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Re: Newbie: Noise on RX when keying TX , try a filter .......



Jon Ogden wrote:
> 
> on 12/4/00 6:32 PM, Scott Townley at nx7u@arrl.net wrote:
> 
> > Sometimes (but not always) when I key my TX I hear about S5 noise
> > "feedback".  NB is off on rig (FT-726R).  Often the noise is loud enough to
> > compromise hearing my downlink sig.
> 
> I don't know why it's worse in daylight hours, but it sounds like you have a
> desense issue.  Solution: Get bandpass filters for each band.  That will
> clear it up.

Gotta agree with Jon on this one , though how you solve it is up to your own
abilities ....

I will give you my own experience and what I ended up doing ...

As a longtime tech in the land mobile radio industry , I have accumulated way to
many bits of radio stuff that was being decommissioned or thrown away because
they wanted to sell new stuff to the customers , so I had a number of uhf and
vhf bandpass cavities , and I thought these would be good clients for just the
problem you are having .....

Well the long and the short of it is , the "bottles" that I had were all too
lossy and narrow in bandwidth , they were made to pass one frequency, not many ,
so I finally made the plunge and bought one of the bandpass filters made by DCI
and stuck it on the tracking generator at work and hey , these puppies are one
fine value for the kind of filtering they do , I was very impressed , they give
you at least 50 db isolation outside of the bandpass  and a relatively flat pass
window ( within a db ) with less than a db loss and hey the vhf one is only $90
!

You cannot go wrong with these filters ! , the main reason I got one is I was
sick and tired of having downlinks killed by the local paging companies , and to
my surprise ( and I didn't have too much hope ) it has eliminated 98% of my
pager garbage , too me that is money well spent!

I also picked up the UHF version , but have not installed it yet , though I
noticed on the tracking generator that it had much better measured specs , ,
whereas the vhf version had a few multiples where it passed stuff at lets say
330 MHz and 670 MHz (not exactly) as well as what it was designed for , the UHF
version only passed the UHF amateur band it was designed for and I swept it all
the way up to 2.2GHz.....

Now I must say that I am in no way connected with these folks , in fact I was
very skeptical of their display at Dayton , but as they say the proof is in the
pudding , these puppies do work and very well at that , they also are rated for
200 watts , and can be purchased with N fittings ( which I desire ) .........

So for all the purists out there who like to roll your own , disregard , but for
you folks out there like me that don't have a machine shop at your disposal nor
full time access to test eqpt. , the DCI filters are a fine thing , and I think
this will cure your desense.

Now I do admit , I am not sure why it only happens at a particular time of the
day , other than that your local "noise floor" may vary and this would give the
impression of the differing levels , lotsa potential variables here , do you
have an mast mount preamp which could be getting overloaded and generating its
own intermod , or some nearby source of rfi that you are not aware of ( I have a
5kw uhf channel 14 tv transmitter just 2 miles away and let me tell you , that
sucks !) there are all sorts of potential sources for noise ( my neighbor has a
phone that puts out a spur right on 50.110! , kinda kills my 6meter dx work on
that 'freq unless he turns it off ) so you may have to do a little "sniffing"
around with a handheld rx or spec analyzer if your one of the lucky ones who has
one at your disposal for field work ;^)


Anyway , just my angle on things ,hope it helps somehow .....

-- 
Douglas Cole
N7BFS
AMSAT#26182 , K2 # 544
http://www.users.qwest.net/~cdoug3
n7bfs@qwest.net
Registered Linux user # 188922
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