Jim wrote:




I have an external RF transistor amplifier (AB1) using 2 MRF455
transistors in common emitter mode. I have a resistive T pad with an
approximate impedance of 50 ohms on the input side of the transistors.
The SWR on the input of the amp shows 1.3:1. When I use a 2 foot
jumper between the radio and the amp the amp puts out about 100 watts
max, but when I switch to a 9 foot jumper the amp puts out about 40
watts max. The receive stays the same no matter which jumper is used.
Why does changing the length of the jumper between the radio and the
amp make such a drastic change in how much the amp puts out?

Are you sure the 9 foot jumper is good?

What frequency is this occurring at?

Is the SWR 1.3:1 for both jumpers? Measure the SWR, forward power, and
reflected power levels at the transmitter end for both jumpers and then
measure it at the amplifier end for both jumpers. See if everything stays
the same for both jumpers.

Do you have an RF probe you can use to measure the voltages at each end of
the link?

Unless you are using rg174 the 7 foot of extra cable should not make this
much difference unless the input impedance of the amplifer is not 50ohm
resistive. If it is not purely resistive then changing the cable length can
impact the SWR seen at the transmitter end significantly. This could cause
foldback in the RF amplifier in the transmitter thus affecting the output
of the amplifier. This would all be seen in the measurements of SWR,
forward power, and reflected power levels in the measurements above.

Let us know what you find.

tim ab0wr

tim gorman wrote:
. . .
Unless you are using rg174 the 7 foot of extra cable should not make this
much difference unless the input impedance of the amplifer is not 50ohm
resistive. If it is not purely resistive then changing the cable length can
impact the SWR seen at the transmitter end significantly. . .

Changing the cable length won't change the SWR on the cable regardless
of the kind of load impedance and, if the SWR meter is designed for the
cable's Z0, it won't change the SWR meter reading, either. Except, of
course, that cable loss will always lower the SWR -- but that shouldn't
be a significant factor with such short cables.

Changing the cable length *will* change the impedance looking into the
cable, whether or not the load is purely resistive. The only exception
to this is if the load is resistive *and equal to the line's
characteristic impedance* in which case the impedance looking in will be
Z0 for any length cable.

Transmitters will often put up with some mismatched impedances better
than others, even if the SWR is the same, and sometimes changing the
cable length between it and a mismatched load will cause it to see a
more or less favorable impedance. But if the SWR really is 1.3:1, I
doubt that's the cause of this problem.

I agree with the suggestion that the OP measure the SWR and if possible
the power at both ends with both cables. Something else is going on,
like maybe a bad cable or connector.

I don't think the OP said what frequency this is happening at. That
might give some additional clues.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
tim gorman wrote:


I agree with the suggestion that the OP measure the SWR and if possible
the power at both ends with both cables. Something else is going on,
like maybe a bad cable or connector.

I am assuming this is VHF and if so I would say that there is a bad
connector at both ends - PL259s. These are not good enough connenctors for
use anywhere above HF, the are simply screened banana plugs. If this is a
ready made jumper for the CB market, the cable might not be anything like 50
ohms.

Mike

From: Roy Lewallen on Jul 13, 12:39 am


tim gorman wrote:
. . .
Unless you are using rg174 the 7 foot of extra cable should not make this
much difference unless the input impedance of the amplifer is not 50ohm
resistive. If it is not purely resistive then changing the cable length can
impact the SWR seen at the transmitter end significantly. . .


I agree with the suggestion that the OP measure the SWR and if possible
the power at both ends with both cables. Something else is going on,
like maybe a bad cable or connector.

I don't think the OP said what frequency this is happening at. That
might give some additional clues.

Good suggestions on simple checking of a cable AND connectors.

