What would cause the SWR to go up drastically when I turn my amp on?
 

This is a very screwy problem. First lets get the set-up of my
radio equipment;

The amp puts out about 100 watts on HF.

RADIO-----2 TRANSISTOR AMP-----SWR METER------ ANTENNA

Note the SWR meter comes AFTER the 2 transistor amp.

Ok, when I check the SWR on my antenna it shows 1.1. When I turn the
amp on it shows over 3.x SWR. The antenna is rated at 5 KW. The amp
only puts out about 100 watts.

Hmmm.....Ok lets see what happens when I use my 1 KW oil filled dummy
load.


RADIO----- 2 TRANSISTOR AMP----SWR METER-------1 KW DUMMY LOAD

Same thing. The SWR is about 1.5 in to the dummy load, but as soon as
I turn the amp on the SWR meter shows 3.x plus.

Ok, Maybe my SWR meter is screwy, so lets use my Dentron MT3000
tuner with a built in 200 watt dry dummy load and built in SWR meter.


RADIO ----2 TRANSISTOR AMP----DENTRON MT3000

Same thing. The SWR is low without the amp on and goes high when the
amp gets turned on.


Ok, Lets see what happens when I use my Drake L-4B amp instead of the
2 transistor amp.


RADIO-----Drake L-4B-----SWR METER-------Dummy load

Now the SWR stays low when I turn on the Drake L-4B for both the 1 KW
dummy load and the external antenna.

The transistor uses MRF455 transistors. Why would the SWR meter go up
significantly when I turn on the 2 transisor amp, but stay low when I
turn on a Drake L-4B.

I've duplicated this scenario with three different SWR meters, and
different patch cables. Lets me say for the record the SWR meter
comes after the amp, so I'm not looking at a high SWR on the input
side of the amp. This is on the output side of the amp.

?????

Joeseph wrote:


This is a very screwy problem. First lets get the set-up of my
radio equipment;

The amp puts out about 100 watts on HF.

RADIO-----2 TRANSISTOR AMP-----SWR METER------ ANTENNA

Note the SWR meter comes AFTER the 2 transistor amp.

Ok, when I check the SWR on my antenna it shows 1.1. When I turn the
amp on it shows over 3.x SWR. The antenna is rated at 5 KW. The amp
only puts out about 100 watts.

Hmmm.....Ok lets see what happens when I use my 1 KW oil filled dummy
load.


RADIO----- 2 TRANSISTOR AMP----SWR METER-------1 KW DUMMY LOAD

Same thing. The SWR is about 1.5 in to the dummy load, but as soon as
I turn the amp on the SWR meter shows 3.x plus.

Ok, Maybe my SWR meter is screwy, so lets use my Dentron MT3000
tuner with a built in 200 watt dry dummy load and built in SWR meter.


RADIO ----2 TRANSISTOR AMP----DENTRON MT3000

Same thing. The SWR is low without the amp on and goes high when the
amp gets turned on.


Ok, Lets see what happens when I use my Drake L-4B amp instead of the
2 transistor amp.


RADIO-----Drake L-4B-----SWR METER-------Dummy load

Now the SWR stays low when I turn on the Drake L-4B for both the 1 KW
dummy load and the external antenna.

The transistor uses MRF455 transistors. Why would the SWR meter go up
significantly when I turn on the 2 transisor amp, but stay low when I
turn on a Drake L-4B.

I've duplicated this scenario with three different SWR meters, and
different patch cables. Lets me say for the record the SWR meter
comes after the amp, so I'm not looking at a high SWR on the input
side of the amp. This is on the output side of the amp.

?????




Check the frequency (or frequencies) coming out of the amplifier. I will bet
that what comes out is not equal to what goes in. The antenna is not a
good match for those other frequencies.

Bill k7NOM

Bill Janssen wrote:

Joeseph wrote:


This is a very screwy problem. First lets get the set-up of my
radio equipment;

The amp puts out about 100 watts on HF.

RADIO-----2 TRANSISTOR AMP-----SWR METER------ ANTENNA

Note the SWR meter comes AFTER the 2 transistor amp.

