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 > From:          tvr@value.net
 > Date:          Sun, 12 Sep 1999 10:01:53 -0700 (PDT)
 > To:            amsat-bb@AMSAT.Org
 > Cc:            Dave Metz WA0AUQ <davemetz@muscanet.com>

 > Mark West <MarkW@instantdocuments.com>,  
 >     You are only slightly right on theroretical grounds.  You will find 
 >     that in practice the mismatch between a 50 ohm and 75 ohm system is 
 >     trivial at 2M and 70cm.  Check the math on a Smith Chart.  The mismatch 
 >     gives you a SWR of 1:1.5 !  Pretty low I would say in any real world 
 >     application.
 > Most, but not all by my experience.  I'm running a dual-band 50 ohm direct
 > feed LPVA into an Alinco DJ-5G.  In my local area, if i run 5W and my SWR
 > at 2 meters is 1:1.5, then i'm not going to hear my own downlink at 70cm.
 > I'm not completely sure why and it might have to do with non-linearities
 > in the antenna from things like imperfect solder joints, but the closer
 > i come to 1:1, the better i hear my own downlink.  (Now, this may not be
 > an issue outside of an urbanized area, by the way and this may have to do 
 > with local condition).  The general rule of thumb is that pattern is much 
 > more important than SWR, but there are exceptions.

Someone did post the actual loss.  At 70cm, with 100' of CATV coax, the 
loss due to a mismatch of 1:1.5 is only 0.09db!  That's right, 0.09db.  In your
case, the loss might be a few hundreths more due to the coax you are 
using.  The point is, your desense is NOT caused by the SWR.  If it was
none of our stations would work!  

Lets look at some other factors that come into play here.

First, the radio.  I had an Alinco once.  It had such poor RF design that
I sold it at the next hamfest and took a $100 loss.  Honest.  I can't speak
for the radio you have, but the one I had used a receiver front end design
the same as is used in cheap scanners.  In other words, this was not a 
radio that you would use in any high RF enviroment.  And that is exactly
what you are doing when running duplex with a dual band radio.

You might be interested to find that I have never had a desense problem
with my station.  I use a seperate radio for each band.  A lot of us have to
use 70cm traps in the two meter transmitter feed line to eliminate the
third harmonic from the transmitter.  [ 144 x 3 = 432Mhz ]  Sadly, you can't 
filter a dual band radio to eliminate problems of this type.

Non-linear elements in the system can also generate harmonics.  Bad
solder joints, corroded UHF connectors (common problem) and oxidized
aluminum antenna parts can all cause problems.  Your 2M RF goes into 
the non-linear junction say in a corroded antenna part, and generates 
harmonics on 70cm.  There goes your reception!
 > I'm NOT an engineer (at least, not in the hardware realm).  But if the 
 > reflection at 2 meters is causing de-sense at 70cm, that matters to me. 

Its not!
 > A better diplexer will probably deal with that, but that's not much help 
 > to me if it's inside the radio.

Your correct to a point.  One problem a lot l of our modern 
radios have is that they are too damn small.  That is, the box is too 
small to fit high Q band pass filters and a high Q diplexer in it.  This is why you see guys
at hamfests with external bandpass filters on their HT's!  Note that the filter
is larger then the HT in some cases :-).
 > I think the more important point is that the 50/75 ohm loss to due mismatch
 > is *much* smaller than the difference between a 100' of 50 ohm feedline of
 > any type that i can afford, and 100' of 75 ohm hardline that might become
 > available to me.

In our case the 75 ohm line is free.  Compare $0.00 with $5.00 a foot for 
50 ohm hardline when you need 250' of it!  Suddenly minor theoretical
considerations become very unimportant :-)!  You will find that in real world
RF engineering, practical considerations often override theory.
 > What i don't understand is why people are designing antennas and perhaps
 > also PA's, to be 75 ohms?  At least then, the mismatch isn't on both ends
 > of the feedline.  Maybe the engineers around here can explain that one to
 > me??

The TV industry went with 75 ohms simply because the losses in the coax
are less.  That's the only reason I have ever heard.  Also most PA designs 
using tubes will match a surprisingly wide range of loads.  If the feed line is
50 or 75 ohms makes no difference to the transmitter.  Solid state PA's are 
a little more fussy, but then, none of mine seem to mind running into 75 ohm

The same is true for antennas.  The matching devices on them have 
surprisingly wide ranges as well.  A Gamma match for example will match
damned near anything!  The bottom line, don't worry about 25 ohm mismatchs
the problem is trivial at best.  The problems with your system are elsewhere.

Please don't take offense, but if I were you, I would try a better receiving 
system.  I would investigate a converter going into a 28Mhz IF.  A 
cheap way to go would be a Hamtronics converter going into a Midland 
2510 or 2600 10 meter rig.  This is what I first used on AO-13 and it worked.

 >                             -- KD6PAG  (Networking Old-Timer, "RF newbie")
--73-- Dave WA0AUQ 

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