system, and not  have TOO MUCH gain in your
>system.  Yes, it's possible . .. .
>
>One gotcha:  I have 2 friends with them that have run into a problem
>with excessive transmit loss with the preamp switched on and using
>RF-sensed bypass.  I suspect bad relay contacts due to "hot" switching.
>I switch my preamps with the transmitter PTT line, not RF, and have not
>had the problem.  In a mostly receive only mode for mode J birds, you
>should be OK.  Good Luck!!
>
>73, Jim
>wb4gcs@amsat.org
>
>
>PAT McGRATH wrote:
>
>> Help
>>
>> Looking to buy a mast mount 70 cm preamp.
>>
>> What, Why  and Where should I buy?
>>
>>
>>
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