[Elecraft] XV-50 LNA problems

[G3VVT at aol.com]

In a message dated 31/03/05 19:42:07 GMT Daylight Time,  
traymer at mail.state.mo.us writes:

Not  inexpensive, but works  well:

http://www.dci.ca/

Specifically:

http://www.dci.ca/?Section=Amateur

We  installed one of the 2M units on our repeater to eliminate bandpass and 
to  give a DC ground to our preamp on our repeater system.  It was  
intolerant of the energy from Lightning.  One of these solved all our  
problems.  Looks like the specs say > -72dBm @ 135Mhz on the 2Mhz  wide 
unit.  The 4Mhz wide unit is better documented, and shows >  -89dBm @ 126 
Mhz. That should knock down any nastys that will get your 6m  XV-50.



Have been using the DCI-145-2H, 4 section helical filter since 1999 on the  
local 2m repeater GB3LD which I hold the license. Without this we would be  
swamped by pagers in particular located on ours and adjacent masts. The normal  
bandpass and notches found in the repeater duplexer whilst they are efficient 
in  preventing energy from it's own TX entering the RX, work relatively poorly  
in keeping out energy from external sources. Notch filters are only really  
successful when a single fixed frequency is involved. 
 
The bandpass filter on the repeater worked so well that I ended up  buying 
one for my shack when 153 MHz pagers were installed close to the home  location.
 
For these devices to work you need to have a bandpass filter of this type  on 
the receiver that is being affected and not the TX that is causing the  
interference, unless the TX output purity is sufficiently poor to warrant  this. In 
other words if the 6m receiver is being affected, a 6m bandpass filter  is 
needed and not really much use in installing one on the 2m TX. Bandpass  filters 
incur a penalty in that they are lossy and usually mean a  loss of about 1dB 
in the case of the 2m unit and greater at UHF. If it is  installed in the 
common antenna feed of the TX/RX mean the same loss on TX  power.
 
All too often the designers of normal every day amateur receivers for  
VHF/UHF skimp on the selectivity of the front end to achieve a better  sensitivity 
and the price is blocking or even damage if the local field  strengths from 
other sources are too high. By comparison commercial 2 way radios  by virtue of 
performance regulations are far superior in the selectivity stakes,  quite 
often using miniature helical filters at the front end to achieve the  performance.
 
Good bandpass filters are expensive, but if transmitters on other  
frequencies are causing a problems, are a necessary evil.
 
Bob G3VVT
Keeper GB3LD/GB3LF