Ok, when I check the SWR on my antenna it shows 1.1. When I turn the
amp on it shows over 3.x SWR. The antenna is rated at 5 KW. The amp
only puts out about 100 watts.

Hmmm.....Ok lets see what happens when I use my 1 KW oil filled dummy
load.


RADIO----- 2 TRANSISTOR AMP----SWR METER-------1 KW DUMMY LOAD

Same thing. The SWR is about 1.5 in to the dummy load, but as soon as
I turn the amp on the SWR meter shows 3.x plus.

Ok, Maybe my SWR meter is screwy, so lets use my Dentron MT3000
tuner with a built in 200 watt dry dummy load and built in SWR meter.


RADIO ----2 TRANSISTOR AMP----DENTRON MT3000

Same thing. The SWR is low without the amp on and goes high when the
amp gets turned on.


Ok, Lets see what happens when I use my Drake L-4B amp instead of the
2 transistor amp.

RADIO-----Drake L-4B-----SWR METER-------Dummy load

Now the SWR stays low when I turn on the Drake L-4B for both the 1 KW
dummy load and the external antenna.

The transistor uses MRF455 transistors. Why would the SWR meter go up
significantly when I turn on the 2 transisor amp, but stay low when I
turn on a Drake L-4B.

I've duplicated this scenario with three different SWR meters, and
different patch cables. Lets me say for the record the SWR meter
comes after the amp, so I'm not looking at a high SWR on the input
side of the amp. This is on the output side of the amp.
?????




Check the frequency (or frequencies) coming out of the amplifier. I will
bet
that what comes out is not equal to what goes in. The antenna is not a
good match for those other frequencies.

Bill k7NOM

What Bill is trying to tell you is that your amplifier is probably
oscillating or putting out way bad harmonics. That certainly fits the
symptoms with the antenna attached; it's a bit surprising with your
dummy load unless the dummy load is only good to two meters (chances are
your oscillation is higher than 2M, and harmonics certainly would be).
You could also see odd results if the SWR meters aren't good beyond 2M,
but I don't know which direction they tend to go with way high frequencies.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Thu, 14 Jul 2005 15:31:14 GMT, Bill Janssen
wrote:



Check the frequency (or frequencies) coming out of the amplifier. I will bet
that what comes out is not equal to what goes in. The antenna is not a
good match for those other frequencies.

Bill k7NOM


Smart guy you are! I never thought of that, but it makes perfect
sense. I don't have a spectrum analyzer, but since the 2nd harmonic
and higher lands above 30 MHz then I should be able to place my Barker
and Williamson low pass filter (30 MHz cut off) between the amp and
the SWR meter. Hopefully the low pass filter will allow the
fundamental through, and stop the higher frequencies (assuming they
are normal multiples of the fundamental frequency). If the
attenuation is great enough the SWR meter should not show the SWR jump
since the unwanted frequencies should be attenuated before they get to
the SWR meter.
Assuming you are correct my next problem will stopping the extra
frequencies from getting out of the amp in the first place.

J.

Your transistor amp is putting out garbage; transmitting harmonics that your
SWR Meter is responding to. You need tuning/lowpass filters on both input
and output.



"Joeseph" wrote in message
...



This is a very screwy problem. First lets get the set-up of my
radio equipment;

The amp puts out about 100 watts on HF.

RADIO-----2 TRANSISTOR AMP-----SWR METER------ ANTENNA

Note the SWR meter comes AFTER the 2 transistor amp.

Ok, when I check the SWR on my antenna it shows 1.1. When I turn the
amp on it shows over 3.x SWR. The antenna is rated at 5 KW. The amp
only puts out about 100 watts.

Hmmm.....Ok lets see what happens when I use my 1 KW oil filled dummy
load.


RADIO----- 2 TRANSISTOR AMP----SWR METER-------1 KW DUMMY LOAD

Same thing. The SWR is about 1.5 in to the dummy load, but as soon as
I turn the amp on the SWR meter shows 3.x plus.

Ok, Maybe my SWR meter is screwy, so lets use my Dentron MT3000
tuner with a built in 200 watt dry dummy load and built in SWR meter.


RADIO ----2 TRANSISTOR AMP----DENTRON MT3000

Same thing. The SWR is low without the amp on and goes high when the
amp gets turned on.