The six-decade-old design of the PL-259 is not the best on
"wiping" action of contacts on the sleeve (outer conductor
portion). It is a license-free standard design, standard
because it is relatively cheap. Cheap silver plating can
corrode fairly easily (rhodium flash over silver plating is
much better but costs more)

Yesterday my "ancient" HP-722 inkjet printer had no primary
power. Check of the AC cord, external supply, said that was
okay. Turns out that the coaxial connection to the back of the
printer had developed some kind of minor corrosion. Simple
unplugging and re-plugging that DC connector brought back
primary power.

Three decades ago I was involved in a seemingly "unsolveable"
problem in Navy-flown L-Band R&D system. Signals would just
cut out at altitude, said altitude varying depending on day
of flight test. System AND RF cables (to top and bottom
fuselage antennas) all checked out fine on the ground. Nothing
intermittent. Blade antennas were taken off and checked
okay, put back. All type N connectors, good ones. Trouble was
in an unlikely form of a "doubler plate."

"Doubler plates" are often used in retro-fitting antennas and
other things on aircraft, just a sheet of metal to re-enforce
strength of the metal skin. The doubler plate drawings had
clearance holes just too close to type N connector sleeve
outer diameter. Connectors mated, but NOT fully. As altitude
increases, temperature drops. The not-fully mated center
conductor pin just contracted until it lost contact at cold
temps. Enlarging the doubler plate clearance hole allowed
full mating, no shrinkage of contacts. Unlikely problem
solved at about quarter to 8 PM in a cold hangar. :-(

Sometimes the "unlikely" not-described-in-text things are to
blame.

wrote:
. . .
"Doubler plates" are often used in retro-fitting antennas and
other things on aircraft, just a sheet of metal to re-enforce
strength of the metal skin. The doubler plate drawings had
clearance holes just too close to type N connector sleeve
outer diameter. Connectors mated, but NOT fully. As altitude
increases, temperature drops. The not-fully mated center
conductor pin just contracted until it lost contact at cold
temps. Enlarging the doubler plate clearance hole allowed
full mating, no shrinkage of contacts. Unlikely problem
solved at about quarter to 8 PM in a cold hangar. :-(

Sometimes the "unlikely" not-described-in-text things are to
blame.

I was in Anchorage when color TV was first being broadcast there
(mid-'60s) and people were using outside antennas with coax feed for the
first time. The center pin of a male type F connector is just the center
conductor of the RG-59 cable, and on a cold day the center conductor
would shrink enough on a long cable run to pull the pin out of the
female connector.

Anybody who has spent some time as a technician (as I did) has a long
list of tales to tell about strange problems. Among my favorites are a
car with the battery installed backward (a Corvair -- car started and
ran, but gauges were funky and the radio blew), and a piece of copper
pipe with 300 ohms DC resistance. But those are just a couple. . .

Roy Lewallen, W7EL

Roy Lewallen wrote:



tim gorman wrote:
. . .
Unless you are using rg174 the 7 foot of extra cable should not make this
much difference unless the input impedance of the amplifer is not 50ohm
resistive. If it is not purely resistive then changing the cable length
can impact the SWR seen at the transmitter end significantly. . .

Changing the cable length won't change the SWR on the cable regardless
of the kind of load impedance and, if the SWR meter is designed for the
cable's Z0, it won't change the SWR meter reading, either. Except, of
course, that cable loss will always lower the SWR -- but that shouldn't
be a significant factor with such short cables.

Changing the cable length *will* change the impedance looking into the
cable, whether or not the load is purely resistive. The only exception
to this is if the load is resistive *and equal to the line's
characteristic impedance* in which case the impedance looking in will be
Z0 for any length cable.



Roy Lewallen, W7EL


I repeat, changing the cable length will change the swr *seen* at the
transmitter end significantly. As w7el says, changing the cable length
will change the impedance looking into the cable. Since most common,
inexpensive swr meters are calibrated for use with a 50ohm load, the actual
swr readings will be dependent upon the impedance presented to the swr
meter while looking into the cable. Although the actual swr may not change,
the swr that you see may very well change. A 7 foot stub represents an
eighth wavelength all the way down to 60m. This is enough to present an
impedance change.

tim ab0wr