With the mfj meter you should plug the stub straight into the meter and just
read the max or min impedance points for the pass and stop bands. Cut the
stub to tune the If you add a piece of cable and try to measure the dips
at the reject points, that is where the stub looks like a short circuit, you
are measuring the very low impedance transformed by the length of cable,
which will be different than the real impedance. You should be able to read
the pass band ok, but there you are going to see a very wide peak because of
the high impedance presented by the stub in parallel with the dummy load.
David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of Felipe Ceglia - PY1NB
> Sent: Tuesday, August 19, 2008 01:30
> To: firstname.lastname@example.org
> Subject: [TowerTalk] stubs for the SO2R setup
> I am trying to make a set of stubs to be used on my SO2R station setup.
> Those stubs should be plugged to a six pak box.
> When I measure the 1/4 wave shorted 40m stub, plugged directly on the
> MFJ-259, with a dummy load paralelled by a tee connector, I can see the
> nulls on 14 and 28mhz.
> When I plug that stub and that very dummy load on the six pak 40m port,
> things change. Nulls go to 12.9mhz and 24.9mhz.
> The obvious explanation is that the length of cable between the amp
> output and the six pak input is being added to the stub, right?
> If so, also seem obvious that I should have the same length of cable
> between both of the amps and the six pak, right?
> If so, I should take this length into account, and things will change if
> I turn the amplifier online, offline, right?
> If so, how can I deal with it in situations where I have multiple stubs?
> For example: on 40m, I plan to have 2 1/4 wave shorted stubs nulling 14
> and 28mhz, and the 1/6 + 1/12 wave for nulling 21mhz
> If so, I have quite a terrible problem in hands.
> 73 and thanks in advance for your advice,
> Felipe - PY1NB
> TowerTalk mailing list
TowerTalk mailing list