Ok, Lets see what happens when I use my Drake L-4B amp instead of the
2 transistor amp.


RADIO-----Drake L-4B-----SWR METER-------Dummy load

Now the SWR stays low when I turn on the Drake L-4B for both the 1 KW
dummy load and the external antenna.

The transistor uses MRF455 transistors. Why would the SWR meter go up
significantly when I turn on the 2 transisor amp, but stay low when I
turn on a Drake L-4B.

I've duplicated this scenario with three different SWR meters, and
different patch cables. Lets me say for the record the SWR meter
comes after the amp, so I'm not looking at a high SWR on the input
side of the amp. This is on the output side of the amp.

?????

Joeseph wrote:
On Thu, 14 Jul 2005 15:31:14 GMT, Bill Janssen
wrote:


Check the frequency (or frequencies) coming out of the amplifier. I will bet
that what comes out is not equal to what goes in. The antenna is not a
good match for those other frequencies.

Bill k7NOM



Smart guy you are! I never thought of that, but it makes perfect
sense. I don't have a spectrum analyzer, but since the 2nd harmonic
and higher lands above 30 MHz then I should be able to place my Barker
and Williamson low pass filter (30 MHz cut off) between the amp and
the SWR meter. Hopefully the low pass filter will allow the
fundamental through, and stop the higher frequencies (assuming they
are normal multiples of the fundamental frequency). If the
attenuation is great enough the SWR meter should not show the SWR jump
since the unwanted frequencies should be attenuated before they get to
the SWR meter.
Assuming you are correct my next problem will stopping the extra
frequencies from getting out of the amp in the first place.

J.

Be sure to verify that it's not oscillating, and that any excessive
harmonics don't indicate some other problem with the amp (if you're in
the process of homebrewing it then it just means you forgot the dang
lowpass filter, but if the amp is a commercial unit & you're using it as
intended then there may be something broke).

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Joeseph wrote:
, , ,
Assuming you are correct my next problem will stopping the extra
frequencies from getting out of the amp in the first place.

No, your problem should be figuring out how to prevent them from getting
generated in the first place. If the harmonic or spurious content is
that high, there's something seriously wrong.

Roy Lewallen, W7EL

RADIO-----Drake L-4B-----SWR METER-------Dummy load

Now the SWR stays low when I turn on the Drake L-4B for both the 1 KW
dummy load and the external antenna.

The transistor uses MRF455 transistors. Why would the SWR meter go up
significantly when I turn on the 2 transisor amp, but stay low when I
turn on a Drake L-4B.

If I had to guess, I'd guess that your MRF455 amplifier may have some
instability issues, and may have a nasty parasitic oscillation going
on. You might also want to check its grounding, and the RF relay
which I presume switches it into or out of the circuit. I'm guessing
that you've got RF flowing where it shouldn't.

I picked up a few MRF455 transistors last year and have toyed with the
idea of using them as a PA in a homebrew rig, but from what I can see
in the application notes they require a goodly amount of care in the
amplifier design and circuit layout. If they aren't used properly it
looks to me as if they can be quite prone to misbehave, oscillate, etc.

If you can borrow a spectrum analyzer, take a careful look at the
amp's output, and wave a probe around the amp itself (prior to the
low-pass output filter). It would not surprise me if you find that
you're spurring the horse a bit :-)

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Thu, 14 Jul 2005 10:31:05 -0400, Joeseph
wrote:


Thanks for all your help guys. I think you've hit the nail on the
head. Now it is time for me to do some more reading, and fix this
puppy.

J

After reading all the other replies up to this moment, let me ask if you:
1. remove the radio (i.e. run the amp without the radio connected at all)
what do you get (if any reading of forward and/or reflected power), and
2. leave the radio connected to the amp, but run the amp without drive
power at all, then what do you get?
3. Can you vary the drive level (say down to lower levels) and still see
the SWR go up.
From what was written below, its not clear for sure if you are implying
that the amp did have a proper level of RF drive or not. But, I'd still be
interested in knowing if you get forward power and/or reflected power with
the amp turned on but without drive. I've actually had amplifiers where
the application of power and certain tuning conditions cause extra
frequencies to come out in addition to what was supposed to come out. And,
sometimes I got self-oscillation if the amplifier was NOT being driven
with input drive.

