Johnson Matchbox 250-24 VSWR Bridge Head
 

I have recently purchased two different VSWR Bridges.

One was made by Johnson.

The second is a clone.

Both use a fifty ohm resistor in series with the signal path. This
strikes me as a lossy setup. Is there a theory of operation posted or
a text which can explain the operation of the bridge?

Thanks

Harry

Johnson Matchbox 250-24 VSWR Bridge Head
 

On 24 Feb 2007 08:35:28 -0800, "harry"
wrote:

I have recently purchased two different VSWR Bridges.

One was made by Johnson.

The second is a clone.

Both use a fifty ohm resistor in series with the signal path. This
strikes me as a lossy setup. Is there a theory of operation posted or
a text which can explain the operation of the bridge?

Hi Harry,

You don't offer enough information. Some resistance is necessary in
the bridge to terminate the sample collected. It is often 50 Ohms,
and just as often it is not. The actual value is a product of the
bridge configuration.

73's
Richard Clark, KB7QHC

Johnson Matchbox 250-24 VSWR Bridge Head
 

"harry" wrote in
ups.com:

I have recently purchased two different VSWR Bridges.

One was made by Johnson.

The second is a clone.

Both use a fifty ohm resistor in series with the signal path. This
strikes me as a lossy setup. Is there a theory of operation posted or
a text which can explain the operation of the bridge?

Harry,

I am not familiar with the specific devices you mention, and you have
provided very little info.

I have made and used a device for impedance measurement that basically
had a 50 ohm resistor between tx and antenna sockets, and three
voltmeters to measure the RF voltage across the antenna, the internal 50
ohm resistor, and the combination. To make a measurement, you apply a
small amount of power at the desired frequency and measure the three
voltages. From those, you can calculate the antenna impedance, though
there is ambiguity about the sign of the antenna reactance.

If you have such a thing, they are for measurment only, you do not leave
them inline because of the losses, and of course, they will only
withstand very low power.

Do you think you may have such a thing?

Owen

Johnson Matchbox 250-24 VSWR Bridge Head
 

"harry" wrote in news:1172334927.954451.290220
@q2g2000cwa.googlegroups.com:

I have recently purchased two different VSWR Bridges.

One was made by Johnson.

The second is a clone.

Both use a fifty ohm resistor in series with the signal path. This
strikes me as a lossy setup. Is there a theory of operation posted or
a text which can explain the operation of the bridge?

The bridge is indeed a simple bridge with a single null indicator. It is
for use with less than 1W of RF, and is not intended to be left inline.

ftp://bama.sbc.edu/downloads/johnson/250-24/

Owen

Johnson Matchbox 250-24 VSWR Bridge Head
 

On Feb 24, 3:31 pm, Owen Duffy wrote:
"harry" wrote in news:1172334927.954451.290220
@q2g2000cwa.googlegroups.com:

I have recently purchased two different VSWR Bridges.

One was made by Johnson.

The second is a clone.

Both use a fifty ohm resistor in series with the signal path. This
strikes me as a lossy setup. Is there a theory of operation posted or
a text which can explain the operation of the bridge?

The bridge is indeed a simple bridge with a single null indicator. It is
for use with less than 1W of RF, and is not intended to be left inline.

ftp://bama.sbc.edu/downloads/johnson/250-24/

Owen

Owen. thanks for the link on the BAMA server. It cleared up a lot of
questions.

What threw for the loop was the coupler has to be removed one the
measurement is completed.
I had assumed (and you know what they say about that word) it was like
other couplers and
remained in circuit.

I had not tried either one since obviously the 2 watt resistors were
going to go poof, if used
like its more modern cousins.

I apologize to all for the lack of info and will be more diligent in
the future.

Harry