_____
From: [email protected]
[mailto:[EMAIL PROTECTED] On Behalf Of Kevin Custer
Sent: Friday, February 23, 2007 8:34 PM
To: [email protected]
Subject: Re: [Repeater-Builder] Re: 220 repeater receiver recommendations?
[EMAIL PROTECTED] wrote:
In a message dated 2/23/2007 4:32:54 P.M. Pacific Standard Time,
[EMAIL PROTECTED] writes:
as all better repeaters builders know, the equipment might have to be tuned
as a system using custom cabling lengths or Z Matchers for optimal
performance.
Can you comment on a difference of opinion regarding how a Z matcher should
be adjusted? One school tunes for maximum smoke downstream of the cavities
which is the approach I adopt. The other school tunes for minimum reflected
SWR between the first cavity and the TX. My academic preference is for the
latter, but as a practical matter it's a pain in the buns to do because you
have to account for the line section that the Bird meter adds to the cable.
Beyond that, using just a meter, what is it that is actually being measured
since it could possibly be a composite of both on-channel energy and and
off-channel reflected spurs and harmonics.
I think you answered your own question, however I'll bite....
In *most* of the instances where I employed impedance matching between the
transmitter and first cavity, the place where best return loss and least
insertion loss was found, is very close to one another -tuning wise. I
usually use the maximum smoke approach first and see what I have, then go
from there. In my most critical applications, I use permanent in-line Bird
samplers. Then, cabling lengths don't change because I don't remove the
samplers; and you know exactly what you have and where....
Kevin
Do you use a cable with the bird that makes it a ½ wavelength total line
length when you insert it in line? That should not change things when the
wattmeter/cable are removed.
With a PA that does not exhibit a 50 ohm output I would think that it would
be best to tune the Z match for maximum output looking at the antenna
connection (output of the cavities). The reason being if you put the
wattmeter between the Z match and the radio and tune the Z match for minimum
reflected power as noted on the wattmeter then you are tuning the Z match to
match the cavity input to 50 ohms that the wattmeter line section is setup
for. But that is not what the transmitter really is so you end up with a
flat reflected power between the cavity and the transmitter but it does not
match the transmitter.
73
Gary K4FMX