------------- No change to below, included for reference and context---

On Thu, 14 Jul 2005, Joeseph wrote:

Date: Thu, 14 Jul 2005 10:31:05 -0400
From: Joeseph
Newsgroups: rec.radio.amateur.homebrew, rec.radio.amateur.equipment
Subject: What would cause the SWR to go up drastically when I turn my amp on?




This is a very screwy problem. First lets get the set-up of my
radio equipment;

The amp puts out about 100 watts on HF.

RADIO-----2 TRANSISTOR AMP-----SWR METER------ ANTENNA

Note the SWR meter comes AFTER the 2 transistor amp.

Ok, when I check the SWR on my antenna it shows 1.1. When I turn the
amp on it shows over 3.x SWR. The antenna is rated at 5 KW. The amp
only puts out about 100 watts.

Hmmm.....Ok lets see what happens when I use my 1 KW oil filled dummy
load.


RADIO----- 2 TRANSISTOR AMP----SWR METER-------1 KW DUMMY LOAD

Same thing. The SWR is about 1.5 in to the dummy load, but as soon as
I turn the amp on the SWR meter shows 3.x plus.

Ok, Maybe my SWR meter is screwy, so lets use my Dentron MT3000
tuner with a built in 200 watt dry dummy load and built in SWR meter.


RADIO ----2 TRANSISTOR AMP----DENTRON MT3000

Same thing. The SWR is low without the amp on and goes high when the
amp gets turned on.


Ok, Lets see what happens when I use my Drake L-4B amp instead of the
2 transistor amp.


RADIO-----Drake L-4B-----SWR METER-------Dummy load

Now the SWR stays low when I turn on the Drake L-4B for both the 1 KW
dummy load and the external antenna.

The transistor uses MRF455 transistors. Why would the SWR meter go up
significantly when I turn on the 2 transisor amp, but stay low when I
turn on a Drake L-4B.

I've duplicated this scenario with three different SWR meters, and
different patch cables. Lets me say for the record the SWR meter
comes after the amp, so I'm not looking at a high SWR on the input
side of the amp. This is on the output side of the amp.

?????

Thanks for all your help guys. I think you've hit the nail on the
head. Now it is time for me to do some more reading, and fix this
puppy.

J

I have not seen much description on the solid state amp except that it has
MRF455 transistors. Because these devices were a favorite in the manufacture
of CB amplifiers, I am suspicious that the only output filtering is a HPF
set to cutoff around 30MHz.
So, running this amp on the lower HF bands would allow a lot of harmonic
energy to appear at the output.

Dale W4OP

RADIO-----2 TRANSISTOR AMP-----SWR METER------ ANTENNA

Ok, when I check the SWR on my antenna it shows 1.1. When I turn the
amp on it shows over 3.x SWR. The antenna is rated at 5 KW. The amp
only puts out about 100 watts.

When I had this it was caused by arcing at the antenna. VSWR perfect with
low power, but it even made an audible noise when you screwed the power up.

1 - Try this using a band-pass filter:
RADIO--2 TRANSISTOR AMP--SWR METER--FILTER--DUMMY
If amp churns out **** then the filter will reflect it. If VSWR is good,
then:

2 - Eliminate your feeder (could be breaking down):
RADIO--2 TRANSISTOR AMP--SWR METER--FEEDER--DUMMY
When you do this you may even see a duff contact/joint.
If the feeder insulation is breaking down then VSWR rises again. If not
then:

3 - Check out the antenna / balun / feed points for strands of wire, smoke,
etc.

To think in terms of SWR readings is a waste of time and effort.

You are all confusing yourselves. Furiously digging yourselves even
further into the mire.

Go back to square one and begin again from Ohm's Law.
----
Reg

"Roy Lewallen" wrote in message
...
Joeseph wrote:
, , ,
Assuming you are correct my next problem will stopping the extra
frequencies from getting out of the amp in the first place.

No, your problem should be figuring out how to prevent them from getting
generated in the first place. If the harmonic or spurious content is that
high, there's something seriously wrong.

