If I put a T-connector on my feedline and a 50 ohm dummy load on each
leg, does the transmitter see 50 ohms, or 25 ohms?

If one leg is not being used, what should be put there (if anything)
for the transmitter to see 50 ohms if the impedance of the other leg
is 50 ohms?

Ken KC2JDY

Ken C wrote:
If I put a T-connector on my feedline and a 50 ohm dummy load on each
leg, does the transmitter see 50 ohms, or 25 ohms?

If one leg is not being used, what should be put there (if anything)
for the transmitter to see 50 ohms if the impedance of the other leg
is 50 ohms?

Ken KC2JDY

Ken;

50 ohms.

Jim

On 8 Mar 2006 04:17:08 -0800, "Jim" wrote:


Ken C wrote:
If I put a T-connector on my feedline and a 50 ohm dummy load on each
leg, does the transmitter see 50 ohms, or 25 ohms?

If one leg is not being used, what should be put there (if anything)
for the transmitter to see 50 ohms if the impedance of the other leg
is 50 ohms?

Ken KC2JDY

Ken;

50 ohms.

This might be a solution to all of our mismatch problems. Repeal
Ohm's law.

Ken C wrote:

If I put a T-connector on my feedline and a 50 ohm dummy load on each
leg, does the transmitter see 50 ohms, or 25 ohms?

If one leg is not being used, what should be put there (if anything)
for the transmitter to see 50 ohms if the impedance of the other leg
is 50 ohms?

Ken KC2JDY

Case #1 two 50 ohms in parallel plus a transmission line 25 to 100
ohms and everything in between [VSWR = 2:1]. If there are no
transmission line effects [very short line] then 25 ohms.

Case #2 one 50 ohm = 50 ohms including transmission line effects.

Ken,

I guess that you are using the T-connector simply as an elbow? If this
is the case, and you put a 50 ohm load on one leg, you should leave the
other leg *open* to make the impedance be about 50 ohms. The
T-connector will give you a little bit of capacitance due to the
concentric conductors on the unused leg, but it will make little to no
difference until you hit UHF or so.

If you put 50 ohms on one leg and a 50 ohm load on the other, Amos's
post will apply. The impedance seen by the transmitter will depend on
the length of transmission line between the T and your transmitter.
This is because the line between the T and your transmitter is no
longer properly terminated at the T end.

With very short transmission lines, the situation is just like parallel
resistors. This means that the two loads on the T will share the
power.

This is good for things like phased yagis, but you wouldn't want to
hook an antenna to one port and a dummy to another...

Dan,
N3OX
www.n3ox.net

On Wed, 08 Mar 2006 06:48:29 -0500, Ken C
wrote:

If I put a T-connector on my feedline and a 50 ohm dummy load on each
leg, does the transmitter see 50 ohms, or 25 ohms?

Not necessarily either.

The T connects the branches in parallel, so if you have a 50+j0 load
and any length of 50 ohm coax connected to each of two branches, the
impedance at the T is 25 ohms. If you connect to T to something else
(eg a transmitter) with 50 ohm coax, the SWR on that section will
result in impedance transformation that depends on the coax (vf),
frequency, and length. For example, if you had 3m of RG58C/U at 7MHz,
the impedance looking into the cable would be 36+j24.

You can solve the transmission line transformation with the calculator
at http://www.vk1od.net/tl/tllce.php .

If one leg is not being used, what should be put there (if anything)
for the transmitter to see 50 ohms if the impedance of the other leg
is 50 ohms?

You know the answer to that now, the branches of the T are in
parallel, leave one branch open circuit.

Owen
--

wrote in message
oups.com...
Ken,

I guess that you are using the T-connector simply as an elbow? If this
is the case, and you put a 50 ohm load on one leg, you should leave the
other leg *open* to make the impedance be about 50 ohms. The
T-connector will give you a little bit of capacitance due to the
concentric conductors on the unused leg, but it will make little to no
difference until you hit UHF or so.

If you put 50 ohms on one leg and a 50 ohm load on the other, Amos's
post will apply. The impedance seen by the transmitter will depend on
the length of transmission line between the T and your transmitter.
This is because the line between the T and your transmitter is no
longer properly terminated at the T end.

With very short transmission lines, the situation is just like parallel
resistors. This means that the two loads on the T will share the
power.


Regardless of the T-line length, it will be a 2:1 mismatch. If the line
is 1/4 wave long, then the Tx will "see" 100 ohms.

Leaving one end of the "T" open will provide no significant effects up
through UHF, but it does represend a very short section of open line in
parallel, which, as Dan reports, will show as a small capacitance.


This, by the way is a good way to check an SWR meter for accuracy,
providing, of course, the loads you use are pretty good as well. You can
easily do 2:1 and 4:1 (with two "T"s and four loads) if you have them.

P.s. If you needed to, and it was a good 50 ohms, you could use an antenna
for one of the loads.

73, Steve, K9DCI



This is good for things like phased yagis, but you wouldn't want to
hook an antenna to one port and a dummy to another...

Dan,
N3OX
www.n3ox.net