Nice answer, Roy. That was my knee-jerk, too.

Joseph, remember, the problem isn't only "stopping them from getting out of
the amp...". Those other frequencies are some sort of parasitic in the
amplifier. Besides being stuff you don't want going to the antenna, they
are robbing your amp of the ability to amplify the frequencies you DO want
amplified, and they are unnecessarily heating up your transistors and
sapping their life.

So first, keep them from being generated, then stiffen up that low pass
filter so anything that does get created is way, way down. Far lower than
what will show up as an apparent SWR problem.

...

On Fri, 15 Jul 2005 19:12:06 +0000 (UTC), "Reg Edwards"
wrote:

To think in terms of SWR readings is a waste of time and effort.

You are all confusing yourselves. Furiously digging yourselves even
further into the mire.

Go back to square one and begin again from Ohm's Law.
----
Reg


Actually they are correct. I ran an experiment using a Barker and
Williamson low pass filter with a 32 MHz cutoff.

With the set-up below the SWR went high when the amp was turned on.

RADIO===AMP===SWR METER====ANTENNA

With the set-up below the SWR stayed at 1.1:1 with and without the amp
on.

RADIO===AMP===LOW PASS FILTER===SWR METER===ANTENNA

Also the output of the amp showed about 110 watts without the low pass
filter, but with the low pass filter the watt meter installed after
the low pass filter showed about 75 watts. That means about 30 watts
was being transmitted above 30 MHz even through the fundamental was at
28 MHz.

I doubled the value of the capacitors going form the collector to
ground, and the SWR dropped to 1.5:1 with the amp on and 1.1:1 with
the amp off. Then I add 470 pf capacitors from the trasnsitor base to
ground and the SWR with the amp dropped to 1.3:1 with the amp on.
With the TVI filter in-line the SWR is 1.1:1 with and without the amp
on.

A lack of working space inside the amp case made it difficult to
install a pi-network on the input and output side of the transistor
finals.

Thanks for everyone's help. The SWR increased was caused by harmonics
above the fundamental frequency, and they were outside the bandwidth
of the antenna.

Thanks for sharing your results with the rest of us. That's a good
experiment you described. The only "final clincher" would be if you had a
receiver that covered the spectrum where you might find out what the
spurious signal was that was being generated. Most power transistor
circuits I've seen have all kinds of weird bypassing, ferrite beads on
power-carrying lines, funny adjustable variable capacitors in funny places
in the circuit. Good luck otherwise.

===== no change to below, included for reference and context =====

On Mon, 25 Jul 2005, Joeseph wrote:

On Fri, 15 Jul 2005 19:12:06 +0000 (UTC), "Reg Edwards"
wrote:

To think in terms of SWR readings is a waste of time and effort.

You are all confusing yourselves. Furiously digging yourselves even
further into the mire.

Go back to square one and begin again from Ohm's Law.
----
Reg


Actually they are correct. I ran an experiment using a Barker and
Williamson low pass filter with a 32 MHz cutoff.

With the set-up below the SWR went high when the amp was turned on.

RADIO===AMP===SWR METER====ANTENNA

With the set-up below the SWR stayed at 1.1:1 with and without the amp
on.

RADIO===AMP===LOW PASS FILTER===SWR METER===ANTENNA

Also the output of the amp showed about 110 watts without the low pass
filter, but with the low pass filter the watt meter installed after
the low pass filter showed about 75 watts. That means about 30 watts
was being transmitted above 30 MHz even through the fundamental was at
28 MHz.

I doubled the value of the capacitors going form the collector to
ground, and the SWR dropped to 1.5:1 with the amp on and 1.1:1 with
the amp off. Then I add 470 pf capacitors from the trasnsitor base to
ground and the SWR with the amp dropped to 1.3:1 with the amp on.
With the TVI filter in-line the SWR is 1.1:1 with and without the amp
on.

A lack of working space inside the amp case made it difficult to
install a pi-network on the input and output side of the transistor
finals.

Thanks for everyone's help. The SWR increased was caused by harmonics
above the fundamental frequency, and they were outside the bandwidth
of the antenna.